i NICKEL OXIDE DOPED NOBLE METALS SUPPORTED CATALYSTS FOR

NICKEL OXIDE DOPED NOBLE METALS SUPPORTED CATALYSTS FOR

CARBON DIOXIDE METHANATION AND DESULFURIZATION REACTIONS

SUSILAWATI BT TOEMEN

UNIVERSITI TEKNOLOGI MALAYSIA i

NICKEL OXIDE DOPED NOBLE METALS SUPPORTED CATALYSTS FOR

CARBON DIOXIDE METHANATION AND DESULFURIZATION REACTIONS

SUSILAWATI BT TOEMEN

A thesis submitted in fulfilment of the requirements for the award of the degree of

Master of Science (Chemistry)

Faculty of Science

Universiti Teknologi Malaysia

JANUARY 2011 i

iii

BISMILLAHIRROHMANIRROHIM.....

A Special Dedication to my beloved family especially mum and dad for their everlasting support

iv

ACKNOWLEDGEMENTS

In the name of Allah, the Most Gracious, the Most Merciful,

All praise to Allah, for His Mercy has given me patience and strength to complete this work. All the praise to Allah again.

Special thanks go to my supervisor, Prof. Dr. Wan Azelee Wan Abu Bakar as my main supervisor, and Associate Professor Dr. Rusmidah Ali as my co-supervisor for their invaluable advice and encouragement during the course of this research.

Without their time and patience much of this work could not have been accomplished.

A million thanks also go to all lecturers and laboratory staffs of Faculty

Science, Faculty of Mechanical Engineering and Institute Ibnu Sina also to my entire fellow friends especially to Fatirah, Nooradilah, Mariami Alisa and Noor Safina

Sulaiman for their technical cooperation, knowledge, encouragement and guidance throughout this research.

I am grateful to Universiti Teknologi Malaysia and Ministry of Science, Technology and Innovation Malaysia for financial support.

Last but not least, I wish to express my sincere appreciation to my beloved family and Megat Mohd Hazwan Yahya for their continuous support, advices and motivation for me to complete my research. Thanks to the love pouring into my life.

Thank you so much.

v

ABSTRACT

Malaysia has one of the most extensive natural gas pipeline networks in Asia.

Using pipeline system, the gas will be channelled to the onshore station where the natural gas will undergo separation of acidic gases. Nowadays, the removal of toxic gases such as H

2

S and CO

2

via chemical conversion attracted many researchers due to the effectiveness of the technique. In this research, new catalysts of high industrial impact that can catalyze the reactions of CO

2 methanation and H

2

S desulfurization were developed. A series of catalysts based on nickel oxide doped with ruthenium, rhodium, palladium and platinum were prepared. Then, the best two catalysts were subjected to undergo several optimizations such as different calcination temperature of catalysts, calcinations temperature of alumina, different support materials, preparation method, reproducibility testing and regeneration testing. Pd/Ru/Ni

(2:8:90)/Al

2

O

3 and Rh/Ni (30:70)/Al

2

O

3

catalysts exhibited the most potential catalysts resulted from the activity testing monitored by FTIR and GC. These catalysts were prepared using wetness impregnation technique, aging at 85 o

C and followed by calcination at 400 o

C. Both catalysts achieved 100% H

2

S desulfurization below 200 o

C. In the presence of H

2

S gas, only 3.64% CH

4

was produced over

Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst from 57.31% CO

2

conversion, while 0.5% CH

4

was obtained from 4.53% CO

2

conversion over Rh/Ni (30:70)/Al

2

O

3

at temperature of

400 o

C, respectively. However, the methane percentage increased to 39.73% for


2

O

3


2

conversion and 70.75% for

Rh/Ni (30:70)/Al

2

O

3


2

conversion during testing without flowing of H

2

S gas. Moreover, the XRD diffractogram showed that both catalysts are highly amorphous in structure with BET surface area in the range of 220-270 m

2 g

-1

.

FESEM analysis indicated a rough surface morphology and non-homogeneous spherical shape with the smallest particles size in the range 40-115 nm for fresh


2

O

3

catalyst and formation of aggregates with rough surface morphology for fresh Rh/Ni (30:70)/Al

2

O

3

catalyst. The elemental analysis performed by EDX confirmed the presence of Ni, Ru, Pd, Al and O in the Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst while, Ni, Rh, Al and O in the Rh/Ni (30:70)/Al

2

O

3

catalyst.

Characterization by FTIR and TGA-DTG revealed the existence of nitrate and hydroxyl ions on the catalysts surface.

vi

ABSTRAK

Malaysia mempunyai rangkaian pempaipan gas asli yang luas di Asia.

Dengan menggunakan sistem pempaipan, gas asli akan dibawa ke daratan bagi menjalankan proses pemisahan gas berasid. Pada masa kini, penyingkiran gas toksik seperti H

2

S dan CO

2

menggunakan penukaran kimia telah menarik perhatian banyak penyelidik kerana keberkesanannya. Dalam penyelidikan ini, mangkin baru berimpak industri yang tinggi telah dibangunkan bagi memangkinkan tindak balas methanasi

CO

2

dan penyahsulfuran H

2

S. Siri mangkin berasaskan nikel didopkan dengan rutenium, rodium, paladium dan platinum telah disediakan. Kemudian, dua mangkin terbaik akan menjalani beberapa pengoptimisinan seperti suhu pengkalsinan mangkin yang berbeza, suhu pengkalsinan bahan penyokong, pelbagai bahan penyokong, pelbagai kaedah penyediaan, ujian pemboleh ulangan dan ujian penjanaan semula.


2

O

3 dan Rh/Ni (30:70)/Al

2

O

3

merupakan mangkin yang berpotensi yang dikenal pasti daripada ujian aktiviti yang dipantau oleh FTIR dan

GC. Mangkin-mangkin ini disediakan menggunakan kaedah pengisitepuan, dikeringkan pada 85 o

C diikuti pengkalsinan pada suhu 400 o

C. Mangkin-mangkin ini mencapai 100% penyahsulfuran pada suhu bawah 200 o

C. Dalam kehadiran gas H

2

S ini, hanya 3.64% gas metana dihasilkan daripada 57.31% penukaran CO

2

bagi mangkin Pd/Ru/Ni (2:8:90)/Al

2

O

3

manakala, 0.5% gas metana dihasilkan oleh Rh/Ni

(30:70)/Al

2

O

3

daripada 4.53% penukaran CO

2

pada suhu 400 o

C, masing-masing.

Walau bagaimanapun, peratusan gas metana ini meningkat ke 39.73% bagi mangkin


2

O

3


2

dan 70.75% bagi mangkin

Rh/Ni (30:70)/Al

2

O

3


2

semasa ujian tanpa aliran gas

H

2

S. Selain itu, difraktogram XRD menunjukkan kedua-dua mangkin ini mempunyai struktur sangat amorfos dengan luas permukaan BET di dalam anggaran 220-270 m

2 g

-1

. Analisis FESEM mempamerkan struktur permukaan yang kasar dan ketidaksekataan bentuk dengan saiz zarah yang kecil di dalam julat 40-115 nm bagi mangkin Pd/Ru/Ni (2:8:90)/ Al

2

O

3

dan pembentukan agregat serta struktur permukaan yang kasar bagi mangkin Rh/Ni (30:70)/Al

2

O

3

. Analisis EDX telah mengenal pasti kewujudan element Ni, Ru, Pd, Al dan O dalam mangkin Pd/Ru/Ni

(2:8:90)/Al

2

O

3

manakala elemen Ni, Rh, Al dan O dalam mangkin Rh/Ni

(30:70)/Al

2

O

3

. Pencirian oleh FTIR dan TGA menunjukkan kewujudan ion nitrat dan hidroksil di atas permukaan mangkin.

vii

1

CHAPTER

2

TABLE OF CONTENTS

TITLE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

LIST OF APPENDICES

INTRODUCTION

1.1

Natural Gas

1.2

Acid Gas Treating Processes

1.3

Problem Statement

1.4

Significant of Study

1.5

Objectives of the Research

1.6

Scope of the Research

LITERATURE REVIEW

2.1

Introduction

2.2

Nickel Based Methanation Catalysts

2.3

Noble Metals Used in Methanation Reaction

2.4

Supports for the Methanation Catalysts

PAGE

7

9

1

4 xviii xx

9

10 vi vii xi xiv ii iii iv v

11

11

18

24

3 EXPERIMENTAL

3.1

Introduction

3.2

Chemicals and Reagents

3.3

Catalysts Preparation

3.4

Catalytic Reaction Conditions

3.5

Optimization Parameters

3.5.1

Amount of Nickel Loading

3.5.2

Calcination Temperature of Supported

Catalysts

3.5.3

Type of Support Materials

3.5.3.1

Preparation of Carbon Support from

Palm Kernel Shell (PKS)

3.5.4

Pre-Calcination Temperature of alumina support

3.5.5

Different Preparation Techniques

3.5.6

H

2

S Testing

3.5.7

Reproducibility Testing

3.5.8

Regeneration Activity

3.6

Methane Measurement via Gas Chromatography

3.7

Characterization

3.7.1

X-Ray Diffraction Spectroscopy (XRD)

3.7.2

Field Emission Scanning Electron Microscopy

- Energy Dispersive X-Ray (FESEM-EDX)

3.7.3

Nitrogen Absorption Analysis (NA)

3.7.4

Fourier Transform Infrared Spectroscopy

(FTIR)

3.7.5

Thermogravimetry Analysis-Differential

Thermal Analysis (TGA-DTA) viii

29

29

30

31

40

41

39

40

37

37

38

39

35

36

36

37

34

34

33

34

34

4 RESULTS AND DISCUSSION

4.1

Catalytic Testing Measurement

4.1.1

Catalytic Performance of Supported NiO

Based Catalyst with Ruthenium as a First

Dopant

4.1.2

Catalytic Performance of Supported NiO

Based Catalyst with Rhodium as a First

Dopant.

4.2

Optimization Parameter of Catalytic Performance

4.2.1

Effect of Nickel Loading

4.2.2

Effect of Different Calcination Temperature towards Supported Catalyst

4.2.3

Effect of Different Support Materials

4.2.4

Effect of Different Calcination Temperature towards Alumina Support

4.2.5

Effect of Different Methods of catalyst

Preparation

4.2.6

Effect of H

2

S Gas on the Alumina Supported

Catalyst

4.2.7

Reproducibility Testing towards Potential

Catalyst

4.2.8

Regeneration Testing on the Potential Catalyst

4.3

Methane Gas Measurement via Gas Chromatography

4.4

Catalyst Testing of CO

2

Methanation Reaction using

Two Reactors over Pd/Ru/Ni (2:8:90)/Al

2

O

3

Catalyst

4.5

Characterization of the Potential Catalysts

4.5.1

X-Ray Diffraction Analysis (XRD)

4.5.1.1

X-Ray Diffraction (XRD) Analysis over Pd/Ru/Ni (2:8:90)/Al

2

O

3

Catalyst

4.5.1.2

X-Ray Diffraction (XRD) Analysis over Rh/Ni (30:70)/Al

2

O

3

Catalyst

42

42

76

76

69

71

74

76

81

64

67

59

61

49

52

56

45

49 ix

5 x

4.5.2

Field Emission Scanning Electron Microscopy and Energy Dispersive X-Ray

4.5.2.1

FESEM Analysis over Pd/Ru/Ni

(2:8:90)/Al

2

O

3

Catalyst

4.5.2.2

FESEM Analysis over Rh/Ni

(30:70)/Al

2

O

3

Catalyst

4.5.3

Nitrogen Absorption Analysis (NA)

4.5.4

Fourier Transform Infra-Red (FTIR) Analysis

4.5.5

Thermogravimetry Analysis – Differential

Thermal Analysis (TGA-DTA)

CONCLUSIONS AND RECOMMENDATIONS

5.1

Conclusions

5.2

Recommendations

REFERENCES

APPENDICES (A-E) 115-119

97

99

88

91

94

100

102

84

84

xi

LIST OF TABLES

TABLE NO. TITLE

1.1

3.1

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

Chemical composition of crude natural gas from Telaga

Bergading, Petronas Carigali Sdn. Bhd.

Wavenumber of CO

2

, H

2

S, CO and CH

4

gasses in FTIR spectra

Percentage conversion of CO

2

from methanation reaction over various alumina supported nickel oxide based catalysts with ruthenium as a dopant and co-dopant


2

from methanation reaction over various alumina supported nickel oxide based catalysts with rhodium as a dopant and co-dopant

Comparison of CO

2

conversion from methanation reaction over Pd/Ru/Ni/Al

2

O

3

catalyst with different loading of nickel calcined at 400 o

C for 5 hours

Comparison of CO

2

conversion from methanation reaction over Rh/Ni/Al calcined at 400

2 o

O

3

catalyst with different loading of nickel

C for 5 hours

Comparison of CO

2

conversion from methanation reaction over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst calcined for 5 hours at four different calcination temperatures

Comparison of CO

2

conversion from methanation reaction over Rh/Ni (30:70)/Al

2

O

3

catalyst calcined for 5 hours at different calcination temperatures

Comparison of CO

2

conversion from methanation reaction over Pd/Ru/Ni catalyst with the ratio of 2:8:90 coated on various support materials then calcined at 400 o

C for 5 hours

Comparison of CO

2

conversion from methanation reaction over Rh/Ni catalyst with the ratio of 30:70 coated on various support materials then calcined at 400 o

C for 5 hours

PAGE

4

33

43

46

50

52

54

54

56

58

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

Comparison of CO

2

conversion from methanation reaction over Pd/Ru/Ni (2:8:90) catalyst coated on alumina which has been calcined for 5 hours at different temperature

Comparison of CO

2

conversion from methanation reaction over Rh/Ni (30:70) catalyst coated on alumina which has been calcined for 5 hours at different temperature

Comparison of CO

2


2

O

3

and Rh/Ni (30:70)/Al

2

O

3 catalysts prepared by different preparation methods and calcined at 400 o

C


2

from methanation reaction and H

2

S desulfurization over Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst calcined at 400 o

C for 5 hours testing with and without the presence of H

2

S gas


2


2

S desulfurization over Rh/Ni (30:70)/Al

2

O

3

catalyst calcined at 400 o


2

S gas

The product and by-product of CO

2

methanation reaction over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst detected via GC


2

methanation reaction over Rh/Ni (30:70)/Al

2

O

3



2

methanation reaction using two reactors over Pd/Ru/Ni (2:8:90)/Al

2

O

3


Peaks assignment in the X-ray diffraction patterns of


2

O

3


C for 5 hours

Peaks assignment in the X-ray diffraction patterns of Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours


(30:70)/Al

2

O

3


C for 5 hours


(30:70)/Al

2

O

3


C for 5 hours

EDX analysis of fresh and spent Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalysts, calcined at 400 o

C for 5 hours

80

83

84

87

72

73

66

75

79

65

62

59

60 xii

4.22

4.23

4.24

EDX analysis of fresh and spent Rh/Ni (30:70)/Al

2

O


C for 5 hours

BET surface area and average pore diameter of the fresh and spent Pd/Ru/Ni (2:8:90)-Al

2

O

3


2

O

3 catalysts calcined at 400 o

C for 5 hours

FTIR analysis of Pd/Ru/Ni (2:8:90)/Al

2

O

3

and Rh/Ni

(30:70)/Al

2

O

3

catalysts calcined at 400 o

C for 5 hours

90

91

96 xiii

LIST OF FIGURES

FIGURE NO.

1.1

TITLE

World gas reserves (6102 tcf)

1.2 Domestic, commercial and industrial utilization of natural gas

3.1

3.2

3.3

Uncoated and coated of alumina support

Schematic diagram of home-built micro reactor

Schematic diagram of glass tube for home-built micro

3.4

3.5

3.6

4.1

4.2 reactor

Diagram of FTIR sample cell

Carbon support from Palm Kernel Shell (PKS)

Calibration graph of standard 99.0% pure methane

FTIR spectra of gaseous products obtained from catalytic screening over i) Pd/Ru/Ni (2:8:90)/Al

2

O

3 ii) Rh/Ni

(10:90)/Al

2

O

3

catalysts during CO

2

methanation reaction

Catalytic performance of CO

2


2

O

3


C for 5 hours with various loading of nickel: i) 90 wt% (Pd/Ru/Ni (2:8:90)/Al

2

O

3

), ii) 80 wt%

(Pd/Ru/Ni (5:15:80)/Al

2

O

3

) and iii) 70 wt% (Pd/Ru/Ni

(5:25:70)/Al

2

O

3

)

4.3 Catalytic performance of CO

2

conversion from methanation reaction over Rh/Ni/Al

2

O

3


C for 5 hours with various loading of nickel: i) 90 wt% (Rh/Ni (10:90)/Al

2

O

3

), ii) 80 wt% (Rh/Ni

(20:80)/Al

2

O

3

) and iii) 70 wt% (Rh/Ni (30:70)/Al

2

O

3

)

PAGE

2

3

30

31

31

32

35

38

48

50

51 xiv

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12


2


2

O

3 catalyst calcined for 5 hours at different calcination temperatures: i) 400

1000 o

C o

C, ii) 500 o

C, iii) 700 o

C and iv)


2


2

O

3

catalyst calcined for 5 hours at different calcination temperatures: i) 400 o

C, ii) 500 o

C and iii) 700 o

C


2

conversion from methanation reaction over Pd/Ru/Ni (2:8:90) catalyst with various support materials: i) Al

2

O

3

beads ii)

TiO

2

/SiO

2 beads and iii) Carbon chips from PKS calcined at 400 o

C for 5 hours


2

conversion from methanation reaction over Rh/Ni (30:70) catalyst with various support materials: i) Al

2

O

3

beads ii) TiO

2

/SiO

2 beads and iii) Carbon chips from PKS and calcined at

400 o

C for 5 hours


2

conversion from methanation reaction over Pd/Ru/Ni (2:8:90) catalyst coated on alumina calcined for 5 hours at different temperatures: i) 700 o

C and ii) 1000 o

C


2

conversion from methanation reaction over Rh/Ni (30:70) catalyst coated on alumina calcined for 5 hours at different temperatures: i) 700 o

C and ii) 1000 o

C


2


2

O

3 catalyst prepared by different preparation methods i) wetness impregnation method and, ii) wetness impregnation modification method iii) Sol-gel method


2


2

O

3

catalyst prepared by different preparation methods i) wetness impregnation method and, ii) wetness impregnation modification method iii) sol-gel method


2


2

O



2

S gas

58

57

61

60

53

55

63

64

63 xv

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

4.22

4.23

4.24

4.25


2


2

O

3



2

S gas

The trend plot of reproducibility testing over Pd/Ru/Ni

(2:8:90)/Al

2

O

3


C for 5 hours towards CO

2

conversion from methanation reaction

The trend plot of reproducibility testing over Rh/Ni

(30:70)/Al

2

O

3



2


Regeneration catalytic testing over Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst for 3 hours at various temperatures towards CO

2


Regeneration catalytic testing over Rh/Ni (30:70)/Al

2

O

3 catalyst at various temperatures and various times towards CO

2



2


2

O

3


C for 5 hours and testing simultaneously with H

2

S gas

XRD diffractograms of Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst calcined at a) 400 o

C, b) 700

o

C and c) 1000 o

C for 5 hours

XRD diffractograms of Rh/Ni (30:70)/Al

2

O

3


C, b) 700

o

C and c) 1000 o

C for 5 hours

FESEM micrographs of fresh and spent Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalysts, calcined at 400 o

C for 5 hours with magnification 5000X and 50000X

EDX Mapping over Pd/Ru/Ni (2:8:90)/Al

2

O

3


C for 5 hours

FESEM micrographs of fresh and spent Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours with magnification 5000X

EDX Mapping over Rh/Ni (30:70)-Al

2

O

3 calcined at 400 o

C for 5 hours

catalyst

Isotherm plot of fresh Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst

66

81

85

86

88

89

92

67

68

70

71

75

77 xvi

4.26

4.27

4.28

4.29

4.30

4.31

4.32

Isotherm plot of spent Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst

Isotherm plot of fresh Rh/Ni (30:70)/Al

2

O

3

catalyst

Isotherm plot of spent Rh/Ni (30:70)/Al

2

O

3

catalyst

FTIR spectra of (a) fresh (b) spent (c) regenerated


2

O

3


C for 5 hours

FTIR spectra of (a) fresh (b) spent (c) regenerated Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours

Thermogram of Pd/Ru/Ni (2:8:90)/Al aging in an oven for 24 hours at 80 o

2

O

3

C-90 o

catalyst after

C

Thermogram of Rh/Ni (30:70)/Al in an oven for 24 hours at 80 o

C-90

2 o

O

3

C

catalyst after aging

93

95

97

98

94

94

95 xvii

xviii

LIST OF ABBREVIATIONS

- Absorbance ABS

BET - Brunnauer, Emmet and Teller

BJH - Barret-Joyner-Halenda

Btu - British thermal unit

Cu Kα - X-ray diffraction from Copper K energy levels rate of conversion (percentage)

- diameter

DTA

EDX

-

-

Differential

Energy Dispersive X-ray Analysis fcc - cubic

FESEM

FID

-

-

Field Emission Scanning Electron Microscope

Flame Ionization Detector

FTIR - Fourier Transform Infrared

GC - Chromatography

GHSV - Gas Hourly Space Velocity

ΔH - change

IWI

LNG

-

-

Incipient Wetness Impregnation techniques.

Liquefied Natural Gas

MgKα - X-ray diffraction from Magnesium K energy levels rate of conversion (percentage)

MS - Spectroscopy

NA - Adsorption

xix

P/ P o

- Relative pressure; obtained by forming the ratio of the equilibrium pressure and vapour pressure P o

of the adsorbate at the temperature where the isotherm is measured

PDF - Powder

PKS - Palm ppm - Part

PROX

RWSG

-

-

Prefential oxidation

Reverse Water Gas Shift

SMSI - Strong metal support interaction

SDS - Sodium

SNG - Substitute natural gas tcf -

TGA

UGC

VOC

- Analysis

-

-

Urea gelation co-precipitation

Volatile Organic Compounds wt% - percentage

WGS

XRD

-

-

Water Gas Shift

Diffraction

θ - Half angle of diffraction beam

λ -

B

C

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Preparation of Alumina Supported Nickel Oxide Based

Catalysts and Its Ratio

115

D

E

Calculation of Methane Percentage

Schematic Diagram of Home Built Micro Reactor

Connected using Two Isothermal Furnaces

Calculation of atomic weight percentage ratio of element in catalyst preparation

Calculation of weight loss over Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst

116

117

118

119 xx

1

CHAPTER 1

INTRODUCTION

Natural gas is a vital component of the world’s supply of energy and one of the most useful of all energy sources. It is considered as an environmental friendly clean fuel, offering important environmental benefits when compared to other fossil fuels. Natural gas is colorless, odorless, tasteless, shapeless and lighter than air.

Natural gas is the flammable gaseous mixture that occurs alone or with petroleum in underground reservoirs. It is predominantly methane (CH

4

) and some higher molecular weight paraffins (C n

H

2n+2

) generally containing up to four carbon atom.

Natural gas is the result of the decay of animal and plant remains (organic debris) that has occurred over millions of years. Over time, the mud and soil that covered the organic debris changed to rock and trapped the debris beneath the newlyformed rock sediment. Pressure and to some extent, heat changed some of the organic material into coal, some into oil (petroleum) and some into natural gas.

Whether or not the debris formed coal, petroleum or gas depended upon the nature of the debris and the localized conditions under which the changes occurred (James,

2007).

Natural gas is produced in many countries around the world and most of those countries produce both oil and natural gas, a few produce only natural gas

(James, 2007). Figure 1.1 shows natural gas reserves around the world. As illustrated in the figure, most of the world supply is in Eastern Europe and the Middle East. The

2

Eastern European reserves are dominated by Russian gas reserves (85% of the

Eastern European gas reserves), which exceed 1,680 trillion cubic feet (tcf). Some of the undeveloped fields may contain more than 1,000 tcf of gas. In the Middle East, the major reserves are in Iran (940 tcf). Qatar, in a series of gas fields known as the

“North Field”, has more than 910 tcf of proven r eserves (Seddon, 2006).

2.90%

41.30%

7.30%

7.80%

8.40%

32.30%

Asia Pacific

Africa

Americas

Eastern Europe

Middle East

Western Europe

Figure 1.1

World gas reserves (6102 tcf) (Taken from Gas Usage and Value: The

Technology and Economics of Natural Gas Use in the Process

Industries, 2004)

Malaysia i s the world’s 13 th

largest natural gas reserves and 24 th

largest crude oil reserves. Ma laysia is also the world’s 3 rd

largest producer of Liquefied Natural

Gas (LNG) with production capacity of 23 million metric ton per year. Our country is important to world energy markets because of its 75 tcf of natural gas reserves and its net oil exports of over 940,000 barrels per day. Natural gas consumption in 2002 was estimated at 1.0 tcf, with LNG exports of around 0.7 tcf mostly to Japan, South

Korea and Taiwan (Ching, 2008).

When cooled to -160°C, natural gas enters the liquid phase, in which it occupies six hundred times less volume than in its gaseous state. The importance of liquefied natural gas (LNG) thus lies in transportation whereas natural gas requires long, expensive pipelines for global export and import while, LNG can be transferred in secure tanks by ship or truck. In order to produce liquefied natural gas (LNG), the

3 gas from offshore platform as the raw material needs to undergo various processes.

Using pipeline system, the gas will be channel to the onshore station where the natural gas will undergo separation of acidic gas. This separation involved the removal of toxic gases such as H

2

S and CO

2

. Technology used for liquefying natural gas so that it can be transported in tankers is improving. As technology continues to expand the options for gas transportation, demand for natural gas is expected to grow.

Gas customers or end users, fall into three broad classes which are residential, commercial and industrial. In addition, utilities and power suppliers use gas to generate electricity and some fleets of vehicles use gas as a transportation fuel.

Figure 1.2 shows the domestic, commercial and industrial utilization of natural gas.

Homeowners and other residential customers use gas for heat, hot water, cooking and cloth drying as well as in gas fireplaces and logs. Commercial businesses which account for about 14% of gas consumption use gas mainly for heat and hot water.

Some larger buildings are also air-conditioned by gas powered equipment and many restaurants use gas to cook. Industrial customers who represent about 44% of all gas consumed use the fuel in tens of thousands of factories and mills to manufacture a great variety of product, from paper to cars (Busby, 1999).

50%

45%

40%

35%

30%

25%

20%

15%

10%

5%

42%

14%

44%

0%

Domestic Commercial

Figure 1.2

Domestic, commercial and industrial utilization of natural gas (Taken from Natural Gas in Non-Technical Language, 1999)

Industrial

4

This means that people tends to use more energy rather than conserve it because demand is strongly affected by price. Future supplies of natural gas and other limited energy resources will depend on how much energy we consume, how much we attempt to preserve for future generations and how well we learn to develop and use our remaining resources efficiently.

1.2 Acid Gas Treating Processes

Growing demand for natural gas is leading to an increase in the production of standard and contaminated natural gas. Malaysia large reserves and the existence of huge gas reserves in the nearby countries have ensured that natural gas is the future energy resource for Malaysia (Md. Yasin, 1987). Malaysia also has one of the most extensive natural gas pipeline networks in Asia. Table 1.1 shows the chemical composition of Malaysian crude natural gas which was analyzed by using Gas

Chromatography-Mass Spectroscopy (GC-MS). It is consists mainly of simplest hydrocarbon which is methane (CH

4

), along with heavier hydrocarbons such as ethane (C

2

H

6

) and propane (C

3

H

8

). Often, natural gas also contains impurities such as carbon dioxide, hydrogen sulfide and water as well as nitrogen, helium and other trace gases. The composition of a natural gas as it comes from the well varies from one location to another. It must be purified before it can be sold, liquefied or fed to a gas to liquids process.

Table 1.1

Chemical composition of crude natural gas from Telaga Bergading,

Petronas Carigali Sdn. Bhd. (Taken from Laboratory Services Unit

(UNIPEM), 2003)

Gases Composition (%)

47.9

5.9

CH

4

C

2

H

6

C

3

H

8

CO

2

H

2

S

Others (CO, O

2

, N

2

, H

2

O)

3.2

23.5

5.4

24.1

5

Gas processing is necessary to ensure that the natural gas intended for use is clean-burning and environmentally acceptable. Natural gas used by consumers is composed almost entirely of methane but natural gas that emerges from the reservoir at the wellhead contains many components that need to be extracted. Although, the processing of natural gas is less complicated rather than the processing and refining of crude oil, it is equal and necessary before it can be used by end user.

One of the most important parts of gas processing is the removal of carbon dioxide and hydrogen sulfide. The removal of acid gases (CO

2

, H

2

S and other sulfur components) from natural gas is often referred to as gas sweetening process. There are many acid gas treating processes available for removal of CO

2

and H

2

S from natural gas. These processes include Chemical solvents, Physical solvents,

Adsorption Processes Hybrid solvents and Physical separation (Membrane) (Kohl and Nielsen, 1997).

According to previous research done by Hao et al.

(2002), there are ways to upgrading the low quality natural gas with selective polymer membranes. The membrane processes were designed to reduce the concentrations of CO

2

and H

2

S in the natural gas to US pipeline specifications. However, this technique incurs high cost and low selectivity towards toxic gas separation. This technique also needs further development because the performance of membrane depends upon the specific characteristics of flue gas composition, and the specific features of the separation (i.e. large volumetric flow rate, low pressure source, high temperature, and the relative low commodity value of H

2

S and CO

2

) (Rangwala, 1996).

Another method of H

2

S removal and one that leaves the CO

2

in the natural gas is called the Iron Sponge process. The disadvantage of this is that it is called a batch-type function and is not easily adapted to continuous operating cycle. The Iron

Sponge is simply the process of passing the sour gas through a bed of wood chips that have been impregnated with a special hydrated form of iron oxide that has a high affinity for H

2

S. Regeneration of the bed incurs excessive maintenance and operating costs, making this method inconsistent with an efficient operating program. If there are any real advantages in using this process, it is a fact that CO

2

remains in the gas,

6 thereby reducing the shrinkage factor which could be significant for very large volumes with an otherwise high CO

2

content (Curry, 1981).

Chemical absorption processes with aqueous alkanolamine solutions are used for treating gas streams containing H

2

S and CO

2

. They offer good reactivity at low cost and good flexibility in design and operation. However, depending on the composition and operating conditions of the feed gas, different amines can be selected to meet the product gas specification (Mokhatab et al., 2006). Some of the commonly used alkanol-amine for absorption desulfurization are monoethanolamine

(MEA), diethanolamine (DEA), triethanolamine (TEA), diglycolamine (DGA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA). MDEA allows the selective absorption of H

2

S in the presence of CO

2

but can be use effectively to remove CO

2

from natural gas in present of additives (Salako and Gudmundsson,

2005).

On the other hand, CO

2

and H

2

S can be removed from natural gas via chemical conversion techniques. Catalysts for CO

2

methanation had been extensively studied because of their application in the conversion of CO

2

gas to produce methane, which is the major component in natural gas (Wan Abu Bakar et al.

,

2008a). Usually, the catalysts were prepared from the metal oxide because of the expensiveness of pure metal. This process can increase the purity and quality of the natural gas without wasting the undesired components but fully used them to produce high concentration of methane (Ching, 2008). Methane (CH

4

) gas was formed from the reaction of hydrogen gas and carbon dioxide gas through methanation process by reduction reaction as shown in Equation 1.1 below:-

CO

2 (g)

+ 4H

2 (g)

→ CH

4 (g)

+ 2H

2

O

(l)

(1.1)

This reaction is moderately exothermic, H o

= -165 kJ/mol. Meanwhile, H

2

S can also be reduced to elemental sulfur simultaneously by oxidation reaction as in Equation

1.2 below:-

H

2

S

(g)

+ ½ O

2 (g)

→ S

(s)

+ H

2

O

(l)

(1.2)

7

In order for this method to be effective, a suitable catalyst must be applied to promote selective CO

2

methanation because of the main side product under this reaction also will be form (Eq. 1.3), which obviously should be avoided. Thus, high selectivity of the catalyst in promoting CO

2

methanation is paramount importance. In

Equation 1.3, carbon monoxide produced by this reaction can also be used to form methane by reaction in Equation 1.4 with hydrogen.

CO

2 (g)

+ H

2 (g)

→ CO

(g)

+ H

2

O

(l)

CO

(g)

+ 3H

2 (g)

→ CH

4 (g)

+ H

2

O

(l)

(1.3)

(1.4)

Since the catalytic process through methanation and desulphurization reactions offers the best way to remove CO

2

and H

2

S in the natural gas, therefore, the present study is to develop a catalyst based on nickel oxide with modifying the dopants using noble metal in order to fully remove these sour gasses at high conversion percentage possibly at low temperature.

1.3 Problem

Carbon dioxide and hydrogen sulfide are commonly referred to as acid gases because they form acids or acidic solution (corrosive compounds) in the presence of water. These gases are generally undesirable components of natural gas. Natural gas can contain up to 28% of H

2

S gas, consequently, it maybe an air pollutant near petroleum refineries and in oil and gas extraction areas. H

2

S itself is colorless and flammable gas. It smells like rotten eggs that is toxic at extremely low concentration and can cause loss of the sense of smell. Inhalation of excessive levels of H

2

S gas in confined space can result in unconsciousness, respiratory failure and death. It also has potential to corrode pipelines, storage tanks and other ship component which may cause operability problems including engine breakdown (Fred and Hans,1998).

Besides that, the removal of H

2

S gas from natural gas often necessary before combustion. The combustion of natural gas in the presence of H

2

S yielded sulfur dioxide which when dissolved in water turn to very corrosive sulfuric acid or sulfurous acid. This may cause the acid rain phenomena to the earth which leads to

8 the increase of acidity of the soil and affect the chemical balance of lakes and streams.

Carbon dioxide is also referred to as diluents because none of them burn and thus they have no heating value. This gas is not very harmful and may actually be used sometimes as “fillers” to reduce the heat content of the supply energy. Thus, its removal may be required in some instances merely to increase the energy content of the gas per unit volume. CO

2

removal may also be required because it forms a complex, CO

2

·CO

2

, which is quite corrosive in the presence of water. For gas being sent to cryogenic plants, removal of CO

2

may be necessary to prevent solidification of the CO

2

(Sanjay, 1987). Since the melting points of CO

2

and H

2

S are higher than the boiling point of methane, it can potentially freeze during the cryogenic process.

Freezing can lead to clogged LNG pipelines and storage vessels, producing various maintenance issues and hindering overall process efficiency. Similar to the H

2

S gas,

CO

2

in the presence of water may enhance the production of carbonic acid which leads to the acid rain phenomena. Furthermore, carbon dioxide is one of the greenhouse gases. As the level of carbon dioxide increases the warming of the earth ’s surface will also increase (Schneider, 1989). This phenomenon is called as greenhouse effect thus, can cause global warming. However, if none of these situations are encountered, there is no need to remove CO

2

.

The existence of both carbon dioxide and hydrogen sulphide will reduce the quality of natural gas and at the same time increase the production cost of natural gas to fulfill the US Pipeline Specification, in which, the natural gas should only consist of 2 w/w % CO

2

and 4 ppm H

2

S before it can be distributed to world market

(Echterhoff and McKee, 1991). These gaseous are needed to be removed in order to prevent or minimize the release of hazardous gases into environment. This will helps to reduce problems of acid rain, ozone layer depletion or greenhouse effect. Many scientists believe that increasing levels of carbon dioxide in the earth’s atmosphere will change the global climate.

9

1.4 Significant of Study

In this research, the potential catalyst that can be used to remove CO

2

and

H

2

S simultaneously which are present in wet natural gas consisting of 23% CO

2

and

5% H

2

S was developed based on nickel oxide doped with noble metal. This catalyst offers very promising techniques for natural gas purification since H

2

S gas is being converted to stable element of sulfur and unwanted CO

2

gas is being converted to the product, CH

4 thus will enhance the methane production.

The purification technique via this catalyst can remove acid gases in wet natural gas which is hazardous to the environment. This will help to prevent phenomena of acid rain, ozone layer depletion or greenhouse effect. Besides, it may be necessary to avoid the corrosion and clogging to the delivery pipeline. This purification method will certainly improve the quality and quantity of Malaysian natural gas and increase the market price of our natural gas that will benefit to our country. The utmost important, the potential catalyst will contribute to the growth of the national economy and create green and sustainable environment.

The catalyst is easily prepared, environmental friendly and reusable. All the ingredients in the fabrication of the catalyst are easily available, cheap and stable.

The beauty of the catalyst is safer to handle because it can be used at low reaction temperature. It requires minimum modification to the already existing system and offers cost effective operating system.

1.5 Objectives of the Research

The main goal of this research is to develop the catalyst that can catalyze methanation and desulfurization reactions effectively and simultaneously at a very low temperature.

10

The objectives of this research are :-

1.

To synthesize the best supported nickel oxide based for carbon dioxide methanation and H

2

S desulphurization reactions.

2.

To test the catalytic activity of the prepared catalysts by using in-house built micro reactor coupled with FTIR.

3.

To conduct several optimization parameters of the best prepared catalyst.

4.

To characterize the best prepared catalysts in order to understand the physical properties of the prepared catalyst.

1.6 Scope of the Research

A series of alumina supported catalyst based on nickel oxide doped with noble metal were prepared using wetness impregnation techniques. Ruthenium chloride, nitrate salts of rhodium, palladium and platinum had been used as precursor for the dopants in this research. Meanwhile, catalytic testing was carried out by using house-built micro reactor coupled with FTIR towards the prepared catalysts. The production of methane gas was determined by Gas Chromatography analysis. All the prepared catalysts were tested based on trial and error basis where different mixed metal oxides with 90 wt% nickel loading were tested until the best performance catalyst was obtained. Then, the best two prepared catalysts were optimized according to the amount of nickel loading, different calcination temperatures of catalysts, different calcination temperatures of alumina support, various support materials and various preparation method. Lastly, the potential catalysts were tested on its reproducibility and regeneration activity. The characterization of the potential catalysts were conducted by various techniques in order to understand the physical properties of the catalysts such as X-Ray Diffraction for bulk structure, Field

Emission Scanning Electron Microscope for morphology study, Energy Dispersive

X-Ray Analysis for elemental composition study, Nitrogen Adsorption for pore texture and surface area of the catalyst. Meanwhile, Thermal Gravimetric Analysis to study the mass loss of the catalyst during temperature change and Fourier Transform

Infrared Spectroscopy for the functional group study present in the catalyst.

11

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

Metal oxide supported catalysts have been widely used in research for investigating the CO and CO

2

methanation reaction. Depending on the metal used and the reaction conditions, a variety of products may be formed including methane.

However, fewer researches on the catalyst for in-situ reactions of CO

2

methanation and H

2

S desulfurization have been carried out. In fact, there is also presence of H

2

S in real natural gas. Therefore, H

2

S should be considered in invention of methanation catalyst, since it could cause poisoning of the nickel catalyst (Wan Abu Bakar et al.

,

2008b). As been said by Xu et al. (2003), a good methanation catalyst is physically durable and reducible at temperature not more than 300 o

C with high performance ability and these properties should retained in the catalyst while in use with a life span up to 10 years.

2.2 Nickel Based Methanation Catalysts

The methanation of carbon dioxide on Ni catalysts was studied in detail by fewer researchers because of the theoretical significance and possible practical application of this reaction. The methanation activity of Ni/Al

2

O

3

catalyst depended intimately on the surface chemical state of Ni and different active phases formed from the reduction of different nickel species in the oxidated states. Nickel oxides

12 appeared in Ni/Al

2

O

3 in two forms prior to reduction as “free” and “fixed oxide”, and formed large and small crystallites, respectively, when reduced (Zielinski, 1982).

Studied done by Rodriguez et al.

(2001) showed that NiO catalyst has ability to give higher catalytic activity with higher methane formation due to the malformation sites which converted to active sites on the surface of nickel oxide. This property is important as reference to construct excellent catalysts for CO

2

conversion

Previously, it was shown that nickel particles change their morphology during catalytic reactions by cluster growth processes and that part of the active clusters are lifted from the support due to carbon deposition and carbon whisker formation

(Czekaj et al.

, 2007). Earlier study by Takahashi et al.

(2007) found that Ni catalysts are promising catalysts since they are active and more resistant to sulfur poisoning thus high dispersion of Ni and is expected to be used in catalytic reaction that proceeds at relatively low temperature. Moreover, Inui (1996) claimed that NiO has a bimodal pore structure, which will enhance the higher activity for CO

2

methanation.

A bimodal pore structure was found to be beneficial to catalyst preparation and methanation rate (Inui, 1979) which will serve as an optimum pore size for the adsorption of both the reactants. Therefore, Ni based catalyst are commonly used as catalysts in hydrogenation and hydrogenolysis reaction.

Aksoylu and Onsan (1997) reported that 5.5 × 10

-5

% of CH

4

was produced at

250 o

C over the Ni/ Al

2

O

3

catalyst prepared by conventional impregnation method at

350 o

C for 3 hours under reduction environment. They also investigated the 15%-

Ni/Al

2

O

3

prepared by coprecipitation method for methanation of carbon dioxide. The result achieved 30% of conversion with 99.7% selectivity towards methane at 510 K

(Aksoylu et al.

, 1996). Some previous research was only focused on conversion of

CO

2

without mentioned the yielded of CH

4

. Similarly to Chang et al.

(2003) who had investigated CO

2

methanation over NiO supported on rice husk ash-Al

2

O

3

and SiO

2

-

Al

2

O

3

which had been synthesized by impregnation method and calcined at 500 o

C.

At reaction temperature of 400 o

C, there were 30% conversion of CO

2

over the rice husk ash-Al

2

O

3

supported catalyst, while only 5% conversion of CO

2

over the SiO

2

-

Al

2

O

3

catalyst.

13

Moreover, Ni/SiO catalyst prepared by conventional impregnation method was also studied by Shi and Liu, (2009). The sample was treated by glow discharge plasma for 1 hour and followed by calcinations thermally at 500 o

C for 4 hours. Such prepared catalyst presents smaller metal particles (17.5 and 7.9 nm) and higher conversion of CO at 400 o

C around 90% for methanation reaction. However, Ni/SiO

2 catalyst prepared by a sol gel process showed better quality when compared to the

Ni/SiO

2

catalyst prepared by conventional impregnation (Tomiyama et al.

, 2003).

Thus, Takahashi et al.

(2007) investigated the bimodal pore structure of

Ni/SiO

2

prepared by the sol-gel method of silicon tetraethoxide and nickel nitrate in the presence of poly(ethylene oxide) (PEO) and urea. They found that the catalyst shows steady activity around 30-40% of CO conversion without decay within the reaction period until 240 min with total flow rate of 360 cm

3

/min. The performance of the catalyst influenced strongly by Ni surface area rather than the presence of macropores. As been shown, nickel oxide can be prepared through various methods such as wetness impregnation, co-precipitation, sol gel method, ion-exchange, adsorption, deposition-precipitation and else. These preparation methods are, however very complicated and difficult to control except for wetness impregnation method. Therefore, most of the work published has focused on the use of impregnation technique for their catalyst preparation.

Research done by Liu et al.

(2008) on the removal of CO contained in hydrogen-rich reformed gases was conducted by selective methanation over Ni/ZrO

2 catalysts prepared by conventional wetness impregnation method. The catalyst achieved CO conversion of more than 96% and held a conversion of CO

2

under 7% at temperature range 260 o

C-280 o

C. The results showed that only methane was observed as a hydrogenated product. Furthermore, the maximum of CO

2

conversion was found by Perkas et al.

(2009) which achieved about 80% at 350 o

C on the

Ni/meso-ZrO

2

catalyst. Around 100% selectivity to CH

4

formation was obtained at the same reaction temperature. This catalyst was prepared by an ultrasound-assisted method and testing with gas hourly space velocity (GHSV) of 5400 h

-1

at all temperatures. They also reported that non modified mesoporous Ni/ZrO

2

catalyst and with the Ni/ZrO

2

modified with Ce and Sm did not effect the conversion of CO

2

.

14

Previous work by Sominski et al.

(2003) reported that a Ni catalyst supported on a mesoporous yttria-stabilized-zirconia composite was successfully prepared by a sonochemical method using templating agent of sodium dodecyl sulfate (SDS).

However, the result is not as good as the catalyst that had been obtained by Perkas et al. (2009).

In a study done by Rostrup-Nielsen et al. (2007), supported nickel catalyst containing 22 wt% Ni on a stabilized support was exposed to a synthesis gas equilibrated at 600 o

C and 3000 kPa for more than 8000 h. The CO

2

conversion is

57.87% while methane formed is 42.76%. The result showed that at 600 o

C, loss of active surface area proceeds via the atom migration sintering mechanism. The methanation reaction is structure sensitive and it was suggested that atomic step sites play the important role as the active sites of the reaction. High temperature methanation may play a role in manufacture of substitute natural gas (SNG). The key problem is resistance to sintering, which results in a decrease of both the metal surface area and the specific activity.

Modification of the catalyst by some appropriate additives may effect the conversion of CO

2

which then methane production. Ni catalysts were modified by alkali metal, alkaline earth metals, transition metal, noble metal or rare earth metal just to select which promoters could increase the conversion of CO

2

as well as the methane formation. The effect of cerium oxide as a promoter in supported Ni catalysts was studied by Xavier et al.

(1999). They claimed that the highest activity of CeO

2

promoter for Ni/Al

2

O

3

catalysts could be attributed to the electronic interactions imparted by the dopant on the active sites under reducing conditions.

The testing was evaluated in a high pressure catalytic reactor consists of a stainless steel reactor of 25 mm diameter and 180 mm length which is mounted vertically inside a furnace. Methanation activity and metal dispersion was found to decrease with increasing of metal loading. It is observed that the catalyst doped with

1.5 wt% CeO

2 exhibited highest conversion of CO and CO

2 with percentage of conversion increase 3.674 moles/second, which is 86.34%. They found that the addition of CeO

2 into Ni/γ -Al

2

O

3

catalysts leads to easier interaction between nickel

15 and alumina support hence increase its reduction ability from CO

2

to CH

4

during the catalytic testing and gave well nickel dispersion on the surface of the catalyst

(Zhuang et al.

, 1991). It showed a beneficial effect by not only decreasing the carbon deposition rate but also increasing and maintaining the catalytic activity.

The study of Yoshida et al.

(1997) in a bench scale test at ambient temperature and 350 o

C for carbon recycling system using Ni ferrite process was carried out in LNG power plant. The feed gas was passed at a flow rate of 10 mL/min. They found that the amount of methane formed after CO

2

decomposition was 0.22 g (conversion of CO

2

to CH

4

: 77%) in the latter and 0.49 g (conversion of

CO

2

to CH

4

: 35%) in the former. According to their study, the methanation and carbon recycling system could also be applied to other CO z

sources such as IGCC power plant and depleted natural gas plant. Hence, pure CH

4

gas can be theoretically synthesized from CO

2

with low concentration in flue gas and H

2

gas with the minimum process energy loss, while conventional catalytic processes need an additional separation process of CH

4

gas formed.

Hashimoto et al. (2002) revealed that the catalysts obtained by oxidationreduction treatment of amorphous Ni-Zr alloys exhibited high catalytic activity with

100% selectivity formation of CH

4

at 1 atm. Around 80% of CO

2

was converted at

573 K. They found the number of surface nickel atom decreases with nickel content of catalyst, because of coagulation of surface nickel atoms leading to a decrease in dispersion of nickel atoms in the catalysts. Moreover, Habazaki et al.

(1998) reported that over the catalysts prepared from amorphous Ni-Zr (-Sm) and Co-Zr, nickelcontaining catalysts show higher activity than the Co-Zr catalyst. CO reacted preferentially with H

2

and was almost completely converted into CH

4

at or above 473

K in the CO-CO

2

-H

2

. The maximum conversion of carbon dioxide under the present reactant gas composition is about 35% at 300 o

C.

Most of the previous work used rare earth oxide as a dopant over Ni/Al

2

O

3 catalysts for hydrogenation reaction. Su and Guo (1999) also reported an improvement in catalytic activity and resistance to Ni sintering of doped with rare earth oxides. The growth of Ni particles and the formation of inactive NiO and

16

NiAl

2

O

4

phases were suppressed by addition of rare earth oxides. The combinations of two oxides lead to creation of new systems with new physicochemical properties which may exhibit high catalytic performance as compared to a single component system (Luo et al., 1997). However, the catalytic and physicochemical properties of different oxide catalysts are dependent mainly on the chemical composition, method of preparation and calcination temperatures (Selim and El-Aihsy, 1994).

Ando and co-workers (1995) had studied on intermetallic compounds synthesized by arc-melting metal constituent in a copper crucible under 66.7 kPa argon atmospheres. The hydrogenation of carbon dioxide took place under 5 Mpa at a reaction temperature at 250 o

C over LaNi

4

X. They found that the conversion of CO

2 was 93% over LaNi

5

and the selectivities to methane and ethane in the product were

98% and 2%, respectively. The source of activity can be attributed to the new active sites generated by decomposition of the intermetallic compounds. However, even under atmospheric pressure, 56% of CO

2

converted to CH

4

and CO with selectivities of 98% and 2%, respectively.

The promotion of lanthanide to the nickel oxide based catalyst gives positive effects which are easier reduction of oxide based, smaller particles size and larger surface area of active nickel (Zhang et al.

, 2001). Moreover, the highly dispersed nickel crystallites are obtained over nickel catalyst containing lanthanide promoter

(Rivas et al.

, 2008). Furthermore, the methanation of carbon dioxide over Niincorporated MCM-41 catalyst was carried out by Du et al. (2007). At 600 o

C, 1 wt% of Ni-MCM-41 with space velocity of 115001 kg

-1 h

-1

showed only 46.5% CO

2 conversion and a selectivity of 39.6% towards CH

4

. Almost no catalytic activity was detected at 100 – 200 o

C and only negligible amounts of products were detected at

300 o

C. However, this catalyst structure did not change much after CO

2

methanation for several hours, producing the high physical stability of this catalytic system.

In addition, nickel based catalysts that used more than one dopants had been studied by Liu et al.

(2009). Ni-Ru-B/ZrO

2

catalyst was prepared by means of chemical reduction and dried at 80 o

C for 18 h in air with total gas flow rate of 100 cm

3

/min. They found that CO

2

methanation occurred only when temperature was

17 higher than 210 o

C. At reaction temperature of 230 o

C, the CO conversion reached

99.93% but CO

2

conversion only 1.55%. Meanwhile, Ni-Fe-Al oxide nanocomposites catalyst prepared by the solution-spray plasma technique for the high temperature water-gas shift reaction was investigated by Watanabe et al.

(2009). The

CO conversion over 39 atom% Ni-34 atom% Fe-27 atom% Al catalyst achieved around 58% and yielded about 6% of methane at 400 o

C.

On the other hand, Kodama et al.

(1997) had synthesized ultrafine Ni x

Fe

3x

O

4 with a high reactivity for CO

2

methanation by the hydrolysis of Ni

2+

, Fe

2+ and Fe

3+ ions at 60-90 o

C followed by heating of the co-precipitates to 300 o

C. At reaction temperature of 300 o

C, the maximum yield (40%) and selectivity (95%) for CH

4

were obtained. Moreover, the conversion of CO

2

over NiO-YSZ-CeO

2

catalyst prepared by impregnation method was 100% at temperature above 800 o

C. This catalyst was investigated by Kang et al.

(2007). No NiC phase was detected on the surface of

NiO-YSZ-CeO

2

catalyst. Yamasaki et al.

(1999) reported that amorphous alloy of

Ni-25Zr-5Sm catalyzed the methanation reaction with 90% conversion of CO

2

and

100% selectivity towards CH

4

at 300 o

C.

Furthermore, Ocampo et al.

(2009) had investigated the methanation of carbon dioxide over 5 wt% nickel based Ce

0.72

Zr

0.28

O

2

catalyst which was prepared by pseudo sol-gel method. The catalyst exhibited high catalytic activity with 71.5%

CO

2

conversion and achieved 98.5% selectivity towards methane gas at 350 o

C.

However, it never stabilized and slowly deactivated with a constant slope and ended up with 41.1% CO

2 conversion and its CH

4

selectivity dropped to 94.7% after 150 h on stream. Catalytic testing was performed under operating conditions at pressure of

1 atm and a CO

2

/H

2

/N

2

ratio is 36/9/10 with a total gas flow of 55 mL/min.

Meanwhile, Kramer et al.

(2009) also synthesize Re

2

Zr

10

Ni

88

O x

catalyst by modified sol gel method based on the molar ratio metal then dried for 5 days at room temperature followed by 2 days at 40 o

C and lastly calcined at 350 o

C for 5 h. The catalytic performance was carried out by the reactant gas mixture of CO/CO

2

/N

2

/H

2

= 2/14.9/19.8/63.3 enriched with water at room temperature under pressure of 1 bar and total flow rate of 125 mL/min. At reaction temperature of 230 o

C, almost 95%

18 conversion of CO was occurred and less than 5% for conversion of CO

2

over this catalyst.

The novel catalyst development to achieve both low temperature and high conversion of sour gases of H

2

S and CO

2

present in the natural gas was investigated by Wan Abu Bakar et al. (2008c). It was claimed that conversion of H

2

S to elemental sulfur achieved 100% and methanation of CO

2

in the presence of H

2

S yielded 2.9% of CH

4

over Fe/Co/Ni-Al

2

O

3

catalyst at maximum studied temperature of 300 o

C.

This exothermic reaction will generate a significant amount of heat which caused sintering effect towards the catalysts (Hwang and Smith, 2009). Moreover, exothermic reaction is unfavorable at low temperature due to its low energy content.

Thus, the improvement of catalysts is needed for the in-situ reactions of methanation and desulfurization to occur at lower reaction temperature.

2.3 Noble Metals Used in Methanation Reaction

Nickel oxide will lose its catalytic ability after a few hours when it undergoes carbon formation process. The carbon formation can be avoided by adding dopants towards the Ni catalyst. Therefore, incorporating of noble metals will overcome this problem. Noble metals such as rhodium, ruthenium, platinum and iridium exhibit promising CO

2

/H

2

methanation performance, high stability and less sensitive to coke deposition. However, from a practical point of view, noble metals are expensive and little available. In this way, the addition of dopants and support is good alternative to avoid the high cost of this precious metal. For the same metal loading, activity is mainly governed by the type of metal but also depends on precursor selection

(Yaccato et al., 2005). While, the reaction selectivity depends on support type and addition of modifier (Kusmierz, 2008).

Methane production rates for noble metals based catalysts were found to decrease in order Ru > Rh > Pt > Ir ~ Pd. It may be suggested that the high selectivities to CH

4

of Ru and Rh are attributed to the rapid hydrogenation of the

19 intermediate CO, resulting in higher CO

2

methanation activities. Panagiotopoulou et al.

(2008) had claimed the selectivity towards methane which is typically higher than

70%, increases with increasing temperature and approaches 100% when CO

2 conversion initiated at above 250 o

C. A different ranking of noble metals is observed with respect to their activity for CO

2

hydrogenation, where at 350 o

C decreases by about one order of magnitude in the order of Pt > Ru > Pd ~ Rh. From the research of

Ali et al. (2000), the rate of hydrogenation can be increased by loading noble metals such as palladium, ruthenium and rhodium. The results showed that all of them perform excellently in the process of selective oxidation of CO, achieving more than

90% conversion in most of the temperature region tested between 200 to 300 o

C.

Finch and Ripley (1976) claimed that the noble metal promoters may enhance the activity of the cobalt supported catalysts to increase the conversion to methane. In addition, the noble metals promoted catalysts maintained greater activity for methane conversion than the non-promoted catalysts in the presence of sulfur poison. The addition of small contents of noble metals on cobalt oxides has been proposed in order to increase the reduction degree on the catalytic activity of Co catalysts (Profeti et al.

, 2008). Research done by Miyata et al. (2006) revealed that the addition of Rh,

Pd and Pt noble metals drastically improved the behavior of Ni/Mg(Al)O catalysts.

The addition of noble metals on Ni resulted in a decrease in the reduction temperature of Ni and an increase in the amount of H

2

uptake on Ni on the catalyst.

It well known that ruthenium is the most active methanation catalyst and highly selective towards methane where the main products of the reaction were CH

4 and water. However, the trace amount of CO was present among the products and methanol was completely absent (Kusmierz, 2008). Takeishi and Aika (1995) who had studied on Raney Ru catalysts found a small amount of methanol was produced on supported Ru catalyst but the methane gas was produced thousands of times more than the amount of methanol from CO

2

hydrogenation. The selectivity to methane was 96-97% from CO

2

. Methane production rate from CO

2

and H

2

at 230 o

C on their

Raney Ru was estimated to be 0.25 mol g

-1 h

-1

. The activity for methane production from CO

2

± H

2

at 160 o

C under 1.1 MPa was much higher than that under atmospheric pressure. The rate of methane synthesis was 3.0 mmol g

-1

h

-1

and the

20 selectivity for methane formation was 98% at 80 o

C, suggesting the practical use of this catalyst (Takeishi et al ., 1998).

Particularly suitable for the methanation of carbon dioxide are Ru/TiO

2 catalysts. Such catalysts display their maximum activity at relatively low temperatures which is favorable with respect to the equilibrium conversion of the strongly exothermic reaction and form small amount of methane even at room temperature (Traa and Weitkamp, 1999). It can be prove by VanderWiel et al.

(2000) who had studied on the production of methane from CO

2

via Sabatier reaction. The conversion reaches nearly 85% over 3 wt% Ru/TiO

2

catalyst at 250 o

C and the selectivity towards methane for this catalyst was 100%.

Meanwhile, a microchannel reactor has been designed and demonstrated by

Brooks et al.

(2007) to implement the Sabatier process for CO

2

reduction of H

2

, producing H

2

O and CH

4

. From the catalyst prepared, the powder form of Ru/TiO

2 catalyst is found to provide good performance and stability which is in agreement with Abe et al.

(2008). They claimed that the CO

2

methanation reaction on Ru/TiO

2 prepared by barrel-sputtering method produced a 100% yield of CH

4

at 160 o

C which was significantly higher than that required in the case of Ru/TiO

2

synthesize by wetness impregnation method and Gratzel method. Barrel-sputtering method gives highly dispersed Ru nano particles deposited on the TiO

2

support which then strongly increase its methanation activity.

Another research regarding CO-selective methanation over Ru-based catalyst was done by Galletti et al. (2009). The γ -Al

2

O

3

to be used as Ru carrier was on purpose prepared through the solution combustion synthesis (SCS) method. The active element Ru was added via the incipient wetness impregnation (IWI) technique by using RuCl

3

as precursor. Three Ru loads were prepared: 3%, 4% and 5% by weight. All of the catalysts reached complete CO conversion in different temperature ranges where simultaneously both the CO

2

methanation was kept at a low level and the reverse water gas shift reaction was negligible. The best results were obtained with 4% Ru/ γ -Al

2

O

3

in the range of 300 – 340 o

C, which is 97.40% of CO conversion.

21

For further understanding about methanation over Ru-based catalysts, Dangle et al. (2007) conducted a research of selective CO methanation catalysts prepared by a conventional impregnation method for fuel processing applications. It well known that metal loading and crystallite size have an effect towards the catalyst activity and selectivity. Therefore, they studied the crystallite size by altering the metal loading, catalyst preparation method, and catalyst pretreatment conditions to suppress CO

2 methanation. These carefully controlled conditions result in a highly active and selective CO methanation catalyst that can achieve very low CO concentrations while keeping hydrogen consumption relatively low. Even operating at a gas hourly space velocity as high as 13500 h

-1

, a 3% Ru/Al

2

O

3

catalyst with a 34.2 nm crystallite was shown to be capable of converting 25 – 78% of CO

2

to CH

4

over a wide temperature range from 240 to 280 o

C, while keeping hydrogen consumption below 10%.

In addition, Gorke and Co-workers (2005) had carried out research on the microchannel reactor which was coated with a Ru/SiO

2

and a Ru/Al

2

O

3

catalyst.

They found that the Ru/SiO

2

catalyst exhibits its highest CH

4

selectivity of only 82% with 90% CO

2

conversion at a temperature of 305 o

C, whereas a selectivity of 99% is obtained by the Ru/Al

2

O

3

catalyst at 340 o

C with CO

2

conversion of 78%. However,

Weatherbee and Bartholomew (1984) achieved a CH

4

selectivity of 99.8% with CO

2 conversion of only 5.7% at reaction temperature of 230 o

C using Ru/SiO

2

catalyst.

Mori et al.

(1996) investigated the effect of reaction temperature on CH

4 yield using Ru-MgO under mixing and milling conditions at initial pressures of 100

Torr CO

2

and 500 Torr H

2

. No CH

4

formation was observed at the temperature below

80 o

C under mixing conditions over Ru-MgO catalyst. It reached 31% at 130 o

C but leveled off at 180 o

C. CH

4

formation over this catalyst under milling condition increased from 11% at 80 o

C to 96% at 180 o

C. They found that incorporation of

MgO, a basic oxide to the Ru, promotes the catalytic activity by strongly adsorbing an acidic gas of CO

2

. According to Chen et al.

(2007), Ru impregnated on alumina and modified with metal oxide (K

2

O and La

2

O

3

) showed that the activity temperature was lowered approximately 30ºC compared with pure Ru supported on alumina. The conversion of CO on Ru-K

2

O/Al

2

O

3

and Ru-La

2

O

3

/Al

2

O

3

was above 99% at 140 –

22

160°C, suitable to remove CO in a hydrogen-rich gas. However, the formation of methane was observed only at temperature above 200 o

C and the selectivity of Ru-

La

2

O

3

/Al

2

O

3

was higher than that of Ru-K

2

O/Al

2

O

3

in the active temperature range.

Other than that, Szailer et al.

(2007) had studied the methanation of CO

2

on noble metal supported on TiO

2

and CeO

2

catalysts in the presence of H

2

S at temperature 548 K. It was observed that in the reaction gas mixture containing 22 ppm H

2

S, the reaction rate increased on TiO

2

and on CeO

2

supported metals (Ru, Rh,

Pd) but when the H

2

S content up increased to 116 ppm, the all supported catalysts was poisoned. In the absence of H

2

S, the result showed that 27% conversion of CO

2 and 39% conversion of CO

2

to methane with the presence of 22 ppm H

2

S after 4 hours of the reaction.

Moreover, the addition of Rh strongly improves the activity and stability of the catalysts (Wu and Chou, 2009), resistance to deactivation and carbon formation can be significantly reduced (Jozwiak et al.

, 2005). Erdohelyi et al . (2004) studied the hydrogenation of CO

2

on Rh/TiO

2

. The rate of methane formation was unexpectedly higher in the CO

2

+ H

2

reaction on Rh/TiO

2

in the presence of H

2

S. At higher temperature of 400 o

C, around 75% of selectivity for CH

4

formation and CO was also formed from the reaction. Choudhury et al . (2006) presented the result of an

Rh-modified Ni-La

2

O

3

-Ru catalyst for the selective methanation of CO. The CO

2 conversion was observed less than 30% over this prepared catalyst after CO was completely removed.

It had been reported that the addition of Pd had a positive effect for hydrogenation of CO or CO

2

because of its higher electronegativity with greater stability of Pd

0

species compared to those of Ni

0

under on stream conditions (Castaño et al.

, 2007). In contrast, Pd/SiO

2

and Pt/SiO

2

catalysts showed poor activities at temperature lower than 700 K with the CO conversion was not greater than 22% at temperature 823 K over these catalysts (Takenaka et al.

, 2004). A Pd – Mg/SiO

2 catalyst synthesized from a reverse microemulsion has been found to be active and selective for CO

2

methanation (Park et al.

, 2009). At 450 o

C, the Pd-Mg/SiO

2

catalyst had greater than 95% selectivity to CH

4

at a carbon dioxide conversion of 59%. They

23 claimed that the similar catalyst without Mg has an activity only for CO

2

reduction to

CO. these results support a synergistic effect between the Pd and Mg/Si oxide.

Furthermore, platinum-based catalysts present an activity and a selectivity that are almost satisfactory. Finch and Ripley (1976) claimed that the tungstennickel-platinum catalyst was substantially more active as well as sulfur resistant than the catalyst in the absence of platinum. It was capable to show a conversion of 84% of CO after on stream for 30 minutes in the presence of less than 0.03% CS

2

. No catalytic activity was observed under the poison of 0.03% CS

2

without the addition of Pt. The platinum group promoters enabled the catalysts to maintain good activity until the critical concentration of poison was reached. Pt catalysts were most well known as effective desulfurizing catalysts. Panagiotopolou and Kodarides (2007) found that the platinum catalyst is inactive in the temperature range of 200 o

C-400 o

C, since temperatures higher than 450 o

C are required in order to achieve conversion above 20%.

Moreover, Nishida et al.

(2008) found that the addition of 0.5 wt% Pt towards cp -Cu/Zn/Al (45/45/10) catalyst was the most effective for improving both activity and sustainability of the catalyst. At 250 o

C, the conversion of CO was achieved around 77.1% under gas mixture of CO/H

2

O/H

2

/CO

2

/N

2

= 0.77/2.2/4.46/0.57/30 mL/min. Pierre et al (2007) found that the conversion of CO over 5.3% PtCeOx catalyst prepared by urea gelation co-precipitation (UGC) which was calcined at

400 o

C reached about 92%. This catalyst is more active and shows excellent activity and stability with time on stream at 300 o

C under water gas shift reaction.

Recently, Bi et al.

(2009) found that Pt/Ce

0.6

Zr

0.4

O

2

catalyst exhibited a markedly higher activity with 90.4% CO conversion at 350 o

C for the water gas shift

(WGS) reaction. The methane selectivity was only 0.9% over this catalyst which has been prepared by wetness impregnation method. Meanwhile, Utaka et al.

(2003) examined the reaction of a simulated reforming gas over Pt-catalysts. At temperatures from 100 o

C to 250 o

C, high CO conversions of more than 90% were obtained but most of the conversion was caused by water gas shift reaction. The use of platinum catalyst in conversion of cyclohexane was conducted by Songrui et al.

24

(2006). They found that the cyclohexane conversion over Pt/Ni catalyst prepared by impregnation method (55%-53%) was obviously higher than that over Pt/Al

2

O

3 catalyst (30%-20%).

2.4 Supports for the Methanation Catalysts

The presence of the support was recognized to play an important role since it may influence both the activity and selectivity of the reaction as well as control the particle morphology. Insulating oxides such as SiO

2

, γ -Al

2

O

3

, V

2

O

5

, TiO

2

and various zeolites are usually used as material supports. These supports are used to support the fine dispersion of metal crystallites, therefore preparing them to be available for the reactions. These oxides will possess large surface area, numerous acidic/basic sites and metal-support interaction that offer particular catalytic activity for many reactions (Wu and Chou, 2009).

Alumina is often used as support for nickel catalyst due to its high resistance to attrition in the continuously stirred tank reactor or slurry bubble column reactor and its favorable ability to stabilize a small cluster size (Xu et al.

, 2005). Alumina does not exhibit methanation activity, it was found to be active for CO

2

adsorption and the reverse spillover from alumina to nickel increases the methane production especially for co-precipitated catalyst with low nickel loading (Chen and Ren, 1997).

It has a high surface area and a strong acidity favourable for hydrogenation reactions and commonly used as a carrier to form a uniform monolayer (Riedel and Schaub,

2003). Happel and Hnatow (1981) also said that alumina could increase the methanation activity although there was presence of low concentration of H

2

S.

Additionally, Chang et al.

(2003) believed that Al

2

O

3

is a good support to promote the nickel catalyst activity for CO

2

methanation by modifying the surface properties.

Alumina supported NiO based catalysts in the powdered form have been widely used by many researchers. Another alternative of alumina beads as a support material are rarely explored by researchers in their studies.

25 on nickel oxide catalyst prepared using impregnation technique. They revealed that the reactivity of the catalysts depended on the type of supports used which follow the order of Al

2

O

3

> SiO

2

> TiO

2

> SiO

2

·

Al

2

O

3

. The reason for the higher activity of

Ni/Al

2

O

3

catalyst with 70% methanation of CO

2

at 500 o

C was attributed to the basic properties of the Al

2

O

3

support on which CO

2

could be strongly adsorbed and kept on the catalyst even at higher temperatures. However, Takenaka et al.

(2004) found that the conversion of CO at 250 o

C were higher in the order of Ni/MgO (0%) <

Ni/Al

2

O

3

(7.9%) < Ni/SiO

2

(30.0%) < Ni/TiO

2

(42.0%) < Ni/ZrO

2

(71.0%). These results implied that Al

2

O

3

support was not appropriate for the CO conversion but suitable support for CO

2

conversion over Ni catalyst.

In addition, Seok et al. (2002) also investigated the effects of supports

(Al

2

O

3

, ZrO

2

, CeO

2

, La

2

O

3

and MnO) and preparation methods (co-precipitated and impregnated) on the catalytic activity and stability of Ni catalyst. Catalytic activity and stability were tested at 923 K with a feed gas ratio CH

4

/CO

2

of 1 without a diluent gas. Co-precipitated Ni/Al

2

O

3

, Ni/ZrO

2

, and Ni/CeO

2

showed high initial activities but reactor plugging occurred due to the formation of large amounts of coke. The gradual decrease in the activity was observed for Ni/La

2

O

3

and Ni/MnO, in which smaller amounts of coke were formed than in Ni/Al

2

O

3

, Ni/ZrO

2

, and

Ni/CeO

2

. The catalyst deactivation due to coke formation also occurred for impregnated 5 wt% Ni/

γ

-Al

2

O

3

catalysts. Addition of MnO onto this Ni/

γ

-Al

2

O

3 catalyst decreased the amount of deposited coke drastically and 90% of initial CO

2 conversion was maintained after 25 h.

Zhou et al.

(2005) had synthesized the Co-Ni catalyst support with activated carbon for CO selective catalytic oxidation. More than 97% conversion of CO was observed over Co-Ni/AC catalyst at reaction temperature between 120-160 o

C.

However, the conversion was decreasing dramatically with the increasing temperature when the reaction temperature is above 160 o

C. Activated carbons are used efficiently in many environmental remediation processes due to their high adsorption capacity, which makes their use possible in the removal of great variety of pollutants present in air and aqueous medium. This is because, besides their high

26 surface area, they possess several functional surface groups with an affinity for several adsorbates, justifying the extreme relevance of this adsorbent for the treatment of the pollutant (Avelar et al.

, 2010). Kiennemann et al.

(1981) found that the amorphous carbon material made of peat could increase the activity of the catalyst but high operating temperature is required.

2

amount with hydrogen (low CO x

/H

2

ratios) are dependent on the Ru dispersion and the kind of support for the metal. Among the supports used in their study (low and high surface area graphitized carbons, magnesia, alumina and a magnesium – aluminum spinel), alumina was found to be the most advantageous material. For similar Ru dispersions, CO methanation over Ru/Al

2

O

3

at 220 o

C was about 25 times and CO

2 methanation was about 8 times as high as ruthenium deposited on carbon B

(Ru3/CB). For high metal dispersion, the following of sequence was obtained:

Ru/Al

2

O

3

> Ru/MgAl

2

O

3

> Ru/MgO > Ru/C, both for CO and CO

2

methanation. It is suggested that the catalytic properties of very small ruthenium particles are strongly affected by metal – support interactions. In the case of Ru/C systems, the carbon support partly covers the metal surface, thus lowering the number of active sites (site blocking).

Takenaka et al.

(2004) also found that at 200 o

C, Ru/TiO

2

catalyst showed the highest activity among all the catalysts but when the reaction temperature was increased to 250 o

C, the CO conversion follows the order of Ru/MgO (0%) <

Ru/Al

2

O

3

(62.0%) < Ru/SiO

2

(85.0%) < Ru/ZrO

2

(100.0%) = Ru/TiO

2

(100.0%).

Ruthenium on the MgO, Al

2

O

3

, SiO

2

, ZrO

2

and TiO

2

support showed similar sequence of CO methanation activity and selectivity as reported by Görke et al.

(2005). They found that Ru/SiO

2

catalyst exhibits higher CO conversion compared to

Ru/Al

2

O

3

. Meanwhile, Panagiotopolou and Kodarides (2007) demonstrated that

Pt/TiO

2

is the most active catalyst at low temperatures exhibiting measurable CO conversion at temperature as low as 150 o

C. Conversion of CO over this catalyst increases with increasing temperature and reach 100% at temperature of 380 o

C.

While, platinum catalyst supported on Nd

2

O

3

, La

2

O

3

and CeO

2

become active at temperature higher than 200 o

C and reach 100% above 400 o

C. MgO and SiO

2

27 supported platinum catalysts are practically inactive in the temperature range of interest.

The activity of CO

2

oxidation had been investigated by Ali et al.

(2000) increases in the order of Pt/Al

2

O

3

< Pt/aerogel – SiO

2

< Pt/C. The order remains the same throughout the whole temperature range during the experiment, which was between 200 o

C to 300 o

C. According to the research of Suh et al. (2004), when platinum is supported on different supports, the activity for carbon monoxide removal slightly increases in the decreasing order of metal-support interaction, i.e.,

Pt – Al

2

O

3

composite aerogel (37.86% CO

2

conversion) < Pt/Al

2

O

3

(54.89% CO

2 conversion) < Pt/aerogel-SiO

2

(57.13% CO

2

conversion) < Pt/C. (70.46% CO

2 conversion). Then, addition of base metal to Pt/Al

2

O

3

enhances their performances for prefential oxidation (PROX) to a great extent. The formation of a new active phase by the modification contributes to the enhancement of catalytic activity.

The detailed studies on the SiO

2

and Al

2

O

3

support was reported by

Nurunnabi et al.

(2008) who investigated the performance of γ -Al

2

O

3

, α -Al

2

O

3

and

SiO

2

supported Ru catalysts prepared using conventional impregnation method. They found that γ -Al

2

O

3

support is more effective than α -Al

2

O

3

and SiO

2

supports for

Fischer-Tropsch synthesis under reaction condition of P=20 bar, H

2

/CO=2 and

GHSV=1800/h. γ -Al

2

O

3 showed a moderate pore and particle size around 8 nm which achieved higher catalytic activity about 82.6% with 3% methane selectivity than those of α -Al

2

O

3

and SiO

2

catalysts. Pentasil-type zeolite could also be used as a support for the catalysts which exhibited high activity and stability due to not only the basicity of alkaline promoters but also the incorporation with zeolite support

(Park et al ., 1995).

In the investigation done by Stoop et al.

(1986), the Ru catalysts supported on the SiO

2

in CO hydrogenation was the most active and stable catalyst compared with

Al

2

O

3

, TiO

2

and V

2

O

3

. Supports such as TiO

2

and V

2

O

3

exhibit Strong Metal

Support Interaction (SMSI) which might be modified the Ru particles or take part in the reaction. Furthermore, Solymosi et al.

(1981) reported a sequence of activity of supported rhodium catalysts of Rh/TiO

2

> Rh/Al

2

O

3

> Rh/SiO

2

. This order of CO

2

28 methanation activity and selectivity was the same as observed for Ni on the same support by Vance and Bartheolomew et al.

(1983). These phenomena can be attributed to the different metal-support electronic interactions which affects the bonding and the reactivity of the chemisorbed species.

Although, noble metal catalysts deposited on different supports such as alumina, titania and silica have been extensively studied, the effect of the addition of

Ni as a based for that catalyst has not been explored especially in the case of methanation in purification of natural gas. Thus, the objective of the present study is to develop the catalyst based on nickel oxide doped with noble metal to achieve high conversion of CO

2

methanation and desulfurization simultaneously at low temperature possibly at 200 o

C. The conversion at low temperature is more likely to applied in gas industry because the temperature higher than 300 o

C is unfavoured for the removal of CO

2

and H

2

S in the industry from the safety point of view.

29

CHAPTER 3

EXPERIMENTAL

3.1 Introduction

This chapter describes in detail about the chemicals and reagents, catalytic testing and characterization techniques. The catalyst preparation is also given in this chapter. Meanwhile, the various supported mixed metal oxide catalysts and its ratio prepared is shown in Appendix A.

3.2 Chemicals and Reagents

Nickel nitrate hexahydrate with chemical formula of Ni(NO

3

)

2

.6H

2

O produced by GCE Laboratory Chemicals was used as a based in this research.

Meanwhile, ruthenium(III) chloride hydrate (RuCl

3

.xH

2

O), palladium(II) nitrate hydrate (Pd(NO

3

)

2

.xH

2

O) and tetraamineplatinum nitrate ((NH

3

)

4

Pt(NO

3

)

2

) produced by Sigma Aldrich Chemical, rhodium(III) nitrate hydrate (Rh(NO

3

)

3

.xH

2

O) from

Fluka Analytical, copper(II) nitrate hexahydrate (Cu(NO

3

)

2

.6H

2

O) from Riedel-de-

Haën and manganese acetate tetrahydrate Mn(CH

3

COO)

2

.4H

2

O from Rinting

Scientific were used as a dopants. In addition, aluminium oxide beads (Al

2

O

3

) produced by MERCK Eurolab and photocatalytic ceramic beads (SiO

2

/TiO

2

) produced by Titan PE Technology Inc. were used as the support materials for the preparation of catalysts. Polyethylene glycol 2000 with chemical formula of

H(OCH

2

CH

2

) n

OH (Fluka Chemicals) and diethanolamine with chemical formula of

HN(CH

2

CH

2

OH)

2

(Merck-Schuchardt) were used for sol-gel preparation.

30

3.3 Catalysts

The metal precursors used in this research are nitrate and chloride salts.

Generally, all the samples in this research were prepared by aqueous incipient wetness impregnation method. Each of metal salts were weighed in a beaker according to the desired ratio (Appendix A) and dissolved it in small amount of distilled water. The nickel loading used was 90 wt%. Then, the solutions were mixed together and stirred continuously by magnetic bar for 30 minutes at room temperature to homogenize the mixture. For catalyst coated using noble metal chloride precursor, it was stirred in doubly distilled water and dispensed and this process was repeated at least three times in order to remove chloride ion. In Ni based catalyst, Al

2

O

3

is the most widely used support material. Thus, alumina beads with diameter of 4 mm to 5 mm were used as support material in this study. The support catalysts were immersed into the catalysts solution and left it for 24 hours. Then, the supported catalyst was transferred onto glass wool. It was then aged inside an oven at

80 – 90 o

C for 24 hours to remove water to allow good coating of the metal on the surface of the supported catalysts. It was then followed by calcination in the furnace at 400 o

C for 5 hours using a ramp rate of 10 o

C/min to eliminate all the metal precursor and excess of water or impurities (refer Figure 3.1). Similar procedure was repeated for the other ratio of catalysts.

(a) (b)

Figure 3.1 a) Uncoated and b) coated of alumina support

31

Reaction

The catalytic reaction of CO

2

methanation and H

2

S desulfurization was performed under atmospheric pressure in a fixed bed micro reactor and analyzed via online Fourier Transform Infrared Spectrometer as illustrated in Figure 3.2. The supported catalyst was placed in the middle of the glass tube made of Pyrex glass with diameter 10 mm and length of 360 mm. It was then secure with glass wool at both ends. Figure 3.3 shows the schematic diagram of glass tube for home-built micro reactor.

Pressure gauge To atmosphere

Mixture cylinder 3 way valve

3 way valve cell

FTIR

Furnace

CO

2

H

2

H

2

S

Figure 3.2 Schematic diagram of home-built micro reactor

360 mm

Supported Catalyst

10 mm

Glass wool

Figure 3.3

Schematic diagram of glass tube for home-built micro reactor

The reaction gas mixture consisting of CO

2

and H

2

in a molecular ratio of 1:4 was passed continuously through the catalysts and was heated in an isothermal tube furnace. The gas composition in the reactor was similar to the composition of crude natural gas which consisted of 20% CO

2

. A flow rate of CO

2

/H

2

= 50.00 cm

3

/min

32 was used. The feed gas flow rate was adjusted with a mass flow controller and the reaction temperature was performed from 60 o

C up to 400 o

C with the increment temperature rate of 5 o

C/min. Heating of the reactor was provided by an electric furnace controlled by a programmable controller which was connected via a thermocouple placed in the middle of the furnace.

Each testing experiment was conducted on a freshly prepared catalyst sample with the aim of avoiding any misleading result due to activity loss or any change that may occur during the reaction which cannot be recovered by regeneration. Prior to the start of each catalytic testing, the catalyst was first subjected to an air pretreatment at 100 o

C for 30 minutes to activate the catalyst sample. After cooling the catalyst down to room temperature, the reactant gas was introduced into the micro reactor system without passing through the catalyst as a calibration. After that, the reactant gas was passed through the catalyst in the sample tube. The product stream composition was collected in a FTIR sample cell (Figure 3.4) with attached

KBr windows and scanned using FTIR Nicolet Avatar 370 DTGS

Spectrophotometer. The FTIR spectra was recorded in the range of 4000 - 450 cm

-1 at the resolution of 4 cm

-1

and 5 scans to ensure better signal to noise ratio.

Figure 3.4

Diagram of FTIR sample cell

The catalytic measurement data were collected in terms of the peak area of the absorption band of the reactant gases in the range of wave number as demonstrated in Table 3.1. Meanwhile, the percentage conversion of CO

2

was calculated by using Equation 3.1.

33

% Conversion = Peak Area of CO

2 calibration

– Peak Area of CO

2 Experiment

x 100%

of CO

2

Peak area of CO

2 calibration

(3.1)

Table 3.1

Wavenumber of CO

2

, H

2

S, CO and CH

4

gases in FTIR spectra

Gases

H

2

S

CO

CO

2

CH

4

3.5

Optimization Parameters

Wavenumber (cm

-1

)

Stretching mode

2087 - 2022

Deformation mode

-

2100 - 2200

2397 - 2275

3200 - 2850

-

800 - 600

1400 - 1300

The optimization parameters that have been used on two potential catalysts in this study were amount of Ni loadings, calcination temperature of supported catalyst, type of support materials, pre-calcination temperature of alumina support, method of preparation, H

2

S testing, reproducibility testing and finally regeneration testing.

3.5.1

Amount of Nickel Loading

The parameter was carried out to determine the effect of nickel loading on the catalyst. Overall, the catalysts were prepared based on the 90 wt% of nickel loading.

After the best two catalysts were chosen from the catalytic testing, these catalysts were undergone for another loading of nickel which are 80 wt% (80:20 for ratio one dopant and 80:15:5 for ratio two dopants) and 70 wt% (70:30 for ratio one type dopant and 70:25:5 for ratio two type dopants).

34

3.5.2 Calcination Temperature of Supported Catalysts

This parameter was conducted to determine the optimum calcination temperature for the catalyst to obtain high performance in conversion of CO

2 methanation. Firstly, the catalysts were coated on alumina support using wetness impregnation method as explained in Section 3.3 above then the catalysts were calcined at temperature of 400 o

C for 5 hours using a ramp rate of 10 o

C/min. The same procedure was repeated by setting the calcination temperatures at 500 o

C and

600 o

C for 5 hours.

3.5.3 Type of Support Materials

The potential catalyst solution were prepared and impregnated on different catalyst supports to obtain a higher performance of CO

2

conversion and H

2

S desulfurization. Furthermore, the supported catalyst used must be more flexible when applied in industry, easy to handle and reusable. The supports used in this research include Al

2

O

3

beads, SiO

2

/TiO

2

beads and carbon form palm kernel shell (PKS).

After introducing the metals precursor on the catalyst supports, catalysts samples were dried in the oven at 80-90 o

C for 24 hours. After that, the potential catalysts were subjected to final calcination at optimum temperature.

3.5.3.1 Preparation of Carbon Support from Palm Kernel Shell (PKS)

One of the support materials used in this research is carbon. It was prepared from palm kernel shell. Raw PKS was washed using distilled water 3 times to remove the existence of impurities such as dead leaf. It was left to dry at room temperature. After that, the raw PKS was fully soaked in 1 M of HCl for one day to remove the volatile organic compound (VOC). The treated PKS was then washed and stirred for many times with distilled water until the final pH was 7. Then, it was

35 allowed to dry in an oven 80 o

C before calcined in the furnace for 4 hours at 500 o

C

(refer Figure 3.5). a) Untreated PKS b) Treated PKS

Figure 3.5 Carbon support from Palm Kernel Shell (PKS)

3.5.4 Pre-Calcination Temperature of alumina support

The supported catalyst was prepared by a similar procedure as described in

Section 3.3 above but the support used was the alumina that was already calcined for

5 hours at 700 o

C and 1000 o

C. The supported catalyst was then allowed to dry in an oven at 80 o

C and calcined using furnace for another 5 hours at 400 o

C using a ramp rate of 10 o

C/min.

The purpose of catalyst preparation was to develop an active, stable and to obtain supported catalysts that are able to operate at temperature below 400 o

C. Three techniques were used in this research to synthesize the catalysts which include incipient wetness impregnation (IWI) method, sol gel method and the modified IWI method. This sol-gel process has been used to prepare supported metal catalysts with higher thermal stability, higher resistance to deactivation while allowing a better

36 flexibility in controlling catalyst properties such as particles size, surface area and pore distribution.

The catalyst prepared by incipient wetness impregnation method was explained as above in Section 3.3 while, sol gel method was prepared by dissolving the 3 g of Ni(NO

3

)

2

.6H

2

O and 1.5 g of diethanolamine (DEA) with 10 mL of ethanol.

The solution was stirred continuously for 15 minutes. In other beaker, 8.75 mL of ethanol was added into 0.44 g of polyethylene glycol (PEG). Then, the two solutions were mixed together before the addition of 0.4 mL of H

2

O to it. The solution was stirred for 1 hour continuously to achieve the sol of Ni. The procedure above was repeated for each dopant salts in a different beaker. After that, the sol of dopants were mixed with the sol of Ni and stirred for about 10 minutes to homogenize the solution. Finally, the alumina was dipped into the solution and left for 24 hours then continued by aging at 24 hours in an oven and calcined at 400 o

C for 5 hours.

For the modification of IWI technique, 3 g of Ni(NO

3

)

2

.6H

2

O was dissolved in 3 mL of distilled water in a beaker and stirred for 15 minutes. Then, the alumina support was impregnated with a Ni solution at room temperature and left for 4 hours.

After that, the supported catalyst was dried at 80 o

C overnight. On the next day, the first dopant salt was weighed and transferred in a beaker, dissolved in 3 mL of distilled water and stirred for 15 minutes. Then, the support that was prepared before was impregnated in a solution for 4 hours and dried at 80 o

C overnight. The procedure was repeated for second dopant salt. Finally, the supported catalyst was calcined at

400 o

C for 5 hours.

3.5.6 H

2

S Testing

The catalytic reaction condition for H

2

S testing was the same as described in

Section 3.4. However, the reaction gas mixture consisting of CO

2

and H

2

in the molecular ratio 1:4 was mixed with H

2

S gas. The flow rate of H

2

S gas used was 1 mL/min.

37

Catalytic testing using in house built micro reactor explained in Section 3.4 was repeated using the potential catalyst several times until the catalyst was deactivated.

Regeneration activity was conducted on the spent catalyst that has been deactivated in Section 3.5.6. It was then regenerated by heating the catalyst at 200 o

C for 3 hours and blowing compressed air simultaneously. The catalyst was then undergone catalytic testing again to check whether the catalyst will produce the same conversion as a fresh one. If not, the procedure was repeated by heating the catalyst at 300 o

C and 400 o

C for 3 hours, 5 hours and 7 hours until the performance activity similar to the fresh catalyst.

3.6 Methane Measurement via Gas Chromatography

The gas product was periodically analyzed with Gas Chromatography in order to determine the formation of the methane from the carbon dioxide conversion reaction. The Hewlett Packard 6890 Series Gas Chromatography System with capillary column (Brand: Ultra 1 932530) with 25.0 m × 200 μm × 0.11 μm nominal column was used in this research. The initial temperature was 40 o

C for 7 minutes and the injection temperature was 150 o

C. The detection temperature for this analysis was

300 o

C. The entire gas samples were then analyzed via Flame Ionization Detector

(FID).

The components present in outlet stream from the column were detected by

Flame Ionization Detector (FID) after the analysis was done. Before the analysis,

38 calibration of methane was carried out using standard 99% pure methane gas. Figure

3.6 shows the calibration graph of standard methane with peak area against volume, mL. The concentration of methane produced by the catalytic testing is calculated by referring to the peak area of chromatogram and this standard methane graph. The calculation of methane yield and selectivity to methane is defined by the formula below and shown in Appendix B:-

% Yield of CH

4

= [CH

4

] from GC × 100%

[Converted CO

2

] from FTIR

% Selectivity to CH

4

= [CH

4

] from GC × 100

% Conversion of CO

2

(3.2)

(3.3)

600000

average

y = 56025x

R² = 0.978

500000

400000

300000

200000

100000

0

0 2 4 6 volume, mL

8 10

Figure 3.6 Calibration graph of standard 99.0% pure methane

12

3.7 Characterization

Potential catalyst was subject to several characterization techniques to study its chemical and physical properties. The information obtained is highly useful in order to understand the relationship between the properties and its catalytic performance towards the methanation activity. In this research, the characterization

39 techniques that used were X-Ray Diffraction Spectroscopy (XRD), Field Emission

Scanning Electron Microscopy (FESEM), Energy Dispersive X-Ray (EDX),

Nitrogen Absorption (NA), Fourier Transform Infrared (FTIR) and

Thermogravimetry Analysis-Differential Thermal Analysis (TGA-DTA).

3.7.1 X-Ray Diffraction Spectroscopy (XRD)

X-Ray diffraction was used to reveal the information about the material structure (its symmetry in transformational structure that occurred in the crystal).

This technique can also be used to determine any changes in phase or crystal’s shape.

In this analysis, the sample was crushed manually into fine powder and then was placed into the sample holder with diameter of 10 - 15 mm and approximate depth of

1 mm in a 40 × 33 × 2 mm glass plate. A glass slide was used to flatten the surface of the powder to ensure the sample height was within the diffractometer. In this research, XRD pattern was recorded by the Diffractometer D5000 Siemens

Crystalloflex using Cu Kα radiation (λ=1.54060 Å). Patterns were recorded for 2θ values comprised between 10 o

to 80 o

and the data obtained was analyzed by a PC interfaced to the diffractometer using software called Diffrac Plus. Peaks position, width and intensity were then identified by a comparison done by an accumulated

Powder Diffraction File (PDF) data which comes with the software used in this technique.

3.7.2 Field Emission Scanning Electron Microscopy - Energy Dispersive X-

Ray (FESEM-EDX)

The surface morphology of the potential catalyst was obtained using scanning electron microscopy Zeiss Supra 35VP FESEM with energy of 15.0 kV coupled with

EDX analyzer. Energy Dispersive X-Ray was used in this study to determine the elemental composition on the submicron scale. For both purposes, the supported catalyst sample was mounted on special platform called stub, coated with gold using

40 a gold sputter at 10

-1

Mbar using a Bio Rad Polaron Division SEM coating system machine. The sample was bombarded using an electron gun with a tungsten filament under 25 kV resolutions to get the required magnification image. The images were observed at different points along the platform.

3.7.3 Nitrogen Absorption Analysis (NA)

Nitrogen absorption analysis was measured using a Micromeritics ASAP

2010 in order to determine the catalyst surface area, average pore sizes, pore type, pore shape and total pore volumes. 0.2-0.3 g of powder sample was degassed at

120 o

C to remove the previously adsorbed gasses and evacuating the dead space by vacuum pump to cool at room temperature before the analysis was done. The specific surface area was calculated from the adsorption curve according to the BET method.

The pore size distribution was calculated from the desorption curve according to the

Dollimore-Heal method. The data that was obtained by this analysis are average pore diameter, Isotherm, Langmuir Surface Area, BET Surface Area and t-Plot.

3.7.4 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR analysis was used to determine the absorbed species present on the surface of the catalyst sample. The sample was mixed and grounded together with

KBr according to the ratio of 1:10. After that, 10 tones of pressure was applied around 5 minutes before a thin transference film was formed. The analysis of transference film was conducted using FTIR Nicolet Avatar 370 DTGS

Spectrometer. A FTIR spectrum was plotted according to the percentage transmittance versus wavenumber (cm

-1

) in a range of 4000 - 400 cm

-1

at the resolution of 4 cm

-1

using 5 scans.

41

3.7.5 Thermogravimetry Analysis-Differential Thermal Analysis (TGA-DTA)

Thermogravimetric analysis (TGA) is a thermal analysis technique which measures the weight change in a material as a function of temperature and time while, Differential thermal analysis (DTA) is a calorimetric technique, recording the temperature and heat flow associated with thermal transitions in a material. This enables phase transitions to be determined. The TGA-DTA curves were obtained by

TGA-SDTA 851 Mettler Toledo simultaneous thermal analyzer. 0.3-0.5 g of catalyst sample in the powder form was weighed in 900 μL alumina crucible and was then heated at temperature of 60 to 900 o

C at 15 o

C/ min in the flows of nitrogen gas (50

μL/ min). The micro balance was connected to the thermal analysis processing system to get the plot of weight loss versus temperature.

42

CHAPTER 4

RESULTS AND DISCUSSION

In order to investigate the potential catalyst for simultaneous CO

2

/H

2 methanation and H

2

S desulphurization reactions, a series of nickel oxide based catalyst with different ratios of dopants were prepared using wetness impregnation method followed by calcination at 400 o

C for 5 hours. In this research, the catalytic activity of all the prepared catalyst was tested by using an in-house built micro reactor coupled with FTIR. The in-situ reactions were performed at 60 o

C up to

400 o

C. However, the reaction temperatures lower than 200 o

C is favorable in industry from the safety point of view and cost effective plant operation.

4.1.1 Catalytic Performance of Supported NiO Based Catalyst with

Ruthenium as a First Dopant

Al

2

O

3

supported NiO based catalyst was modified by incorporating with different dopants (Ru, Pd, Rh) to study their effect on the catalytic activity. Some dopants are found to be active to enhance the conversion of CO

2

. Table 4.1 shows the catalytic performance of alumina supported nickel oxide based catalyst with ruthenium as a dopant and co-dopant. Based on theoretical calculation, the nickel loading was fixed to 90 wt% and the ratio for one dopant is 10:90 while two dopants is 2:8:90.

43

Table 4.1


2

from methanation reaction over various alumina supported nickel oxide based catalysts with ruthenium as a dopant and co-dopant

Alumina Supported

Catalysts

Ni (100%)/Al

2

O

3

100 o

C

6.35

Reaction Temperature

200 o

C 300 o

C

% Conversion of CO

2

10.86 18.15

400 o

C

24.58

Ru/Ni (10:90)/Al

2

O

3

1.26 1.89 3.94 7.21

Pd/Ru/Ni (2:8:90)/Al

Pt/Ru/Ni (2:8:90)/Al

2

2

O

O

3

3

Rh/Ru/Ni (2:8:90)/Al

2

O

3

Ru/Mn/Ni (2:8:90)/Al

2

O

3

Ru/Cu/Ni (2:8:90)/Al

2

O

3

37.94

10.75

3.23

7.74

14.62

43.60

18.03

5.67

22.19

24.02

45.77

32.33

11.00

26.31

24.88

52.95

34.74

19.29

34.86

25.87

Ru/ Pd/ Ni (2:8:90)/ Al

2

O

3

17.30 22.58 28.77 35.64

Table 4.1 illustrates the catalytic performance of supported nickel oxide based catalysts which were calcined at 400 o

C for 5 hours. According to the results,

Ni/Al

2

O

3

catalyst gives only 24.58% conversion of CO

2

at reaction temperature of

400 o

C. Incorporating ruthenium into this catalyst further lowered the catalytic performance towards CO

2

conversion to 7.21%. The decreasing performance of this catalyst could be due to the Ru precursor, RuCl

3

.nH

2

O used in this study. This is in a good agreement with Nurunnabi et al.

(2008) who had found that the small amount of chloride ion in Ru/Al

2

O

3

catalyst could lead to the decrease of active sites of Ru catalyst surface. The residual chloride ions presence formed partition between the support and the metal and therefore, inhibits both CO and hydrogen chemisorptions phenomena on the catalyst surface.

However, the addition of palladium into Ru/Ni catalyst, Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst coincidentally enhanced the catalytic activity for the conversion of CO

2

of the prepared catalysts. It showed 37.94%, 43.60%, and 45.77% of CO

2

conversion at reaction temperatures of 100 o

C, 200 o

C and 300 o

C, respectively.

While, 52.95% CO

2

conversion was achieved at maximum studied temperature of

400 o

C. This suggests that small amounts of Pd addition can play an important role on

44 the improvement of catalyst activity. This is in accordance with Baylet et al.

(2008) finding who studied the catalytic activity and stability of Pd doped hexaaluminate catalysts for the CH

4

catalytic combustion. They found that the addition of palladium to the alumina support material gives a sufficient absorption for CO

2

dissociation process which is due to the increasing of active sites created on the catalyst surface.

When platinum was added into Ru/Ni catalyst, Pt/Ru/Ni (2:8:90)/Al

2

O

3 catalyst did not perform well and managed to convert only 34.74% at maximum reaction temperature, 400 o

C. While at 200 o

C, the conversion of CO

2

was around

18.03%. Bi et al . (2009) reported that Pt catalyst generally effective for the reverse water-gas shift (RWGS) reaction. It can be seen from the Table 4.1, the result of

Ru/Ni doped with Pt was not as good as Ru/Ni doped with Pd. It showed Pd is better than Pt. Our finding was in agreement with Lapisardi et al.

(2006) who revealed that the conversion rate of Pd catalyst was four times higher than on Pt catalyst for the production of syngas. As previously reported by Gelin et al.

(2003), Pd/Al

2

O

3

did not exhibit any deactivation with time on stream under experimental conditions but

Pt/Al

2

O

3

slowly deactivates with time. The decreasing activity of this catalyst mostly due to the sintering of Pt particles exaggerated by the local hot spots owing to the highly exothermic CO

2

methanation reaction which responsible for the loss of Pt dispersion (Yaccato et al., 2005).

The presence of rhodium in the Rh/Ru/Ni (2:8:90)/Al

2

O

3

catalyst was even worst with poor activity and selectivity in comparison to Ni/Al

2

O

3

. Only 19.29%

CO

2

conversion was observed in the similar experimental conditions even at the highest reaction temperature of 400 o

C. The finding might be explained that the addition of Rh in this catalyst caused poor metal dispersion. This was also reported by Wachs (2005) and the reason given was due to the poor interaction between active basic metal oxide and support material. Thus, the metals are easily migrated and sintered to form large metal particles (Wu and Chou, 2009) leading to the agglomeration on the surface of the catalyst which will reduce degree of dispersion.

Furthermore, when Ru was added into Mn/Ni/Al

2

O

3

catalyst to form Ru/Mn/Ni

(2:8:90)/Al

2

O

3

catalyst, it did not show a good performance in its catalytic activity which only gave 22.19% of CO

2

conversion at reaction temperature of 200 o

C.

45

Meanwhile, the CO

2

conversion of Ru/Cu/Ni/Al

2

O

3

catalyst with the ratio of 2:8:90 gave not more than 25.87% of conversion even at maximum reaction temperature of

400 o

C.

Pd and Ru are a good combination for Ni based only when the ratio of Ru is higher than that of Pd. It can be observed from the Table 4.1, only 35.64% of CO

2 conversion was obtained at reaction temperature of 400 o

C for the Ru/Pd/Ni

(2:8:90)/Al

2

O

3 catalyst compared to the Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst. From the results, it was suggested that Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst is the potential catalyst for CO

2

conversion. Generally, the conversion rate of CO

2

for all the prepared catalysts increased with the increasing of reaction temperature.

4.1.2 Catalytic Performance of Supported NiO Based Catalyst with Rhodium as a First Dopant.

From the results discussed in Section 4.1.1 above, it was proven that the reducibility of the catalyst and its metal dispersion can be improved with the incorporation of suitable promoters. Therefore, rhodium as first dopant was also prepared in this study to examine the effect of the rhodium on the Ni based catalyst.

Table 4.2 shows the performances of alumina supported nickel oxide based catalysts with rhodium as a dopant and co-dopant.

As can be seen that Rh/Ni (10:90)/Al

2

O

3 catalyst which was calcined at

400 o

C shows the best catalytic performance for CO

2

conversion among all the catalysts prepared. At reaction temperature of 100 o

C and 200 o

C, the catalytic activity of this catalyst showed 21.37% and 29.20% of conversion, respectively. When the temperature was increased from 300 o

C to 400 o

C, the CO

2

conversion increased significantly from 33.60% to 89.03%. The catalyst containing of Rh when reacting with H

2

will reduce the highest oxidized state, Rh

3+

to more stable Rh

0

even at room temperature (Paál et al ., 2007). The finding may indicate that the presence of Rh

0 strongly enhances the catalytic performance of the Ni based catalyst. There are three steps of reaction temperature regions for Rh catalyst to be activated. The first step

46 was formation of Rh metal from Rh

2

O

3

by reduction reaction, the second step was the sintering or coalescence of the tiny Rh crystallites into the original particles and the third step was the homogenization of the segregated metal particles as reported by Wang and Schmidt (1981). Rhodium segregation usually emerged at temperature above than 530 o

C. It seems possible to suggest that incorporating of Rh into the catalyst system gives a positive effect on the methanation reaction because H

2

and

CO

2

are easily chemisorbed and activated on the surface of the catalyst.

Table 4.2


2

from methanation reaction over various alumina supported nickel oxide based catalysts with rhodium as a dopant and co-dopant

Alumina supported

Catalysts

100 o

C


200 o

C 300 o

C 400 o

C

Ni (100%)/Al

2

O

3

6.35


2

10.86 18.15 24.58

Rh/Ni (10:90)/Al

2

O

3

Pt/Rh/Ni (2:8:90)/Al

2

O

3

Pd/Rh/Ni (2:8:90)/Al

2

O

3

21.37

2.86

10.78

29.20

5.48

20.35

33.60

13.60

30.05

89.03

78.38

84.92

Ru/Rh/Ni (2:8:90)/Al

2

O

3

Rh/Mn/Ni (2:8:90)/Al

2

O

3

Rh/Cu/Ni (2:8:90)/Al

2

O

3

17.84

15.80

11.57

22.00

21.19

13.70

26.31

26.84

12.55

44.14

31.12

23.72

In contrast, the rate of CO

2

conversion over Rh/Ni (10:90)/Al

2

O

3

was lower after the addition of Pt to form Pt/Rh/Ni (2:8:90)/Al

2

O

3

catalyst. The conversion is lower compared to the non doping catalyst of Ni/Al

2

O

3

which was converting only

5.48% of CO

2

at 200 o

C. It might be explained that the active sites of Rh and Ni was covered by less active Pt particles during preparation which could be the reason for the lower activity on CO

2

methanation of Pt containing catalysts. Therefore, the active species on the surface of catalyst was decreased with the addition of Pt. This finding was in good agreement with Panagiotopolou et al.

(2008) who claimed that,

Pt was not active enough to convert CO

2

to CH

4

since the temperature higher than

400 o

C is required to obtain the higher conversion. This is because the Pt containing catalyst is more active towards the reverse water-shift gas (RWSG) reaction as

47 reported earlier. The existence of CO and H

2

O species will react together resulting the increasing of CO

2

in the product stream that can be detected by the FTIR.

Furthermore, the CO

2

conversion only achieved 10.78% at 100 o

C over

Pd/Rh/Ni (2:8:90)/Al

2

O

3 catalyst. When the temperature was raised up to 200 o

C, the

CO

2

conversion reached 20.35% and the conversion only increased to 22% when Pd was replaced by Ru in the catalyst to form Ru/Rh/Ni (2:8:90)/Al

2

O

3

. These results showed that the addition of palladium and ruthenium in the catalyst which contains

Rh and Ni does not promote the catalytic activity. It is probably due to the strong chemical interaction of the metal oxide with the alumina support which leads to the difficulty of reduction over supported catalysts. So, the active surface of this catalyst was not homogeneously dispersed for methanation reaction. Therefore, palladium and ruthenium are not a good combination for the Rh/Ni/Al

2

O

3

catalyst under methanation reaction.

Similarly, the addition of Mn as a second dopant to form Rh/Mn/Ni/Al

2

O

3 catalyst gave only 21.19% CO

2

conversion at 200 o

C. In the presence of Cu in the

Rh/Cu/Ni (2:8:90)/Al

2

O

3

, the catalyst was not performing very well as for the based catalyst Ni/Al

2

O

3

. The CO

2

conversion was only 23.72% even at higher reaction temperature of 400 o

C. From this comparison, it can be concluded that the doping of transition metal of Cu and Mn as second dopant did not effect the catalytic activity of the Rh/Ni/Al

2

O

3

catalyst.

In summary, results listed in Table 4.1 and Table 4.2 suggest that increasing the reaction temperature up to 400 o

C also increased the CO

2

conversion. It also showed that the dopant introduced to the nickel oxide based catalyst gave different effects on the reaction of catalytic activity. In this research, the effect of addition of

Rh is more prominent at higher temperature due to the interaction between noble metal and Al

2

O

3

support which is one of the most important factors that determined the redox ability and thus will increase the catalytic activity, as suggested by Miao et al.

, (1997).

48

Figure 4.1 shows the FTIR spectra of gaseous products resulted from catalytic screening of Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O

3

catalysts starting from 100 o

C until the maximum studied temperature of 400 o

C. The CO

2

stretching was monitored at region between 2274.8 cm

-1

to 2397.4 cm

-1

. At the beginning of the catalytic activity measurement (100 o

C), the gaseous mixture gave the maximum CO

2 stretching and bending peaks but no peak was observed at ~3000 cm

-1

due to no CH

4 formation. As the catalytic activity temperature was increased, the intensity of the

CO

2

peak was decreased. Interestingly, at 200 o

C small peak due to CH

4

stretching was observed and the intensity peak for CH

4

increased as the reaction temperature was increased. i) ii)

Figure 4.1 FTIR spectra of gaseous products obtained from catalytic screening over i) Pd/Ru/Ni (2:8:90)/Al

2

O during CO

2

methanation reaction

3 ii) Rh/Ni (10:90)/Al

2

O

3

catalysts

49

Based on the results taken from catalytic activity testing in Table 4.1 and

Table 4.2, Pd/Ru/Ni (2:8:90)/Al

2

O


2

O

3

catalysts were found as the most potential catalysts for CO

2

conversion. Several optimization parameters were conducted on these catalysts including the amount of Ni loadings, calcination temperature of supported catalyst, type of support materials, pre-calcination temperature of alumina support, method of preparation, H

2

S testing, reproducibility testing and finally regeneration testing .

The catalytic activity and selectivity of supported catalysts are mainly effected by the the size of dispersed metal particles, the amount of metal employed in the catalyst and the used of the support material as well as the metal-support interactions. Therefore, to promote the catalyst activity, it is necessary to modify the catalyst’s surface properties .

Table 4.3 compares the amount of Ni loading towards the percentage CO

2 conversion by the Pd/Ru/Ni/Al

2

O

3

catalyst. The detailed trend plot of the catalytic activity is as shown in Figure 4.2. The Ni loading used were 90 wt%, 80 wt% and 70 wt%. The catalytic activity of the Pd/Ru/Ni (5:25:70)/Al

2

O

3

catalyst is only slightly lower than that observed on the same catalyst with Ni loading of 90 wt% and 80 wt%. The CO

2

conversion over Pd/Ru/Ni (5:25:70)/Al

2

O

3

catalyst reached their maximum conversion at 400 o

C around 32.88% and achieved 45.06% for Pd/Ru/Ni

(5:15:80)/Al

2

O

3

catalyst at the same reaction temperature. The lowest activity of Ni catalyst with lowest Ni amount was due to the partial Ni particles located in the pores, thus leading to the lesser active sites for reduction process as mentioned by

Perkas et al.

(2009). From this observation, it can be concluded that the catalytic activity of Ni loading follows the trend in the order of 90 wt% > 80 wt% > 70 wt%.

50

Table 4.3

Comparison of CO

2

conversion from methanation reaction over

Pd/Ru/Ni/Al

2

400 o

O

3

catalyst with different loading of nickel calcined at

C for 5 hours


Catalysts

Ni (100%)/Al

2

O

3

100 o

C

6.35


200 o

C 300 o

C


2

10.86 18.15

400 o

C

24.58

Pd/Ru/Ni (2:8:90)/Al

2

O

3

Pd/Ru/Ni (5:15:80)/Al

2

O

3

Pd/Ru/Ni (5:25:70)/Al

2

O

3

37.94

29.55

22.63

43.60

37.57

32.05

45.77

41.22

32.84

52.95

45.06

32.88

60

50

40

30

20

10

0

0 100 200

Reaction Temperature ( o

300

C)


2

O

3

400

Pd/Ru/Ni (5:15:80)/Al

2

O

3

Pd/Ru/Ni (5:25:70)/Al

2

O

3

Ni (100%)/Al

2

O

3

Figure 4.2 Catalytic performance of CO

2


2

O

3


C for 5 hours with various loading of nickel: i) 90 wt% (Pd/Ru/Ni (2:8:90)/Al

2

O

3

), ii) 80 wt% (Pd/Ru/Ni (5:15:80)/Al

2

O

3

) and iii) 70 wt% (Pd/Ru/Ni

(5:25:70)/Al

2

O

3

)

Figure 4.3 shows the trend effect of nickel loading towards catalytic activity over Rh/Ni/Al

2

O

3

catalyst and the data of the catalytic activity is shown in Table 4.4.

The performance of Rh/Ni (10:90)/Al

2

O

3

catalyst was described in Section 4.1.2 above. The CO

2

conversion by this catalyst reached 29.20% at 200 o

C. With the decrease in the Ni loading to 80 wt%, the CO

2

conversion of Rh/Ni (20:80)/Al

2

O

3 achieved 33.65% at 200 o

C. The performance of this catalyst is better when the Ni

51 content reached 70 wt% where it was able to convert around 43.02% of CO

2

at temperature of 200 o

C and 90.1% at 400 o

C. The effect of the Ni content on the catalytic activity of Rh/Ni/Al

2

O

3

catalyst changed in the following order: 70 wt% >

80 wt% > 90 wt%. The highest amount of nickel can possibly caused the blocking on the catalyst pores structure, thus decrease the porous volume and size as had been explained by Perkas et al ., (2009) in their Ni-Zr-Ce catalyst. In addition, according to

Natesakhawat et al.

(2005) the decreasing of catalyst surface area is more obvious when Ni loading increased owing to the aggregation of Ni to form larger particles on the surface of Al

2

O

3

support, thus reduced the performance of the catalyst.

100

90

80

70

60

50

40

30

20

10

Rh/Ni (10:90)/Al

Rh/Ni (20:80)/Al

Rh/Ni (30:70)/Al

Ni Ni (100%)/Al

2

O

3

2

2

2

O

O

O

3

3

3

0

0 100 200 300

Reaction Temperature ( o C)

400

Figure 4.3


2

conversion from methanation reaction over Rh/Ni/Al

2

O

3


C for 5 hours with various loading of nickel: i) 90 wt% (Rh/Ni (10:90)/Al

2

O

3

), ii) 80 wt% (Rh/Ni

(20:80)/Al

2

O

3

) and iii) 70 wt% (Rh/Ni (30:70)/Al

2

O

3

)

52

Table 4.4

Comparison of CO

2


Rh/Ni/Al

2

O

3

catalyst with different loading of nickel calcined at

400 o

C for 5 hours


Catalysts

Ni (100%)/Al

2

O

3

100 o

C

6.35


200 o

C 300 o

C


2

10.86 18.15

400 o

C

24.58

Rh/Ni (30:70)/Al

Rh/Ni (20:80)/Al

Rh/Ni (10:90)/Al

2

2

2

O

O

O

3

3

3

39.81

25.37

21.37

43.02

33.65

29.20

48.27

40.50

33.60

90.10

85.00

89.03

From Figures 4.2 and 4.3, it can be concluded that the best ratio for nickel based catalyst doped with ruthenium and palladium is 2:8:90 while, 30:70 is the best ratio for nickel catalyst doped with rhodium . The selection of metal loading is tremendously important in order to properly balance the activity with selectivity.

Then, these catalysts were further analyzed by varying the calcination temperature of supported catalyst.

4.2.2 Effect of Different Calcination Temperature towards Supported Catalyst

This parameter was investigated to determine the effect of calcination temperature on alumina supported catalyst towards CO

2 conversion. Pd/Ru/Ni

(2:8:90)/Al

2

O


2

O

3

catalysts were coated on alumina and aged in an oven for 24 hours before calcined the catalyst at four different temperatures of

400 o

C, 500 o

C, 700 o

C and 1000 o

C. Figure 4.4 indicates the trend plot of catalytic activity over Pd/Ru/Ni (2:8:90)/Al

2

O

3

at various calcination temperatures and the data of the catalytic activity is shown in Table 4.5.

53

60

50

40

30

20

10

400 C

500 C

700 o

700oC C

1000 o

C

0

0 100 200 300


400

Figure 4.4


2


2

O

3


C, ii) 500 o

C, iii) 700 o

C and iv)

1000 o

C

From Figure 4.4 above, it can be observed that the highest CO

2

conversion was obtained from Pd/Ru/Ni (2:8:90)/Al

2

O

3


C with 43.60% conversion, followed by Pd/Ru/Ni (2:8:90)/Al

2

O

3


C and

700 o

C which achieved only 24.1% and 10.36% of CO

2

conversion at 200 o

C, respectively. This phenomena could be explained by the drastic increasing of calcination temperature which will decrease the specific surface area of the solids and then the catalytic activity. This is probably due to the deep encapsulation of sintered noble metal particles which limit its diffusion by reducing the pore size as stated in the research of Ruckenstein and Hu (1995). As expected, further increased of the calcination temperature to 1000 o

C, induces the decreasing of catalytic activity over this Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst. It can be concluded in this research that

400 o

C was the best calcination temperature over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst.

54

Table 4.5

Comparison of CO

2



2

O

3

catalyst calcined for 5 hours at four different calcination temperatures

Calcination Temperature of Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst

100 o

C

37.94


200 o

C 300 o

C


2

43.60 45.77

400 o

C

52.95 400

o

C

500 o

C

700 o

C

17.31

4.16

24.1

10.36

32.51

11.76

35.65

23.27

1000 o

C 2.48 3.16 7.49 10.48

Figure 4.5 shows the comparison trend plot of calcination temperature over

Rh/Ni (30:70)/Al

2

O

3

catalyst. Similar results were obtained for Rh/Ni (30:70)/Al

2

O

3 catalyst as for Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst. It can be seen that the highest conversion of CO

2

was obtained at calcination temperature of 400 o

C which achieved

43.02% then followed by 37.55% and 20.97% for the catalyst calcined at 500 o

C and

700 o

C at reaction temperature 200 o

C, respectively. The percentages of CO

2 conversion over the respective catalysts are shown in Table 4.6.

Table 4.6

Comparison of CO

2

conversion from methanation reaction over Rh/Ni

(30:70)/Al

2

O

3

catalyst calcined for 5 hours at different calcination temperatures

Calcination

Temperature of Rh/Ni

(30:70)/Al

2

O

3 catalyst

100 o

C


200 o

C 300 o

C 400 o

C

400 o

C

500 o

C

39.81

31.29


2

43.02 48.27

37.55 42.03

90.10

95.25

700 o

C 14.86 20.97 25.67 91.80

55

100

90

80

70

60

50

40

30

20

400 o

400oC C

500 C

700 C

10

0

0 50 100 150 200 250 300 350 400


Figure 4.5

Catalytic CO

2


2

O

3


C, ii) 500 o

C and iii) 700 o

C

As expected, the increasing calcination temperature has decreased the catalytic activity which probably due to the changes that occurred on the surface of the catalyst. It is in good agreement with Perego and Villa (1997) who claimed that several processes may occur during calcinations that includes modification of structure and texture through the sintering process, active phase generation, loss of chemically bonded carbon dioxide or water and stabilization of chemical properties.

The high calcination temperature could cause agglomeration of catalyst particles thus decreasing the surface area consequently produces less active catalyst. According to

Oh et al.

(2007) the growth in crystallite size and the morphology on the surface of catalysts have strong relationship with these calcination temperatures. It can be proven by the XRD analysis (Section 4.5.1) which will be discussed in characterization part.

From Figure 4.4 and 4.5, it can be summarized that the calcination temperature of 400 o

C is the best for both catalysts. In other words, the conversion of

CO

2

over alumina supported catalyst in this research with different calcination temperatures is in the increasing order as follows: 400 o

C > 500 o

C > 700 o

C. It was then further optimized by varying the support materials.

56

4.2.3 Effect of Different Support Materials

As the acid or base properties of the support may influence the catalysts either electronically or structurally, potential Pd/Ru/Ni (2:8:90) and Rh/Ni (30:70) catalysts calcined at 400 o

C were chosen to be supported on various supports such as alumina beads (Al

2

O

3

)

,

TiO

2

/SiO

2

beads and carbon chips from palm kernel shell to compare the compatibility and suitability of the supports towards the catalysts. The comparison of the support effect towards the catalytic performance of Pd/Ru/Ni

(2:8:90) catalyst is summarized in Table 4.7.

Table 4.7

Comparison of CO

2


Pd/Ru/Ni catalyst with the ratio of 2:8:90 coated on various support materials then calcined at 400 o

C for 5 hours

Support Materials 100 o

C


200 o

C 300 o

C


2

400 o

C

Alumina beads (Al

2

O

3

) 37.94 43.60 45.77 52.95

C chips

TiO

2

/SiO

2

beads

14.05

3.13

25.54

4.80

32.38

10.30

33.44

17.87

It could be seen that catalyst on the Al

2

O

3

support gave the highest CO

2 conversion. It still appeared as the most suitable support as it showed stable performance on CO

2

conversion. The carbon support from palm kernel shell was found to convert 25.54% of CO

2

at reaction temperature 200 o

C. Meanwhile,

TiO

2

/SiO

2

support gives very low conversion of CO

2

(4.8%) at 200 o

C and achieved maximum CO

2

conversion of 17.87% at maximum studied reaction temperature of

400 o

C. These results implied that TiO

2

/SiO

2

support was not appropriate for the CO

2 conversion as also suggested by Takenaka et al . (2004). They found that TiO

2

/SiO

2 supported catalysts are effective for the complete removal of CO through methanation because it can react more strongly with metal surface compared to CO

2

.

De Boer et al.

(1991) reported that less than 100% metal oxides dispersion on the

SiO

2

support obtained because of the lower reactivity and more acidic character of this support. Such properties will decrease its reduction ability of CO

2

to CH

4

during

57 the catalytic testing. In other words, the conversion of CO

2

over Pd/Ru/Ni (2:8:90) with different support is in the increasing order as follows : Pd/Ru/Ni (2:8:90)/ Al

2

O

3

> Pd/Ru/Ni (2:8:90)/ Carbon > Pd/Ru/Ni (2:8:90)/ TiO

2

. The detailed trends of the catalytic activity of the respective catalysts are shown in Figure 4.6.

60

50

40

30

O

3

/SiO

2

20

10

0

0 100 200 300 400 500


Figure 4.6


2

conversion from methanation reaction over Pd/Ru/Ni (2:8:90) catalyst with various support materials: i)

Al

2

O

3

beads ii) TiO

2

/SiO

2 beads and iii) Carbon chips from PKS calcined at 400 o

C for 5 hours

From Figure 4.7 and Table 4.8, it can be observed that the Rh/Ni (30:70) catalyst supported on alumina still showed the highest conversion of CO

2 with

43.02% and 50.74 % at 200 o

C and 350 o

C, respectively. On the other hand, the similar catalyst (Rh/Ni (30:70)) supported on TiO

2

/SiO

2

beads showed 21.05% of

CO

2

conversion at 200 o

C. However, this catalyst achieved 95.75% CO

2

conversion at

350 o

C. It is indicated that the catalyst used being activated at higher temperature.

Similar result was obtained by VanderWiel et al ., (2000) in their study. At temperatures below 250 o

C, the conversion remained below 50% but at higher temperatures, the conversion rapidly maximized to about 85%. It might be due to the catalyst which started to react with support material producing the tertiary compound that enhanced the catalytic performance at higher temperature. Meanwhile, the result for TiO

2

/SiO

2

support was much better than the catalyst support with carbon which only able to convert CO

2

around 11.45% at 200 o

C and 14.43% at 350 o

C. The

58 conversion of CO

2

over Rh/Ni (30:70) with different support is in the increasing order as follows: Rh/Ni (30:70)/Al

2

O

3

> Rh/Ni (30:70)/TiO

2

> Rh/Ni (30:70)/carbon.

Table 4.8

Comparison of CO

2

conversion from methanation reaction over Rh/Ni catalyst with the ratio of 30:70 coated on various support materials then calcined at 400 o

C for 5 hours


Support Materials 100 o

C 200 o

C 300 o

C 350


2 o

C 400 o

C

Alumina beads (Al

2

O

3

) 39.81 43.02 48.27 50.74 90.10

TiO

2

/SiO

C chips

2

beads 11.86

5.70

21.05

11.45

30.41

13.21

95.75

14.43

99.12

17.44

100 O

3

2

/SiO

2

80

60

40

20

0

0 100 200 300


400

Figure 4.7

Catalytic of

2

conversion from methanation reaction over Rh/Ni (30:70) catalyst with various support materials: i) Al

2

O

3 beads ii) TiO calcined at 400 o

2

/SiO

2 beads and iii) Carbon chips from PKS and

C for 5 hours

Therefore, the catalysts coated on alumina shows the best performance compared to the other support materials (C and TiO

2

/SiO

2

). Further, both catalysts were undergoing other optimization parameters which are calcination temperatures of alumina support.

59

4.2.4 Effect of Different Calcination Temperature towards Alumina Support

The effects of calcination temperature towards alumina support on the physical properties of potential catalysts are reported in Table 4.9 and Table 4.10 while, the comparison of the trend plot of CO

2

conversion over the catalysts are shown in Figure 4.8 and Figure 4.9. At reaction temperature of 200 o

C, it showed that the percentage conversion of CO

2

over Pd/Ru/Ni (2:8:90) catalyst towards precalcined alumina at 700 o

C is 15.43% while, 13.68% CO

2

conversion was obtained towards pre-calcined alumina at 1000 o

C. Whereas, the percentage of CO

2

conversion over Rh/Ni(30:70) catalyst towards pre-calcined alumina at 700 o

C is only 6.95% and the conversion was increased up to 77.38% at reaction temperature of 350 o

C. It showed that the catalyst was activated at higher temperature, thus the application of this catalyst in industry is not practical. However, the reaction temperature above

200 o

C was applied to study the trend of CO

2

conversion for each catalyst.

Table 4.9

Comparison of CO

2


Pd/Ru/Ni (2:8:90) catalyst coated on alumina which has been calcined for 5 hours at different temperature

Calcination Temperature of Al

2

O

3

Support

100 o

C

37.94


200 o

C 300 o

C


2

43.60 45.77

400 o

C

52.95 Non Calcined

700 o

C

1000 o

C

12.33

9.36

15.43

13.68

20.91

18.79

57.64

45.44

60

60

50

40

30

20

10 cal. 700 o

C cal. 1000 o

C

0

0 100 200 300 400


Figure 4.8

Catalytic of

2

conversion from methanation reaction over Pd/Ru/Ni (2:8:90) catalyst coated on alumina calcined for 5 hours at different temperatures: i) 700 o

C and ii) 1000 o

C

Table 4.10

Comparison of CO

2

conversion from methanation reaction over Rh/Ni

(30:70) catalyst coated on alumina which has been calcined for 5 hours at different temperature


Calcination

Temperature of 100 o

C 200 o

C 300 o

C 350 400 o

C

Al

2

O

3

Support % Conversion of CO

2

Non Calcined

700 o

C

1000 o

C

39.81

4.04

3.27

43.02

6.95

4.69

48.27

34.95

18.00

50.74

77.38

31.10

90.10

97.76

79.70

Both figures reveal that the conversion of CO

2

decreases with a rising calcination temperature on alumina support. Non-calcined alumina is better than precalcined alumina (700 o

C and 1000 o

C) for CO

2

conversion at reaction temperature

200 o

C. This might due to the large crystallites of metal formed at higher calcination temperature as has been suggested by Chang et al.

(1997). The increase in precalcined temperature of alumina also led to a significant decrease in surface area value and thus decreased the catalytic performance. This is in a good agreement with

El-Shobaky (2004) who had reported on the decreasing of catalytic performance of

61

NiO/CuO catalyst due to the effective sintering of this solid. Therefore, catalyst with pre-calcined support showed low catalytic activity at low reaction temperature.

100

80

60

40

20 cal. 1000oC C

0

0 100 200 300


400

Figure 4.9

Catalytic CO

2

conversion from methanation reaction over Rh/Ni (30:70) catalyst coated on alumina calcined for 5 hours at different temperatures: i) 700 o

C and ii) 1000 o

C

4.2.5 Effect of Different Methods of Catalyst Preparation

This parameter was carried out to examine the effect of preparation methods on the stability and catalytic activity performance. The two best catalysts which are


2

O

3


2

O

3

were synthesized by three different preparation techniques. The usually used preparation method is incipient wetness impregnation (IWI) and this method was applied also in this research. The other two techniques used for comparison are sol gel method and modification of

IWI method. Table 4.11 indicates the catalytic activity performance over potential


2

O

3


2

O

3

catalysts prepared by different methods and calcined at 400 o

C.

It is noted that Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O

3 catalysts prepared by sol-gel method give the higher CO

2

conversion than modified IWI

62 method which was able to convert around 30.82% and 27.56% at reaction temperature 200 o

C, respectively. Most of the literatures found that the active catalyst can be synthesized by sol-gel method because of its smaller particles size in nano level. Oh et al.

(2007) proposed that reducing the particle size of the metal oxide catalyst to the nano level would have an effect on the reactivity since the chemical and physical properties of nano-sized materials are different from their bulk

(intrinsic) properties. However, reducing the particles size by this technique had decreased its catalytic performance compared to the conventional impregnation one.

Previous research by Tomiyama et al.

(2003) revealed that the higher conversion of

CO

2

can be achieved using catalyst in the powder form rather than in the form of thin film. In this research, the catalyst coated on the alumina support was in the form of thin film before aging in an oven. It probably reduced the surface area thus reduced the effectiveness of CO

2

conversion over these catalysts.

Table 4.11

Comparison of CO

2



2

O

3


2

O

3 by different preparation methods and calcined at 400 catalysts prepared o

C

Catalysts

Preparation

Methods

100 o

C


200 o

C 300 o

C


2

400 o

C

Pd/Ru/Ni

(2:8:90)/Al

2

O

3

IWI 37.94 43.60 45.77 52.95

Sol-gel 24.06 30.82 34.66 37.61

Modified IWI 8.52 10.43 17.95 23.22

Rh/Ni

(30:70)/Al

2

O

3

IWI

Sol-gel

39.81

17.65

43.02

27.56

48.27

29.13

90.10

45.57

Modified IWI 8.32 20.13 23.34 97.08

On the other hand, the modified IWI method gives the lowest results with poor activity performance compared to the IWI and sol-gel techniques. The conversion of CO

2

over Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O

3

catalysts increased from 10.43% to 23.22% and 20.13% to 97.08%, respectively as the reaction temperature was increased from 200 o

C to 400 o

C. By preparing the catalysts via this method, sintering is more likely to occur and eventually causes the loss of surface area and then decrease the catalytic activity of the catalyst. This reason might

63 be due to the preparation method of the catalyst in which the catalysts was calcined at 400 o

C more than two times. The detailed trend plot are shown in Figure 4.10 and

4.11.

60

50

40

30

20

10

0

0 100 200 300


400

Figure 4.10


2


2

O

3

catalyst prepared by different preparation methods i) wetness impregnation method and, ii) wetness impregnation modification method iii) Sol-gel method

120

(IWI)

100

80

60

40

20

0

0 100 200 300


400

Figure 4.11


2


2

O

3

catalyst prepared by different preparation methods i) wetness impregnation method and, ii) wetness impregnation modification method iii) sol-gel method

64

It can be clearly observed in both figures above, the catalysts that were prepared by IWI technique show the best performance compared to the other methods. This suggests that IWI method produces an excellent catalyst for the hydrogenation of CO

2

. The IWI technique also has several advantanges such as simplicity, easy to control and a low cost method.

4.2.6 Effect of H

2

S Gas on the Alumina Supported Catalysts

The potential catalysts were further tested for CO

2

methanation reaction in the presence of H

2

S to check the durability of the catalysts. Under the presence of sulfur compound in the gas stream, nickel catalysts for CO

2

methanation are easily deactivate. Thus, the toughness of catalyst towards the H

2

S attack is an important factor for the practical use of catalysts as has been suggested by Habazaki et al.

(1998). Figure 4.12 indicates the comparison of catalytic activity over Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst in the presence of H

2

S gas.

100

80

60

40

20

0

0 100 200 300


400

conv. without H

2

S conv. with H

2

S

Figure 4.12

` Catalytic over Pd/Ru/Ni (2:8:90)/Al

CO

2


2

O

3



2

S gas

65

As can be seen in the above figure, Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst had achieved 100% H

2

S desulfurization at 140 o

C to sulfur and remains constant until reaction temperature of 300 o

C. Its started to decrease from 100% to 57.31% at temperature 300 o

C to 400 o

C. The catalyst lost its H

2

S activity probably due to the high concentration of S which will form an external layer on the catalyst surface preventing the next coming H

2

S to pass and continuing its reaction with active sites of the catalyst as suggested by Karim (2010). The percentage conversion of CO

2

and

H

2

S desulfurization over this catalyst is shown in Table 4.12.

Table 4.12


2


2

S desulfurization over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst calcined at

400 o


2

S gas

Catalyst Conversion

100 o

C


200 o

C 300 o

C 400 o

C

Pd/Ru/Ni

(2:8:90)/Al

2

O

3

CO

2

without H

2

S

37.94 43.60 45.77 52.95

CO

2

with H

2

S 11.54 13.53 25.97 35.03

H

2

S 98.78 100 100 57.31

It seems that in the presence of H

2

S, the conversion of CO

2

was decreased around 30.07% of its performance from 43.6% to 13.53% at 200 o

C indicating that the catalyst was poison by the sulfur compound. This sulfur compound is more active to make an interaction between the catalyst and blocked the pore on the surface of catalyst, hence, retards the reduction of CO

2

during the reaction. This is in a good agreement with the results reported previously that the adsorption of H

2

S on Ni is very strong and then deactivates the nickel based samples by chemisorption on the metal catalyst (Erdohelyi et al ., 2004 and Rostruo-Nielsen, 1968).

66

100

90

80

70

60

50

40

30

20

10

0

0 100 200 300 400


conv. without H

2

S

Figure 4.13

Catalytic over Rh/Ni (30:70)/Al

2

O

3

CO

2




2

S gas

Meanwhile, H

2

S was completely removed by Rh/Ni (30:70)/Al

2

O

3

catalyst at reaction temperature of 180 o

C and its still remains 100% even at higher temperature of 400 o

C (Figure 4.13 and Table 4.13). However, the percentage conversion of CO

2 was slighly lower in the presence of H

2

S. Its reaches the maximum conversion of

19.6% at 100 o

C and decreases gradually after this. The deposition of larger amount of sulfur elements on the catalyst surface would block the active sites presence which leads to the decreasing of the activity. This phenomenon may induces the changes occurred in the form of electronic and geometric on the catalyst structure.

Table 4.13


2

from methanation reaction and H desulfurization over Rh/Ni (30:70)/Al

2

O

3

catalyst calcined at 400

2 o

S


2

S gas

Catalyst Conversion

100 o

C


200 o

C 300 o

C 400 o

C

Rh/Ni

(30:70)/Al

2

O

3

CO

2

without H

2

S

39.81 43.02 48.27 90.10

CO

2

with H

2

S 19.60 4.95 4.51 4.53

H

2

S 87.52 100 100 100

67

From the results above, it can be concluded that Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst is more stable catalyst compared to the Rh/Ni (30:70)/Al

2

O

3

catalyst. So, this catalyst will undergo further testing using two reactors that would be discussed in

Section 4.4. The following testing is to investigate the activity performance when using two reactors running in series.

4.2.7 Reproducibility Testing towards Potential Catalyst

The reproducibility catalytic activity of the catalyst was tested by using the same catalyst several times until the catalyst was deactivated. Figure 4.14 shows the trend plot of reproducibility testing over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst.

60

50

40

30

20

10

1st test (fresh)

2nd test

3rd test

4th test

0

0 100 200 300


400

Figure 4.14

The trend plot of reproducibility testing over Pd/Ru/Ni (2:8:90)/Al

2

O



2


It can be seen that the conversion of CO

2

over the fresh catalyst is 43.60% at reaction temperature of 200 o

C. However, the conversion was slightly decreased to

35.10% after second testing using the spent catalyst and continuously decreased to

26.17% for third testing over same spent catalyst at the same reaction temperature.

68

For fourth testing, the CO

2

conversion was lower eventhough the conversion of catalyst keeps increasing from temperature of 60 o

C to 240 o

C then slowly deactivate until the maximum studied temperature of 400 o

C.

Similar pattern was observed for Rh/Ni (30:70)/Al

2

O

3

catalyst as plotted in the Figure 4.15. However, deactivation of Rh/Ni is more significant than Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst. About 50% of catalyst deactivation was observed in Rh/Ni

(30:70)/Al

2

O

3

catalyst in comparison between first testing (43.02%) and second testing (19.8%). For the third testing using the same spent catalyst, the catalyst decreases to 6.74% at 200 o

C and then started to deteriote at 350 o

C.

120

100

1st test (fresh)

2nd test

3rd test

80

60

40

20

0

0 100 200 300 400


Figure 4.15

The trend plot of reproducibility testing over Rh/Ni (30:70)/Al

2

O



2


From the observation in Figures 4.14 and 4.15, it can be suggested that


2

O

3


2

O

3

catalysts started to deactivate after first testing. The decreases in catalytic activity observed in this study were clearly due to the carbon deposition on the surface of the catalyst as proven from EDX analysis discussed in Section 4.5.2.1. The metal carbon interaction inducing the changes in surface composition which affect the catalytic activity and selectivity.

However, the catalyst can be regenerated to regain its original activity as the fresh by

69 heating the catalyst under compressed running air. This will be explained later in

Section 4.2.8.

4.2.8 Regeneration Testing on the Potential Catalyst

This experiment was demonstrated to determine the optimum operating conditions to regenerate the spent catalyst and re-test its activity. The spent catalyst from Section 4.2.7 was used to carry out this experiment. Figures 4.16 and 4.17 show the trend of regenerated catalyst testing of Pd/Ru/Ni (2:8:90)/Al

2

O

3

and Rh/Ni

(30:70)/Al

2

O

3

catalysts at various temperatures and times. There are two methods of catalyst regeneration processes which are the oxidative regeneration and nonoxidative regeneration as classified by Furimsky and Massoth (1993). In this research, the regeneration process is categorized as oxidative regeneration because the waste catalyst was exposed to the oxygen. The Equation 4.1 below seems to be the idea of using compressed air in this research.

NiC

(p)

+ 3O

2 (g)

→

2NiO

(p)

+ 2CO

2 (g)

(4.1)

The use of compressed air in this research is the most practical approach since industrial equipment used for the regeneration temperature is usually limited to

430 o

C as suggested by Trimm, (1980). Figure 4.16 revealed that the carbon was removed from the Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst surface by heating the catalyst at

200 o

C for 3 hours in the flow of compressed air since the conversion of CO

2

was comparable to that fresh catalyst which was 42%. The carbon deposition problem can also be avoided by increasing the hydrogen content of the feed stream so that kinetics or thermodynamic equilibria are unfavorable towards the deactivation as been stated by Gardner and Bartholomew (1981).

70

60

50

40

30

Al2O3 catalyst

Regenerate 100 o

C, 3h

20

10

Regenerate 200 o

C, 3h

0

0 100 200 300


400 500

Figure 4.16

Regeneration catalytic testing over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst for 3 hours at various temperatures towards CO

2


Approximately 98.99% recovery was achieved over Rh/Ni (30:70)/Al

2

O

3 catalyst which showed that the C was released from the catalyst in the flow of compressed air after heating the catalyst at higher temperature of 400 o

C for 4 hours

(Figure 4.17). The conversion significantly increased in activity from 6.74% (before regeneration) to 42.01% (regeneration) at reaction temperature of 200 o

C. The value is very close to the fresh one (43.02%) meaning that the catalyst has regained its activity. The treatment of deactivated catalyst with an oxygen containing gas at elevated temperature is preferably carried out at a temperature in the range of 300 o

C-

550 o

C as in accord by Henni and Herman, (1991). However, the regeneration activity more than 400 o

C is not compatible to the catalyst calcined at 400 o

C since the collapsed of catalysts structure could be occurred.

71

100

80

60

40

20

0

0 100 200 300


400

Regenerate 200 o regenerate 200oC, 3h

Regenerate 400 o

Figure 4.17

Regeneration catalytic testing over Rh/Ni (30:70)/Al

2

O

3

catalyst at various temperatures and various times towards CO

2


From these investigations, Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O

3 catalysts which were calcined at 400 o

C are regenerable and can be reused without loosing its good catalytic activity.

4.3 Methane Gas Formation Measurement via Gas Chromatography

The reactor gas product from FTIR cell was collected and analysed for CH

4 formation. The methane formation was determined via GC because of the low sensitivity of FTIR spectroscopy towards the methane stretching region. Tables 4.14 and 4.15 show the testing results of CO

2

/H

2

methanation over the potential alumina supported catalyst. Pd/Ru/Ni (2:8:90)/Al

2

O


2

O

3

catalysts are the most potential for the CO

2

methanation according to the results obtained in Table

4.1 and Table 4.4.

72

Table 4.14 The product and by-product of CO

2

methanation reaction over


2

O

3


Catalyst Reactant

Reaction

Temp ( o

C)

CO

2

Conversion (%) Selectivity

Product

CH

4

By-product

CO + H

2

O

(%)

Unreacted

CO

2

(%)

Pd/Ru/Ni CO

2

100 2.78 35.16 7.33 62.06

(2:8:90) 200 6.82 36.78 15.64 56.40

/Al

2

O

3 300 15.95 29.82 34.85 54.23

400 39.73 13.22 75.03 47.05

CO

2

+ H

2

S 100 0.35 11.19 3.03 88.46

200 1.73 11.80 12.79 86.47

300

400

2.61

3.64

23.36

31.39

10.05

10.39

74.03

64.97

*The unreacted CO

2

gas was calculated using FTIR analysis

There are three possible products obtained during the CO

2

methanation reaction namely carbon monoxide, water and methane. From the Table 4.14 above, it can be seen that the percentage of unreacted CO

2

decreases as the CO

2

was converted into CH

4

, CO and H

2

O. While, CH

4

content was increased as temperature is increased. However, the CO

2

methanation in this research could be considered as partial oxidation reaction. This is because the formation of CO and the amount of unreacted CO

2

is higher than the formation of CH

4

.

Even at reaction temperature of 200 o

C, the conversion of CO

2

did not yield

100% CH

4

but tend to form CO and H

2

O. These results are in a good agreement with other researchers. Yaccato et al . (2005) found that when methanation process tested, the main product observed was CO at low temperature and when using higher temperature, CH

4 was formed. This is due to the indirect conversion of CO

2

into C

1 hydrocarbons, through the formation of intermediate CO as suggested by Silver et al.

(1988). Only 39.73% of methane was formed over this catalyst at maximum studied temperature.

The methane production lowered when the CO

2

methanation was conducted in the presence of H

2

S over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst. At 200 o

C, the methane

73 production decreased from 6.82% to 1.73% while, at 400 o

C the formation of methane dropped around 91% from 39.73% to only 3.64% indicating that the catalyst was possibly been poisoned by the presence of H

2

S in the gas stream. The active sites of the catalyst were blocked by the sulfur compound. This phenomenon prevents the CO

2 and H

2

from being absorbed and converted to CH

4

on the surface of the catalyst.


2

methanation reaction over Rh/Ni

(30:70)/Al

2

O

3


CO

2

Conversion (%) Selectivity

Reaction Unreacted


Temp ( o

C)

Product

CH

4

By-product

CO + H

2

O

(%)

CO

2

(%)

Rh/Ni CO

2

100 0.7 39.11 3.28 60.19

(30:70) 200 3.8 39.22 13.01 56.98

/Al

2

O

3 300 31.29 16.98 93.13 51.73

400 70.75 19.35 79.48 9.9

CO

2

+ H

2

S 100 4.89 14.71 24.95 80.40

200 0.1 4.85 2.02 95.05

300

400

0.3

0.5

4.21

4.01

6.65

11.04

95.49

95.49

*The unreacted CO

2

gas was calculated using FTIR analysis

A similar result was also observed over Rh/Ni (30:70)/Al

2

O

3

catalyst (Table

4.15). The methane production has increased as temperature is increased. The higher methane formation was reached at 400 o

C with 70.75%. It may be attributed to the rapid hydrogenation of intermediate CO species resulting in higher CO

2

methanation activities at this temperature. However, this percentage droped to only 0.5% after testing was conducted simultaneously with H

2

S.

74

Testing CO

2

Methanation Reaction using Two Reactors over

Pd/Ru/Ni (2:8:90)/Al

2

O

3

Catalyst

From the results discussed in Sections 4.2.6, 4.2.8 and 4.3 above, Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst was found as the most stable catalyst. Its simultaneous reaction with H

2

S gives higher CO

2 methanation compared to Rh/Ni (30:70)/Al

2

O

3 catalyst. Furthermore, this catalyst can only be regenerated in the flow of compressed air, heating at 200 o

C for 3 hours. Figure 4.18 shows the performance activity using two reactors over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst. Meanwhile, the methane production over this catalyst was demostrated in Table 4.16. The testing was carried out simultaneously with H

2

S gas using two isothemal furnaces connected in series

(Appendix C).

From the Figure 4.18, simultaneous reaction with H

2

S over Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst using two reactors started to achieve 100% H

2


C and maintained constant until maximum studied reaction temperature of 400 o

C. In the meantime, the conversion of CO

2

at 200 o

C was 44.29% and only produced 3.63% of CH

4

. At temperature of 400 o

C, the yield of methane was

7.64% over 68.07% CO

2 conversion. However, the methane formation was observed at temperature as low as 100 o

C. It is clear that most of the converted CO

2

does not form CH

4

but instead formed CO and H

2

O. Takeishi and Aika, (1995) had claimed that the CO

2

cannot be adsorbed on the surface of the catalyst under the existence of

CO. Therefore, the intermediate species of CO will inhibit the formation of CH

4

. As can be seen in Figure 4.12 and Figure 4.18, it can be concluded that the formation of methane increased around 50% by using the two reactors.

75

120

100

80

60

40

20

0

0 100 200

Reaction

300 400


Figure 4.18


2


2

O

3


C for 5 hours and testing simultaneously with H

2

S gas


2


2

O

3


CO

2

Conversion (%)


Unreacted

Temp ( o

C)

Product

CH

4

By-product

CO + H

2

O

CO

2

(%)

Pd/Ru/Ni CO

2

+ H

2

S 100 0.35 30.51 69.14

(2:8:90) 200 3.63 40.66 55.71

/Al

2

O

3

300 5.71 48.80 45.49

400 7.64 60.43 31.93

In order to further understand the performance of the potential prepared catalyst, characterization process was carried out. The best potential prepared catalyst that was identified from the catalytic activity testing were chosen. Pd/Ru/Ni/Al

2

O

3 catalyst with ratio loading of 2:8:90 and 30:70 for Rh/Ni/Al

2

O

3

catalyst which were prepared by incipient wetness impregnation method and calcined at 400 o

C for 5 hours were characterized.

76

4.5 Characterization of the Potential Catalysts

Characterization techniques used were X-ray Diffraction (XRD), Field

Emission Scanning Electron Microscopy-Energy Dispersion X-Ray (FESEM-EDX),

Fourier Transform Infra-Red (FTIR) and Thermogravimetry Analysis – Differential

Thermal Analysis (TGA-DTG).

4.5.1 X-Ray Diffraction Analysis (XRD)

The XRD analysis was conducted to investigate the catalysts structure, to examine the species and the phases responsible for crystallinity. Information regarding the crystalline phases was obtained by comparing the d value of the material with those of phases from the powder diffraction file (PDF).

4.5.1.1 X-Ray Diffraction (XRD) Analysis over Pd/Ru/Ni (2:8:90)/Al

2

O

3

Catalyst

Figure 4.19 shows the diffractograms of XRD analysis for the Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalysts which were calcined at 400 o

C, 700 o

C and 1000 o

C while the peaks assignments are shown in Table 4.17 and Table 4.18.

77

Figure 4.19

XRD diffractograms of Pd/Ru/Ni (2:8:90)/Al

2

O

3


C, b) 700

o

C and c) 1000 o

C for 5 hours

The XRD diffractogram for the potential catalyst that was calcined at 400 o

C showed very low degree of crystallinity and high noise to signal ratio. It can be seen that the slight significant peaks in diffractogram could all attribute to the presence of

Al

2

O

3 as the support for the catalyst which occurred centered at 2θ = 67 .000

o

and

37.000

o

. Research done by Wang et al.

(1999) also reveals no crystalline phases was detected in the Cu – Mn – O/Al

2

O

3

catalyst that was calcined at 400 o

C by XRD because the active components of metal oxides are highly dispersed in the alumina support which is highly amorphous.

However, at calcination temperature of 700 o

C, the intensity of alumina peaks over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst was slightly increased and became more profound. This revealed that the intermediate crystallinity was observed in this catalyst. The phase which was dominated by alumina support was revealed as cubic

Al

2

O

3 at 2θ = 67.114 (I

100

), 45.450 (I

100

) and 37.505

o

(I

90

) with d spacing value of

78

1.393, 1.994 and 2.396 Å (PDF d values for cubic Al

2

O

3

(Å) = 1.403, 1.985 and

2.394 Å). From the calculation based on relative intensity ratio, the overlapping of cubic NiO diffraction peaks with those of the support peaks was assumed to be occurred at 2θ of 45.450

o

(I

100

) and 37.505 (I

68

) with d spacing values of 1.994 and

2.396 Å (PDF d values for cubic NiO (Å) = 2.088 and 2.412 Å).

Interestingly, four new peaks were also observed in the XRD pattern. Among the four peaks, one peak was obtained as cubic NiO at 2θ of 62.878

o

(I

44

) with d spacing value of 1.463 Å (PDF d values for NiO (Å) = 1.477 Å). The other two peaks were obtained at 2θ of 28.198 (I

100

) and 34.066

o

(I

77

) with d spacing value of 3.162 and 2.630 Å (PDF d values for RuO

2

(Å) = 3.182 and 2.690 Å) and were assigned for the tetragonal of RuO

2

. As shown in Figure 4.19 (b), the pattern show very small peaks which was hardly distinguished from the background noise, possibly indicating that there were only a very small amount of RuO

2

present on the surface or the particles dispersed on the support were relatively small. The similar species of tetragonal RuO

2

phase was also observed by Chen et al.

(2007) at diffraction peaks of 2θ = 28.0

00 o

and 35.100

o

over Ru-La

2

O

3

/Al

2

O

3

catalyst even calcination at temperature of 500 o

C. It can be suggested that the presence of RuO

2

is not resolved over the catalyst.

The cubic phase of PdO peaks was assumed in the enveloped of RuO peaks at

2θ of 34.066

o

(I

100

) or d spacing values of 2.630 Å (PDF d value for PdO (Å) = 2.820

Å ). Meanwhile, the emergence of a new broad peak at 2θ of 54.177

o

(I

70

) with d spacing value of 1.692 (PDF d value for PdO (Å) = 1.630 Å) was also observed for the same compound.

All the peaks observed become more intense, sharper and narrower when the catalyst was calcined at 1000 o

C (Figure 4.19 (c)) indicating that the degree of crystanallity over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst increased with increasing calcination temperature. The highest intensity was also due to the cubic phase of

Al

2

O

3 at 2θ = 66.671 (I

100

), 45.648

o

(I

100

) and 37.416 (I

90

) or d spacing values of

1.402, 1.986 and 2.402 Å (PDF d values for cubic Al

2

O

3

(Å) =1.403, 1.985 and 2.394

Å).

79

In addition, one of the peak corresponding to the cubic NiO was covered with alumina support at 2θ of 37.416

(I

68

) or d spacing values of 2.402 Å (PDF d values for cubic NiO (Å) = 2.412 Å). Another two peaks was appeared sharply at 2θ of

43.294 (I

100

) and 62.902

o

(I

44

) or d spacing values of 2.088 and 1.476 Å (PDF d values for cubic NiO (Å) = 2.088 and 1.477 Å). Nickel oxide cubic phase is thermostable and it can prevent the changing phase to be occurred as suggested by

Richardson et al.

(2003).

Furthermore, peaks assigned to the tetragonal of RuO remained similar at 2θ of 27.265

o

(I

100

) and 32.131 (I

77

) or d spacing values of 3.268 and 2.783 Å (PDF d values for RuO

2

(Å) = 3.182 and 2.690 Å). In the meantime, the broad peaks of PdO cubic phase have been transformed into high intensity and shaper peaks at 2θ of

31.702 (I

100

) and 56.820

o

(I

90

) or d spacing values of 2.820 and 1.619 Å (PDF d values for PdO (Ǻ) = 2.820 and 1.630 Å).

Table 4.17 Peaks assignment in the X-ray diffraction patterns of Pd/Ru/Ni

(2:8:90)/Al

2

O

3


C for 5 hours

Compound

2θ d ( Å ) d

Al

2

O

3

(c) 67.114

45.450

37.505

1.393

1.994

2.396

1.403(I

100

)

1.985(I

100

)

2.394(I

90

)

RuO

2

(t)

37.505

62.878

28.198

34.066

2.396

1.463

3.162

2.630

1.692

2.088(I

100

)

2.412(I

68

)

1.477 (I

44

)

3.182(I

100

)

2.690(I

77

)

2.820 (I

100

)

1.630 (I

70

) 54.177

*c = cubic and t = tetragonal

*d = distance between two lattice planes

80

Table 4.18 Peaks assignment in the X-ray diffraction patterns of Pd/Ru/Ni

(2:8:90)/Al

2

O

3


C for 5 hours

Compound

2θ d ( Å ) d

Al

2

O

3

(c) 66.671

45.648

37.416

1.402

1.986

2.402

1.403(I

100

)

1.985(I

100

)

2.394(I

90

)

RuO

2

(t)

37.416

62.902

27.265

32.131

2.402

1.476

3.268

2.783

2.088(I

100

)

2.412 (I

68

)

1.477 (I

44

)

3.182(I

100

)

2.690(I

77

)

56.820 1.619

2.820(I

100

)

1.630(I

70

)

* c = cubic; t = tetragonal


However, all the NiO, RuO

2

and PdO species were not observed in the catalyst that was calcined at 400 o

C (Figure 4.19 (a)). It can be suggested that these species are presence with small crystallite size and highly dispersed so that they could not be detected due to the insensitivity of XRD instrument. This was confirmed by EDX analysis which showed the existence of Ni, Ru, Pd, Al and O elements in the

Pd/Ru/Ni (2:8:90)-Al

2

O

3

catalyst itself. Therefore, it can be concluded that these species can lead to the higher conversion of CO

2

at reaction temperature of 200 o

C as shown in Figure 4.4. The highest CO

2

conversion was obtained for Pd/Ru/Ni

(2:8:90)-Al

2

O

3


C (43.60%) followed by Pd/Ru/Ni (2:8:90)-

Al

2

O

3


C (24.10%), 700 o

C (10.36%) and 1000 o

C (3.16%).

The formation of active species such as cubic phase of NiO also leads to higher catalytic activity towards CO

2

methanation reaction.

81

4.5.1.2 X-Ray Diffraction (XRD) Analysis over Rh/Ni (30:70)/Al

2

O

3

Catalyst

Figure 4.20 shows the diffractograms of XRD analysis for the Rh/Ni

(30:70)/Al

2

O

3

catalyst which was calcined at 400 o

C, 700 o

C and 1000 o

C. From the figure, the Rh/Ni (30:70)/Al

2

O

3


C is relatively amorphous as shown by the presence of broad peaks that correspond to Al

2

O

3 phases at 2θ =

36.000

o

and 67.000

o

. This result is in good agreement with Park and McFarland

(2009) who found that no crystalline phase of NiO was found in the Ni/SiO

2

catalysts even calcined at 550 o

C. No diffraction peaks are assigned for the catalyst metal oxide phases in the Figure 4.20 (a) implying that metal particles of the catalyst are very small and could be well dispersed over the alumina support.

Figure 4.20

XRD diffractograms of Rh/Ni (30:70)/Al

2

O

3

catalyst calcined at a)

400 o

C, b) 700 o

C and c) 1000 o

C for 5 hours

When the calcination temperature was increased to 700 o

C, the diffraction peaks assigned for the cubic Al

2

O

3 at 2θ = 67.053 (I

100

), 46.157 (I

100

) and 36.997

o

(I

90

) with d spacing value of 1.395, 1.965 and 2.428 Ǻ (PDF d values for cubic Al

2

O

3

82

(Å) = 1.403, 1.985 and 2.394 Å) were slightly increased. In the meantime, the peaks correspond to the cubic NiO was also found in the broad shoulder of alumina support at 2θ = 43.286 (I

100

) and 36.997

o

(I

68

) with d spacing values of 2.089 and 2.428 Å

(PDF d values for cubic Al

2

O

3

(Å) = 2.088 and 2.411 Å).

One peak of rhombohedral Rh

2

O

3 was assumed overlapped at 2θ = 36.997

(I

100

) or d spacing value of 2.428 Å (PDF d value for rhombohedral Rh

2

O

3

= 2.564

Å). In addition, a new peak which assigned to the same phases appeared at 2θ =

32.769 (I

87

) with d spacing value of 2.731 Å (PDF d value for rhombohedral Rh

2

O

3

=

2.731 Å).

Similar pattern was observed for Rh/Ni (30:70)-Al

2

O

3

catalyst, as the calcination temperature increased, all the peaks observed become more intense and sharper indicating that the degree of crystanallity was increased. The crystallite growth during the calcination process results from an increase in the average crystallite size because of a tendency for minimization of the interfacial surface energy as had been explained by Oh et al.

(2007). However, the diffractogram in

Figure 4.20 (c) showed that the catalyst which was calcined at 1000 o

C having moderate degree of crystallinity. All the peaks assignment of Al

2

O

3

in the diffractogram remained almost unchanged except for the Rh

2

O

3

phases.

The emergence of a new broad peak assigned for rhombohedral Rh

2

O

3 at 2θ of 34.563 (I

100


2

O

3

= 2.564 Å) was observed. The peak for the same compound at 2θ of 31.988 (I

87


2

O

3

= 2.731 Å) was also observed, similar as at 700 o

C calcinations temperature. It can be suggested that the presence of Rh

2

O

3

is not resolved over the catalyst.

The peaks assigned for the cubic NiO was no longer observed in the diffractogram of Figure 4.20 (c). However, the mixture of NiO and alumina support produced a peak in the difractogram indicated that there were the peaks of cubic

NiAl

2

O

4 at 2θ = 3 7.570 (I

100

), 67.009 (I44) and 45.656 (I

40

) with d spacing values of

83

2.392, 1.395 and 1.985 Å (PDF d values for cubic NiAl

2

O

4

= 2.462, 1.422 and 2.011

Å). Natesakhawat et al.

(2005) claimed that the formation of nickel aluminate spinel species in their research was only observed at higher calcination temperatures above

700 o

C. It is noted that the presence of this phases causing the difficulty of catalyst to be reduced during the catalytic testing due to the stronger interaction between nickel oxide and Al

2

O

3

support in this catalyst. This was the main reason for the reduction in the catalytic activity of the catalyst towards CO

2

conversion as shown in Figure

4.5.

Therefore, the NiO species is considered as the active species for Rh/Ni

(30:70)-Al

2

O

3

catalyst which responsible for the higher conversion of CO

2

activity since it cannot be observed at calcination temperature of 1000 o

C. It can be suggested that the catalyst calcined at 400 o

C might have this species which was highly dispersed on the surface. This proved that cubic nickel oxide play the important roles in increasing the catalytic activity of the catalyst. The peaks assignment for all species is presented in Table 4.19 and Table 4.20.

Table 4.19 Peaks assignment in the X-ray diffraction patterns of Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours

Compound

2θ d ( Å ) d

Al

2

O

3

(c) 67.053

46.157

36.997

1.395

1.965

2.428

1.403(I

100

)

1.985(I

100

)

2.394(I

90

)

NiO (c) 43.286

36.997

2.089

2.428

2.428

2.731

2.088 (I

2.412 (I

100

68

)

)

2.564 (I

100

)

2.731 (I

87

)

Rh

2

O

3

(r) 36.997

32.769

*c = cubic and r = rhombohedral


84

Table 4.20 Peaks assignment in the X-ray diffraction patterns of Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours

Compound

2θ d ( Å ) d

Al

2

O

3

(c) 67.009

45.656

37.570

1.395

1.985

2.392

1.403(I

100

)

1.985(I

100

)

2.394(I

90

)

NiAl

2

O

4

67.009

45.656

1.395

1.985

2.426 (I

100

)

1.422 (I

44

)

2.011 (I

40

)

Rh

2

O

3

(r) 34.563

31.988

2.593

2.796

2.564 (I

100

)

2.731 (I

87

)

*c = cubic and r = rhombohedral


4.5.2 Field Emission Scanning Electron Microscopy and Energy Dispersive X-

Ray

The potential catalysts in fresh and spent form were characterized by

FESEM-EDX analysis in order to study the morphology of the catalyst. The distribution and the composition of elemental catalyst can be determined from this analysis.

4.5.2.1 FESEM-EDX Analysis over Pd/Ru/Ni (2:8:90)/Al

2

O

3

Catalyst

Figure 4.21 shows the FESEM micrographs of fresh and spent Pd/Ru/Ni

(2:8:90)/Al

2

O

3


C for 5 hours with magnification of 5000X and 50000X. The fresh Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst showed rough surface morphology with inhomogeneous spherical shape and comes with small particles sizes. The morphology of fresh catalyst changed significantly after the hydrogenation of methanation reaction which showed the formation of aggregated and agglomerated undefined shape on the surface of spent catalyst (Figure 4.21 (b)). This observation

85 was possibly due to the heat generated during the catalytic reaction which caused the catalyst to agglomerate thus decreasing the activity as shown in Figure 4.14.

(a) Fresh (b) Spent

Figure 4.21

FESEM micrographs of fresh and spent Pd/Ru/Ni (2:8:90)/Al

2

O


C for 5 hours with magnification 5000X and

50000X

In this research, it was found that the particle size of fresh Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst is categorized in nano (< 100 nm) level which varies from 40 nm to 115 nm. The smaller particles size plays an important role to exhibit the higher catalytic activity. This result is consistent to the results of XRD analysis which revealed very broad peaks denoting an amorphous state observed in the diffratogram calcined at 400 o

C caused by the very small nanocrystallites size. The smaller particle size of the catalyst will leads to the higher dispersion of the catalyst and large surface

86 area of supported nickel oxide based catalyst. One of the most efficient ways to improve the reactivity for CO

2

methanation is to use materials with a large surface area and high dispersion as had been explained by Kodama et al.

(1997). The distribution of the elements on the surface of Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst can be seen in Figure 4.22. Each element is well distributed on the surface of the catalyst representing that the catalyst have higher dispersion.

Figure 4.22

EDX Mapping over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst calcined at

400 o

C for 5 hours

Furthermore, Table 4.21 shows the EDX analysis for the fresh and spent of


2

O

3

catalysts. The elemental analysis performed by EDX confirmed the presence of Ni, Ru, Pd, Al and O in the potential catalyst.

87

Table 4.21

EDX analysis of fresh and spent Pd/Ru/Ni (2:8:90)/Al

2

O

3


C for 5 hours

Catalyst

Al O

Weight Ratio (%)

Ni Ru Pd C

Fresh catalyst 38.46 50.69 3.33 4.64 2.89 -

Spent catalyst 36.33 44.58 4.33 7.77 3.19 3.80

*Fresh – Catalyst after calcined at 400 o

C and before catalytic testing activity

*Spent – Deactivated catalyst after catalytic testing.

From EDX analysis, it can be observed that the composition of Ru was higher than the composition of Ni on the fresh and spent Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalysts.

This phenomenon can be explained due to the incorporation of Ni into support after calcined the respected catalyst at 400 o

C. This was in good agreement with the finding observed by Nurunnabi et al.

(2008) who claimed that Ni may have been adsorbed into the porous support, hence lower the concentration of Ni on the catalyst surface that can be detected by EDX.

The migration of Ni atom from the bulk matrices to the surface of the catalyst making the higher composition of Ni was detected for the spent catalyst.

Moreover, spent Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst confirms the existence of

3.80% carbon on the surface of the catalyst. The different of fresh and spent catalyst surface was due to the presence of carbon as showed in FESEM analysis (Figure

4.21). The formation of agglomerated catalyst particles in spent Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst was observed. As mentioned by Hu and Lu (2009), the catalyst containing nickel is subjected to deactivation by carbon after running the catalytic testing. The low activation energy also can be related to the deactivation by coke formation at higher temperature as explained by Paál et al.

(2007). However, the surface state under these conditions is regenerable as confirmed by regeneration reaction on this catalyst as discussed in Section 4.2.8.

88

4.5.2.2 FESEM Analysis over Rh/Ni (30:70)/Al

2

O

3

Catalyst

Figure 4.23 shows the FESEM micrographs of fresh and spent Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours. The magnification used is

5000X and 25000X. As can be seen from the micrographs, the fresh catalyst showed the aggregated with rough surface morphology and having average particle sizes.

However, by increasing the magnification to 25000X, the micrograph showed that the catalyst has many pores that will assist the CO

2

methanation activity. The moderate particle size and pore diameter of the catalyst is considered as important factors for the increasing activity of CO

2

methanation. The obvious changes in the morphology of the spent catalyst were observed as shown in Figure 4.23 below. The formation of aggregated and agglomerated species making the spent catalyst increased its particle size and having larger interaction between metal and support catalyst.

(a) Fresh (b) Spent

Figure 4.23

FESEM micrographs of fresh and spent Rh/Ni (30:70)/Al

2

O


C for 5 hours with magnification 5000X and

25000X

89

From the results presented in Figure 4.24, it can be concluded that the active surface of Ni oxide species was homogeneously distributed on the surface of the supported catalyst which is good for the methanation reaction activity. Anyway, nonhomogeneous particle distribution indicated that the Rh element adsorbed on the surface of the catalyst made the reaction happened easier.

Figure 4.24

EDX Mapping over Rh/Ni (30:70)/Al

2

O

3


C for 5 hours

Furthermore, Table 4.22 shows the EDX analysis for the fresh and spent of

Rh/Ni (30:70)/Al

2

O

3

catalysts. The elemental analysis performed by EDX confirmed the presence of Ni, Rh, Al and O in the potential catalyst.

90

Table 4.22

EDX analysis of fresh and spent Rh/Ni (30:70)/Al

2

O

3


C for 5 hours

Catalyst

Al

Weight Ratio (%)

O Ni Rh C

Fresh catalyst 44.56 41.73 5.50 8.21 -

Spent catalyst 41.65 38.60 5.75 11.25 2.76

*Fresh – Catalyst after calcined the catalyst at 400 o

C and before catalytic testing activity

*Spent – Deactivated catalyst after catalytic testing

Similar to the results presented for the EDX analysis of Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst, the composition of Ni in the Rh/Ni (30:70)/Al

2

O

3

catalyst was also revealed as lower than Rh. This is because Ni catalyst was not just deposited on the surface but was incorporated into the bulk of the support or adsorb into the pores of the support due to the high porosity of alumina support as explained in Section 4.5.2.1. It might probably one of the reasons why the composition of Ni is lower than the other elements. In spite of this, the atomic ratio of Ni in spent catalyst

(5.75%) is similar to their fresh catalyst (5.50%) indicating that the catalyst is stable with its similar composition of active species.

However, the atomic ratio of the Rh increases from 8.21% to the 11.25% after reaction with H

2

and CO

2

. This might be due to the migration of the Rh element from the pore of alumina support to the surface of the support. There also have 2.76% carbon deposition on the surface of this spent catalyst after undergone the catalytic testing. This result proved that the declining activity of the catalyst has been caused by the deactivation of carbon deposition. It can be seen that the percentage of Al was dominated in the fresh and spent catalyst with 44.56% and 41.65% since it is the support for the catalyst.

91

4.5.3 Nitrogen Absorption Analysis (NA)

The potential catalysts, Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O

3

in fresh and spent form were characterized by nitrogen adsorption analysis. Table 4.23 summarizes the BET surface area and average pore diameter.

Table 4.23 BET surface area and average pore diameter of the fresh and spent

Pd/Ru/Ni (2:8:90)-Al

2

O

3


2

O at 400 o

C for 5 hours

3 catalysts calcined

Catalyst Condition S

Bet

(m

2 g

-1

)

Average pore diameter (

Å

)

Pd/Ru/Ni (2:8:90) -Al

2

O

3

Fresh 266.10 50.3841

Spent 221.97 58.3265

Rh/Ni (30:70) - Al

2

O

3

Fresh

Spent

269.91

235.38

50.2171

52.9980

From BET surface area analysis of Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst, it can be seen that surface area of fresh catalyst was 266.10 m

2 g

-1

with 16.58% higher than its spent catalyst (221.97 m

2 g

-1

). According to Wan Abu Bakar et al.

(2009), the BET surface area is presumed to be reduced when there is no generation of new active sites and no transformation of active species occurred during the catalytic reaction.

By referring to the EDX analysis, the existence of carbon also might reduced the surface area by blocking the pores of the spent catalyst. As a result, the conversion of

CO

2

over spent Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst was decreased to 35.10% compared to the fresh catalyst (43.60%) at 200 o

C.

However, the surface area of the Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst is considered higher. The higher surface area of catalyst denoted the increase active sites of the catalyst. This result agrees well with the particle size of Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst obtained from FESEM micrograph, where the catalyst showed nano particles level. This smaller particle size presumed to contribute the increment of the surface area. This may contribute to the increasing of catalytic activity over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst. While, the average pore diameter of

92 fresh and spent Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst is around 50 Å. Preferable effect of pore structure of the support could also enhance the conversion of CO

2

as claimed by

Chang et al.

(1997). Unfortunately, nitrogen analysis for the catalyst calcined at

700 o

C and 1000 o

C cannot be carried out due to instrument breakdown.

Figures 4.25 and 4.26 for fresh and spent catalysts exhibit isotherms which shows the Type IV characteristic which features its hysteresis loop of type H3. Type

IV represents stepwise multilayer adsorption on a uniform non-porous surface. It also indicates the presence of typical mesopores.

250

200

150

100

50

0

0 0.2

0.4

0.6

0.8

1

Relative Pressure P/P o

Figure 4.25

Isotherm plot of fresh Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst

93

250

200

150

100

50

0

0 0.2

0.4

0.6

0.8

1


Figure 4.26

Isotherm plot of spent Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst

For the Rh/Ni (30:70)/Al

2

O

3 catalyst, the surface area of fresh catalyst

(269.91 m

2 g

-1

) was 12.79% larger than that of spent catalyst (235.38 m

2 g

-1

). This loss of catalyst surface area is undoubtedly a consequence of sintering and even loss of the catalyst crystallites after running the testing at reaction temperature of 400 o

C.

The sintering effect not just resulted in a decrease of catalyst surface area but also in an activity as can be seen in Section 4.2.7. The catalytic activity of spent catalyst

(19.80%) was not comparable to that of fresh catalyst (43.02%) at reaction temperature of 200 o

C. BET surface area of fresh and spent Rh/Ni (30:70)/Al

2

O

3 catalysts can also be considered higher. The results showed that there is an interaction between the support catalyst with rhodium and nickel which resulted an increasing of surface area thus increase the catalytic activity of the catalyst.

The N

2

adsorption-desorption isotherms of the Rh/Ni (30:70)/Al

2

O

3

catalyst demonstrated an isotherm Type IV with hyteresis loop of type H3 for that of mesoporous materials (Figure 4.27). The type of isotherm and pore distribution of

Rh/Ni (30:70)/Al

2

O

3

catalyst is similar to their spent catalyst (Figure 4.28). The mesopores structure of nickel oxide catalyst led to the increment of the CO

2 performance which has been mention previously by Inui (1996). The existence of the mesopores structure give the optimum pore size in helping to adsorb the reactant gasses on the surface of the catalyst itself. The average pore diameter of the catalyst

94 is summarized in Table 4.23 above. This catalyst showed the optimum pore diameter which around 50 Å.

250

200

150

100

50

0

0 0.2

0.4

0.6


0.8

1

Figure 4.27

Isotherm plot of fresh Rh/Ni (30:70)/Al

2

O

3

catalyst

250

200

150

100

50

0

0 0.2

0.4

0.6


0.8

1

Figure 4.28

Isotherm plot of spent Rh/Ni (30:70)/Al

2

O

3

catalyst

4.5.4 Fourier Transform Infra-Red (FTIR) Analysis

Figure 4.29 and Figure 4.30 show the comparison of the FTIR spectra analysis of fresh and spent Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O


C for 5 hours.

95

Figure 4.29

FTIR spectra of (a) fresh (b) spent (c) regenerated Pd/Ru/Ni

(2:8:90)/Al

2

O

3


C for 5 hours

Figure 4.30

FTIR spectra of (a) fresh (b) spent (c) regenerated Rh/Ni

(30:70)/Al

2

O

3


C for 5 hours

96

In addition, there was small amount of nitrate residues left in the catalyst due to the absorption peaks at 1384-1399 cm

-1

. Carpentier et al.

(2007) had suggested that calcination at temperature of 400 o

C did not completely remove nitrate originated from the metal precursors. However, the intensity of the nitrate peaks decreased after underwent several calcination starting from the fresh to regenerate catalysts. The absorption bands below 1071 cm

-1

for fresh and spent catalysts were due to the stretching mode of metal oxide (M=O) groups. The FTIR analyses for both catalysts are tabulated in Table 4.24 below.

Table 4.24 FTIR analysis of Pd/Ru/Ni (2:8:90)/Al

2

O

3


2

O


C for 5 hours

Catalyst Wave number (cm

-1

) Assignment

Pd/Ru/Ni (2:8:90)-

Al

2

O

3

(fresh, spent and regenerated)

3444.61 - 3448.01 Asymmetric stretching mode of

OH group from adsorbed water molecule.

1638.10 - 1638.32

1384.59 – 1399.10

Bending mode of adsorbed water molecule.

Stretching mode of free nitrate

(NO

3

-

) group from metal precursors.

Rh/Ni (30:70)-

Al

2

O

3

(fresh, spent and regenerated)

1070.91 – 610.96

3444.41 - 3448.39

1638.17

1384.60

1071.02

–

–

–

1638.83

1384.83

602.96

Stretching mode of M=O

Asymmetric stretching mode of

OH group from adsorbed water molecule.

Bending mode of adsorbed water molecule.

Stretching mode of free nitrate

(NO

3

-

) group from metal precursors.

Stretching mode of M=O

97

4.5.5 Thermogravimetry Analysis Differential Thermal Analysis (TGA-

DTA)

After aging the catalysts which had been prepared by wetness impregnation method in an oven for 24 hours at 80 o

C-90 o

C, the catalyst was sent for characterization using TGA-DTG. Figure 4.31 shows the TGA thermogram of


2

O

3

catalyst.

Figure 4.31

Thermogram of Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst after aging in an oven for 24 hours at 80 o

C-90 o

C

It showed two significant weight lost curves which occurred at 60 o

C and

280 o

C. The total weight loss is 14.66% (Appendix E). Starting from temperature of

60 o

C until 280 o

C, free water molecule and nitrate compound from the supported catalysts were removed, while, 280 o

C onwards, nitrate compound and surface hydroxyl molecule were decomposed from the samples. Savva et al.

(2008) who studied on Ni/Al

2

O

3

catalyst prepared by conventional impregnation and sol-gel methods found that the physisorbed water was completely removed up to 150 o

C and

200 o

C. Meanwhile, the decomposition of nitrate was occurred at temperature range

98 of 190-360 o

C. Finally, they observed weight loss at temperature higher than 360 o

C should be attributed to the removal of structural water from the alumina.

From this investigation, the calcination temperature of 400 o

C is insufficient to remove all the nitrate compounds that were originated from metal precursor. The presence of nitrate compounds in this catalyst was also observed by FTIR analysis which detected the stretching mode of free nitrate group. It can be seen that all the impurities was removed from the catalysts at temperature higher than 800 o

C meaning that pure metal oxide had obtained. Thus, the catalyst calcined at 1000 o

C should be good for this catalyst. However, the investigation found that calcination temperature higher than 400 o

C decreased its catalytic activity.

Figure 4.32 Thermogram of Rh/Ni (30:70)/Al

2

O

3

catalyst after aging in an oven for 24 hours at 80 o

C-90 o

C

Figure 4.32 shows the TGA thermogram of Rh/Ni (30:70)/Al

2

O

3 catalyst.

Thermogravimetric analyses of Rh/Ni (30:70)/Al

2

O

3

catalyst revealed that this catalyst lost a 19.0155% of weight between 25 o

C and 400 o

C, due to the release of adsorbed water and hydroxyl anions. It can be concluded from both figures above, there was no significant differences in weight loss observed between Pd/Ru/Ni

(2:8:90)/Al

2

O

3


2

O

3

catalysts.

99

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

Overall performance from the catalytic activity studies did not yield any catalyst that gives 100% conversion of CO

2

at lower reaction temperature. However,


2

O

3


2

O

3

were assigned as the most potential catalysts resulted from the catalytic activity measurement from FTIR and

GC. These catalysts were prepared using wetness impregnation technique, aging at

85 o

C and followed by calcination at 400 o

C for 5 hours. Pd/Ru/Ni (2:8:90)/Al

2

O

3 catalyst shows 43.60% of CO

2

conversion with 6.82% of methane formation at

200 o

C. This catalyst had highest percentage of 52.95% CO

2

conversion and yield

39.73% methane at maximum temperature 400 o

C. In the presence of H

2

S in the gas stream, the conversion dropped to 35.03% with 3.64% yield of methane. However, this catalyst achieved 100% H

2


C and remains constant until reaction temperature of 300 o

C.

Alumina supported NiO which was doped with rhodium, Rh/Ni

(30:70)/Al

2

O

3

gave 3.8% of CH

4

at 200 o

C while the conversion of CO

2

was 43.02%.

This catalyst was further increased its CO

2

conversion to 90.10% and yielded 70.75%

CH

4

at 400 o

C. As the temperature increases, the amount of CH

4

formed also increased. However, the methane percentage decreased to 0.5% with 4.53% CO

2 conversion for Rh/Ni (30:70)/Al

2

O

3

catalyst during testing with flowing of H

2

S gas.

100

Meanwhile, H

2

S was completely removed at reaction temperature of 180 o

C and still remains 100% even at temperature of 400 o

C.

The characterization of these two potential catalysts was studied in depth.

From XRD results, these supported catalysts showed very low degree of crystallinity at calcination temperature of 400 o

C, indicating that they are highly amorphous in structure. Besides that, FESEM images of Pd/Ru/Ni (2:8:90)/Al

2

O

3

and Rh/Ni

(30:70)/Al

2

O

3

catalysts indicated the inhomogeneous spherical shape and a rough surface morphology of the catalysts, respectively. However, both fresh and spent of these catalysts exhibited high surface area which are more than 200 m

2

/g and displayed small nano particle size with undefined shape, uniformly distributed on the surface of the catalyst material and having Type IV isotherm. The elemental analysis performed by EDX confirmed the presence of Ni, Ru, Pd, Al and O in the Pd/Ru/Ni

(2:8:90)/Al

2

O

3

catalyst while, Ni, Rh, Al and O in the Rh/Ni (30:70)/Al

2

O

3

catalyst.

Characterization by FTIR and TGA-DTG revealed the existence of nitrate and hydroxyl compounds on the catalyst surface.

5.2 Recommendations

In order to get an excellent catalyst that has larger surface area with high stability for high effectiveness conversion, amorphous alloy of Ni containing catalyst can be used for the methanation catalyst as suggested by previous research.

Furthermore, MCM-41 can be the other alternative for the support material over Ni containing catalyst doped with noble metals. MCM-41 posseses uniform pore size dimensions with very high specific surface are. Therefore, it can be expected that this support can enhance the activity of the catalyst. Moreover, the exact ratio of catalyst solution to alumina beads needs to determine in order to find the gas hourly space velocity (GHSV) for the testing purpose and it can be used as one of the optimization parameter.

101

For the highly dispersed catalyst, TEM is often used for the observation.

Therefore, the use of TEM in future research is important in order to determine the dipersion of the catalyst surface. Furthermore, the catalyst calcined at 700 o

C and

1000 o

C should be sent for FESEM-EDX analysis to investigate the changes that occur on the catalyst surface during the calcination process. Lastly, all the catalysts should be further tested with real natural gas in order to get the real conversion of the methanation and desulfurization reaction.

102

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O

3

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2

in flue gas in

LNG power plant. Energy Conversion Management. 38. 44 – 448.

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2

O

3

catalysts. Applied Catalysis A: General. 205, 279-284.

Zhou, G., Jiang, Y., Xie, H. and Qiu, F. (2005). Non-noble metal catalyst for carbon monoxide selective oxidation in excess hydrogen. Chemical Engineering

Journal . 109, 141-145.

Zhuang, Q., Qin, Y. Chang, L. (1991). Promoting effect of cerium oxide in supported nickel catalyst for hydrocarbon steam-reforming. Applied Catalyst . 70 (1), 1-

8.

Zielinski, J. (1982). Morphology of nickel / alumina catalyst. Journal of catalysis . 76

(1), 157-163.

115

APPENDIX A

Preparation of Alumina Supported Nickel Oxide Based Catalysts and Its Ratio

Catalyst

Ni /Al

2

O

3

Ratio

100

Ru/Ni /Al

2

O

3

10:90

Pd/Ru/Ni /Al

2

O

3

2:8:90

Pd/Ru/Ni /Al

2

O

3

5:25:70

Pd/Ru/Ni /Al

2

O

3

Pt/Ru/Ni /Al

2

O

3

Rh/Ru/Ni /Al

2

O

3

5:15:80

2:8:90

2:8:90

Ru/Mn/Ni /Al

2

O

3

2:8:90

Ru/Cu/Ni /Al

2

O

3

2:8:90

Ru/ Pd/ Ni / Al

Rh/Ni /Al

2

O

3

Rh/Ni /Al

2

O

3

2

O

3

2:8:90

10:90

Rh/Ni /Al

2

O

3

30:70

20:80

Pt/Rh/Ni /Al

2

O

3

Pd/Rh/Ni /Al

2

O

3

Ru/Rh/Ni /Al

2

O

3

Rh/Mn/Ni /Al

2

O

3

Rh/Cu/Ni /Al

2

O

3

2:8:90

2:8:90

2:8:90

2:8:90

2:8:90

APPENDIX B

Calculation of Methane Percentage

Sample catalyst : Rh/Ni (30:70)/Al

2

O

3

At reaction temperature of 400 o

C,

Conversion CO

2

= 80.10% = 801000

Peak area from GC = 158749

Equation from calibration of methane standard : y = 56025x

y = 56025x

158749 = 56025x

x = 158749

56025

= 2.833

ppm = 2.833 x 2 x 10

5

10

6

5 mL x 2 x 10

5

= 566707.7198 ppm

% Yield of CH

4

= [CH

4

] from GC × 100%

[Converted CO

2

] from FTIR

=

801000

= 70.75%

116

APPENDIX C

Schematic Diagram of Home Built Micro Reactor Connected using Two

Isothermal Furnaces

To atmosphere

Pressure Gauge

Mixture cylinder 2 way valve

117

3 way valve Cell

FTIR

CO

2

H

2

H

2

S

Furnace 1 Furnace 2

118

APPENDIX D

Calculation of atomic weight percentage ratio of element in catalyst preparation

Example: Rh/Ni with the ratio of 30:70

Weight of nickel salt, Ni(NO

3

)

2

.

6

H

2

O used = 2 g

Weight of Ni

2+

= 2 g________ x AW Ni

2

O

= 2 g________ x 58.64 g/mole

290.83 g/mole

= 0.404 g

Weight of Rh

3+

= g_ × 30%

70%

Weight of rhodium salt (Rh(NO

3

)

3

.xH

2

O) used

= 0.173 g__ × MW Rh(NO

3

)

3

AW Rh

3+

.xH

2

O

= 0.173 g___ × 324.951 g/ mole

102.905 g/ mole

= 0.546 g

APPENDIX E

Calculation of weight loss over Pd/Ru/Ni (2:8:90)/Al

2

O

3

catalyst

119

(a)

(b)

Stage (a) :- 100% - 92.3% = 7.7%

Stage (b) :- 92.3% - 85.1% = 7.2%

Stage (a) + (b) = 14.9% ~ 14.66%

120

PUBLICATIONS AND PRESENTATIONS

Wan Azelee Wan Abu Bakar, Mohd. Yusuf Othman, Rusmidah Ali, Ching Kuan

Yong and Susilawati Toemen. (2009). The Investigation of Active Sites on

Nickel Oxide Based Catalysts towards the In-Situ Reactions of Methanation and Desulfurization. Modern Applied Science. Vol. 3. No. 2. Feb 2009.

Wan Azelee Wan Abu Bakar, Rusmidah Ali and Susilawati Toemen. (2010). Noble

Metal for the Enhancement of CO

2

Methanation Reaction on Nickel Oxide

Based Catalysts. Chinese Journal of Catalysis. (Article Submitted).

Wan Azelee Wan Abu Bakar, Rusmidah Ali and Susilawati Toemen. (2011).

Catalytic Methanation Reaction over Supported Nickel-Ruthenium Oxide based for the Purification of Simulated Natural Gas. Scientia Iranica. (Article

Submitted-Manuscipt Number: 15.185.110112).


Catalytic Methanation Reaction over Supported Nickel-Rhodium Oxide based for the Purification of Simulated Natural Gas. Journal of Natural Gas

Chemistry. (Article Submitted)

Wan Azelee Wan Abu Bakar, Rusmidah Ali and Susilawati Toemen. (2009). Noble

Metal for the Enhancement of CO

2

Methanation Reaction on Nickel Oxide

Based Catalysts. (SECOND INTERNATIONAL CONFERENCE AND

WORKSHOPS ON BASIC AND APPLIED SCIENCES & REGIONAL

ANNUAL FUNDAMENTAL SCIENCE SEMINAR (03-04 June 2009) The

Zone Regency Hotel, Johor).


Methanation Reaction over Nickel Oxide based Catalysts for the Purification on Natural Gas. (2 nd

JUNIOR CHEMIST COLLOQUIUM (01-02 July 2009)

Universiti Malaysia Sarawak, UNIMAS).

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