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Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported
Ruthenium Catalysts
Article in Journal of Nanoscience and Nanotechnology · July 2015
DOI: 10.1166/jnn.2015.9872
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Article
Journal of
Nanoscience and Nanotechnology
Copyright © 2014 American Scientific Publishers
All rights reserved
Printed in the United States of America
Vol. 14, 1–7, 2014
www.aspbs.com/jnn
Vapor Phase Hydrogenation of Nitrobenzene to Aniline
Over Carbon Supported Ruthenium Catalysts
Chakravartula S. Srikanth, Vanama Pavan Kumar, Balaga Viswanadham,
Amirineni Srikanth, and Komandur V. R. Chary∗
Catalysis Division, Indian Institute of Chemical Technology, Hyderabad, India
A series of Ru/Carbon catalysts (0.5–6.0 wt%) were prepared by impregnation method. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), CO-chemisorption,
surface area and pore-size distribution measurements. The catalytic activities were evaluated for
the vapor phase hydrogenation of nitrobenzene. The dispersion measured by CO-uptake values
suggests that a decrease of dispersion is observed with increasing Ru loading on carbon support.
These findings are well supported by the crystallite size measured from XRD measurements. XPS
study reveals the formation of Ru0 after reduction at 573 K for 3 h. The catalysts exhibit high conversion/selectivity at 4.5 wt% Ru loading during hydrogenation reaction. The particle size measured
from CO-chemisorption and TEM analysis are related to the TOF during the hydrogenation reaction.
Ru/C catalysts are found to show higher conversion/selectivities during hydrogenation of nitrobenzene to aniline.
Keywords: Activated Carbon, Ruthenium Catalysts, Hydrogenation of Nitrobenzene and Active
Sites.
1. INTRODUCTION
Aniline is a valuable chemical for the plastics, rubber processing, herbicides, dyes and pigments industries. About
85% of global aniline is produced by catalytic hydrogenation of nitrobenzene. Supported metal catalysts are widely
being used in various organic transformation reactions,
especially hydrogenation. The supported metal systems
are usually characterized by several techniques to measure metal dispersion and its availability since the activity and selectivity of hydrogenation reaction depends on
them.
Recently, carbon nanotube supported platinum catalyst,1
platinum nano-particle core-polyaryl ether trisacetic acid
ammonium chloride dendimer shell nano-composites,2
Pd/active carbon,3 Pt/C catalysts in supercritical carbon dioxide and ethanol,4 Pd supported on hydrotalcite
catalysts5 and Pd-B/SiO2 amorphous catalyst6 are used
for nitrobenzene hydrogenation. Among supported noble
metal catalysts, ruthenium is one of the most frequently
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2014, Vol. 14, No. xx
used metal for hydrogenation reactions at laboratory level
as well as industrial scale.7–11 Ruthenium has been used
as the most promising catalyst7 in partial hydrogenation of
benzene to cyclohexene. Ru is well known as very active
catalyst for the hydrogenation of CO to hydrocarbons8
and ammonia synthesis.9 Ruthenium is very selective in
hydrogenation of C O group in the vicinity of conjugated
or isolated double bonds or an aromatic ring.10 11 Literature reports suggest that hydrogenation of nitrobenzene
produces aniline along with several intermediates such as
hydrazobenzene, azobenzene, azoxybenzene, nitrosobenzene and phenylhydroxylamine.4 However, on Ru/SBA-15
catalysts in our earlier report showed the selectivity for the
formation of aniline was 100%.12 Due to its wide spread
utility of Ru in hydrogenation reactions in this present
work we used Ru/C catalysts for the hydrogenation of
nitrobenzene. The catalytic activity results were discussed
in terms of characteristics such as Ru particle size and
metal dispersion. The catalysts were also characterized by
an array of complimentary spectroscopic and adsorption
techniques like BET surface area, pore size distribution,
XRD, TEM, TPR and CO chemisorption.
1533-4880/2014/14/001/007
doi:10.1166/jnn.2014.9872
1
Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported Ruthenium Catalysts
2. EXPERIMENTAL DETAILS
2.1. Preparation of Catalyst
Activated carbon of commercial origin (NORIT, BET
surface area = 1340 m2 /g, crushed and sieved to 18/25
BSS) was used as a support material for synthesizing
Ru/activated carbon catalysts. A series of ruthenium catalysts with different ruthenium loadings (0.5 to 6 wt%)
were prepared by simple wet impregnation method using
aqueous solution of RuCl3 on activated carbon support.
The samples were dried at 383 K for 12 h in air. Prior
to the catalyst characterization the catalysts are reduced
under pure H2 flow (50 ml/min) at 573 K for 3 h.
2.2. X-Ray Diffraction Studies
X-ray diffraction patterns were obtained on Rigaku miniflex diffractometer using graphite filtered Cu K (K =
0.15406 nm) radiation. Determination of the ruthenium
phase was made with the help of JCPDS data files.
2.3. TEM Analysis
The morphological analysis was carried out using transmission electron microscopy (TEM) on a JEOL 100S
microscope at high resolution (HR) on a JEOL 2010
microscope). Samples for both TEM analysis were prepared by adding about 1 mg of pre-reduced sample to
5 mL of methanol followed by sonication for 10 min.
A few drops of suspension were placed on a hollow copper
grid coated with a carbon film made in the laboratory.
2.4. BET Surface Area and Pore Size Distribution
The surface area, pore size distribution studies of the
pre-reduced catalysts was estimated using N2 adsorption
isotherms at 77 K by the multipoint BET method taking
0.0162 nm2 as its cross-sectional area using Autosorb-1
(Quanta chrome instruments).
2.5. Temperature Programmed Reduction (TPR)
Temperature programmed reduction (TPR) experiment
were carried out on Auto Chem 2910 (Micromeritics,
USA) instrument. In a typical experiment ca. 100 mg of
oven dried Ru/C sample (dried at 383 K for 12 h) was
taken in a U-shaped quartz sample tube. Prior to TPR
studies the catalyst sample was pretreated in an inert gas
(Argon, 50 mL/min) at 473 K. After pretreatment, the sample was cooled to ambient temperature and the carrier gas
consisting of 5% hydrogen balance argon (50 mL/min) was
allowed to pass over the sample raising the temperature
from ambient to 673 K heating at the rate of 10 K/min.
The HCl produced during the reduction was condensed in
a cold trap immersed in liquid nitrogen and isopropanol
slurry. The hydrogen concentration in the effluent stream
was monitored with the TCD and the areas under the peaks
were integrated using GRAMS/32 software.
2.6. CO-Chemisorption
CO-chemisorptions measurements were carried out on
AutoChem 2910 (Micromeritics, USA) instrument. Prior
2
Srikanth et al.
to adsorption measurements, ca. 100 mg of the sample
was reduced in a flow of hydrogen (50 mL/min) at 673 K
for 3 h and subsequently passed with pure helium gas
flow for an hour at 673 K. The sample was subsequently
cooled to ambient temperature in the same He stream. CO
uptake was determined by injecting pulses of 9.96% CO
balanced helium from a calibrated on-line sampling valve
into the helium stream passing over the reduced samples at
673 K. Ruthenium surface area, percentage dispersion and
Ru average particle size were calculated assuming the stoichiometric factor (CO/Rus ) as 1. Adsorption was deemed
to be complete after three successive runs showed similar
peak areas.
2.7. X-Ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy was used to study the
chemical composition and oxidation state of catalyst surfaces. The XPS spectra of the catalysts were measured on
a XPS spectrometer (Kratos-Axis 165) with Mg K radiation (h = 12536 eV) at 75 W. The Ru 3d and 3p corelevel spectra were recorded and the corresponding binding
energies were referenced to the C 1s line at 284.6 eV
(accuracy within (0.2 eV)). The background pressure during the data acquisition was kept below 10−10 bar.
2.8. Catalytic Activity Studies
Hydrogenation of nitrobenzene (C99.9% Aldrich chemicals) was carried out over the catalysts in a vertical
down-flow glass reactor at 548 K and operating under
atmospheric pressure. Ca. 100 mg of the catalyst, diluted
with double the amount of quartz grains was packed
between the layers of quartz wool. The upper portion of
the reactor was filled with glass beads, which served as
pre-heater for the reactants. Prior to the reaction, the catalyst was reduced in a flow of hydrogen (50 mL/min)
at 573 K for 3 h. After reduction the reactor was
fed with nitrobenzene at 523 K (WHSV = 3612 h−1 ;
H2 /Nitrobenzene = 4; Residence time: 0.0276 h). The reaction products were analyzed by HP-6890 gas chromatograph equipped with a HP-5 capillary column with a
flame-ionization detector (FID). The products were also
identified using HP-5973 quadrupole GC-MSD system
using HP-1MS capillary column.
3. RESULTS AND DISCUSSION
3.1. X-Ray Diffraction
Figure 1 shows the XRD patterns of pure activated carbon and different loadings of ruthenium supported on
activated carbon catalysts exhibited no peaks due to Ru
except 6.0 wt% Ru/C catalysts corresponding to Ru (110)
plane.12 13 The Ru/C catalyst showed an increase in the
intensity of the broad peak centered at 2 = 44 , from
3 wt% Ru catalyst characteristic of amorphous metallic
ruthenium species.14 This clearly indicates the formation
J. Nanosci. Nanotechnol. 14, 1–7, 2014
Srikanth et al.
Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported Ruthenium Catalysts
Ru Wt%
Intensity(a.u.)
6.0
4.5
3.0
1.5
0.5
Activated carbon
10
20
30
40
50
60
70
80
2 Theta
Figure 1. X-ray diffraction patterns of Ru/C catalysts.
of Ru0 due to reduction of catalysts at 573 K in pure H2
flow prior to the XRD analysis. We could observe an XRD
peak a 2 = 694 for 6.0 wt% ruthenium loading.12 13
The catalysts with 3 wt% Ru loading also showed a
broad reflection at 2 = 694 indicating the presence of
crystalline Ru. However, the same was not observed in
4.5 wt% catalysts; hence the peak in 3 wt% catalysts is
attributed as artifact of the instrument. The absence of
XRD reflections corresponding to crystalline Ru0 in the
present catalytic system suggests that the high dispersion
of metallic ruthenium on carbon support. However, it cannot be ruled out the presence of Ru0 crystallites having
size less than 4 nm, which is beyond the detection limit
of powder XRD technique. This implies that the smaller
metal crystallites either interacts strongly with the support
surface or get immobilized in to the high surface area of
the activated carbon support.
3.2. Transmission Electron Microscopy
Figure 2 shows the morphology of Ru supported on activated carbon catalyst, investigated by the transmission
electron microscopy (TEM). The micrographs of 0.5 wt%
Ru/C shows no visible images for the presence of Ru
on the carbon support, which suggest the presence of Ru
particles as highly dispersed state in amorphous form.
This clearly indicates that Ru at lower loadings is present
in highly dispersed state with very small particle sizes
or immobilized into the porous network of the carbon
support. The TEM image of 3.0 and 6.0 wt% catalyst
show that the uniform distribution of larger number of
small ruthenium particles. These results clearly suggest
the highly dispersed state of ruthenium at lower Ru loadings and with increasing Ru content the particle size is
increased. The presence of larger number of smaller particle of ruthenium agrees well with the absence of Ru particle below 4.5 wt% of Ru on activated carbon support
in XRD.
3.3. BET Surface Area and Pore Size Distribution
The surface areas determined by nitrogen physisorption
for pure carbon and various Ru/C catalysts are given in
Table I. The BET surface area of the pure carbon was
found to be 1340 m2 /g. Impregnation of Ru on the carbon decreased the surface area of the support which can
be attributed to filling of the pores of the support evidenced by the pore diameter and pore volume measurements. Figure 3(a) show the nitrogen adsorption/desorption
isotherm and 3(b) show the Barrett Joyner Halenda (BJH)
pore size distribution of Ru/activated carbon catalysts. The
pure activated carbon support shows the presence of both
micro and mesorporus nature with the average pore diameters in the range of ∼ 15 and ∼ 50 Å. The presence of
micropores can be observed by the presence of shoulder peak at 20 Å, however not able to measure in the
present system. The addition of Ru to carbon has no significant effect on the isotherms and pore size distribution.
The pure activated carbon support shows the presence of
both micro and mesorporus nature with the average pore
diameters in the range of ∼ 15 and ∼ 36 Å. The addition
of Ru to carbon has a significant effect on the isotherms
and pore size distribution. A well-defined step occurs in
P /P0 range of 0.4–0.5 represents the spontaneous filling
of the mesopores due to capillary condensation, which is
also evidenced by the BJH distribution Figure 3(b). The
amount of physisorbed nitrogen decreased at the higher
loading of Ru on activated carbon support. The intensity
of pore size distribution peak 3(b) decreases with increase
of Ru loading due to the presence of Ru particles inside
the pores of activated carbon. The results of BET surface area and pore volume of the sample decrease after
impregnation of Ru onto the activated carbon are showed
Figure 2. TEM images of various Ru/C catalysts.
J. Nanosci. Nanotechnol. 14, 1–7, 2014
3
Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported Ruthenium Catalysts
Srikanth et al.
Table I. Results of ICP analysis and BET surface area of various ruthenium supported on carbon.
Ru loading (wt%)
Ru (ICP) loading (wt%)
BET surface area (m2 /g)
Total pore volume (mL/g)
Average pore diameter (Å)
—
0.38
1.24
2.60
4.26
5.74
1340
1322
1322
1288
1233
1209
0.883
0.874
0.861
0.842
0.778
0.732
26.48
26.25
25.98
25.77
25.33
24.22
0.0
0.5
1.5
3.0
4.5
6.0
in Table I. The decrease in surface area and pore volume
are attributed to the intra-pore formation of the ruthenium
nano-particles within the pores reduces the surface area
and pore volume. The introduction of metallic elements
onto activated carbon decreases the surface area can be
also ascribed to an increase in the density of the composites after loading of ruthenium.
3.4. Temperature Programmed Reduction
The reducibility of ruthenium species in Ru/C catalysts
with different ruthenium loading was investigated by the
temperature programmed reduction (TPR) method and the
profiles are shown in Figure 4. All the samples show a
main reduction peak around 450–480 K with a shoulder at
lower reduction temperature. This can be assigned to the
stepwise reduction of Ru3+ /Ru0 .12 15–17 This peak is broad,
(a)
Ru wt%
6.0%
Volume (cc/g)
4.5%
3.0%
1.5%
3.5. CO Chemisorption
The dispersion of Ru was calculated from COchemisorption using the following equation assuming the
0.5%
0.0%
0.0
0.2
possibly due to the reduction of Ru3+ ions located in different environments. However, this peak is less intense and
is shifted to lower reduction temperatures (426–411 K) at
higher loadings. The Ru/C catalysts with 4.5 and 6.0 wt%
the main reduction peak is more intense, possibly due to
the poor dispersion of the ruthenium salt in this solid,
leading to the formation of bigger particles where the diffusion of H2 is more difficult, and thus appearing at higher
temperatures (> 460 K). The third H2 consumption peak
appears at around 540–510 K, which could be assigned to
the reduction of Ru3+ strongly interacting with the support.
This high temperature peaks can also possibly associated
with to the reduction of Ru present in the narrowest pores
or reduction of ruthenium oxide (RuO2 ) or ruthenium oxychloride formed by exposition and drying in air during the
preparation of the samples.12 18 The positions of Tmax and
the H2 consumptions are presented in the Table II showed
slightly high amounts than the amount of metal precursor present on the support. The excess H2 consumption
might be due to the reduction of oxygen containing groups
present on the carbon support.19
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
(b)
Ru wt%
Pore volume (cc/g)
6.0%
4.5%
3.0%
1.5%
0.5%
Activated Carbon
20
40
60
80
100
120
140
160
Pore Diameter (Å)
Figure 3. (a) N2 adsorption–desorption isotherms (b) BJH pore size
distribution of pure carbon support and Ru/C catalysts.
4
Figure 4.
catalysts.
Temperature programmed reduction profiles of various Ru/C
J. Nanosci. Nanotechnol. 14, 1–7, 2014
Srikanth et al.
Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported Ruthenium Catalysts
Table II. Temperature programmed reduction results of various Ru/C catalysts.
Ru wt%
0.0
0.5
1.5
3.0
4.5
6.0
1
Tmax
(K)
H2 uptake (mol/g)
2
Tmax
(K)
H2 uptake (mol/g)
3
Tmax
(K)
H2 uptake (mol/g)
Total H2 uptake (mol/g)
—
–
–
426
411
409
–
–
–
277.4
367.0
450.5
433
460
467
473
475
478
85.5
134.7
371.2
554.5
740.6
901.5
–
–
543
521
515
513
–
–
25.0
166.4
178.4
187.4
85.5
134.7
396.2
998.3
1286.0
1539.4
cubic particle with five sides exposed to the gas plane,
%Dispersion = (number of surface Ru atoms × 100/
Total number of Ru atoms
Average particle size (nm)
= 6000/Ru metal area per gram of Ru × Ru density
The ruthenium metal areas were determined using the
equation SCO = nmS Xm ns−1 , where SCO is the total metallic surface area, nmS is the CO consumption and Xm is
chemisorptions stoichiometry at monolayer coverage, and
ns−1 is the number of ruthenium atoms per unit surface
area. The results of CO-chemisorption dispersion, metal
area and particle size are presented in Table III. Dispersion of ruthenium on activated carbon varied from 84.3%
to 23.8% for 0.5 to 6.0 wt% ruthenium loading. The high
dispersion of ruthenium at lower loadings of ruthenium is
probably attributed to strong interaction of ruthenium with
the oxygen containing groups of activated carbon. It is
likely that as Ru content increases; the deposition may be
more on the external surface of the carbon support. This
will reduce the distance between metallic species, thereby
promoting agglomeration leading to the decrease in dispersion of Ru. The results showed that irreversible CO
uptakes and number of active sites increases up to 4.5 wt%
and decrease slightly at 6.0 wt% ruthenium on activated
carbon. The particle size of ruthenium also increases linearly up to 4.5 wt% and exponentially at 6.0 wt%. The
results suggest that ruthenium is present in highly dispersed form at lower loadings and agglomerization takes
place at higher loadings. The results of CO-chemisorption
are in good agreement with the results of XRD and TEM
where the increase in the particle size was observed only
at 6 w% Ru/C catalysts.
3.6. X-Ray Photoelectron Spectroscopy
Table IV presents the information obtained from XPS
results such as the binding energy values of C 1s, its
FWHM values, deconvoluted binding energies of C 1s and
its ratio. All the catalysts are showing the C 1s binding
energy values around ∼ 284.8 eV. The binding energy of
284.8 eV is well fit with the corresponding binding energy
of carbon. With increase of ruthenium loading there is a
slight shift in the C 1s B.E values towards lower energy.
However, the decrease in binding energy of C 1s, might
be due to decrease of ruthenium dispersion i.e., interaction
between the metal and the support decreased. The constant FWHM values are around 1.8, implying that only
Table III. Results of CO chemisorption dispersion and metal area on various Ru/C catalysts.
Ru loading
(wt%)
0.5
1.5
3.0
4.5
6.0
Dispersion
(%)
COirr uptake
(mol/g)
Metal area
(m2 /gRu )
Metal area
(m2 /gcat )
No. of Ru sites
g(cat)−1 (x1019 Particle size
(nm)
84.3
73.3
50.0
35.3
23.8
48.6
108.8
148.1
157.0
141.4
478.6
357.2
243.3
172.1
116.0
2.39
5.53
7.30
7.74
6.95
2.92
6.55
8.92
9.45
8.51
1.0
1.3
2.0
2.8
4.2
Table IV. Binding energies (eV), FWHM of Ru 3p and deconvoluted BE and its ratio of C 1s of Ru/C catalysts.
BE (eV) C 1s (deconvoluted)
Ru loading
(wt%)
0.5
1.5
3.0
4.5
6.0
BE (eV) and
FWHM of C 1s
284.8
284.8
284.8
284.7
284.7
(1.8)
(1.8)
(1.7)
(1.9)
(1.9)
CC–O
CC–C
284.3
284.4
284.4
284.4
284.4
(60)∗
(60)
(61)
(62)
(61)
285.8
285.7
285.7
285.7
285.7
Notes: ∗ Corresponding to % of peak; # corresponding to FWHM values C
oxygen groups.
J. Nanosci. Nanotechnol. 14, 1–7, 2014
(40)∗
(40)
(39)
(38)
(39)
Ratio of
CC–C /CC–O
1.25
1.25
1.56
1.63
1.56
BE (eV) and FWHM
of Ru 3d5/2
463.0
463.2
463.4
463.5
463.5
C corresponding to carbon bonded with carbon; C
(1.8)
(1.9)
(2.1)
(2.2)
(2.4)
BE (eV) and
FWHM of Ru 3d3/2
483.7
483.9
484.0
484.1
484.2
(0.8)
(1.1)
(1.2)
(1.2)
(1.3)
O corresponding to carbon bonded with
5
Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported Ruthenium Catalysts
one type of doublet is present. According to the literature14
the peaks of C 1s deconvolution can be ascribed to carbon
atoms bonded to the following functionalities. The carbon
in graphite at ∼ 284.4 eV (CC–C ), carbon would be linked
to ketone, quinine at 287.5 (Cc o) hydroxyl and phenol groups have peak at ∼ 286.5 eV (CC–O ). These results
clearly demonstrated that the activated carbon is formed
in a frame like morphology containing different oxygen
functional groups. The ratio of the carbon bonded to carbon and the carbon bonded to oxygen is increasing with
increase of ruthenium loading, which might be due to formation bonding between ruthenium and oxygen (Ru–O) or
coverage of ruthenium on oxygen containing carbon surface. The intensity of C 1s core level spectra does not
change much with the increase of ruthenium loading.
Figure 5 shows the XPS spectrum of Ru 3p regions of
Ru/AC, catalysts, indicating the electronic state and the
environment of Ru species present in supported catalysts.
The binding energies and their FWHM values are reported
in Table IV. Ru 3p signals were considered instead of 3d,
due to overlaps of Ru 3d signal with C 1s signal which created ambiguity in determining the exact oxidation state of
ruthenium.20 Generally Ru0 possesses binding energy values in the range of 462–463 eV. The 3p signal of Ru shows
a doublet at B.E. (3P3/2 ) 462.8 and (3P5/2 ) 484.3 (eV),
which are characteristic of Ru0 .12 16 The B. E. (462.8) of
Ru 3p signal at 3p3/2 remained constant in all loadings
showing the absence of any specific interaction between
the metal and support. The intensity of XPS peak corresponding to 3p5/2 increases with ruthenium loading on carbon support. Further, the XPS analysis did not shown any
signal for the presence of residual chloride after reduction
of the catalyst samples at 573 K for 3 h, prior to the analysis. This further confirms the absence of any surface Cl
species.12 TPR results also show that the reduction of the
Srikanth et al.
catalysts was complete before 573 K indicating complete
reduction of ruthenium to its metallic state (Ru0 ).
3.7. Hydrogenation of Nitrobenzene
Figure 6 shows the results for the effect of ruthenium loading in the hydrogenation of nitrobenzene to aniline on carbon support at 548 K. The conversions of nitrobenzene to
aniline increased with ruthenium loading up to 4.5 wt%
and leveled off at 6 wt% Ru catalysts. The leveling off
of the hydrogenation activity at higher loadings is can be
attributed to increase in the particle size of ruthenium particles at higher loadings.21 22 The conversion of 0.5 wt%
ruthenium loading was 20% increases up to 90% as the
ruthenium loadings increase up to 4.5 wt%. The selectivity for the formation of aniline remained > 99% for all
the catalysts with trace amounts of benzene formation.
The CO-uptake values also increased up to 4.5 wt% and
showed a slight decrease at 6 wt% higher ruthenium loading. No. of surface active sites also decreased at higher
loading due to agglomerization of Ru particles.
Figure 7 show the relationship between TOF and
particle size of Ru/C catalysts for the hydrogenation
of nitrobenzene to aniline. TOF is defined as the rate
of nitrobenzene molecules converted per unit time per
exposed site of ruthenium. The TOF’s are calculated
form chemisorption data as the number of molecules of
nitrobenzene converted by one surface Ru atom per second
using following equation:
Rate = Volume of reactant fed
× Fractional conversion/weight of the catalyst
TOF = Rate/CO − uptake
The correlation in Figure 7 suggests that structure-activity
relationship exists between ruthenium dispersion and
hydrogenation activity. The TOF increased with increase
% Conversion/Selectivity
100
90
80
70
60
50
40
Conversion of nitrobenzene
30
Selectivity for aniline
20
10
0
0
1
2
3
4
5
6
Ru loading (Wt%)
Figure 5. Ru 3p XPS spectra of various Ru/C catalysts.
6
Figure 6. Hydrogenation of nitrobenzene to aniline over Ru/C catalysts reaction conditions: weight of the catalyst = 100 mg; reaction
temperature = 548 K; WHSV = 3612 h-1; H2 /Nitrobenzene = 4; residence time: 0.0276.
J. Nanosci. Nanotechnol. 14, 1–7, 2014
Srikanth et al.
Vapor Phase Hydrogenation of Nitrobenzene to Aniline Over Carbon Supported Ruthenium Catalysts
TOF results suggest that the activity of the catalysts is
dependent of the particle size of ruthenium and the reaction is found to be structure sensitive.
5
4.8
3
4.0
2
3.6
TOF
Particle size
Particle size (nm)
TOF x 10–4(s–1)
4
4.4
1
3.2
0
1
2
3
4
5
6
Ru loading (wt%)
Figure 7. Correlation between turn over frequency, particle size and Ru
loading over Ru/C catalysts.
of ruthenium loading up to 4.5 wt%, and less significant
beyond this loading. There is a substantial change of per
site activity (TOF) with respect to the Ru particle size.
This suggests that there is structure-sensitive nature of
the reaction over ruthenium supported on activated carbon
catalysts. The increase in hydrogenation activity with Ru
loadings on Ru/C catalysts is probably due to a high dispersion of Ru and the constant activity at higher loadings
is due to formation of Ru particle. These catalytic results
are in good agreement with the results of dispersion, and
the information derived from XRD and TEM.
4. CONCLUSIONS
The results of TEM suggest that ruthenium is highly dispersed on the carbon support up to 4.5 wt% and formed
large particles at 6 wt% which is good agreement with
CO-chemisorption data. TPR results showed that reduction of RuCl3 takes place in two steps and the reduction
is complete by 573 K. XPS results showed the formation
of Ru0 species and absence of chloride ions in the catalysts after reducing 573 K and for 3 h. High dispersion of
Ru can be achieved on carbon support below 4.5 wt% of
Ru. Catalytic activity for the hydrogenation of nitrobenzene to aniline on Ru/C catalysts showed optimum activity
obtained at 4.5 wt%. Selectivity for the formation of aniline was found to be > 99% for all the catalysts tested.
Acknowledgments: The authors thank Director, IICT,
Hyderabad for her encouragement.
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Received: 5 March 2014. Accepted: 24 May 2014.
J. Nanosci. Nanotechnol. 14, 1–7, 2014
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