copper electroless

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Structured Copper-Based Catalyst on an Aluminum Plate
Prepared by Electroless Plating for a Wall-Type Methanol
Reformer and CO Shift Converter
On-line Number 498
Choji Fukuhara 1 , Hiromichi Ohkura 1 , Yoshiyuki Kamata 1 and Akira Igarashi 2
1
Department of Chemical Engineering on Biological Environment
Faculty of Engineering
Hachinohe Institute of Technology
88-1 Myo Ohbiraki, Hachinohe, Aomori 031-8501, Japan, e-mail: [email protected]
2
Department of Environmental Chemical Engineering
Faculty of Engineering
Kogakuin University
2665-1 Nakano-machi, Hachioji, Tokyo 192-0015, Japan, e-mail: [email protected]
ABSTRACT
A wall reactor system, in which a metallic wall is directly catalyzed, is a reaction system that
enables an efficient transfer of thermal energy, a rapid response to fluctuating loads. Application
of the system in a reactor involving reformers and water gas shift converters for power generator
system by fuel cells is expected. In order to develop a high-performance plate-type catalyst for
wall-type methanol reformer and CO shift converter, a structured copper-based catalysts were
prepared by electroless plating, consisting of a displacement plating of zinc, an intermediate
chemical plating of various metals, and a chemical plating of copper. The reforming and shift
performances and the physicochemical properties of the plated catalysts were investigated.
Results showed that the prepared catalysts had different performance depending on the metal
species used in the intermediate plating. Among them, a plate-type Cu-Fe/Zn catalyst on an
intermediate iron plating exhibited high reforming and shift performances. The performances of
the Cu-Fe/Zn catalyst made much progress by pre-oxidizing in air stream before reaction, which
were nearly the same and/or higher with that of a commercial granular catalyst. Characterization
including an X-ray diffraction (XRD) suggested that the oxidation treatment produced a copperzinc alloy on the plated surface where zinc atoms existed in proximity of copper atoms. The
existence of copper and zinc atoms in proximity might have formed an active site that
accelerated the formation and decomposition of an intermediate species in both reactions, which
led to the increased catalytic activity. It was also demonstrated that the declined activity of the
Cu-Fe/Zn catalyst was repeatedly recovered by oxidizing, which is practically convenient in
using the Cu-Fe/Zn catalyst as a plate-type catalyst for a wall-type methanol reformer and shift
converter.
KEYWORDS
structured copper-based catalyst, steam reforming of methanol, CO shift reaction,
electroless plating
1
1. INTRODUCTION
In hydrogen production process by steam reforming of methanol and succeeding CO shift
reaction, the reactions involved a relatively large amount of reaction energy, so that a reduction
of the thermal time constant is required of the reformer and shift converter for efficient exchange
of heat energy. It is also envisioned to apply the fuel cell power generation system to the onboard or portable type (Lindstrom, 2001). Therefore, the reformer and converter need to be
compact and respond quickly to load fluctuations (Wild, 2002). However, with the fixed-bed
reaction system which has been used for the reformer and converter in the main, these
requirements cannot be met because of the presence of film resistance and the temperature
gradient caused by convectional heat transfer as well as the increase of pressure loss caused by
packing the catalyst (Fukuhara and Igarashi, 2000). Thus, development of a reformer and
converter equipped with a new conception for reaction system is needed.
A wall reactor system, in which a metallic wall is directly catalyzed, is a reaction system that
enables an efficient transfer of thermal energy by conductional heat transfer, a rapid response to
fluctuating loads by a lower pressure loss, and a downsizing of the reactor dimension. We have
previously estimated by numerical simulation that the wall reactor system demonstrates such
performance even when a reaction proceeds with high reaction heat and/or with rapid change of
feed rate (Fukuhara et al., 1993, 1995). If the reformer and converter are successfully constructed
with a wall-type reaction system, substantial improvement of the hydrogen production process in
the chemical industry can be expect as well as further promotion of deployment for fuel cell
power generation systems in a wide variety of fields.
In our previous works, we had deposited a nickel component on an aluminum substrate by
electroless plating to prepare a plate-type catalyst and examined its methanol decomposition
properties (Fukuhara et al., 1994, 2002, 2003, 2004). Results demonstrated the effectiveness of
the electroless plating for the preparation of wall-type catalysts used in a wall reactor system.
Further, it was demonstrated that the composition of the bath used in electroless plating and
changes in plating conditions have a large influence on the catalytic properties of the prepared
plate-type catalyst, and preparation of a plate-type catalyst with high performance is possible by
controlling the plating conditions. Electroless plating makes uniform deposition of catalyst
components and in-situ catalyst regeneration possible. Electroless plating is thought to be an
advantageous method also for the preparation of catalysts for wall-type methanol reformer and
CO shift converter.
In this study, with a view to develop a high performance plate-type catalyst for wall-type
methanol reformer and CO shift converter, a structured copper-based catalysts were prepared by
electroless plating, consisting of a displacement plating of zinc, an intermediate chemical plating
of various metals (iron, nickel, cobalt, tin), and a chemical plating of copper. The reforming and
shift performances of the plated catalysts were investigated. In addition, the influence of
different plating conditions and reaction conditions on the reforming and shift properties of the
plated catalyst were also examined, and the factors necessary for the reforming and shift
properties to show themselves were studied, combining with measurements of physicochemical
properties for the plated layers.
2. EXPERIMENTAL
2.1 Preparation of Plate-Type Copper-Based Catalyst
A plate-type copper-based catalyst was prepared on an aluminum plate by the electroless
plating, which consisted of a displacement plating of zinc, an intermediate plating of metal (iron,
2
nickel, cobalt, tin, respectively), and a chemical plating of
Pretreatment of Al corrugated
copper. The aluminum plate (JIS A1100P-H24,
fin by 3N-HCl aq.
thickness:0.4mm) was formed into a pentagonal prism
shape, of which sectional view resembled a star. Its
Displacement of Al by Zn
maximum diameter was 21 mm and its length was 120 mm.
( standard bath temp. : 20˚C )
The apparent total surface area of electrolessly plated
(two times )
catalyst composition was 330 cm2. The procedure of
Washing in water
preparing catalyst by the electroless plating is shown in
Figure1. In order to remove impurities and activate the
Each metals (Fe, Ni, Co, Sn)
surface, the aluminum plate was first immersed in 3N
deposited by chemical reduction
hydrogen chloride solution. The plate was then immersed in
a zinc oxide plating bath (ZnO:50g/l, NaOH:60g/l, alkaline,
Electroless Cu deposition by
bath temperature:20 ℃ , time:3.0min) to displace surface
chemical reduction
( bath temp. : 24˚C )
aluminum with zinc, and washed in a water bath. The
displacement and washing procedures were repeated two
times, although the immersion time of the second
Drying in air (about 12h)
displacement was only 1.5min. Subsequently, the plate was
immersed in various metals plating baths. Four kinds of Fig.1 Procedure of electroless plating
metal plating baths were used : (a) an iron bath (FeSO4・
7H2O:30g/l, C4H4KNaO6・4H2O:50g/l, NaH2PO2・H2O:10g/l,
bath temperature:20℃, time:20min), (b) a nickel bath (NiCl2・6H2O:45g/l, NaH2PO2・H2O:50g/l,
CH3COONa ・ 3H2O:60g/l, bath temperature:60 ℃ , time:3.0min), (c) a cobalt bath (CoCl2 ・
6H2O:45g/l, NaH2PO2 ・ H2O:50g/l, C6H5Na3O7 ・ 2H2O:35g/l, NaOH:10g/l, CH3COONa ・
3H2O:60g/l, bath temperature:95 ℃ , time:7.0 min), and (d) a tin bath (SnCl2 ・ 2H2O:18g/l,
TiCl3:6.2g/l, C6H5Na3O7・2H2O:98g/l, EDTA・2Na:30g/l, bath temperature:80℃, time:3.0 min).
Thereafter, the plate was immersed in a copper plating bath (Cu(NO3)2 ・ 3H2O:15g/l,
Na2CO3:10g/l, C4H4KNaO6 ・4H2O:30g/l, NaOH:20g/l, HCHO(37%aq.):100ml/l, alkaline, bath
temperature:24℃) to deposit copper component on the surface by chemical reduction, containing
formaldehyde solution as a reducing agent. After washing in a water bath, the plate was dried in
air for about 12 h to prepare a plate-type copper-based catalyst.
2.2 Methanol Steam Reforming and CO Shift Reaction over
the Plated Catalyst
Methanol steam reforming and CO shift reaction over the prepared catalyst were conducted at
atmospheric pressure using a conventional flow reactor. After placing the plated catalyst in the
reactor, the catalyst was reduced in a hydrogen stream (100 ml/min) at 300°C for 1.0h or
oxidized in air stream (100 ml/min) at 300°C for 1.0h. In steam reforming, methanol and water
was pumped into the reactor. Flow rate of methanol was 5.7x10-3 mol/min, partial pressure of
feed 0.85 atm (diluted by helium), ratio of steam to carbon (S/C) 1.0, space velocity (LHSV
based on the net volume of the plated layer) 10.1h-1 , and reaction temperature 200-350 °C. In
CO shift reaction, carbon monoxide and water were fed into the reactor under the conditions that
flow rate of carbon monoxide was 1.0x10-3 mol/min, partial pressure of feed 1.0 atm, ratio of
steam to carbon (S/C) 2.0, space velocity (GHSV based on the net volume of the plated layer)
2,120 h-1, and reaction temperature 150-300 °C. In both reactions, conversion and selectivity of
products were calculated on the basis of carbon.
3
2.3 Characteristics of the Plated Layer
The physicochemical properties of the plated catalysts were characterized using some
instruments for analysis. The surface morphology and the sectional view of the plated layers
were observed using scanning electron microscopy (SEM), and the elemental profiles in the
same fields analyzed using energy dispersion X-ray (EDX). The crystal structures for each plated
layer on the catalyst were measured by X-ray diffraction (XRD) with CuKα radiation. The
specific surface areas of the plated catalysts were measured by the BET one point method using
nitrogen at its liquid temperature. The net weight of the plated components on the aluminum
substrate was estimated from the difference in weight of the catalyst sample between before and
after immersing in concentrated nitric acid solution.
3. RESULTS AND DISCUSSION
3.1 Effect of Intermediate Plating on Catalytic Property
Table 1 shows the properties
of methanol steam reforming
(M.S.R.) and CO shift reaction
(CO S.R.) of various plate-type
copper-based catalysts prepared
by changing the metal species in
the intermediate plating. Of the
element symbols indicating the
kind of catalyst here, the middle
element appearing between Cu
and Zn indicates the metal
species used in the intermediate
plating. In reforming, none of
the catalysts were reduced by
hydrogen stream or oxidized by
air stream. As shown in the table, the reforming and shift properties of each plated catalyst
largely differ depending on the metal species used in the intermediate plating. Namely, the CuNi/Zn catalyst on an intermediate nickel plating has high catalytic activity, however, the
decomposition is dominant over the reforming in M.S.R. and methane is produced in some
amounts as a by-product in CO S.R. The catalyst on an intermediate cobalt or tin plating does not
have a high activity and is poor in selectivity of the reforming. On the other hand, the Cu-Fe/Zn
catalyst on an intermediate iron plating has high catalytic activity and selectively produces
carbon dioxide in both reactions. It is suited as a plate-type catalyst for reforming and shift
reaction. The intermediate plating is a process carried out to increase the adhesion of the copper
component, however, the metal species used for that has a large influence on the performance of
the plated catalyst.
3.2 Effect of Pretreatment before Reaction
We took up a Cu-Fe/Zn catalyst that had high catalytic performance and subjected it to a
reduction treatment by hydrogen and/or oxidation treatment by air before the reaction. Table 2
shows the difference in catalytic properties due to the treatments. As shown in this table, the CuFe/Zn catalyst oxidized by air is improved in reforming and shift activity more than the catalyst
4
reduced by hydrogen. Besides the former exhibits higher reforming activity than the catalyst not
pre-treated before reaction shown in Table 1, on the other hand, the activity is lowered below the
level of the catalyst not pre-treated if the Cu-Fe/Zn catalyst was reduced by hydrogen. The
selectivity of products does not show a very large change for both treatments. Takezawa et al.
(1982, 1985, 1995) and Jang et al. (1993) indicated that the reforming reaction proceeds on a
copper-based reforming catalyst by the reaction mechanism via formation of a formate, and
deduced that the metallic copper provides
active sites for the formaldehyde formation
step, which is the rate-determining step.
Therefore, the reduction of the catalyst is
thought to have a positive effect on the
reforming reaction and the oxidation is
thought to have negative effect. In reports
about CO S.R. over copper-based catalysts by
other researchers, a reduction by hydrogen
usually takes place prior to the reaction.
However, the activity change of the plate-type
Cu-Fe/Zn catalyst due to the oxidation or
reduction demonstrates an opposite tendency.
The result is very interesting from the
viewpoint of the catalytic chemistry. The
factors for the activity improvement due to
the oxidation will discussed in section 3.4.
Figure 2 compares the reforming and shift activity of the oxidized Cu-Fe/Zn catalyst with that
‥
of reduced commercial granular catalysts (SUD CHEMIE Co., MDC-3(reforming catalyst), C187(shift catalyst)) under the same reaction conditions. For the comparison, the amount of packed
granular catalyst was made equal to the net weight of the plated layer on substrate for the platetype catalyst. As shown in the figure, the reforming and shift activity of the plate-type catalyst is
nearly equal to the industrial catalyst in low temperature regions while it is far superior to the
industrial catalyst in high temperature regions. It might be inferred that the active site on the
plate-type catalyst is qualitatively different from that of the commercial catalysts. At any rate, it
is apparent that the oxidized Cu-Fe/Zn catalyst has the high performance for M.S.R. and CO S.R.
100
(a)
Conversion [ % ]
Conversion [ % ]
100
80
60
40
20
●
0
Commercial catalyst
▲
( Reduction by H2 )
Plate-type Cu・Fe/Zn
catalyst (Pre-oxidation)
(b)
80
60
40
●
200 250 300 350
Reaction temperature [ ℃ ]
0
Commercial catalyst
( Reduction by H2 )
Plate-type Cu・Fe/Zn
catalyst (Pre-oxidation)
▲
20
150 200 250 300
Reaction temperature [ ℃ ]
Fig.1 Comparison of (a) M.S.R. and (b) CO S.R. performance of
Fig.2
Cu・Fe/Zn catalyst with that of commercial granular catalyst
5
3.3 Morphology of the Plated Catalyst
Figure 3 shows SEM photographs of a section of the plated layer for the Cu-Fe/Zn and CuNi/Zn catalysts oxidized by air and elemental profiles in a sectional field obtained by EDX
analysis (analyzed area indicated on the SEM photographs). In the figure, the thickness of the
plated layer on substrate is about 50-60 micrometers for both catalysts and the plated surface is
considerably rugged. In both plated layers, intense peaks of zinc besides copper are detected
from the bulk to the surface, that is, zinc is distributed widely in the plated layer. The detected
zinc comes from the zinc oxide used in the displacement plating. In the Cu-Fe/Zn catalyst, small
peaks of the iron used in the intermediate plating are detected near the surface, while in the CuNi/Zn catalyst, peaks of nickel are detected somewhat strongly, indicating a large amount of
nickel having gotten mixed in the plating. Nickel is an active component for methanol
decomposition (Fukuhara et al., 1994, 2003, 2004), and the reaction rate of the water gas shift
reaction on a transition metal is slow (Takezawa et al., 1985, 1995). Therefore, as indicated by
the selectivity of products in Table 1, it can be inferred that, on the Cu-Ni/Zn catalyst, the
decomposition is promoted more than the reforming. On the other hand, because the Cu-Fe/Zn
catalyst contains a small amount of mixed iron, it exhibits the catalytic properties proper to the
copper-zinc catalyst and high reforming and shift performance. For the Cu-Co/Zn and Cu-Sn/Zn
catalyst, a similar elemental analysis of a section of the plated layer was also conducted and
differences in the amounts of copper and zinc present and the amounts of cobalt and tin mixed in
were found. These differences were thought to bring about the catalytic properties shown in
Table 1. The results suggest the possibility of preparing the plate-type catalyst to meet the
targeted reaction by controlling the preparation conditions of electroless plating.
Aluminum
base
Plated area
Aluminum
base
20μm
Plated area
20μm
a)
Analytical
area
Intensity(cps)
a)
500
Al
Zn
1600 500
b)
Cu
800
250
Fe
0
0
250
Al
b)
Zn Ni
0
Cu
5000
2500
0
Cu・Ni/Zn catalyst
Cu・Fe/Zn catalyst
Fig.3 a) SEM photographs and b) elemental profiles measured by EDX for
section of the plated layers
3.4 Effects of Oxidation on Physicochemical Properties of
the Cu-Fe/Zn Catalyst
6
a)
Element Concentration
(Mass) [%]
Al 0.0 Zn
96.9
Fe
0.4
Cu
2.7
Element
Al Zn
Fe
Cu
Concentration
(Mass) [%]
0.0 61.6
2.7
35.7
Element Concentration
(Mass) [%]
Al 0.0 Zn
0.0
Fe
0.0
Cu
100
Element Concentration
(Mass) [%]
Al 0.0 Zn
26.7
Fe
0.0
Cu
73.3
Element Concentration
(Mass) [%]
Al 0.2 Zn
0.0
Fe
0.0
Cu
99.8
Element Concentration
(Mass) [%]
Al 0.0 Zn
0.0
Fe
0.0
Cu
100
b)
500μm
Element Concentration
(Mass) [%]
Al 0.0 Zn
98.0
Fe
0.2
Cu
1.8
Surface area :
48.7m2/g-deposit
Element Concentration
(Mass) [%]
Al 0.0 Zn
48.3
Fe
1.1
Cu
50.6
500μm
Surface area :
49.6m2/g-deposit
Fig.4 SEM photographs and EDX analyses of the Cu-Fe/Zn catalyst
a) reduced by hydrogen and b) oxidized by air at 300 °C
7
10,000
○
(e)
△
○
△
(d)
Intensity (cps)
△
△
○
(c)
△
(b)
○
(a)
○
41
42
43
44
▼
5,000
◆
■
●
(e)
■
▼
◆
Intensity (cps)
In this section, we examine the effect of oxidation on
the physicochemical properties of the plated layer for the
Cu-Fe/Zn catalyst and discuss activity-improving factors
and active sites on the surface.
Figure 4 shows representative SEM photographs of
the Cu-Fe/Zn catalyst surface which was reduced by
hydrogen and oxidized by air, together with results of
elemental analysis by EDX. As a trait of the Cu-Fe/Zn
catalyst, white deposits appeared in a large number of
spots on the surface during the drying of the catalyst in
air after plating. The circular formations in the figure are
the deposits and were found from elemental analysis to
be zinc. In comparison of the surface of the catalysts
between reduced and oxidized, the two are the same in
that copper and zinc were detected inside the circular
deposits. On the reduced surface, however, almost
copper only was detected outside the circular deposits,
while on the oxidized surface, zinc was also detected
outside the circular deposits in parts. On the oxidized
surface, circular deposit spots are numerous. In Fig.4,
the BET specific surface areas for the reduced and
oxidized layers were also shown. Both of the layers have
some surface area, and no difference in them was
observed. It was though that the reforming and shift
activity of the Cu-Fe/Zn catalyst was little related with
the surface area.
Figure 5 shows XRD profiles of the Cu-Fe/Zn
catalyst that was oxidized at different temperature.
Results for the reduced catalyst also given in the figure
for comparison. In the profiles of the reduced catalyst, a
distinct peak assigned to the Cu (111) is observed and a
●
(d)
■
◆
◆
◆
30
32
●
▼
(c)
■
●
●
■
34
36
2θ (deg)
(b)
(a)
38
(a) reduced by hydrogen at 300°C,
(b) oxidized by air at 200°C, (c) at 300°C,
(d) at 350°C, (e) at 400°C
△ :Cu(111), ○:Cu-Zn alloy, ■:ZnO(101)
●:ZnO(002),◆:ZnO(100), ▼:CuO(002)
Fig.5 XRD profiles of the Cu-Fe/Zn
catalyst with pre-treatment
peak assigned to the zinc oxide mixed in during the plating operations can also be observed
slightly. On the other hand, if the catalyst oxidized, the peak assigned to the Cu (111) lowers and
the peak assigned to the copper-zinc alloy becomes distinct as the treatment temperature rises.
The peak of zinc oxide also increases in intensity as the treatment temperature rises. The peak of
zinc oxide was detected slightly for the hydrogen reduction, but it significantly increased for the
oxidation. This indicates that the zinc mixed in the bulk layer moves to the surface of the plated
layer by oxidation. Further, the formation of copper-zinc alloy by oxidation indicates that the
zinc having moved to the surface comes to exist in proximity of copper. Nakamura et al.
(1997,1998a, 1998b) reports that the methanol synthesis of carbon monoxide and hydrogen on a
Cu/ZnO catalyst proceeds via the formation of the intermediates of formate- and methoxygroups, and the existence of copper and zinc atoms in proximity forms a reaction site that
activates further the reaction step between the intermediate of formate- and methoxy-groups. The
existence of zinc and copper in proximity at the plated Cu-Fe/Zn surface due to the oxidation can
be thought to form a reaction site that promotes further the intermediate forming step in M.S.R.
and CO S.R., thus bringing about the improvement in catalytic activity shown in Table 2.
3.5 Durability Performance of the Oxidized Cu-Fe/Zn Catalyst
In relation to a copper-based catalyst, sintering of copper particles and partial surface
oxidation generally take place during the reaction, causing an aging deterioration of activity. We
examined the aging deterioration of the oxidized Cu-Fe/Zn catalyst in M.S.R.
Figure 6 shows the results of performance change of the oxidized Cu-Fe/Zn catalyst with time,
regarding to its reforming activity at a reaction temperature of 300°C for five days. In the figure,
the activity is maintained constant to 1200 minutes but gradually declines after that and to 60%
in 4800 minutes. This is the same behavior of activity deterioration as ordinary copper-based
catalysts. As a great feature of the Cu-Fe/Zn catalyst, however, it restored its initial activity when
the catalyst was reoxidized by flowing oxygen after the activity deterioration. Activity
restoration of the catalyst by reoxidation could take place repeatedly. This property of the CuFe/Zn catalyst will provide convenience in actual use. Namely, after the reforming takes place on
the Cu-Fe/Zn catalyst, one may let flow air at the same temperature to give an oxidation. Thus,
the initial activity will be restored again by the next occasion of use. In addition, as shown in
Table 2, there is no need for taking precautions against oxidation of catalyst during storage, if
Conversion (%)
100
80
60
40
Oxidized
(300℃,1h)
20
0
Oxidized
(300℃,1h)
React. Temp. : 300℃
1200
2400
3600
4800
6000
7200
Time on stream (min)
Fig.6 Durability performance of the oxidized Cu-Fe/Zn catalyst for five days
8
anything, oxidation is rather favorable. Exfoliation or removal of catalyst components was not
observed in the catalyst after the aging deterioration test. Thus, one can judge that the plate-type
Cu-Fe/Zn catalyst is a catalyst having great advantages from the viewpoint of practical use.
CONCLUSIONS
The plate-type copper-based catalysts were prepared on the aluminum substrate by electroless
plating and examined their catalytic properties for steam reforming of methanol and CO shift
reaction. The prepared catalysts had different catalytic properties depending on the metal species
used in the intermediate plating. The Cu-Fe/Zn catalyst on the intermediate iron plating has high
catalytic properties. The reforming and shift activity of the Cu-Fe/Zn catalyst were improved by
oxidation before reaction, which were nearly equal to that of the reduced commercial catalyst in
low temperature regions and higher in high regions. Even after the activity of the Cu-Fe/Zn
catalyst was declined, it restored its initial activity by reoxidation and the activity restoration by
reoxidation could be repeated.
From the measurements of physicochemical properties of the plated catalyst, the zinc, mixing
in the plated layer, moved from the bulk to the surface of the plated layer by oxidation. It was
inferred that the active sites for reforming and shift reaction were formed because of the presence
of zinc in proximity of copper, which is a improving factor by oxidation for catalytic activity of
the Cu-Fe/Zn catalyst.
ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas from the
Ministry of Education, Culture, Sports, Science and Technology (13126221).
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