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(Ce,Gd)O2−x catalysts
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Hydrogen production via steam reforming of methanol over Cu/(Ce,Gd)O2Lx catalysts Ta-Jen Huang*, Hsiao-Min Chen Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC
article info
abstract
Article history:
Hydrogen production via steam reforming of methanol is carried out over Cu/(Ce,Gd)O2x
Received 14 December 2009
catalysts at 210e600 C. The CO content in reformate is about 1% at 210e270 C, which are
Received in revised form
the typical temperature for hydrogen production via steam reforming of methanol. Largest
12 March 2010
H2 yield and CO2 selectivity and smallest CO content are obtained at 240 C. The formation
Accepted 18 March 2010
rate of CO increases with increasing temperature. The average formation rate of CO becomes larger than that of CO2 at about 450 C. The H2 yield, the CO2 selectivity and the
Keywords:
CO content become constant at about 550 C. At 240 C, the smallest CO content is obtained
Steam reforming of methanol
with a catalyst weight of 0.5 g and a Cu content of 3 wt%. The H2 yield, defined as H2/
Hydrogen production
(CO þ CO2) in formation rates, at 240 C is always 3 and not affected by the variations of
CO content
either the catalyst weight or the Cu content.
Gadolinia-doped ceria
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Catalyst weight Cu content
1.
Introduction
The issue of global warming has provided strong incentive to develop energy-efficient electrical power sources for vehicles. Currently, the proton exchange membrane fuel cell (PEMFC) appears as the most viable technology for powering vehicles. However, PEMFC needs high-purity hydrogen as the fuel. To overcome the difficulties of hydrogen distribution and storage as well as limited driving range with the use of pressurized hydrogen tanks on vehicles using the PEMFCs, hydrogen production on-board from liquid fuels becomes an alternative [1,2]. Among various high energy density liquid fuels, methanol remains prominent because of its low conversion temperature, high hydrogen to carbon ratio, and no CeC bond thereby lessening soot formation. For automotive PEMFC applications,
hydrogen can be extracted from methanol through steam reforming of methanol [3e5]. The reactions for steam reforming of methanol are considered as: CH3OH / CO þ 2H2
(1)
CO þ H2O / CO2 þ H2
(2)
For the produced hydrogen to be used in the PEMFCs, CO is a poison. To reduce the CO amount, the process of steam reforming of methanol can be followed by treatments of high-temperature and low-temperature wateregas shift reactions as well as preferential CO oxidation. If the CO2 selectivity of steam reforming of methanol is high enough, the CO content would be low enough and only preferential
* Corresponding author. Tel.: þ886 3 5716260; fax: þ886 3 5715408. E-mail address: [email protected] (T.-J. Huang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.082
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CO oxidation is needed; thus, the hydrogen production cost can be reduced substantially. Therefore, the CO2 selectivity is an important factor for steam reforming of methane to produce hydrogen. Copper-containing catalysts are usually used for steam reforming of methanol [6e13]. Oxygen-ion conducting materials, such as ceria [11,12], have been used as the support of copper-containing catalysts for steam reforming of methanol; however, gadolinia-doped ceria (GDC) has not yet been used for this purpose. Notably, GDC has higher activity than ceria for CO2 reforming of methane [14]; this is attributed to the fact that Gd doping increases the oxygen-ion conductivity and thus the amount of surface oxygen vacancy, which results in an increase of the reforming activity [15]. Therefore, GDC has been used as a support material of the nickel catalyst for steam reforming of methane [15e17]; notably, also, GDCsupported Ni catalysts can have minor C deposition problem with methane as reactant e this is attributed to the fact that the lattice oxygen of GDC can enhance the oxidation reaction of the deposited C species to form CO2 [18,19]. It is thus interesting to see whether GDC can have good effect as the support of copper-containing catalysts for steam reforming of methanol. In this work, Cu/GDC catalysts were used for hydrogen production via steam reforming of methanol. The effects of reaction temperature, catalyst weight and Cu content on hydrogen yield, CO2 selectivity and CO content were studied. Optimum conditions on reaction temperature, catalyst weight and Cu content were obtained.
2.
Experimental
2.1.
Preparation of Cu/GDC catalysts
GDC was prepared by co-precipitation. The details of the method have been presented elsewhere [14]. The GDC powders were calcined by heating at 10 C min1 to 400 C and held for 2 h, and then to 750 C and held for 4 h before cooling down. GDC is Ce0.9Gd0.1O1.95d. The Cu/GDC catalyst was prepared by impregnating the GDC powders (325e270 mesh, i.e. 44e53 mm diameter) with an aqueous solution of copper nitrate Cu(NO3)2$3H2O (99% purity, SHOWA). The mixture was placed in a heated stirrer to remove excess water and then in a vacuum oven to dry overnight. The dried Cu/GDC powder was heated at 10 C min1 to 400 C and held for 6 h, and then cooled down. In this work, the percentage of Cu content denotes the weight of Cu with respect to that of GDC.
2.2.
Fig. 1 e Effect of reaction temperature on product profiles with 0.1 g of 5% Cu/GDC. (a) 210 C, (b) 240 C, (c) 270 C.
Activity tests
The activity of the catalyst for steam reforming of methanol was measured under atmospheric pressure in a continuous flow reactor charged with 0.1 g of catalyst, if not specified otherwise. The catalyst sample was reduced at 400 C with 30 ml min1 of H2 (99.999% purity) for 1 h. Then, the catalyst was purged by Ar, and then heated to the designated temperature with a heating rate of 10 C min1. Then, the test was carried out with feeding of 100 ml min1 of a gas mixture
of CH3OH:H2O:Ar ¼ 9.83:9.83:80.34 e that is, the molar ratio of H2O:CH3OH in feed is 1, which is the ratio of a stoichiometric reaction for steam reforming of methanol with complete CO2 formation. The reactor outflow was analyzed on-line by gas chromatograph (GC, China Chromatography 8900), CO-NDIR (non-dispersive infrared, Beckman 880) and CO2-NDIR (NGA 2000) analyzers.
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Table 1 e Variations of conversion, formation rates of H2, CO2 and CO, H2 yield, CO2 selectivity and CO content with reaction temperature over 0.1 g of 5% Cu/GDC. Temp ( C)
Conversiona (%)
H2b (103)
CO2b
COb
H2 yieldc
CO2 selectivityd
CO contente (%)
7.25 29.30 40.32 52.04 67.27 71.90 76.87 84.95 86.58 89.80 94.87 99.04
0.372 1.50 2.05 2.63 3.36 3.50 3.59 3.83 3.70 3.68 3.79 3.95
119.3 486.5 664.4 834.3 1051 1022 953.9 923.2 717.7 596.0 525.2 546.5
4.89 14.70 25.45 56.06 100.4 208.6 361.4 530.2 763.5 940.4 1098 1148
2.99 3.00 2.97 2.95 2.92 2.84 2.73 2.64 2.49 2.40 2.33 2.33
0.961 0.971 0.963 0.937 0.913 0.830 0.725 0.635 0.485 0.388 0.324 0.323
1.30 0.97 1.23 2.09 2.90 5.62 9.15 12.2 17.1 20.4 22.5 22.5
210 240 270 300 330 360 390 420 450 500 550 600 a b c d e
Conversion was calculated by (COx formed/CH3OH fed), where COx denotes CO2 plus CO. Formation rate in mmol*g1.min1. Average value of the data taken during the test from 30 to 180 min. H2 yield ¼ H2/(CO þ CO2). CO2 selectivity ¼ CO2/(CO þ CO2). CO content ¼ CO/(H2þCO).
3.
Results
3.1.
Effect of reaction temperature
Fig. 1 shows the profiles of H2, CO2 and CO formation rates over 0.1 g of 5% Cu/GDC catalyst at the reaction temperatures of 210e270 C. The formation rates of H2 and CO2 went up to the maximum and then decreased with time; the extents of both going up and coming down of the rates increase with increasing reaction temperature. The formation rate of CO is very small at these reaction temperatures. Table 1 indicates that the CO content is about 1% at these temperatures of 210e270 C; notably, the CO content is defined as the percentage of CO in the hydrogen-rich reformate after the removal of the remaining methanol and water and also the formed CO2. These temperatures are the typical ones for hydrogen production via steam reforming of methanol; thus, only preferential CO oxidation would be needed to reduce the
Fig. 2 e Effect of reaction temperature on CO2/CO and H2/CO ratios, respectively, with 0.1 g of 5% Cu/GDC.
Fig. 3 e Effect of reaction temperature on product profiles with 0.1 g of 5% Cu/GDC. (a) 360 C, (b) 390 C.
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Table 2 e Variation of the on-stream time when the formation rate of CO became larger than that of CO2 with reaction temperature over 0.1 g of 5% Cu/GDC. Temperature ( C) 390 or lower 420 450 500 550 600
Fig. 4 e Effect of reaction temperature on product profiles with 0.1 g of 5% Cu/GDC. (a) 450 C, (b) 500 C, (c) 550 C.
CO content in reformate to an acceptable level for the usage in the PEMFCs. Notably, also, with a CO content of about 1%, tests of selective CO oxidation in rich hydrogen have shown that CO conversion close to or even being 100% can be obtained [20,21]. Table 1 also shows that the conversion and the average formation rates of H2, CO2 and CO increase with increasing reaction temperature from 210 to 270 C. However, the CO content is smallest at 240 C; this is in accordance with the
Time (min) Longer than 200 168.3 88.4 37.6 21.6 From beginning
largest CO2 selectivity and the highest H2 yield. Fig. 2 shows that both ratios of CO2/CO and H2/CO are maxima at 240 C; additionally, the extents of variations in CO2/CO and H2/CO ratios are almost the same. Notably, the maximum in the CO2/CO ratio results in the maximum in the CO2 selectivity and that in the H2/CO ratio results in the minimum in the CO content. This indicates that 240 C is the optimum reaction temperature, in terms of the smallest CO impurity, for steam reforming of methanol over the Cu/GDC catalyst. Fig. 3a shows the profiles of H2, CO2 and CO formation rates over 0.1 g of 5% Cu/GDC catalyst at 360 C. The formation rate of CO went up to the maximum and then decreased with time; this is the same behavior as those of H2 and CO2. However, Fig. 3b shows that the formation rate of CO at 390 C increases almost continuously instead of coming down with time. Consequently, the formation rate of CO can equal that of CO2 at about 200 min on-stream during the test. Notably, Table 1 also shows that the average rate of CO2 formation is at maximum at 330 C; then, the CO2 rate decreases with increasing temperature before becoming constant at about 550 C. Therefore, the H2 yield, the CO2 selectivity and the CO content become constant at about 550 C. Table 1 also shows that the average formation rate of CO2 becomes smaller than that of CO at 450 C. Fig. 4a shows that, at 450 C, the formation rate of CO reaches a maximum, comes down for a short period, and then increases continuously to become larger than that of CO2. The exact time-on-stream when the formation rate of CO becomes larger than that of CO2 is presented in Table 2. This behavior of the on-stream rate of CO becoming larger than that of CO2 may be an indication of the change of the property of the Cu/GDC catalyst during the test. Fig. 4b shows that, at 500 C, the formation rate of H2 becomes quite stable after going up, and that of CO does not have a maximum but increases continuously with time. Fig. 4c shows a similar behavior of H2 and CO at 550 C; however, the formation rate of CO becomes larger than that of CO2 from almost the beginning of the test. At 600 C, the CO rate is larger than the CO2 rate from the very beginning, as presented in Table 2. Therefore, the on-stream rate of CO formation becoming larger than that of CO2 formation occurs earlier at higher temperature. After the test of steam reforming of methanol, oxidation treatment at 800 C with 100 ml/min of a gas mixture of 20% O2 in argon indicates negligible carbon deposition even after the 600 C test. This is also confirmed by a conversion over 99% at 600 C, as shown in Table 1. Notably, the conversion is
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calculated by (COx formed/CH3OH fed), where COx denotes CO2 plus CO; thus, a conversion over 99% means that the CH3OH fed has been almost completely converted to the gaseous products of CO2 and CO e that is, there is no or negligible carbon deposition.
3.2.
Effect of catalyst weight
Fig. 5 shows the profiles of H2, CO2 and CO formation rates over 0.3e0.8 g of 5% Cu/GDC catalyst at 240 C, which is the reaction temperature for the lowest CO content as indicated in the above. Fig. 5a shows that the formation rates of H2 and CO2 went up to the maximum and then decreased with time, a similar behavior as that with 0.1 g of catalyst; however, the extent of the decrease of the rate with time with 0.3 g of catalyst is smaller in comparison with that of 0.1 g of catalyst. With 0.5 g of catalyst, the formation rates of H2 and CO2 become relatively constant after reaching the maxima, as shown in Fig. 5b. With 0.8 g of catalyst, the maxima in the formation rates of H2 and CO2 disappear e that is, the formation rates increase with time until reaching the steady states, as shown in Fig. 5c. These results indicate that the steady state formation rates of H2 and CO2 can be realized with a catalyst weight of about 0.5 g or more, under the operating conditions in this work; notably, larger amount of catalyst means longer contact time in accordance with the reactor setup of this work. Therefore, enough amount of the Cu/GDC catalyst is needed for the occurrence of the steady state, i.e. constant behavior with the reaction time. On the other hand, if the catalyst weight is not enough, both formation rates of H2 and CO2 decrease with time. Table 3 shows that the conversion increases with increasing catalyst weight; however, the formation rates of H2, CO2 and CO per gram of catalyst decrease as the catalyst weight increases from 0.1 to 0.5 g; then, these rates increase as the catalyst weight increases further to 0.8 g. Notably, a conversion over 90% with a CO content of about 1% was achieved. Additionally, the CO2 selectivity is the largest and the CO content is the smallest with 0.5 g of catalyst. However, at 240 C, the H2 yield is the same at 3 and not affected by the variation of catalyst weight. The above results indicate that 0.5 g of catalyst is optimum in terms of the smallest CO content. Notably, also, the catalyst weight is associated with the contact time. Therefore, for obtaining the smallest CO content, an optimum contact time exists. Fig. 6 shows that the conversion increases with increasing catalyst weight but the H2/CO ratio has a maximum with 0.5 g of catalyst. Notably, the increase in the conversion is accompanied by the increase in the CO2 selectivity as shown also in Table 3. However, as the catalyst weight increases from 0.3 to 0.5 g, the conversion increases moderately but the H2/CO ratio increases dramatically; this results in a very large decrease of the CO content. Then, as the catalyst weight increases from 0.5 to 0.8 g, a large increase of the conversion occurs but is accompanied by a large decrease of the H2/CO ratio.
3.3.
Effect of Cu content
Fig. 7 shows that, with 0.8 g of catalyst at 240 C, the formation rates of both H2 and CO2 over Cu/GDC catalysts with 1 and 3%
Fig. 5 e Effect of catalyst weight on product profiles with 5% Cu/GDC at 240 C. (a) 0.3 g, (b) 0.5 g, (c) 0.8 g.
Cu can reach steady states, similar to those over 5% Cu. Notably, the Cu content is the percentage of the weight of Cu with respect to that of GDC. This indicates that the determining factor for the occurrence of steady state is the catalyst weight, i.e. the contact time; 0.8 g of catalyst is enough for the occurrence of the steady state behavior. Fig. 8 shows that the conversion increases with increasing Cu content but the
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Table 3 e Variations of conversion, formation rates of H2, CO2 and CO, H2 yield, CO2 selectivity and CO contenta with catalyst weight over 5% Cu/GDC at 240 C. Catalyst weight (g)
Conversion (%)
H2 (103)
CO2
CO
H2 yield
CO2 selectivity
CO content (%)
29.30 35.79 48.92 93.26
1.50 0.611 0.502 0.598
486.5 198.4 164.3 192.2
14.70 5.72 3.10 7.19
3.00 3.00 3.00 3.00
0.971 0.972 0.981 0.964
0.97 0.93 0.61 1.19
0.1 0.3 0.5 0.8 a Defined as in Table 1.
H2/CO ratio is at maximum with 3% Cu. Nevertheless, as the Cu content increases from 3 to 5%, the conversion increases only moderately but the H2/CO ratio decreases dramatically. Notably, this behavior of conversion increase accompanied by H2/CO decrease is similar to that during the variation of catalyst weight, although with different extent. Table 4 shows that, as the Cu content increases from 1 to 3%, the formation rates of H2 and CO2 increase but that of CO decreases; with further increase of the Cu content to 5%, all rates increase. Thus, the CO2 selectivity is largest and the CO content is smallest with 3% Cu. Additionally, the H2 yield is the same at 3 and not affected by the variation of the Cu content. Therefore, under the conditions of this work, the H2 yield depends only on the reaction temperature and is always 3 at 240 C. Since the BET surface areas of these catalysts are about the same, as shown in Table 5, the large variations in the conversion and the formation rates of H2, CO2 and CO as shown in Table 4 should be due to the variation of the Cu content alone. The above results indicate that a Cu content of 3% is optimum, in terms of the smallest CO content, for steam reforming of methanol over the Cu/GDC catalyst.
4.
(COx þ 3H2), where COx denotes (CO2 þ CO). This indicates the occurrence of an overall reaction of CH3OH þ H2O / COx þ 2Oads,1x/2 þ 3H2
(3)
Notably, the formation of the adsorbed O species in reaction (3) is associated with that of CO. Since the rate of CO formation at 240 C is very small as shown in Table 3, the amount of the adsorbed O species is also very small. Notably, also, if no CO is formed, reaction (3) becomes CH3OH þ H2O / CO2 þ 3H2
(4)
which is a combination of reactions (1) and (2).
Discussion
The above results indicate that the H2 yield is always 3 at 240 C. Since the H2 yield is defined as (H2 formed)/(COx formed), an H2 yield of 3 means that the reaction product is
Fig. 6 e Effect of catalyst weight on conversion and H2/CO ratio, respectively, with 5% Cu/GDC at 240 C.
Fig. 7 e Effect of Cu content on product profiles with 0.8 g of catalyst at 240 C. (a) 1% Cu/GDC, (b) 3% Cu/GDC.
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Fig. 8 e Effect of Cu content on conversion and H2/CO ratio, respectively, with 0.8 g of catalyst at 240 C.
The product profiles indicate that, at the very beginning, H2 is formed but there is neither CO2 nor CO. This may be attributed to the occurrence of methanol decomposition, i.e. reaction (1) but with the CO species adsorbed: CH3OH / COads þ 2H2
(5)
Then, the adsorbed CO species may desorb to the gas phase: COads / CO
0.3 g of catalyst in Fig. 5a. However, as the catalyst weight increases to 0.5 g, this extra CO2 peak disappears. These results indicate that the occurrence of the extra CO2 peak is a consequence of the accumulation of the adsorbed CO species beforehand; this confirms the existence of the adsorbed CO species and thus the occurrence of reaction (5). On the other hand, the non-occurrence of the extra CO2 peak with larger amount of catalyst can be attributed to the smaller extent of the accumulation of the adsorbed CO species. At 210 C, Fig. 1 indicates that the initial rate of CO2 formation is much smaller than that at 240 C and there is no extra CO2 peak. This is attributed to the fact that the activity of reaction (7) is lower at 210 C than that at 240 C. Thus, CO desorption, reaction (6), can occur at a larger extent; this results in a larger CO content. Additionally, the accumulated amount of the adsorbed CO species is small at this low temperature of 210 C and thus no extra CO2 peak could be formed. On the other hand, at reaction temperature higher than 240 C, the initial rate of CO2 formation becomes much larger than that at 240 C but there is also no appearance of the extra CO2 peak. This indicates that the extent of the accumulation of the adsorbed CO species becomes smaller, a consequence of the increased extent of desorption of the CO species with increasing temperature; this also results in a larger CO content. These explain why the smallest CO content occurs at 240 C. The occurrence of reaction (7) results in the adsorbed C species. However, these C species can be removed by the O species produced via H2O dissociation:
(6)
where the symbol of the species without subscript denotes the species in the gas phase. Notably, a combination of reactions (5) and (6) is reaction (1). However, at the typical temperatures for hydrogen production via steam reforming of methanol, i.e. 210e270 C, the amount of CO formation is very much smaller than that of CO2 formation, as shown in Fig. 1 and also Table 1. Thus, the disproportionation reaction may occur: 2 COads / CO2 þ Cads
(7)
The occurrence of reaction (7) would result in the formation of an extra amount of CO2 in addition to that according to reaction (3). This extra CO2 formation indeed occurs as confirmed by the formation of the CO2 peak at around 9 min on-stream over 0.1 g of catalyst, as shown in Fig. 1b. Notably, a smaller CO2 peak appears at around 11 min on-stream over
H2O / H2 þ Oads
(8)
Cads þ Oads / COads
(9)
where the O species may be associated with the oxygen vacancy of GDC [14]. Thus, there is negligible carbon deposition as reported in the above. On the other hand, when both adsorbed CO and O species exist, which should be the general case during steam reforming of methanol in accordance with the abovedescribed reactions, CO2 may be formed via the following reaction: COads þ Oads / CO2
(10)
Notably, a combination of reactions (8) and (10) leads to reaction (2), the wateregas shift reaction, but with adsorbed
Table 4 e Variations of conversion, formation rates of H2, CO2 and CO, H2 yield, CO2 selectivity and CO contenta with Cu content over 0.8 g of catalyst at 240 C. Cu content (wt%)
Conversion (%)
H2 (103)
CO2
CO
H2 yield
CO2 selectivity
CO content (%)
58.02 85.49 93.26
0.371 0.591 0.598
119.2 179.1 192.2
4.89 3.79 7.19
3.00 3.00 3.00
0.961 0.979 0.964
1.30 0.64 1.19
1 3 5 a Defined as in Table 1.
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Table 5 e BET surface area. Catalyst GDC 1% Cu/GDC 3% Cu/GDC 5% Cu/GDC
BET surface area (m2/g) 53.2 52.1 51.4 50.0
CO species. Notably, also, the adsorbed CO species is produced mainly from CH3OH decomposition reaction (5) and the adsorbed O species from H2O dissociation reaction (8); in addition, the formation of the extra CO2 peak may also be due to the oxidation of the adsorbed CO species via reaction (10). Therefore, it can be confirmed that reaction (3) consists of reactions (5)e(10). The occurrence of reaction (3) has been considered to be at 240 C, which always results in a H2 yield of 3. For H2 yield much different from 3, i.e. at higher temperatures, the overall reaction should also consist of reactions (5)e(10) but with different activities for these reactions. For example, the reaction at 550e600 C results in a constant H2 yield of 2.33; this means that the extent of the involvement of H2O in the overall reaction decreases; restated, the extent of H2O dissociation is smaller at these higher temperatures than that at 240 C. This should result in a decreased amount of the O species; consequently, the CO content can increase, as indeed being the case as shown by the above results. On the other hand, the above results indicate that the extent of the involvement of CH3OH in the overall reaction, i.e. CH3OH decomposition, increases with increasing temperature. The existence of an optimum Cu content of 3 wt% in terms of the smallest CO content can be attributed to the interfacial interaction between the Cu species and the surface oxygen vacancy of GDC. Notably, for CO oxidation over yttriastabilized zirconia supported copper oxide catalyst, the interfacial interaction between the Cu species and the surface oxygen vacancy greatly enhances the catalytic activity [22]. Thus, the interfacial active site consisting of the Cu species and the surface oxygen vacancy can play a key role for the reaction activities. This is indicated by the dramatically smaller extent of the conversion increase as the Cu content increases from 3 to 5 wt% than that from 1 to 3 wt%. This drop of the extent of the conversion increase may be attributed to a decreased amount of the interfacial active sites as a consequence of the cover-up of the surface oxygen vacancy by the Cu species. The variation of the amount of the interfacial active sites, which can be the result of variation in the amount of either the surface oxygen vacancy or the active Cu species, may explain the behavior of the on-stream rate of CO becoming larger than that of CO2 during the test. Notably, this variation can be considered as a change of the property of the Cu/GDC catalyst. Notably, also, when either the surface vacancy or the Cu species are occupied, the interfacial active site becomes inactive and thus its amount for the associated reactions decreases. During the test, either the surface vacancy or the Cu species should be occupied by either adsorbed CO or O species. Since the adsorbed O species is produced in the overall reaction (3), they can accumulate so as to decrease the amount of the interfacial active sites. Consequently, the rate
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of reaction (10), which needs the most of the interfacial active sites, decreases and thus that of CO desorption reaction (6) should increase. This results in an increase of CO formation in association with a decrease of CO2 formation, in agreement with the observation that the on-stream rate of CO formation becomes larger than that of CO2 formation. Additionally, since the extent of CH3OH decomposition increases with increasing temperature, the on-stream rate of CO formation becoming larger than that of CO2 formation can occur earlier at higher temperature, as indeed being the case as shown by the above results. As the catalyst weight increases from 0.1 to 0.8 g, the contact time increases. When the contact time increases, the extent of the formed CO2 being dissociated to produce CO can increase. Notably, CO2 can dissociate according to the following reaction [14,23]: CO2 / CO þ Oads
(11)
This may explain why 0.5 g of catalyst is optimum in terms of the smallest CO content. Notably, the extent of the wateregas shift reaction (2) can also increase with increasing contact time so as to reduce the CO concentration; however, CO2 is the major reaction product while CO is minor and thus CO2 dissociation to produce CO can have much larger effect on the overall CO content. Nevertheless, this needs further studies for clarification.
5.
Conclusions
(1) At the usual temperature for hydrogen production via steam reforming of methanol, i.e. 210e270 C, the CO content is about 1% over Cu/GDC. Thus, only preferential CO oxidation would be needed to reduce the CO content in reformate to an acceptable level for the usage in the PEMFCs. (2) A reaction temperature of 240 C is optimum in terms of the H2 yield, the CO2 selectivity and the CO content. (3) The formation rate of CO increases with increasing temperature. (4) The average formation rate of CO becomes larger than that of CO2 at about 450 C. (5) The H2 yield, the CO2 selectivity and the CO content become constant at about 550 C. (6) A catalyst weight of 0.5 g is optimum in terms of the smallest CO content; that is, an optimum contact time exists. (7) A Cu content of 3 wt% is optimum in terms of the smallest CO content. (8) The H2 yield at 240 C is always 3 and not affected by the variations of either the catalyst weight or the Cu content.
references
[1] de Wild PJ, Verhaak MJFM. Catalytic production of hydrogen from methanol. Catal Today 2000;60:3e10. [2] Lemons RA. Fuel cells for transportation. J Power Sources 1990;29:251e64.
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[3] Pan L, Wang S. Methanol steam reforming in a compact plate-fin reformer for fuel-cell systems. Int J Hydrogen Energy 2005;30:973e9. [4] Sohn JM, Byun YC, Cho JY, Choe J, Song KH. Development of the integrated methanol fuel processor using microchannel patterned devices and its performance for steam reforming of methanol. Int J Hydrogen Energy 2007;32: 5103e8. [5] Iulianelli A, Longo T, Basile A. Methanol steam reforming reaction in a PdeAg membrane reactor for CO-free hydrogen production. Int J Hydrogen Energy 2008;33:5583e8. [6] Takezawa N, Iwasa N. Steam reforming and dehydrogenation of methanol: difference in the catalytic functions of copper and group VIII metals. Catal Today 1997; 36:45e56. [7] Lindstrom B, Pettersson LJ. Hydrogen generation by steam reforming of methanol over copper-based catalysts for fuel cell applications. Int J Hydrogen Energy 2001;26: 923e33. [8] Lindstrom B, Agrell J, Pettersson LJ. Combined methanol reforming for hydrogen generation over monolithic catalysts. Chem Eng J 2003;93:91e101. [9] Houteit A, Mahzoul H, Ehrburger P, Bernhardt P, Le´gare´ P, Garin F. Production of hydrogen by steam reforming of methanol over copper-based catalysts: the effect of cesium doping. Appl Catal A Gen 2006;306:22e8. [10] Papavasiliou J, Avgouropoulos G, Ioannides T. In situ combustion synthesis of structured Cu-Ce-O and Cu-Mn-O catalysts for the production and purification of hydrogen. Appl Catal B 2006;66:168e74. [11] Frank B, Jentoft FC, Soerijanto H, Kro¨hnert J, Schlo¨gl R, Schoma¨cker R. Steam reforming of methanol over copper-containing catalysts: influence of support material on microkinetics. J Catal 2007;246:177e92. [12] Pe´rez-Herna´ndez R, Gutie´rrez-Martı´nez A, Gutie´rrezWing CE. Effect of Cu loading on CeO2 for hydrogen production by oxidative steam reforming of methanol. Int J Hydrogen Energy 2007;32:2888e94.
[13] Udani PPC, Gunawardana PVDS, Lee HC, Kim DH. Steam reforming and oxidative steam reforming of methanol over CuOeCeO2 catalysts. Int J Hydrogen Energy 2009;34:7648e55. [14] Huang TJ, Yu TC. Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst. Catal Lett 2005;102:175e81. [15] Huang TJ, Lin HC, Yu TC. A comparison of oxygen-vacancy effect on activity behaviors of carbon dioxide and steam reforming of methane over supported nickel catalysts. Catal Lett 2005;105:239e47. [16] Huang TJ, Yu TC, Jhao SY. Weighting variation of water-gas shift in steam reforming of methane over supported Ni and Ni-Cu catalysts. Ind Eng Chem Res 2006;45:150e6. [17] Huang TJ, Huang MC. Effect of Ni content on hydrogen production via steam reforming of methane over Ni/GDC catalysts. Chem Eng J 2008;145:149e53. [18] Huang TJ, Wang CH. Factors in forming CO and CO2 over cermet of Ni-gadolinia-doped ceria with relation to direct methane SOFCs. J Power Sources 2006;163:309e15. [19] Huang TJ, Wang CH. Roles of surface and bulk lattice oxygen in forming CO2 and CO during methane reaction over gadolinia-doped ceria. Catal Lett 2007;118:103e8. [20] Wang JB, Lin SC, Huang TJ. Selective CO oxidation in rich hydrogen over CuO/samaria-doped ceria. Appl Catal A Gen 2002;232:107e20. [21] Naknam P, Luengnaruemitchai A, Wongkasemjit S. Preferential CO oxidation over Au/ZnO and Au/ZnOeFe2O3 catalysts prepared by photodeposition. Int J Hydrogen Energy 2009;34:9838e46. [22] Dow WP, Huang TJ. Yttria-stabilized zirconia supported copper oxide catalyst II. Effect of oxygen vacancy of support on catalytic activity for CO oxidation. J Catal 1996;160: 171e82. [23] Wang JB, Hsiao SZ, Huang TJ. Study of carbon dioxide reforming of methane over Ni/yttria-doped ceria and effect of thermal treatments of support on the activity behaviors. Appl Catal A Gen 2003;246:197e211.
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Hydrogen production via steam reforming of methanol over Cu/(Ce,Gd)O2Lx catalysts Ta-Jen Huang*, Hsiao-Min Chen Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC
article info
abstract
Article history:
Hydrogen production via steam reforming of methanol is carried out over Cu/(Ce,Gd)O2x
Received 14 December 2009
catalysts at 210e600 C. The CO content in reformate is about 1% at 210e270 C, which are
Received in revised form
the typical temperature for hydrogen production via steam reforming of methanol. Largest
12 March 2010
H2 yield and CO2 selectivity and smallest CO content are obtained at 240 C. The formation
Accepted 18 March 2010
rate of CO increases with increasing temperature. The average formation rate of CO becomes larger than that of CO2 at about 450 C. The H2 yield, the CO2 selectivity and the
Keywords:
CO content become constant at about 550 C. At 240 C, the smallest CO content is obtained
Steam reforming of methanol
with a catalyst weight of 0.5 g and a Cu content of 3 wt%. The H2 yield, defined as H2/
Hydrogen production
(CO þ CO2) in formation rates, at 240 C is always 3 and not affected by the variations of
CO content
either the catalyst weight or the Cu content.
Gadolinia-doped ceria
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Catalyst weight Cu content
1.
Introduction
The issue of global warming has provided strong incentive to develop energy-efficient electrical power sources for vehicles. Currently, the proton exchange membrane fuel cell (PEMFC) appears as the most viable technology for powering vehicles. However, PEMFC needs high-purity hydrogen as the fuel. To overcome the difficulties of hydrogen distribution and storage as well as limited driving range with the use of pressurized hydrogen tanks on vehicles using the PEMFCs, hydrogen production on-board from liquid fuels becomes an alternative [1,2]. Among various high energy density liquid fuels, methanol remains prominent because of its low conversion temperature, high hydrogen to carbon ratio, and no CeC bond thereby lessening soot formation. For automotive PEMFC applications,
hydrogen can be extracted from methanol through steam reforming of methanol [3e5]. The reactions for steam reforming of methanol are considered as: CH3OH / CO þ 2H2
(1)
CO þ H2O / CO2 þ H2
(2)
For the produced hydrogen to be used in the PEMFCs, CO is a poison. To reduce the CO amount, the process of steam reforming of methanol can be followed by treatments of high-temperature and low-temperature wateregas shift reactions as well as preferential CO oxidation. If the CO2 selectivity of steam reforming of methanol is high enough, the CO content would be low enough and only preferential
* Corresponding author. Tel.: þ886 3 5716260; fax: þ886 3 5715408. E-mail address: [email protected] (T.-J. Huang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.082
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CO oxidation is needed; thus, the hydrogen production cost can be reduced substantially. Therefore, the CO2 selectivity is an important factor for steam reforming of methane to produce hydrogen. Copper-containing catalysts are usually used for steam reforming of methanol [6e13]. Oxygen-ion conducting materials, such as ceria [11,12], have been used as the support of copper-containing catalysts for steam reforming of methanol; however, gadolinia-doped ceria (GDC) has not yet been used for this purpose. Notably, GDC has higher activity than ceria for CO2 reforming of methane [14]; this is attributed to the fact that Gd doping increases the oxygen-ion conductivity and thus the amount of surface oxygen vacancy, which results in an increase of the reforming activity [15]. Therefore, GDC has been used as a support material of the nickel catalyst for steam reforming of methane [15e17]; notably, also, GDCsupported Ni catalysts can have minor C deposition problem with methane as reactant e this is attributed to the fact that the lattice oxygen of GDC can enhance the oxidation reaction of the deposited C species to form CO2 [18,19]. It is thus interesting to see whether GDC can have good effect as the support of copper-containing catalysts for steam reforming of methanol. In this work, Cu/GDC catalysts were used for hydrogen production via steam reforming of methanol. The effects of reaction temperature, catalyst weight and Cu content on hydrogen yield, CO2 selectivity and CO content were studied. Optimum conditions on reaction temperature, catalyst weight and Cu content were obtained.
2.
Experimental
2.1.
Preparation of Cu/GDC catalysts
GDC was prepared by co-precipitation. The details of the method have been presented elsewhere [14]. The GDC powders were calcined by heating at 10 C min1 to 400 C and held for 2 h, and then to 750 C and held for 4 h before cooling down. GDC is Ce0.9Gd0.1O1.95d. The Cu/GDC catalyst was prepared by impregnating the GDC powders (325e270 mesh, i.e. 44e53 mm diameter) with an aqueous solution of copper nitrate Cu(NO3)2$3H2O (99% purity, SHOWA). The mixture was placed in a heated stirrer to remove excess water and then in a vacuum oven to dry overnight. The dried Cu/GDC powder was heated at 10 C min1 to 400 C and held for 6 h, and then cooled down. In this work, the percentage of Cu content denotes the weight of Cu with respect to that of GDC.
2.2.
Fig. 1 e Effect of reaction temperature on product profiles with 0.1 g of 5% Cu/GDC. (a) 210 C, (b) 240 C, (c) 270 C.
Activity tests
The activity of the catalyst for steam reforming of methanol was measured under atmospheric pressure in a continuous flow reactor charged with 0.1 g of catalyst, if not specified otherwise. The catalyst sample was reduced at 400 C with 30 ml min1 of H2 (99.999% purity) for 1 h. Then, the catalyst was purged by Ar, and then heated to the designated temperature with a heating rate of 10 C min1. Then, the test was carried out with feeding of 100 ml min1 of a gas mixture
of CH3OH:H2O:Ar ¼ 9.83:9.83:80.34 e that is, the molar ratio of H2O:CH3OH in feed is 1, which is the ratio of a stoichiometric reaction for steam reforming of methanol with complete CO2 formation. The reactor outflow was analyzed on-line by gas chromatograph (GC, China Chromatography 8900), CO-NDIR (non-dispersive infrared, Beckman 880) and CO2-NDIR (NGA 2000) analyzers.
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Table 1 e Variations of conversion, formation rates of H2, CO2 and CO, H2 yield, CO2 selectivity and CO content with reaction temperature over 0.1 g of 5% Cu/GDC. Temp ( C)
Conversiona (%)
H2b (103)
CO2b
COb
H2 yieldc
CO2 selectivityd
CO contente (%)
7.25 29.30 40.32 52.04 67.27 71.90 76.87 84.95 86.58 89.80 94.87 99.04
0.372 1.50 2.05 2.63 3.36 3.50 3.59 3.83 3.70 3.68 3.79 3.95
119.3 486.5 664.4 834.3 1051 1022 953.9 923.2 717.7 596.0 525.2 546.5
4.89 14.70 25.45 56.06 100.4 208.6 361.4 530.2 763.5 940.4 1098 1148
2.99 3.00 2.97 2.95 2.92 2.84 2.73 2.64 2.49 2.40 2.33 2.33
0.961 0.971 0.963 0.937 0.913 0.830 0.725 0.635 0.485 0.388 0.324 0.323
1.30 0.97 1.23 2.09 2.90 5.62 9.15 12.2 17.1 20.4 22.5 22.5
210 240 270 300 330 360 390 420 450 500 550 600 a b c d e
Conversion was calculated by (COx formed/CH3OH fed), where COx denotes CO2 plus CO. Formation rate in mmol*g1.min1. Average value of the data taken during the test from 30 to 180 min. H2 yield ¼ H2/(CO þ CO2). CO2 selectivity ¼ CO2/(CO þ CO2). CO content ¼ CO/(H2þCO).
3.
Results
3.1.
Effect of reaction temperature
Fig. 1 shows the profiles of H2, CO2 and CO formation rates over 0.1 g of 5% Cu/GDC catalyst at the reaction temperatures of 210e270 C. The formation rates of H2 and CO2 went up to the maximum and then decreased with time; the extents of both going up and coming down of the rates increase with increasing reaction temperature. The formation rate of CO is very small at these reaction temperatures. Table 1 indicates that the CO content is about 1% at these temperatures of 210e270 C; notably, the CO content is defined as the percentage of CO in the hydrogen-rich reformate after the removal of the remaining methanol and water and also the formed CO2. These temperatures are the typical ones for hydrogen production via steam reforming of methanol; thus, only preferential CO oxidation would be needed to reduce the
Fig. 2 e Effect of reaction temperature on CO2/CO and H2/CO ratios, respectively, with 0.1 g of 5% Cu/GDC.
Fig. 3 e Effect of reaction temperature on product profiles with 0.1 g of 5% Cu/GDC. (a) 360 C, (b) 390 C.
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Table 2 e Variation of the on-stream time when the formation rate of CO became larger than that of CO2 with reaction temperature over 0.1 g of 5% Cu/GDC. Temperature ( C) 390 or lower 420 450 500 550 600
Fig. 4 e Effect of reaction temperature on product profiles with 0.1 g of 5% Cu/GDC. (a) 450 C, (b) 500 C, (c) 550 C.
CO content in reformate to an acceptable level for the usage in the PEMFCs. Notably, also, with a CO content of about 1%, tests of selective CO oxidation in rich hydrogen have shown that CO conversion close to or even being 100% can be obtained [20,21]. Table 1 also shows that the conversion and the average formation rates of H2, CO2 and CO increase with increasing reaction temperature from 210 to 270 C. However, the CO content is smallest at 240 C; this is in accordance with the
Time (min) Longer than 200 168.3 88.4 37.6 21.6 From beginning
largest CO2 selectivity and the highest H2 yield. Fig. 2 shows that both ratios of CO2/CO and H2/CO are maxima at 240 C; additionally, the extents of variations in CO2/CO and H2/CO ratios are almost the same. Notably, the maximum in the CO2/CO ratio results in the maximum in the CO2 selectivity and that in the H2/CO ratio results in the minimum in the CO content. This indicates that 240 C is the optimum reaction temperature, in terms of the smallest CO impurity, for steam reforming of methanol over the Cu/GDC catalyst. Fig. 3a shows the profiles of H2, CO2 and CO formation rates over 0.1 g of 5% Cu/GDC catalyst at 360 C. The formation rate of CO went up to the maximum and then decreased with time; this is the same behavior as those of H2 and CO2. However, Fig. 3b shows that the formation rate of CO at 390 C increases almost continuously instead of coming down with time. Consequently, the formation rate of CO can equal that of CO2 at about 200 min on-stream during the test. Notably, Table 1 also shows that the average rate of CO2 formation is at maximum at 330 C; then, the CO2 rate decreases with increasing temperature before becoming constant at about 550 C. Therefore, the H2 yield, the CO2 selectivity and the CO content become constant at about 550 C. Table 1 also shows that the average formation rate of CO2 becomes smaller than that of CO at 450 C. Fig. 4a shows that, at 450 C, the formation rate of CO reaches a maximum, comes down for a short period, and then increases continuously to become larger than that of CO2. The exact time-on-stream when the formation rate of CO becomes larger than that of CO2 is presented in Table 2. This behavior of the on-stream rate of CO becoming larger than that of CO2 may be an indication of the change of the property of the Cu/GDC catalyst during the test. Fig. 4b shows that, at 500 C, the formation rate of H2 becomes quite stable after going up, and that of CO does not have a maximum but increases continuously with time. Fig. 4c shows a similar behavior of H2 and CO at 550 C; however, the formation rate of CO becomes larger than that of CO2 from almost the beginning of the test. At 600 C, the CO rate is larger than the CO2 rate from the very beginning, as presented in Table 2. Therefore, the on-stream rate of CO formation becoming larger than that of CO2 formation occurs earlier at higher temperature. After the test of steam reforming of methanol, oxidation treatment at 800 C with 100 ml/min of a gas mixture of 20% O2 in argon indicates negligible carbon deposition even after the 600 C test. This is also confirmed by a conversion over 99% at 600 C, as shown in Table 1. Notably, the conversion is
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calculated by (COx formed/CH3OH fed), where COx denotes CO2 plus CO; thus, a conversion over 99% means that the CH3OH fed has been almost completely converted to the gaseous products of CO2 and CO e that is, there is no or negligible carbon deposition.
3.2.
Effect of catalyst weight
Fig. 5 shows the profiles of H2, CO2 and CO formation rates over 0.3e0.8 g of 5% Cu/GDC catalyst at 240 C, which is the reaction temperature for the lowest CO content as indicated in the above. Fig. 5a shows that the formation rates of H2 and CO2 went up to the maximum and then decreased with time, a similar behavior as that with 0.1 g of catalyst; however, the extent of the decrease of the rate with time with 0.3 g of catalyst is smaller in comparison with that of 0.1 g of catalyst. With 0.5 g of catalyst, the formation rates of H2 and CO2 become relatively constant after reaching the maxima, as shown in Fig. 5b. With 0.8 g of catalyst, the maxima in the formation rates of H2 and CO2 disappear e that is, the formation rates increase with time until reaching the steady states, as shown in Fig. 5c. These results indicate that the steady state formation rates of H2 and CO2 can be realized with a catalyst weight of about 0.5 g or more, under the operating conditions in this work; notably, larger amount of catalyst means longer contact time in accordance with the reactor setup of this work. Therefore, enough amount of the Cu/GDC catalyst is needed for the occurrence of the steady state, i.e. constant behavior with the reaction time. On the other hand, if the catalyst weight is not enough, both formation rates of H2 and CO2 decrease with time. Table 3 shows that the conversion increases with increasing catalyst weight; however, the formation rates of H2, CO2 and CO per gram of catalyst decrease as the catalyst weight increases from 0.1 to 0.5 g; then, these rates increase as the catalyst weight increases further to 0.8 g. Notably, a conversion over 90% with a CO content of about 1% was achieved. Additionally, the CO2 selectivity is the largest and the CO content is the smallest with 0.5 g of catalyst. However, at 240 C, the H2 yield is the same at 3 and not affected by the variation of catalyst weight. The above results indicate that 0.5 g of catalyst is optimum in terms of the smallest CO content. Notably, also, the catalyst weight is associated with the contact time. Therefore, for obtaining the smallest CO content, an optimum contact time exists. Fig. 6 shows that the conversion increases with increasing catalyst weight but the H2/CO ratio has a maximum with 0.5 g of catalyst. Notably, the increase in the conversion is accompanied by the increase in the CO2 selectivity as shown also in Table 3. However, as the catalyst weight increases from 0.3 to 0.5 g, the conversion increases moderately but the H2/CO ratio increases dramatically; this results in a very large decrease of the CO content. Then, as the catalyst weight increases from 0.5 to 0.8 g, a large increase of the conversion occurs but is accompanied by a large decrease of the H2/CO ratio.
3.3.
Effect of Cu content
Fig. 7 shows that, with 0.8 g of catalyst at 240 C, the formation rates of both H2 and CO2 over Cu/GDC catalysts with 1 and 3%
Fig. 5 e Effect of catalyst weight on product profiles with 5% Cu/GDC at 240 C. (a) 0.3 g, (b) 0.5 g, (c) 0.8 g.
Cu can reach steady states, similar to those over 5% Cu. Notably, the Cu content is the percentage of the weight of Cu with respect to that of GDC. This indicates that the determining factor for the occurrence of steady state is the catalyst weight, i.e. the contact time; 0.8 g of catalyst is enough for the occurrence of the steady state behavior. Fig. 8 shows that the conversion increases with increasing Cu content but the
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Table 3 e Variations of conversion, formation rates of H2, CO2 and CO, H2 yield, CO2 selectivity and CO contenta with catalyst weight over 5% Cu/GDC at 240 C. Catalyst weight (g)
Conversion (%)
H2 (103)
CO2
CO
H2 yield
CO2 selectivity
CO content (%)
29.30 35.79 48.92 93.26
1.50 0.611 0.502 0.598
486.5 198.4 164.3 192.2
14.70 5.72 3.10 7.19
3.00 3.00 3.00 3.00
0.971 0.972 0.981 0.964
0.97 0.93 0.61 1.19
0.1 0.3 0.5 0.8 a Defined as in Table 1.
H2/CO ratio is at maximum with 3% Cu. Nevertheless, as the Cu content increases from 3 to 5%, the conversion increases only moderately but the H2/CO ratio decreases dramatically. Notably, this behavior of conversion increase accompanied by H2/CO decrease is similar to that during the variation of catalyst weight, although with different extent. Table 4 shows that, as the Cu content increases from 1 to 3%, the formation rates of H2 and CO2 increase but that of CO decreases; with further increase of the Cu content to 5%, all rates increase. Thus, the CO2 selectivity is largest and the CO content is smallest with 3% Cu. Additionally, the H2 yield is the same at 3 and not affected by the variation of the Cu content. Therefore, under the conditions of this work, the H2 yield depends only on the reaction temperature and is always 3 at 240 C. Since the BET surface areas of these catalysts are about the same, as shown in Table 5, the large variations in the conversion and the formation rates of H2, CO2 and CO as shown in Table 4 should be due to the variation of the Cu content alone. The above results indicate that a Cu content of 3% is optimum, in terms of the smallest CO content, for steam reforming of methanol over the Cu/GDC catalyst.
4.
(COx þ 3H2), where COx denotes (CO2 þ CO). This indicates the occurrence of an overall reaction of CH3OH þ H2O / COx þ 2Oads,1x/2 þ 3H2
(3)
Notably, the formation of the adsorbed O species in reaction (3) is associated with that of CO. Since the rate of CO formation at 240 C is very small as shown in Table 3, the amount of the adsorbed O species is also very small. Notably, also, if no CO is formed, reaction (3) becomes CH3OH þ H2O / CO2 þ 3H2
(4)
which is a combination of reactions (1) and (2).
Discussion
The above results indicate that the H2 yield is always 3 at 240 C. Since the H2 yield is defined as (H2 formed)/(COx formed), an H2 yield of 3 means that the reaction product is
Fig. 6 e Effect of catalyst weight on conversion and H2/CO ratio, respectively, with 5% Cu/GDC at 240 C.
Fig. 7 e Effect of Cu content on product profiles with 0.8 g of catalyst at 240 C. (a) 1% Cu/GDC, (b) 3% Cu/GDC.
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Fig. 8 e Effect of Cu content on conversion and H2/CO ratio, respectively, with 0.8 g of catalyst at 240 C.
The product profiles indicate that, at the very beginning, H2 is formed but there is neither CO2 nor CO. This may be attributed to the occurrence of methanol decomposition, i.e. reaction (1) but with the CO species adsorbed: CH3OH / COads þ 2H2
(5)
Then, the adsorbed CO species may desorb to the gas phase: COads / CO
0.3 g of catalyst in Fig. 5a. However, as the catalyst weight increases to 0.5 g, this extra CO2 peak disappears. These results indicate that the occurrence of the extra CO2 peak is a consequence of the accumulation of the adsorbed CO species beforehand; this confirms the existence of the adsorbed CO species and thus the occurrence of reaction (5). On the other hand, the non-occurrence of the extra CO2 peak with larger amount of catalyst can be attributed to the smaller extent of the accumulation of the adsorbed CO species. At 210 C, Fig. 1 indicates that the initial rate of CO2 formation is much smaller than that at 240 C and there is no extra CO2 peak. This is attributed to the fact that the activity of reaction (7) is lower at 210 C than that at 240 C. Thus, CO desorption, reaction (6), can occur at a larger extent; this results in a larger CO content. Additionally, the accumulated amount of the adsorbed CO species is small at this low temperature of 210 C and thus no extra CO2 peak could be formed. On the other hand, at reaction temperature higher than 240 C, the initial rate of CO2 formation becomes much larger than that at 240 C but there is also no appearance of the extra CO2 peak. This indicates that the extent of the accumulation of the adsorbed CO species becomes smaller, a consequence of the increased extent of desorption of the CO species with increasing temperature; this also results in a larger CO content. These explain why the smallest CO content occurs at 240 C. The occurrence of reaction (7) results in the adsorbed C species. However, these C species can be removed by the O species produced via H2O dissociation:
(6)
where the symbol of the species without subscript denotes the species in the gas phase. Notably, a combination of reactions (5) and (6) is reaction (1). However, at the typical temperatures for hydrogen production via steam reforming of methanol, i.e. 210e270 C, the amount of CO formation is very much smaller than that of CO2 formation, as shown in Fig. 1 and also Table 1. Thus, the disproportionation reaction may occur: 2 COads / CO2 þ Cads
(7)
The occurrence of reaction (7) would result in the formation of an extra amount of CO2 in addition to that according to reaction (3). This extra CO2 formation indeed occurs as confirmed by the formation of the CO2 peak at around 9 min on-stream over 0.1 g of catalyst, as shown in Fig. 1b. Notably, a smaller CO2 peak appears at around 11 min on-stream over
H2O / H2 þ Oads
(8)
Cads þ Oads / COads
(9)
where the O species may be associated with the oxygen vacancy of GDC [14]. Thus, there is negligible carbon deposition as reported in the above. On the other hand, when both adsorbed CO and O species exist, which should be the general case during steam reforming of methanol in accordance with the abovedescribed reactions, CO2 may be formed via the following reaction: COads þ Oads / CO2
(10)
Notably, a combination of reactions (8) and (10) leads to reaction (2), the wateregas shift reaction, but with adsorbed
Table 4 e Variations of conversion, formation rates of H2, CO2 and CO, H2 yield, CO2 selectivity and CO contenta with Cu content over 0.8 g of catalyst at 240 C. Cu content (wt%)
Conversion (%)
H2 (103)
CO2
CO
H2 yield
CO2 selectivity
CO content (%)
58.02 85.49 93.26
0.371 0.591 0.598
119.2 179.1 192.2
4.89 3.79 7.19
3.00 3.00 3.00
0.961 0.979 0.964
1.30 0.64 1.19
1 3 5 a Defined as in Table 1.
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Table 5 e BET surface area. Catalyst GDC 1% Cu/GDC 3% Cu/GDC 5% Cu/GDC
BET surface area (m2/g) 53.2 52.1 51.4 50.0
CO species. Notably, also, the adsorbed CO species is produced mainly from CH3OH decomposition reaction (5) and the adsorbed O species from H2O dissociation reaction (8); in addition, the formation of the extra CO2 peak may also be due to the oxidation of the adsorbed CO species via reaction (10). Therefore, it can be confirmed that reaction (3) consists of reactions (5)e(10). The occurrence of reaction (3) has been considered to be at 240 C, which always results in a H2 yield of 3. For H2 yield much different from 3, i.e. at higher temperatures, the overall reaction should also consist of reactions (5)e(10) but with different activities for these reactions. For example, the reaction at 550e600 C results in a constant H2 yield of 2.33; this means that the extent of the involvement of H2O in the overall reaction decreases; restated, the extent of H2O dissociation is smaller at these higher temperatures than that at 240 C. This should result in a decreased amount of the O species; consequently, the CO content can increase, as indeed being the case as shown by the above results. On the other hand, the above results indicate that the extent of the involvement of CH3OH in the overall reaction, i.e. CH3OH decomposition, increases with increasing temperature. The existence of an optimum Cu content of 3 wt% in terms of the smallest CO content can be attributed to the interfacial interaction between the Cu species and the surface oxygen vacancy of GDC. Notably, for CO oxidation over yttriastabilized zirconia supported copper oxide catalyst, the interfacial interaction between the Cu species and the surface oxygen vacancy greatly enhances the catalytic activity [22]. Thus, the interfacial active site consisting of the Cu species and the surface oxygen vacancy can play a key role for the reaction activities. This is indicated by the dramatically smaller extent of the conversion increase as the Cu content increases from 3 to 5 wt% than that from 1 to 3 wt%. This drop of the extent of the conversion increase may be attributed to a decreased amount of the interfacial active sites as a consequence of the cover-up of the surface oxygen vacancy by the Cu species. The variation of the amount of the interfacial active sites, which can be the result of variation in the amount of either the surface oxygen vacancy or the active Cu species, may explain the behavior of the on-stream rate of CO becoming larger than that of CO2 during the test. Notably, this variation can be considered as a change of the property of the Cu/GDC catalyst. Notably, also, when either the surface vacancy or the Cu species are occupied, the interfacial active site becomes inactive and thus its amount for the associated reactions decreases. During the test, either the surface vacancy or the Cu species should be occupied by either adsorbed CO or O species. Since the adsorbed O species is produced in the overall reaction (3), they can accumulate so as to decrease the amount of the interfacial active sites. Consequently, the rate
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of reaction (10), which needs the most of the interfacial active sites, decreases and thus that of CO desorption reaction (6) should increase. This results in an increase of CO formation in association with a decrease of CO2 formation, in agreement with the observation that the on-stream rate of CO formation becomes larger than that of CO2 formation. Additionally, since the extent of CH3OH decomposition increases with increasing temperature, the on-stream rate of CO formation becoming larger than that of CO2 formation can occur earlier at higher temperature, as indeed being the case as shown by the above results. As the catalyst weight increases from 0.1 to 0.8 g, the contact time increases. When the contact time increases, the extent of the formed CO2 being dissociated to produce CO can increase. Notably, CO2 can dissociate according to the following reaction [14,23]: CO2 / CO þ Oads
(11)
This may explain why 0.5 g of catalyst is optimum in terms of the smallest CO content. Notably, the extent of the wateregas shift reaction (2) can also increase with increasing contact time so as to reduce the CO concentration; however, CO2 is the major reaction product while CO is minor and thus CO2 dissociation to produce CO can have much larger effect on the overall CO content. Nevertheless, this needs further studies for clarification.
5.
Conclusions
(1) At the usual temperature for hydrogen production via steam reforming of methanol, i.e. 210e270 C, the CO content is about 1% over Cu/GDC. Thus, only preferential CO oxidation would be needed to reduce the CO content in reformate to an acceptable level for the usage in the PEMFCs. (2) A reaction temperature of 240 C is optimum in terms of the H2 yield, the CO2 selectivity and the CO content. (3) The formation rate of CO increases with increasing temperature. (4) The average formation rate of CO becomes larger than that of CO2 at about 450 C. (5) The H2 yield, the CO2 selectivity and the CO content become constant at about 550 C. (6) A catalyst weight of 0.5 g is optimum in terms of the smallest CO content; that is, an optimum contact time exists. (7) A Cu content of 3 wt% is optimum in terms of the smallest CO content. (8) The H2 yield at 240 C is always 3 and not affected by the variations of either the catalyst weight or the Cu content.
references
[1] de Wild PJ, Verhaak MJFM. Catalytic production of hydrogen from methanol. Catal Today 2000;60:3e10. [2] Lemons RA. Fuel cells for transportation. J Power Sources 1990;29:251e64.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 6 2 1 8 e6 2 2 6
[3] Pan L, Wang S. Methanol steam reforming in a compact plate-fin reformer for fuel-cell systems. Int J Hydrogen Energy 2005;30:973e9. [4] Sohn JM, Byun YC, Cho JY, Choe J, Song KH. Development of the integrated methanol fuel processor using microchannel patterned devices and its performance for steam reforming of methanol. Int J Hydrogen Energy 2007;32: 5103e8. [5] Iulianelli A, Longo T, Basile A. Methanol steam reforming reaction in a PdeAg membrane reactor for CO-free hydrogen production. Int J Hydrogen Energy 2008;33:5583e8. [6] Takezawa N, Iwasa N. Steam reforming and dehydrogenation of methanol: difference in the catalytic functions of copper and group VIII metals. Catal Today 1997; 36:45e56. [7] Lindstrom B, Pettersson LJ. Hydrogen generation by steam reforming of methanol over copper-based catalysts for fuel cell applications. Int J Hydrogen Energy 2001;26: 923e33. [8] Lindstrom B, Agrell J, Pettersson LJ. Combined methanol reforming for hydrogen generation over monolithic catalysts. Chem Eng J 2003;93:91e101. [9] Houteit A, Mahzoul H, Ehrburger P, Bernhardt P, Le´gare´ P, Garin F. Production of hydrogen by steam reforming of methanol over copper-based catalysts: the effect of cesium doping. Appl Catal A Gen 2006;306:22e8. [10] Papavasiliou J, Avgouropoulos G, Ioannides T. In situ combustion synthesis of structured Cu-Ce-O and Cu-Mn-O catalysts for the production and purification of hydrogen. Appl Catal B 2006;66:168e74. [11] Frank B, Jentoft FC, Soerijanto H, Kro¨hnert J, Schlo¨gl R, Schoma¨cker R. Steam reforming of methanol over copper-containing catalysts: influence of support material on microkinetics. J Catal 2007;246:177e92. [12] Pe´rez-Herna´ndez R, Gutie´rrez-Martı´nez A, Gutie´rrezWing CE. Effect of Cu loading on CeO2 for hydrogen production by oxidative steam reforming of methanol. Int J Hydrogen Energy 2007;32:2888e94.
[13] Udani PPC, Gunawardana PVDS, Lee HC, Kim DH. Steam reforming and oxidative steam reforming of methanol over CuOeCeO2 catalysts. Int J Hydrogen Energy 2009;34:7648e55. [14] Huang TJ, Yu TC. Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst. Catal Lett 2005;102:175e81. [15] Huang TJ, Lin HC, Yu TC. A comparison of oxygen-vacancy effect on activity behaviors of carbon dioxide and steam reforming of methane over supported nickel catalysts. Catal Lett 2005;105:239e47. [16] Huang TJ, Yu TC, Jhao SY. Weighting variation of water-gas shift in steam reforming of methane over supported Ni and Ni-Cu catalysts. Ind Eng Chem Res 2006;45:150e6. [17] Huang TJ, Huang MC. Effect of Ni content on hydrogen production via steam reforming of methane over Ni/GDC catalysts. Chem Eng J 2008;145:149e53. [18] Huang TJ, Wang CH. Factors in forming CO and CO2 over cermet of Ni-gadolinia-doped ceria with relation to direct methane SOFCs. J Power Sources 2006;163:309e15. [19] Huang TJ, Wang CH. Roles of surface and bulk lattice oxygen in forming CO2 and CO during methane reaction over gadolinia-doped ceria. Catal Lett 2007;118:103e8. [20] Wang JB, Lin SC, Huang TJ. Selective CO oxidation in rich hydrogen over CuO/samaria-doped ceria. Appl Catal A Gen 2002;232:107e20. [21] Naknam P, Luengnaruemitchai A, Wongkasemjit S. Preferential CO oxidation over Au/ZnO and Au/ZnOeFe2O3 catalysts prepared by photodeposition. Int J Hydrogen Energy 2009;34:9838e46. [22] Dow WP, Huang TJ. Yttria-stabilized zirconia supported copper oxide catalyst II. Effect of oxygen vacancy of support on catalytic activity for CO oxidation. J Catal 1996;160: 171e82. [23] Wang JB, Hsiao SZ, Huang TJ. Study of carbon dioxide reforming of methane over Ni/yttria-doped ceria and effect of thermal treatments of support on the activity behaviors. Appl Catal A Gen 2003;246:197e211.