Weighting Variation of Water−Gas Shift in Steam Reforming of

Nov 9, 2005 - at temperatures of 673-823 K. A method for analyzing the weighting of the water-gas shift (WGS) activity in steam reforming is proposed...
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Ind. Eng. Chem. Res. 2006, 45, 150-156

Weighting Variation of Water-Gas Shift in Steam Reforming of Methane over Supported Ni and Ni-Cu Catalysts Ta-Jen Huang,* Tien-Chun Yu, and Shih-Yao Jhao Department of Chemical Engineering, National Tsing Hua UniVersity, Hsinchu, Taiwan 300, R.O.C.

In this work, catalysts of 2 wt % Ni supported on samaria-doped ceria (SDC), gadolinia-doped ceria, and R-Al2O3, as well as SDC-supported Ni-Cu catalysts with 0.5 wt % Ni or Cu and addition of 0.01-0.1 wt % Cu or Ni were prepared. Activity tests for steam reforming of methane over these catalysts were carried out at temperatures of 673-823 K. A method for analyzing the weighting of the water-gas shift (WGS) activity in steam reforming is proposed. The effects of temperature, support, and Ni-Cu ratio on the WGS weighting variations were studied. The results indicate that the weighting of the WGS activity decreases with increasing temperature. The extent of the variation of the WGS weighting is larger with doped ceria as the support than with R-Al2O3. The addition of Cu into a Ni catalyst enhances the WGS activity, and this enhancement effect can be quantitatively related to the amount of the bimetallic Cu-Ni species. The simple analysis of WGS weighting is able to give some ideas on the operation and catalyst design for achieving a good WGS activity. Introduction Steam reforming of methane has been employed for largescale production of hydrogen.1-3 Recently, this reaction has become increasingly important as the fuel processing technology for fuel cells.4 The overall reaction for steam reforming of methane is considered to be

CH4 + 2H2O f CO2 + 4H2

(1)

However, reaction 1 is usually considered to occur via the following reaction steps

CH4 + H2O f CO + 3H2

(2)

CO + H2O f CO2 + H2

(3)

where reaction 2 is the main reaction for the steam reforming of methane and reaction 3, i.e., the water-gas shift (WGS) reaction, is secondary. The amount of carbon monoxide produced via steam reforming of methane is usually quite high, and this is an indication that the WGS reaction is less active than methane reaction 2, because equal activities of reactions 2 and 3 would mean no CO production. Therefore, the weighting of the WGS activity, i.e., the ratio of the rate of reaction 3 to that of reaction 2, would be useful for estimating the amount of CO in the final product from the steam reforming of methane. This information would be useful for the determination of the downstream processes necessary to reduce the CO concentration, especially for use in proton-exchange membrane fuel cells. Nevertheless, although the WGS reaction has been well studied,5-7 the above-described weighting analysis of the WGS activity has not yet been reported, to the best of our knowledge. To reduce the amount of CO and simultaneously produce additional hydrogen, the process of methane steam reforming is usually followed by high-temperature and/or low-temperature WGS treatment. For this, Cu-ZnO catalyst is well-known to have very good low-temperature WGS activity. On the other hand, nickel catalyst has been found to exhibit promising catalytic performance for the steam reforming of methane.3,8 * To whom correspondence should be addressed. Tel: +886-3-5716260. Fax: +886-3-571-5408. E-mail: [email protected].

Because ZnO is usually considered as a promoter for WGS activity and the Cu species is considered as the main catalytic agent in the Cu-ZnO catalyst, the addition of Cu to the nickel catalyst might be able to enhance the activity of the WGS reaction (eq 3) in the overall reaction of methane steam reforming. In addition, the effect of the support on the steam reforming of methane over nickel catalysts has been studied.3,9 Doped-ceria-supported nickel catalysts have been shown to have much higher catalytic activity for the steam reforming of methane than alumina-supported catalysts;10 thus, with doped ceria as the support, the WGS activity might also be affected. On the other hand, the internal steam reforming of methane in solid oxide fuel cells (SOFCs) has recently been studied to an increasing extent.11-14 In this case, the CO concentration has to be reduced, and additional hydrogen must be produced with high WGS activity of the SOFC anode because there is no separate WGS treatment. Thus, the information on the WGS weighting in the steam reforming of methane over the SOFC anode is important for the operation and design of the anode so as to achieve a good WGS activity. In this work, catalysts of Ni supported on samaria-doped ceria (SDC), gadolinia-doped ceria (GDC), and R-Al2O3, as well as catalysts of various Ni-Cu ratios supported on SDC were prepared. Activity tests for the steam reforming of methane over these catalysts were carried out at temperatures of 673-823 K. A method for analyzing the weighting of the WGS activity in steam reforming is proposed. The results show that the WGS weighting in the steam reforming of methane varies with variations in the temperature, support, and Ni-Cu ratio over these supported Ni and Ni-Cu catalysts. Experimental Section Preparation of Doped Ceria. Samaria-doped ceria (SDC) was prepared by a coprecipitation method from reagent-grade (99.9% purity, Strem Chemical) metal nitrates Sm(NO3)3‚6H2O and Ce(NO3)3‚6H2O. Appropriate amounts of samarium nitrate and cerium nitrate, corresponding to an atomic molar ratio of Sm/Ce ) 1:9, were dissolved in deionized water to make 0.08 M solutions. Hydrolysis of the metal salts to hydroxides was obtained by slowly dropping each such solution into NH4OH solution while simultaneously stirring to keep the pH of the

10.1021/ie050744r CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2005

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solution >9. A distinct deep purple color of precipitate/gel was formed when the nitrate solution was dropped into NH4OH. Vacuum filtration was employed to isolate the gel, which was then washed twice by water and ethanol. After being washed, the gel was dried under vacuum at 110 °C for 4 h, calcined in air at 300 °C for 2 h and then at 600 °C for 4 h, and then slowly cooled to room temperature to obtain the SDC powder. Gadolinia-doped ceria (GDC) and yttria-doped ceria (YDC) were prepared by the same method. The atomic molar ratios of Gd/Ce and Y/Ce were also 1:9. Preparation of Ni Catalysts with Various Supports. The doped-ceria-supported nickel catalyst was prepared by impregnating the above-prepared powders of the doped ceria with an appropriate amount of aqueous solution of nickel nitrate trihydrate, Ni(NO3)2‚3H2O (99% purity, SHOWA, Japan), for 7 h. After excess water had been evaporated at 80 °C, the catalysts were dried under vacuum at 80 °C for 12 h and then calcined in air at 260 °C for 1.5 h and then at 500 °C for 3.5 h. The calcination of the supported nickel catalyst was conducted by passing air at a rate of one L/min and by ramping the temperature at a rate of 10 °C/min. The R-Al2O3- (R-alumina-) supported nickel catalyst was prepared with the same impregnation method as described above using the R-alumina powders. Preparation of SDC-Supported Ni-Cu Catalysts. The SDC-supported Ni-Cu catalysts were prepared with the same impregnation method as described above for the doped-ceriasupported nickel catalyst, except that the aqueous impregnation solution was made of both nickel nitrate and copper nitrate at the appropriate amounts and ratio. This preparation method is also called coimpregnation. In this work, the Ni and Ni-Cu catalysts are always supported, and the indicated metal loading is the weight percent with respect to the weight of the support. Activity Tests of Steam Reforming of Methane. The activity tests for the steam reforming of methane were conducted at 673-823 K (400-550 °C) under atmospheric pressure in a continuous-flow reactor charged with 100 mg of sample, which was fixed by quartz wool and quartz sand downstream of the bed. The reactor was made of an 8-mm-i.d. quartz U-tube imbedded in an insulated electric furnace, equipped with a temperature-programmable controller, to achieve an isothermal environment. A K-type thermocouple was inserted into the catalyst bed to measure and control the bed temperature. The steady-state activity results are reported in this work. These steady-state results were obtained generally in about 20 min. Ni Catalysts with Various Supports. Ni/GDC and Ni/SDC catalysts with 2 wt % Ni were used. Ni/R-Al2O3 catalyst with 2 wt % Ni was also tested for comparison. The gas feed was passed through an oxygen filter to eliminate trace amounts of oxygen. The test started with H2 prereduction of the catalyst and followed by purging with argon flow. When the specified temperature was reached, a mixture of CH4/H2O/Ar ) 25:25: 50 was then fed into the catalyst bed at a total flow rate of 100 mL/min. Water was introduced with a syringe pump. The reactor outflow was analyzed on-line by gas chromatograph (China Chromatograph 8900, Taiwan), CO-NDIR (Beckman 880), and CO2-NDIR (Beckman 880) instruments. The rate of carbon deposition was measured by the method of temperatureprogrammed oxidation after each activity test, as detailed below. SDC-Supported Ni-Cu Catalysts. SDC-supported 0.5Ni, 0.5Ni-0.01Cu, 0.5Ni-0.1Cu, 0.5Cu, 0.5Cu-0.01Ni, and 0.5Cu0.1Ni catalysts were used, where 0.5Ni denotes 0.5 wt % Ni,

Figure 1. Methane conversion rates over 2 wt % Ni catalysts supported on various materials as indicated.

0.5Ni-0.01Cu denotes 0.5 wt % Ni plus 0.01 wt % Cu, and so on. The activity test procedure is the same as described above, except that CH4/H2O/Ar ) 11.5:11.5:77 to accommodate the lower metal loading of the catalysts. Temperature-Programmed Oxidation. Temperature-programmed oxidation (TPO) of the reforming tested catalyst was carried out by using 20% oxygen in argon as an oxidizing gas in a reactor made up of an 8-mm-i.d. quartz U-tube with a 100mg sample mounted on loosely packed quartz wool. The flow rate of the oxidizing gas was kept at 100 mL/min by a mass flow controller. The temperature of the reactor was ramped from room temperature to 900 °C at a rate of 10 °C/min by a temperature-programmable controller (Eurotherm, model 815P). The amount of carbon dioxide production was measured by a CO2-NDIR instrument and recorded by an on-line personal computer. The peak area of TPO was integrated using software developed by SISC, Taiwan. Results and Discussion Steam Reforming of Methane over Ni Catalysts with Various Supports. The methane conversion rates recorded over 2 wt % Ni catalysts supported on various materials are presented in Figure 1. It is seen that, as the temperature increases over 723 K (450 °C), the methane activities over the doped-ceriasupported Ni catalysts experience a dramatic increase, whereas those over the R-Al2O3-supported Ni catalyst do not exhibit such a behavior. This behavior of dramatic activity increase has been ascribed to be due to oxygen-ion conductivity of the doped ceria.15 Note that the BET surface areas of GDC and SDC are roughly the same at 40 m2/g, whereas that of R-Al2O3 is 6.3 m2/g, all measured as fresh; thus, the difference between the activities of doped-ceria- and R-Al2O3-supported Ni catalysts at 673 K (400 °C) might be due to the difference in the surface areas. On the other hand, it is seen in Figure 2 that the ratio of the H2 production rate to the CH4 conversion rate decreases with increasing temperature. This ratio decrease is considered to be due to the decrease of the weighting of the water-gas shift (WGS) activity in the overall steam reforming because the WGS activity is related to the production of hydrogen. Because a high rate of hydrogen production is desired for steam reforming, an analysis of the WGS weighting in the overall reaction of the steam reforming of methane would be desired; this analysis would be able to give some ideas on operation and catalyst design for a good WGS activity so as to achieve a high rate of H2 production along with a high rate of CH4 conversion.

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Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 Table 1. Variation of the Ratio of the WGS Reaction Rate to the CH4-H2O Reaction Rate, i.e. the WGS Weighting, over 2 wt % Ni Catalysts

Figure 2. Ratios of H2 production rate to CH4 conversion rate over 2 wt % Ni catalysts supported on various materials as indicated.

WGS Weighting Analysis for Steam Reforming of Methane. For the steam reforming of methane, the overall reaction and the individual reaction steps have been described above as reactions 1-3. Nevertheless, the following reactions might occur to produce carbon

CH4 f C + 2H2

(4)

2CO f C + CO2

(5)

Note that reaction 4 is the direct reaction for carbon production, and reaction 5 is the secondary reaction for carbon production, i.e., producing CO from CH4 first via reaction 2. Thus, the amount of carbon production from CH4 is represented by the total of both reactions 4 and 5; however, there is no distinction between the reactions of carbon production in the following analysis because all carbon comes from methane. The WGS weighting analysis is carried out with the equation

CH4 + aH2O ) bC + cH2 + dCO2 + eCO

(6)

Equation 6 is based on the assumption that reactants converted ) products formed; it states that 1 mol of CH4 conversion is accompanied by 1 mol of H2O conversion along with b moles of carbon production, c moles of H2 production, d moles of CO2 production, and e moles of CO production. To check whether any additional product might have been formed but not detected in the activity measurements of this work, a molar balance of carbon was performed based on the equation 〈CH4〉 ) 〈C〉 + 〈CO〉 + 〈CO2〉, and an oxygen balance was performed based on the equation 〈H2O〉 ) 〈CO〉 + 2〈CO2〉 (where the symbol 〈〉 denotes the mole difference). Calculations for the activity data over the doped-ceria-supported Ni catalysts show that the maximum error in these molar balances is less than 2%; this indicates that additional product formed from the CH4 + H2O reaction is negligible if any, noting that methanol was not detected. In addition, because carbon was formed over the catalyst, deactivation of the catalyst can occur even after the removal of the carbon deposition; however, the activity of the catalyst was tested after a 400-550 °C run, and the result showed negligible deactivation. According to eq 6, the weighting factor for reaction 2 is (1 - b), and that for reaction 3 is [a - (1 - b)]. Note that the weighting factor represents the relative kinetic significance of the individual reaction step in the overall reaction. In the following, reaction 2 is designated as the CH4-H2O reaction and reaction 3 as the WGS reaction. Thus, the weighting of the

temperature (K)

Ni/SDC

Ni/GDC

Ni/R-Al2O3

673 723 773 823

0.89 0.59 0.45 0.39

0.79 0.62 0.44 0.38

0.79 0.71 0.59 0.50

WGS activity, i.e., the ratio of the WGS reaction rate to the CH4-H2O reaction rate, is [a - (1 - b)]/(1 - b). The main purpose of the WGS weighting analysis in this work is to understand the effects of the temperature, support, and Ni-Cu ratio on the WGS weighting variation so as to obtain some ideas on system operation and catalyst design. Effect of Temperature on WGS Weighting over Ni Catalysts. The results of the WGS weighting analysis for the 2 wt % Ni catalysts are presented in Table 1 in terms of the ratio of the WGS reaction rate to the CH4-H2O reaction rate, i.e., the WGS weighting. It is seen that this ratio decreases as the temperature increases; in other words, this means that the reaction rate of CH4-H2O becomes higher than that of WGS as the temperature increases. From a thermodynamic point of view, because the CH4-H2O reaction is endothermic (∆H° ) 206 kJ/mol) whereas the WGS reaction is exothermic (∆H° ) -41 kJ/mol), the CH4-H2O reaction would increase and the WGS reaction rate would decrease as the temperature increased. On the other hand, the WGS reaction might be at or close to equilibrium in the temperature range of this work. Nevertheless, as shown in Table 1, the extent of the weighting variation with temperature is quite different with different supports and thus not determined solely by thermodynamic considerations. In addition, the activity data of this work were taken at the very early stage of steady state so as to reduce the probability of the WGS reaction reaching equilibrium. In this work, the effects of temperature, support, and Ni-Cu ratio on this weighting variation are studied. For the CH4-H2O and CO-H2O (WGS) reactions, H2O is the same reactant, and thus the adsorption ability of CH4 or CO would be a key parameter affecting the reaction rate. Huang et al.15 reported that, over a doped-ceria-supported Ni catalyst, CO is adsorbed onto Ni, but its adsorption is weaker at higher temperature; in addition, as the temperature increases, the CH4 dissociation rate increases, which means that the capacity for the dissociative adsorption of CH4 increases because CH4 dissociation is via dissociative adsorption of CH4.15 Thus, over the doped-ceria-supported Ni catalyst and with increasing temperature, the decrease of the CO adsorption ability and the increase of the CH4 adsorption ability contribute to the decrease of the weighting of the WGS reaction. This might be an explanation for the above-described effect of temperature on WGS weighting. In addition, as the temperature increases, the ration of the CO production rate to the methane conversion rate increases, as shown in Figure 3. This means that the CH4-H2O reaction, which produces CO, becomes more important than the WGS reaction, which consumes CO, as the temperature increases. This result is consistent with that of the above-described WGS weighting analysis. On the other hand, as shown in Figure 2, the ratio of the H2 production rate to the CH4 conversion rate decreases as the temperature increases. This is the same trend as observed for the ratio of the CO2 production rate to the CH4 conversion rate, as shown in Table 2. Because the decreases of both the CO2 and H2 production rates are a consequence of the decrease of

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Figure 3. Ratios of CO production rate to CH4 conversion rate over 2 wt % Ni catalysts supported on various materials as indicated. Table 2. Variation of the Ratio of the CO2 Production Rate to the CH4 Conversion Rate over 2 wt % Ni Catalysts temperature (K)

Ni/SDC

Ni/GDC

Ni/R-Al2O3

673 723 773 823

0.78 0.53 0.41 0.35

0.71 0.56 0.39 0.32

0.62 0.48 0.40 0.34

Table 3. Variation of the Ratio of the H2O Conversion Rate to the CH4 Conversion Rate over 2 wt % Ni Catalysts temperature (K)

Ni/SDC

Ni/GDC

Ni/R-Al2O3

673 723 773 823

1.81 1.51 1.39 1.32

1.70 1.49 1.34 1.24

1.47 1.37 1.16 1.05

the WGS reaction rate, this means that the weighting of the WGS reaction in the overall steam reforming reaction decreases as the temperature increases. This result is also consistent with that of the WGS weighting analysis. Effect of the Support on WGS Weighting over Ni Catalysts. As also shown in Table 1, the extent of the decrease of the WGS/CH4-H2O rate ratio with increasing temperature is higher with doped ceria as the support than with R-Al2O3. This means that the CH4-H2O reaction becomes much more important over the doped-ceria-supported Ni catalysts than over the alumina-supported one. This is considered to be due to the existence of oxygen vacancies in doped ceria with the formation of interfacial Ni-oxygen vacancy sites that aid the dissociation of methane.15 As reported in Table 3, as the temperature increases, the ratio of the H2O conversion rate to the CH4 conversion rate decreases. Because H2O is the same reactant for both reactions 2 and 3 and it is consumed at a 1:1 ratio with CH4 in reaction 2, its variation is a reflection of the variation of the WGS reaction (eq 3), especially for the Ni/SDC catalyst with a very low rate of CH4 conversion to produce carbon. Note that the CH4 conversion rate equals the rate of the CH4-H2O reaction if no carbon is produced. Thus, the trend of the decrease of the H2O conversion rate/CH4 conversion rate ratio with increasing temperature is consistent with that of the WGS/CH4-H2O rate ratio calculated by the method of the WGS weighting analysis and, thus, is a consequence of the decrease of the extent of the WGS reaction. Nevertheless, as noted from Tables 1 and 3, with the same WGS weighting, the H2O conversion rate over the Ni catalyst with doped ceria as the support is higher than that with alumina as the support. This is due to the existence of oxygen

Figure 4. CO2 production rates over 2 wt % Ni catalysts supported on various materials as indicated. Table 4. Variation of the Ratio of the Carbon Production Rate to the CH4 Conversion Rate over 2 wt % Ni Catalysts temperature (K)

Ni/SDC

Ni/GDC

Ni/R-Al2O3

673 723 773 823

0.04 0.05 0.04 0.05

0.05 0.08 0.07 0.10

0.18 0.20 0.27 0.30

vacancies in the doped ceria that enhance H2O adsorption and dissociation15 and, in turn, aids gasification of the surface carbon species

C + H2O f CO + H2

(7)

With a higher rate of reaction 7, the H2O conversion rate becomes higher. In addition, the occurrence of reaction 7 reduces carbon deposition, and thus, the carbon production rate over the doped-ceria-supported Ni catalyst is much lower than that over the alumina-supported one, as shown in Table 4. From Figure 4, it is seen that, during the steam reforming of methane, carbon dioxide (CO2) is produced at a higher rate over the doped-ceria-supported nickel catalyst than over the aluminasupported Ni catalyst; in particular, the difference of the CO2 production rates between doped-ceria- and alumina-supported Ni catalysts increases dramatically as the temperature increases from 450 to 500 °C (from 723 to 773 K). Nevertheless, the WGS weightings of the doped-ceria- and alumina-supported Ni catalysts might actually be the same, as shown in Table 1. Therefore, although the WGS reaction rate equals the CO2 production rate according to the above discussion, the variation of the WGS weighting cannot be reflected by that of the CO2 production rate. Steam Reforming of Methane over SDC-Supported NiCu Catalysts. Because it is well-known that Cu-ZnO catalyst is very effective for the WGS reaction,16 a series of Cu-Ni catalysts were prepared over the SDC support, which is shown in Table 4 to lead to the lowest carbon production. Activity test results indicate that, with 0.5 wt % Ni, the addition of only 0.01 wt % Cu leads to a considerable increase in methane conversion, as shown in Figure 5. This is considered to indicate that the addition of Cu increases the rate of the CO-H2O (WGS) reaction, which consumes CO, and thus that of the CH4-H2O reaction, which produces CO, increases to accommodate the higher CO consumption rate in the preceding WGS reaction. In other words, the increase of the CO removal rate by the WGS reaction (eq 3) increases the rate of the CH4-H2O reaction (eq 2) because CO is the product of reaction 2. Thus, increasing

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Table 5. Variation of the Rate Ratios over SDC-Supported Ni-Cu Catalysts

a

temperature (K)

0.5Nia

673 723

3.45 3.26

673 723 673 723

0.5Ni-0.01Cu

0.5Ni-0.1Cu

0.5Cu-0.01Ni

0.5Cu-0.1Ni

(a) H2 Production Rate/CH4 Conversion Rate 3.46 3.54 3.39 3.43

3.52 3.45

3.49 3.43

0.62 0.45

(b) WGS Reaction Rate/CH4-H2O Reaction Rate 0.68 0.70 0.49 0.57

0.69 0.52

0.70 0.56

1.60 1.42

(c) H2O Conversion Rate/CH4 Conversion Rate 1.68 1.70 1.49 1.56

1.68 1.52

1.70 1.54

Note: There is no detectable activity for the steam reforming of methane over 0.5Cu catalyst.

Figure 5. Methane conversion rates over SDC-supported Ni-Cu catalysts.

Cu to the higher loading of 0.1 wt % increases the CH4 conversion rate even higher. On the other hand, over the 0.5 wt % Cu catalyst, there is no detectable activity for the steam reforming of methane. However, with the addition of only 0.01 wt % Ni, it is seen that CH4 conversion occurs and its rate increases with temperature as shown in Figure 5. This indicates that CH4 conversion occurs over Ni but not over pure copper. In addition, with an increase in Ni loading to 0.1 wt %, the CH4 conversion rate increases to an extent close to that of 0.5 wt % Ni. This is considered to be due to the coexistence of 0.5 wt % Cu, which increases the WGS activity so as to increase the CH4-H2O activity. However, the CH4 conversion rate of the 0.5Cu-0.1Ni catalyst is well below that of the 0.5Ni-0.1Cu catalyst because the CH4-H2O reaction should occur first before any WGS activity can take effect. As shown in Figure 6, the hydrogen production rate increases with the addition of Cu. In addition, as shown in Table 5a, the ratio of the H2 production rate to the CH4 conversion rate also increases with the addition of Cu to the 0.5 wt % Ni catalysts. However, with increasing Ni content in the 0.5 wt % Cu catalysts, the ratio of the H2 production rate to the CH4 conversion rate decreases. This is considered to be due to the direct conversion of CH4 into CO without steam reforming according to the following reactions

CH4 f C + 2H2

(4)

C + O f CO

(8)

In reaction 8, the O species comes from the doped ceria.15 As shown in Figure 7 for methane dissociation over 2 wt % Ni supported over yttria-doped ceria (YDC) catalyst, there is no CO2 formation, which is consistent with reaction 8; in addition,

Figure 6. Hydrogen production rates over SDC-supported Ni-Cu catalysts.

Figure 7. Methane dissociation over 2 wt % Ni/YDC catalyst. Operating conditions: 100 mL/min CH4/Ar ) 25:75 at 500 °C, 30 mg of catalyst.

CO formation can persist for over 12 min. Wang et al.17 have shown that, over Ni/YDC catalyst and with operating conditions identical to those of Figure 7, CO formation can persist for over 60 min. Wang et al.18 showed similar persistence of CO formation over a temperature range of 500-900 °C. Therefore, as the steady state was obtained generally in about 20 min as described in the Experimental Section, steady-state or pseudosteady-state activity results would have been obtained in the case of the occurrence of reaction 8. With the direct conversion of CH4 to carbon as shown by reaction 4, the amount of H2 produced is smaller than that in the CH4-H2O reaction (eq 2). As the Ni loading increases, the rate of reaction 4 increases, and thus, the CH4 conversion rate increases, but the ratio of the H2 production rate to the CH4 conversion rate decreases compared to the ratio to the CH4 conversion in reaction 2. This explains the decrease of the H2 production rate/CH4 conversion rate ratio. Note that the increase

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Figure 8. Ratios of CO2 production rate to CH4 conversion rate over SDCsupported Ni-Cu catalysts.

of the rate of reaction 4 is confirmed by the measurements of the deposited carbon in this work; it is found that the amount of carbon produced over the 0.5Cu-0.1Ni catalyst is much higher, at approximately 3:1 for the conditions in this work, than that over the 0.5Cu-0.01Ni catalyst. WGS Weighting Variation over Ni-Cu Catalysts. The results of the weighting analysis for the SDC-supported NiCu and Cu-Ni catalysts are shown in Table 5b. It is seen that, as the temperature increases, the WGS weighting decreases; this is the same trend as observed for the 2 wt % Ni catalysts with various supports. In addition, as the Cu loading over the 0.5 wt % Ni catalyst increases, the WGS weighting increases; this is as expected, because the addition of Cu enhances the WGS activity as described above. On the other hand, it is seen that, as the Ni loading over the 0.5 wt % Cu catalyst increases, the WGS weighting also increases; this is because the CH4-H2O activity increases with increasing Ni loading, and thus, the amount of CO produced increases, which leads to an increase of the CO-H2O (WGS) reaction activity. A comparison of part c and b of Table 5 shows that the variation trends of the ratios of the H2O conversion rate to the CH4 conversion rate are the same as those of the WGS reaction rate to the CH4-H2O reaction rate. This is due to the very low carbon production rate, so that the CH4 conversion rate approximately equals the CH4-H2O reaction rate; note that the WGS activity contributes additional H2O conversion other than that in the CH4-H2O reaction. In addition, as shown in Figure 8, the ratio of the CO2 production rate to the CH4 conversion rate decreases as the temperature increases. Because the decrease of the CO2 production rate is a consequence of the decrease of the WGS reaction rate, this means that the weighting of the WGS reaction in the overall steam reforming reaction decreases as the temperature increases. This result for SDC-supported Ni-Cu catalyst is the same as that for the 2 wt % Ni catalyst supported on various materials. Nevertheless, it is noted in Figure 8 that the activity profile of the 0.5Ni-0.1Cu catalyst is the same as that of the 0.5Cu-0.1Ni catalyst, and that of 0.5Ni-0.01Cu is the same as 0.5Cu-0.01Ni. This indicates that the WGS activity can be quantitatively related to the amount of the bimetallic Cu-Ni species, noting that CO2 production is due to the WGS reaction; thus, same Cu-Ni amount leads to the same ratio of the CO2 production rate to the CH4 conversion rate, and a higher CuNi amount leads to a higher ratio. Therefore, it is concluded

Figure 9. Ratios of CO production rate to CH4 conversion rate over SDCsupported Ni-Cu catalysts.

that the addition of Cu into Ni catalyst enhances the WGS activity and this enhancement effect can be quantitatively related to the amount of the bimetallic Cu-Ni species. On the other hand, as shown in Figure 9, as the temperature increases, the ratio of the CO production rate to the CH4 conversion rate increases. This means that the CH4-H2O reaction, which produces CO, becomes more important than the WGS reaction, which consumes CO, as the temperature increases. This result for SDC-supported Ni-Cu catalyst is also the same as that for the 2 wt % Ni catalyst supported on various materials. Nevertheless, it is also noted in Figure 9 that the activity profile of the 0.5Ni-0.1Cu catalyst is the same as that of the 0.5Cu-0.1Ni catalyst, and that of 0.5Ni-0.01Cu is the same as 0.5Cu-0.01Ni. Because a decrease of the WGS activity leads to an increase of the amount of CO production, this behavior is consistent with the above-described relation of the WGS activity with the amount of the bimetallic Cu-Ni species. From Table 5b, it is seen that the above-described relation of the WGS activity to the Cu-Ni amount is reflected by the WGS weighting. On the other hand, as shown in Table 5c, the variation of the ratio of the H2O conversion rate to the CH4 conversion rate is also consistent with this relation of the WGS activity to the Cu-Ni amount. In addition, it is seen from parts b and c of Table 5 that the ratio of the H2O conversion rate to the CH4 conversion rate approximately equals that of the WGS reaction rate to the CH4-H2O reaction rate plus one; this is true for the case that the CH4 conversion rate equal to the CH4H2O reaction rate, and this is the case when the carbon production rate is low, as for the SDC-supported Ni catalysts. Conclusions In this work, the effects of temperature, support, and Ni-Cu ratio on the weighting variation of the water-gas shift (WGS) activity in the steam reforming of methane over supported Ni and Ni-Cu catalysts were studied, and the following conclusions were obtained: 1. The decrease of the CO adsorption ability and the increase of the CH4 adsorption ability with increasing temperature contribute to a decrease of the weighting of the WGS reaction in the steam reforming of methane at higher temperature. 2. The extent of the variation of the WGS weighting is larger with the doped ceria as the support than with R-Al2O3. 3. The CH4-H2O reaction is much more important over the doped-ceria-supported Ni catalysts than over the aluminasupported ones.

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4. Carbon dioxide is produced at a much higher rate over the doped-ceria-supported nickel catalysts than that over the alumina-supported ones. 5. The addition of Cu to Ni catalysts enhances the WGS activity, and this enhancement effect can be quantitatively related to the amount of the bimetallic Cu-Ni species. Acknowledgment The assistance of Mr. Han-Jun Lin in the measurement of the BET surface areas and the methane dissociation test of Figure 7 is acknowledged. Literature Cited (1) Hou, K.; Hughes, R. The Kinetics of Methane Steam Reforming over a Ni/R-Al2O3 Catalyst. Chem. Eng. J. 2001, 82, 311. (2) Choudhary, V. R.; Banerjee, S.; Rajput, A. M. Hydrogen from StepWise Steam Reforming of Methane over Ni/ZrO2: Factors Affecting Catalytic Methane Decomposition and Gasification by Steam of Carbon Formed on the Catalyst. Appl. Catal. A 2002, 234, 259. (3) Matsumura, Y.; Nakamori, T. Steam Reforming of Methane over Nickel Catalysts. Appl. Catal. A 2004, 258, 107. (4) Song, C. Fuel Processing for Low-Temperature and High-Temperature Fuel Cells Chanlenges, and Opportunities for Sustainable Development in the 21st Century. Catal. Today 2002, 77, 17. (5) Utaka, T.; Sekizawa, K.; Eguchi, K. CO Removal by OxygenAssisted Water Gas Shift Reaction. Appl. Catal. A 2000, 194-195, 21. (6) Utaka, T.; Okanishi, T.; Takeguchi, T.; Kikuchi, R.; Eguchi, K. Water Gas Shift Reaction of Reformed Fuel over Supported Ru Catalysts. Appl. Catal. A 2003, 245, 343. (7) Goerke, O.; Pfeifer, P.; Schubert, K. Water Gas Shift Reaction and Selective Oxidation of CO in Microreactors. Appl. Catal. A 2004, 263, 11. (8) Roh, H. S.; Jun, K. W.; Dong, W. S.; Chang, J. S.; Park, S. E.; Joe, Y. I. Highly Active and Stable Ni/Ce-ZrO2 Catalyst for H2 Production from Methane. J. Mol. Catal. A 2002, 181, 137.

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ReceiVed for reView June 21, 2005 ReVised manuscript receiVed October 5, 2005 Accepted October 18, 2005 IE050744R