The water gas shift (WGS) reaction (CO + H2O = CO2 + H2) is one of the ... The reaction rates reported in Table 1 per surface area of Cu are the same on all.
Kinetics of water-gas shift reaction on copper catalysts: application to fuel cell reformers N. Koryabkina1, A. A. Phatak1, F. H. Ribeiro1, W.F. Ruettinger2, and R. J. Farrauto2 1 Worcester Polytechnic Institute, Department of Chemical Engineering, 100 Institute Road, Worcester, Massachusetts 01609-2280. 2Engelhard Corporation, 101 Wood Avenue, Iselin, New Jersey 08830. Introduction The water gas shift (WGS) reaction (CO + H2O = CO2 + H2) is one of the reaction steps in the fuel cell processor. Its purpose is to produce hydrogen and to reduce the level of CO for final cleanup by preferential oxidation. The WGS reactor currently represents the largest volume of any catalyst in a fuel processor due to the slow kinetics at temperatures where the equilibrium is favorable. The objective of this study was to determine the kinetic parameters from experiments conducted at conditions close to the ones likely to be encountered in fuel processors for fuel cell applications on Cu based catalysts. The reaction is inhibited by the reaction products (CO2 and H2) and thus the rates reported so far tend to be higher than the values at the conditions of interest. A kinetic model based on the Redox mechanism explains the results satisfactorily. Results and Discussion The reaction rates reported in Table 1 per surface area of Cu are the same on all samples. On the basis of unit of mass of catalyst, the industrial CuO-ZnO-Al2O3 catalyst is the most active. The ceria containing catalysts have lower rates per unit of mass and lower Cu surface area. There is no promotion effect of Ce observed in the WGS reaction at the conditions tested, contrary for example to the promotion of ceria to Pt or Pd [1]. The constancy of rate per unit of Cu surface area in Table 1 indicates that the reaction occurs on Cu only and that ceria and ZnO do not affect the rates. To determine the reaction orders with respect to reactants and products, the kinetic data was fitted to a power rate law expression [2] R= k•(CO)a•(H2O)b•(CO2)c•(H2)d•(1-β) where β=[(CO2)•(H2)]/[K•(CO)•(H2O)] is the approach to equilibrium, k is the rate constant, a, b, c, d are reaction orders, and K is the equilibrium constant for the watergas shift reaction. The values for β were usually of the order of 0.03-0.1, which indicates that the reaction is carried out far from equilibrium. The resulting data are summarized in Table 1. A power rate law expression is very useful for the design of reactors. It also serves as an indication of the prevalent reaction mechanism, especially when the expression can be obtained for different samples at the same conditions of temperature and concentration. An inspection of reaction orders in Table 1 reveals that the mechanism may be similar on all catalysts. The differences in reaction orders are not interpreted as a significant difference. The power rate law expression however does not suggest a simple sequence of reaction steps.
Table 1 Summary for the kinetics for water-gas shift reaction on Cu catalysts Catalyst
Rate per area of Cu* / 10-6 mol m-2 s-1
Reaction order
Rate per mass of catalyst* / 10-6 mol g-1 s-1 COa 0.9 0.7
H2 Ob 0.8 0.6
CO2c -0.7 -0.6
H2 d -0.8 -0.6
8% CuO-Al2O3 0.80 2.4 8% CuO0.83 0.75 15%CeO2-Al2O3 8% CuO-CeO2 0.11 0.9 0.4 -0.6 -0.6 40% CuO-ZnO0.79 7.6 0.8 0.8 -0.9 -0.9 Al2O3 *Rates of reaction for the WGS reaction on Cu based catalysts at 200oC, 1 atm total pressure, 7% CO, 8.5% CO2, 22 % H2O, 37 % H2, and 25% Ar a Concentration range: 5 to 25% CO and balance Ar to 33%; 8.5% CO2, 22 % H2O, 37 % H2. b Concentration range: 10 to 46% H2O and balance Ar to 47.5%; 7% CO, 8.5% CO2, 37 % H2. c Concentration range: 5 to 30% CO2 and balance Ar to 34%; 7% CO, 22 % H2O, 37 % H2. d Concentration range: 25 to 60% H2 and balance Ar 62.5%; 7% CO, 8.5% CO2, 22 % H2O.
Because the partial pressure and temperature ranges used here are not the same as the ones reported in the literature, a comparison of our data with the ones in the literature is difficult. In particular, many of the studies did not consider or measure the inhibing effect of CO2 and H2. In particular, only Ovesen et al. [3] had reactant and products fed simultaneously to the reactor, although the experiments were carried out at a higher pressure. The power rate law and rate (0.91x10-6 mol m-2 s-1) they measured is similar to the ones we report. To study the reaction kinetics we used the method and reaction steps based on the Redox methanism as described in Ovesen et al. [3, 4]. The mechanism predicts that CO∗ + O∗ CO2∗ + ∗ is the rate-determining step. In spite of the good agreement between calculated and measured rates, this mechanism could not predict the negative reaction order for CO2 although the other reaction orders are in good agreement. A better model is necessary. References 1. Bunluesin, T., Gorte, R. J., and Graham, G. W., Appl. Catal., B 15, 107 (1998). 2. Bohlbro, H., in "An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapour over Iron Oxide Based Catalysts." Gjellerup, Copenhagen, 1969. 3. Ovesen, C. V., Clausen, B. S., Hammershoei, B. S., Steffensen, G., Askgaard, T., Chorkendorff, I., Nørskov, J. K., Rasmussen, P. B., Stoltze, P., and Taylor, P., J. Catal. 158, 170 (1996). 4. Ovesen, C. V., Stoltze, P., Nørskov, J. K., and Campbell, C. T., J. Catal. 134, 445 (1992).
