Kinetics of the water–gas shift reaction on Pt catalysts supported on alumina and ceria

We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO2, 22% H2O, 37.3% H2, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate (r) on Pt/alumina catalysts as a function of the forward rate (rf) was found to be: r = rf(1 − β), where rf = kf[CO]0.1[H2O]1.0[CO2]−0.1[H2]−0.5, kf is the forward rate constant, β = ([CO2][H2])/(Keq[CO][H2O]) is the approach to equilibrium, and Keq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO2 and H2. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H2 and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study.