Is the water–gas shift reaction on Pt simple?: Computer-aided microkinetic model reduction, lumped rate expression, and rate-determining step

The water–gas shift (WGS) is an essential process in hydrogen production from hydrocarbon and biomass fuel processing. Recently, it was shown that the chemistry of the WGS reaction on Pt is complex and depends critically on the oxidation of CO by adsorbed OH and H2O, mainly via the carboxyl intermediate [A.B. Mhadeshwar, D.G. Vlachos, J. Phys. Chem. B 108 (2004) 15246]. On the other hand, previous one-step rate expressions in the literature have described experimental data reasonably well. Here, starting from a comprehensive microkinetic model, we derive a reduced microkinetic model consisting of elementary reaction steps using principal component analysis and then develop a one-step rate expression for WGS on Pt using a posteriori analysis. It is shown that the rate-determining step of WGS on Pt is the oxidation of CO by H2O, but the effective reaction rate constant and reaction orders are concentration dependent. Finally, the effect of uncertainty in reaction rate constants on the rate-determining step is discussed.