DFT comparison of intrinsic WGS kinetics over Pd and Pt

• Report consistent DFT calculations of WGS steps on Pd(1 1 1) and Pt(1 1 1).
• Kinetics described using analytical model with variable CO binding energy.
• Reaction proceeds through a carboxyl intermediate, more rapidly on Pt than Pd.
• Rate-limiting step depends on CO binding energy.
• Comparison with Pt/Pd-γ-alumina kinetics implicates support in observed activity.

We report density functional theory (DFT) results for five competing water–gas shift (WGS) pathways on identical Pd and Pt(1 1 1) surface models. We develop analytical kinetic models to compare intrinsic catalytic activity under conditions relevant to low-temperature WGS. The baseline model predicts that WGS proceeds preferentially through a carboxyl intermediate, that water dissociation is rate limiting, and that the intrinsic rate on Pt is greater than on Pd. Absolute rates are unphysically low because of unphysically high CO coverage. Systematically decreasing the CO binding energy in keeping with its known coverage dependence yields more realistic coverages and absolute rates, shifts the rate limiting step to carboxyl formation, and retains the same ordering of activity. Predicted rate orders and activation energies are not consistent with those observed for supported Pt and Pd catalysts, suggesting that even on nominally inert supports, the support plays an active role in WGS activity.

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