A density functional theory study of a water gas shift reaction on Ag(111): potassium effect

Density functional theory (DFT) calculations are executed to investigate the effect of a potassium (K) promoter on the activity of the water gas shift reaction (WGSR) over an Ag(111) surface. It is found that the WGSR proceeds mainly through the OH(O)-assisted carboxy pathway in which H₂O dehydrogenation is the rate-controlling step on both Ag(111) and K/Ag(111) surfaces. Energetic span model analysis shows that K addition can enhance the activity of the WGSR by reducing the apparent activation energy of the whole reaction since it can promote H₂O dissociation and stabilize the adsorption of the reactants (CO and H₂O). Importantly, the K adatom can stabilize the binding of all oxygenates by direct K-O bonding and the stabilizing effect of K on OH adsorption of the transition state (TS) plays a leading role in promoting H₂O dissociation. Moreover, the K-O distance and K coverage are two key factors affecting H₂O activation, that is, the shorter the K-O distance (2-3 Å) the more the K coverage (25%) contributes to the stronger promotion effect. For various metals catalyzing the WGSR, K promotes H₂O dissociation on inert metals like Ag, Au and Cu better than those on reactive metals (Pd and Ni) since the more inert metal surfaces would weaken the K and O binding and accordingly strengthen the interaction between them, resulting in a higher promotion effect.