Water-Gas Shift Reaction Cocatalyzed by the Polyoxometalates (POMs)-gold Composites: The “Magic” Role of the POMs

The water-gas shift reaction (WGSR, CO + H2O ↔ CO2 + H2) is an industrially important process that has been used to achieve high conversions of CO to CO2 for application in proton-exchange membrane fuel cells. Many oxide-supported gold catalysts are identified as effective catalysts for low-tempature WGSR, but the origins for the unique catalytic properties and the role of the Au/oxide interface in this process remains under debate. In the present work, ab initio density functional theory (DFT) calculations combined with the periodic continuum solvation model were applied to provide a mechanistic network of WGSR co-catalyzed by Au(111) and polyoxometalates (POM = [PMo12O40]3- and [PW12O40]3-) in aqueous solution. The contributions of Mo(d) and O(sp) bands near the Fermi level (EF) of PMo12-Au(111) were found to be responsible for the high activity of PMo12 modified gold catalysts, by serving as both electron shuttle and proton acceptors. We propsed a simple route that CO could assist water dissociation to directly form COOHads on POM-Au(111) (POMads + COads +H2Oads COOHads+HPOMe), with barriers lower than 8 kcal mol-1 for the rate-determining step. Fully consistent with the electronic trends (W(d) > Mo(d) above EF), the PW12-Au(111) system is computed to be less active than homologous phosphomolybdate due to the decreased basicity of O sites and lower reduction ability of W(d) orbitals. The functional mechanism and the role of POMs described in this work could be inspiring to desing new active WGSR catalysts.