Water-gas shift reaction co-catalyzed by polyoxometalate (POM)-gold composites: the 'magic' role of POMs

摘要

The water-gas shift reaction (WGSR, CO + H2O CO2 + H-2) 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 the low-temperature WGSR, but the origin of the unique catalytic properties and the role of the Au/oxide interface in this process remain under debate. In the present work, ab initio density functional theory (DFT) calculations combined with a 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 (E-F) of PMo12-Au(111) were found to be responsible for the high activity of the PMo12 modified gold catalyst, by serving as both an electron shuttle and a proton acceptor. We proposed a simple route where 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 E-F), the PW12-Au(111) system is computed to be less active than the 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 inspire the design of new active WGSR catalysts.