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

Abstract

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 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 COOH_ads on POM–Au(111) (POM_ads + CO_ads + H₂O_ads → COOH_ads + HPOM^e), 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.