Quantum Chemical Studies on All-Inorganic Catalyst for the Oxidation of Water to Dioxygen ([{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10 −): Electronic Property and Redox Tunability

Abstract

The development of catalyst for the oxidation of water that operates at PH 7, 1 atm, and room temperature is a fundamental chemical challenge. As a all-inorganic catalyst for the rapid oxidation of water to dioxygen working in aqueous solution at PH 7, ruthenium-containing polyoxometalate (POM), [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10 −, have been reported by two research groups, respectively. But the details of this mechanism are still lacking. A description of the electronic structure and demonstration of redox tenability of this catalyst are herein provided by means of density functional theory (DFT) calculations. The results show that [Ru4O4(OH)2(H2O)4] core in this POM is the withdrawing electron group, the FMO of this POM delocalizes over the [Ru4(μ-O)4(μ-OH)2] core, which indicate that ruthenium and oxygen atoms are the redox centers. The role of POM ligand in this catalyst has been evaluated according to our DFT calculations; the results show that it is the key structural factor in determination of stability for this catalyst. The POM ligand’s effect significantly changes the redox properties of these clusters. A linear dependence between the one-electron-oxidized energy and the anion charge is found, with a slope of average 0.21 V per two unit charges for tungstates, and 0.13 V per two unit charges for molybdates. The first oxidization step becomes more difficult as substitution of POM ligands (X=Al, Si, P, S). For the same total charge of clusters in tungstates and molybdates, the first oxidization step of tungstates becomes more favorable compared with molybdates.