Molecular-Orbital-Based Verification of Water Photo-Splitting on Pt-Loaded TiO2: Effective Formation of HOOH and Reduced Platinum Cluster as Precursors of O2 and H2

Abstract

Sayama and Arakawa reported in 1997 that Pt-loaded nc-TiO2 catalyzes effective H2O photo-splitting occurring in aqueous sodium carbonate solutions (pH8~11.5). Sayama and Arakawa’s H2O photo-splitting is now verified by density-functional theory-based molecular modeling (DFT/MM), where Yamashita-Jono’s nc-TiO2 model of [HOTi9O18H], hydrated hydroxide anion model of [HO-&H2O] and platinum triangular pillar model of [Pt6] are employed. At first, hydroxyl anion of HO-&H2O is verified to undergo endothermic one-electron transfer oxidation, giving hydrogen peroxide of HOOH&(H2O)2 via formation of hydroxyl radical of HO.&H2O and its coupling. As for verification of electron-transfer reduction of Pt6 cluster for the H2O photo-splitting to H2, the van der Waals complex of [Pt6] with nonpolar H2O trimer model of [ts-(H2O)3] is molecular-modeled, and highly exothermic electron transfer reactions verify the effective H2 formation via formation of hydrogen radical on [Pt6]. As the model of Pt-loaded nc-TiO2 catalyst, van der Waals and Coulomb interaction-based complex of [HOTi9O18H] with [Pt6] and [HO-&H2O] is successfully molecular-modeled, and the molecular orbital energy structure rationalizes that the effective water-splitting catalysis should occur via effective electron transfer from HOMO of [HO-&H2O] to LUMO of [Pt6] when the catalyst is photo-energized. DFT/MM also verifies successive oxidation of HOOH to 3O2 and photo-regeneration of HOOH via radical anion of O2 .- that could be produced from 3O2 under UV irradiance of Sayama and Arakawa’s photocatalysis systems.