Critical role of the defect state in the photo-oxidation of methanol on TiO2(110)

Abstract

Photo-oxidation of CH₃OH on TiO₂ has been extensively studied in order to understand the fundamental principles of heterogeneous photocatalysis. However, the roles of the photogenerated hole and the energy barrier in this reaction are still controversial. Using the ab initio many-body Green's function theory, we examine the evolution of excitons along the reaction path of CH₃OH photo-oxidation on the rutile TiO₂(110) surface. We reveal the critical role of the Ti 3d defect state produced by the final product in lowering the energy barrier (by 1.3 eV) and stabilizing the product in the conversion from CH₃O to CH₂O. With the transfer of hydrogen from CH₃O to TiO₂, the hole migrates gradually from the O 2p orbitals of TiO₂ at the valence band edge to the Ti 3d orbitals near the conduction band bottom, with the band-gap hole state localized on the adsorbate as the bridge. We find that whether the hole is initially trapped by the reactant CH₃O is not a crucial issue for the oxidation of CH₃O to occur. Hole trapping required in photocatalytic reactions might just be an event happening in the reaction process. In the oxidation of CH₃OH to CH₃O, the hole remains within TiO₂ in the whole process and this reaction is essentially thermal-driven. The critical role of the defect state clarified in this work provides new insights into understanding photochemical reactions at metal oxide surfaces. Based on our studies, we propose a convenient strategy to judge whether the hole participates in the reaction.