Reinvestigation of the Photocatalytic Reaction Mechanism for Pt-Complex-Modified TiO₂ under Visible Light Irradiation by Means of ESR Spectroscopy and Chemiluminescence Photometry

Abstract

A plausible reaction mechanism for a visible light photocatalyst of TiO₂ modified with platinum­(IV) chloride (PtCl) was proposed on the basis of the measurements with electron spin resonance (ESR) spectroscopy and chemiluminescence photometry. Under visible light (λ > 500 nm) irradiation, the deposited Pt­(IV) chloride is charge-separated into Pt³⁺ and Cl radical by the excitation of the ligand-to-metal charge transfer. The Pt³⁺ gives an electron to the conduction band of TiO₂, which has Pt³⁺ return to Pt⁴⁺. The electron in the conduction band reduces the oxygen molecule into O₂^–. The presence of Pt³⁺ and O₂^– has been elucidated in the present study. Moreover, valence band holes of TiO₂ were detected by ESR spectroscopy under visible light irradiation. Therefore, besides being used to oxidize organic compounds, the photogenerated Cl radicals likely receive electrons from the TiO₂ valence band by visible light excitation, producing the valence band holes. Because the valence band holes have a stronger oxidation power than Cl radicals, the excitation of valence band electrons to Cl radicals would be the origin of the high photocatalytic activity of the PtCl-modified TiO₂ under visible light irradiation