Correlation of the Catalytic Activity for Oxidation Taking Place on Various TiO2 Surfaces with Surface OH Groups and Surface Oxygen Vacancies

Three catalytic oxidation reactions have been studied: The ultraviolet (UV) light induced photocatalytic decomposition of the synthetic dye sulforhodamine B (SRB) in the presence of TiO2 nanostructures in water, together with two reactions employing Au/TiO2 nanostructure catalysts, namely, CO oxidation in air and the decomposition of formaldehyde under visible light irradiation. Four kinds of TiO2 nanotubes and nanorods with different phases and compositions were prepared for this study, and gold nanoparticle (Au-NP) catalysts were supported on some of these TiO2 nanostructures (to form Au/TiO2 catalysts). FTIR emission spectroscopy (IES) measurements provided evidence that the order of the surface OH regeneration ability of the four types of TiO2 nanostructures studied gave the same trend as the catalytic activities of the TiO2 nanostructures or their respective Au/TiO2 catalysts for the three oxidation reactions. Both IES and X-ray photoelectron spectroscopy (XPS) proved that anatase TiO2 had the strongest OH regeneration ability among the four types of TiO2 phases or compositions. Based on these results, a model for the surface OH group generation, absorption, and activation of molecular oxygen has been proposed: The oxygen vacancies at the bridging O2− sites on TiO2 surfaces dissociatively absorb water molecules to form OH groups that facilitate adsorption and activation of O2 molecules in nearby oxygen vacancies by lowering the absorption energy of molecular O2. A new mechanism for the photocatalytic formaldehyde decomposition with the Au/TiO2 catalysts is also proposed, based on the photocatalytic activity of the Au-NPs under visible light. The Au-NPs absorb the light owing to the surface plasmon resonance effect and mediate the electron transfers that the reaction needs.