Correlation between donating or accepting electron behavior of the adsorbed CO or H2 and its oxidation over TiO2 under ultraviolet light irradiation

Although both H2 and CO can be thermodynamically oxidized by O2 over TiO2 under ultraviolet light (UV) irradiation, it was found that CO not H2 could be oxidized over an anatase TiO2 in this work. The chemisorption results of CO and H2 at TiO2 surface under UV irradiation, investigated by a gas sensing testing, showed that CO adsorption at TiO2 would cause the decrease of TiO2 surface impedance, whereas H2 adsorption would cause its increase. It is proposed that the CO adsorbed at TiO2 donate electrons to TiO2 (as a process of CO pre-oxidation), resulting in its oxidation. In contrast, the H2 adsorbed at TiO2 accept electrons from TiO2 (as a process of H2 pre-reduction), which makes it difficult to be oxidized. This result indicates that the photocatalytic oxidation of a reactant over TiO2 not only depends on the formation of the photo-generated carriers and the subsequent activated oxidizing species, but maybe also depends on the electron transfer behavior at the interface of the adsorbed reactant and TiO2.