Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions:: Correlation with electron-hole recombination kinetics

Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (k(r)) of recombination of photoexcited electrons and positive holes determined by femtosecond pump-probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H-2, pipecolinic acid (PCA), and CO2) under UV (> 300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (< 100 fs). The second-order rate constant (k(r)) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H-2 and PCA production under deaerated conditions depended strongly on k(r), but the relation between k(r) and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different k(r) dependences of photoactivity on the reaction kinetics governed by e(-)-h(+) recombination were attributed to the presence of O-2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.