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

Abstract

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 (H₂ , 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 H₂ 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 O₂ 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.