Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions : Correlation with electron–hole recombination kinetics Shigeru Ikeda,ab Noboru Sugiyama,b Bonamali Pal,a Giuseppe Leonardo Palmisano,c Marc•  ,c Hidenori Noguchi,d Kohei Uosakid and Bunsho Ohtaniab a Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan. E-mail : ohtani=cat.hokudai.ac.jp b Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan c Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, di Palermo, Universita  V iale delle Scienze, 90128, Palermo, Italy d Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received 4th October 2000, Accepted 28th November 2000 First published as an Advance Article on the web 21st December 2000 Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded (M- powders TiO 2 TiO 2 ) 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 of recombination of photoexcited (k r ) electrons and positive holes determined by femtosecond pumpÈprobe di†use reÑection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products pipecolinic (H 2 , acid (PCA), and under UV ([300 nm) irradiation, and the e†ect became more intense on increasing the CO 2 ) loading. In PP-DRS, these gave similar decays of absorption at 620 nm arising from excitation by a M-TiO 2 310 nm pulse (\100 fs). The second-order rate constant markedly increased with loading, even at a low (k r ) level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized for and PCA production under deaerated conditions depended strongly on but the M-TiO 2 H 2 k r , relation between and the rate of production by unplatinized under aerated conditions was k r CO 2 M-TiO 2 ambiguous ; other factor(s) might control the rate of the latter. These di†erent dependences of photoactivity k r on the reaction kinetics governed by e~Èh` recombination were attributed to the presence of and Pt O 2 deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the e†ect of recombination kinetics on photoactivity is discussed. Introduction Titanium(IV) oxide has been investigated extensively as (TiO 2 ) one of the most promising candidates for a semiconductor photocatalyst,1 and has been found to be suitable for a wide range of processes, including solar energy conversion and storage,2,3 reductive Ðxation of carbon dioxide,4 organic syn- thesis,5,6 and mineralization and/or detoxiÐcation of organic compounds.7,8 A disadvantage of the use of as a photo- TiO 2 catalyst is that it absorbs only UV light at j\ 400 nm, and therefore a signiÐcant fraction of the solar radiation is not used in photocatalytic reactions, though this photoab- TiO 2 sorption property, as is practically white or colorless, TiO 2 enables us to apply photocatalytic coatings without changing the appearance of the underlying materials. In order to extend the range of photoabsorption to visible light matching the solar radiation, doping of metals, especially transition metals ions, into the crystal lattice or loading them on the TiO 2 TiO 2 surface has been shown to be successful in several studies.9 h14 The absorption spectrum has been extended to the visible region, and photoresponses, i.e., visible light-induced photo- catalytic reactions, on modiÐed have been reported. TiO 2 However, it should be noted that in most cases the fundamen- tal photocatalytic activity of UV light-induced band- TiO 2 , gap excitation seemed to be reduced, in spite of the emphasis on visible-light response. As a possible explanation, some investigators have concluded that the doping or loading of metals, acting as or leading to the formation of defective sites in the crystal lattice, accelerates recombination of photoexcit- ed electrons and positive holes.10,12 However, this is still only speculation, and no clear evidence of an e†ect of transition metal on the photocatalytic activity of due to its band- TiO 2 gap excitation has yet been obtained. Furthermore, to the best of our knowledge, few detailed examinations on the corre- lation between the recombination rate and photocatalytic activity have been performed. In the primary stage of the photocatalytic reaction, the band-gap photoabsorption of generates photoexcited TiO 2 electrons and holes, which can migrate to the surface to drive redox reactions with adsorbed substrates, competing with their disappearance due to mutual recombination. Although the mechanism and/or kinetics of such fast primary events in heterogeneous systems have been poorly understood so far, mainly due to the limitation of time resolution in conventional techniques, recent progress in laser technology has enabled direct measurement of the dynamics of heterogeneous electron transfer with a time resolution even faster than 100 fs.15 h26 For particles and colloids,20 h26 femtosecond PP tran- TiO 2 sient absorption spectroscopy has shown that certain surface sites trap electrons within a few tens of fs after the photoexci- DOI : 10.1039/b008028o Phys. Chem. Chem. Phys., 2001, 3, 267È273 267 This journal is The Owner Societies 2001 (