Investigation of the photocatalytic activity of TiO2-polyoxometalate systems for the oxidation of methanol

We have developed a TiO2-POM co-catalyst system for use in a hybrid photo-electrochemical cell to oxidize methanol. We employed a simple alpha-Keggin-type polyoxometalate (PW12O403-) that is known to adsorb to the surface of positively charged TiO2 particles at low pH. The use of a co-catalyst dramatically improves the separation of photogenerated electron-hole pairs in suspended TiO2 nanoparticles, leading to a 50-fold increase in the observed photocurrent compared to the use of TiO2 alone. The photocurrent densities we observed exceeded those recently reported by Kamat and co-workers [K. Drew, G. Girishkumar, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 109 (2005) 11851]. Due to the improved charge separation offered by the co-catalyst system, electron transfer from the reduced polyoxometalate to the anode now appears to be the kinetic limitation. In addition, we find that the polyoxometalate itself can oxidize methanol efficiently in the absence of TiO2. However, photogenerated holes in the POM are unable to oxidize either formic acid or acetic acid, in contrast to the behavior of TiO2. The dependence of the photocurrent on [PW12O403-] was also investigated. We find that the optimum [PW12O403-] is approximately 2 mM. (c) 2006 Elsevier B.V. All rights reserved.