Investigation of the photocatalytic activity of TiO 2–polyoxometalate systems for the oxidation of methanol

Abstract

We have developed a TiO 2–POM co-catalyst system for use in a hybrid photo-electrochemical cell to oxidize methanol. We employed a simple α-Keggin-type polyoxometalate (PW 12O 40 3−) that is known to adsorb to the surface of positively charged TiO 2 particles at low pH. The use of a co-catalyst dramatically improves the separation of photogenerated electron–hole pairs in suspended TiO 2 nanoparticles, leading to a 50-fold increase in the observed photocurrent compared to the use of TiO 2 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 TiO 2. However, photogenerated holes in the POM are unable to oxidize either formic acid or acetic acid, in contrast to the behavior of TiO 2. The dependence of the photocurrent on [PW 12O 40 3−] was also investigated. We find that the optimum [PW 12O 40 3−] is approximately 2 mM.