Palladium catalysis of O₂ reduction by electrons accumulated on TiO₂ particles during photoassisted oxidation of organic compounds

Our earlier theoretical analysis suggested that the quantum efficiency of photoassisted oxidation of organic compounds in water by O2 on n-TiO2 surfaces can be limited by the kinetics of the reduction of O2. When the rate of O2 reduction is not sufficiently fast to match he rate of reaction of holes, an excess of electrons will accumulate on the TiO2 particles, and the rate of electron–hole recombination will increase. We now show experimentally that electrons do indeed accumulate on slurried TiO2 particles during photoassisted oxidation of 1.6 M aqueous methanol and that electrons on the slurried particles persist for at least ~ 1 min even in O2-saturated solutions. The rate of particle depolarization, i.e. of electron transfer to dissolved O2, is increased and the negative charge on the TiO2 particles is completely eliminated upon incorporation of Pd0 in the surface of the TiO2 particles. We also show that incorporation of Pd0 in the surface increases the quantum efficiency of the photoassisted oxidation of 10–2 M aqueous 2,2-dichloropropionate 3-fold at 0.01 wt % Pd and 7-fold at 2 wt % Pd