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 the 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.