Photo-electro-oxidation of alcohols on titanium dioxide thin film electrodes

The photo-electro-oxidation on titanium dioxide film electrodes of methanol, 2-propanol, and tert-butyl alcohol has been studied by measuring the transient photocurrents observed during the early stages of illumination. Transients and steady-state photocurrents, measured at different applied potentials and methanol concentrations, were compared with model predictions. The numerical solution of the differential equations corresponding to methanol photo-electro-oxidation, as well as the advanced experimental evidence, supports the hypothesis that surface hole trapping as -OH. mediates the charge transfer to methanol. Formation of -OH. accounts for the high initial photocurrents, its rapid decay being due to recombination. The rate of oxidation of methanol is then determined by the rate of reaction between -OH. and CH3OH located in the interfacial region. The oxidation of (CH2OH)-C-. to CH2O, through the injection of an electron into the conduction band (current doubling), gives rise to an increase in photocurrent; steady state values are later attained. As a consequence, a minimum transient is observed. The minimum is marginally observable in tert-butyl alcohol solutions, in line with the properties of the respective radicals.