Comparison of photocatalytic degradation kinetic characteristics of different organic compounds at anatase TiO2 nanoporous film electrodes

Photoelectrochemical method was adopted to investigate the photocatalytic oxidation of different organic compounds. It was found that at the anatase TiO2 nanoporous electrode, the potential bias changes the rate-determining step from electron migration in the film at low potentials to photohole capture at relatively high potentials. When the applied potential bias is sufficient, the steady state photocurrent obtained reflects exclusively the rate of photohole capture at TiO2 surface. Under such conditions, the photocatalytic degradation of various organic compounds with different chemical structures was studied.At very low concentrations, the linear increase of steady state photocurrent with the concentration was observed for all compounds investigated, due to the mass transfer limitation, although the number of electron needed for complete mineralization of these compounds differs markedly. It was demonstrated that the substrate molecules that reaches the electrode surface have been exhaustively mineralized under mass transfer-controlled conditions regardless of their chemical nature.At high concentration, substrate molecules (or intermediates) are accumulated on the surface (or in the reaction zone). As a result, the steady state photocurrent deviated from the linear relationship. Under such conditions, the interaction of substrate molecules and/or partially degraded intermediates with TiO2 determines the overall photohole capture rate. The differences in photohole capture rate among different organic compounds were observed. The kinetic characteristics of different organic compounds at high concentrations were also explained based on the structural differences.