A kinetic model for distinguishing between direct and indirect interfacial hole transfer in the heterogeneous photooxidation of dissolved organics on TiO2 nanoparticle suspensions

A kinetic model for assessing the photocatalytic degradation of water-dissolved pollutant species at suspensions of TiO2 nanoparticles is presented. The model is based on the sequence of reactions occurring at the semiconductor/electrolyte interface under illumination, and emphasizes the degree of electronic interaction of dissolved pollutant species with the semiconductor surface. In the case of weak interaction (nonspecific adsorption), the model establishes that interfacial hole transfer takes place via an isoenergetic, indirect mechanism involving photogenerated surface-bound OH. radicals. In contrast, for strong interaction (specific adsorption), interfacial hole transfer takes place via a mixture of the indirect mechanism and an inelastic, direct one involving photogenerated valence band free holes, which predominates for low enough photon fluxes. Under high illumination intensity (standard experimental conditions), a linear dependence of the photodegradation quantum yield (QY) on the inverse of the square root of the photon flux, phi (i.e., QYinfinity phi(-1/2)), is predicted for indirect hole transfer (nonspecific adsorption), while a phi independent QY is predicted for direct hole transfer. The quantum yield dependence on the pollutant concentration for strong interaction is determined by the type of adsorption. So, for a Langmuir adsorption type, a linear dependence of the inverse of the QY on the inverse of the pollutant concentration (QY(-1) infinity [RH2](aq) (-1)) is predicted. On the other hand, in the absence of specific adsorption a linear dependence of the inverse of the QY on the inverse of the square root of the pollutant concentration (QY(-1) infinity [RH2](aq)(-1/2)) is obtained for high enough illumination intensity. The predicted behaviour has been contrasted with literature experimental data.