The Direct-Indirect model:: An alternative kinetic approach in heterogeneous photocatalysis based on the degree of interaction of dissolved pollutant species with the semiconductor surface

The analysis of photocatalyst kinetics concerning degradation of water dissolved pollutants with TiO2 suspensions has been built on for years in a robotic way on the basis of the Langmuir-Hinshelwood (L-H) model. According to the L-H model the reaction rate is described by the equation: rate = k(LH)K(L)[M]/(1+K-L[M]), where K-L is the Langmuir adsorption constant, k(LH) the apparent Langmuir rate constant and [M] is the reactant concentration. Even in cases where 1/rate versus 1/[M] plots are apparently linear, as predicts the previous equation, it is frequently found that K-L depends on the illumination flux, Phi, which contradicts the L-H model premise that equilibrated adsorption/desorption of reactants is maintained under illumination. Moreover, the L-H model does not define the k(LH) dependence on Phi, so that by itself is unable to predict any existing relationship between Phi and the reaction rate. Here we describe in detail an alternative kinetic approach, the Direct-Indirect (D-I) model, which is based on the degree of electronic interaction of the semiconductor surface with dissolved reactant molecules. The D-I model introduces the systematic use of fundamental concepts like direct, indirect, adiabatic and inelastic interfacial transfer of charge as basic tools, giving a physical meaning to the involved kinetic parameters. Moreover, it is shown to be able to predict the functional dependence of the photooxidation rate on the experimental parameters (photon flux and pollutant concentration), distinguishing between strong (specific adsorption) and weak semiconductor-reactant interaction (absence of specific adsorption). The general believe that OH center dot radicals, either TiO2-adsorbed or free, photogenerated from OH- groups adsorbed on terminal Ti atoms, behave as active species in interfacial oxidation reactions is disregarded by the D-I model, as adsorbed OH- groups cannot be photooxidized with valence band holes. (C) 2007 Elsevier B.V. All rights reserved.