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 = kLHKL[M]/(1 + KL[M]), where KL is the Langmuir adsorption constant, kLH 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 KL depends on the illumination flux, Φ, 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 kLH dependence on Φ, so that by itself is unable to predict any existing relationship between Φ 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 OHradical 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.