Comprehensive kinetic and mechanistic analysis of TiO<inf>2</inf> photocatalytic reactions according to the direct-indirect model: (I) theoretical approach

The photocatalytic oxidation kinetics of organic species in semiconductor (sc) gas phase and liquid semiconductor suspensions, strongly depends on the electronic interaction strength of substrate species with the sc surface. According to the Direct-Indirect (D-I) model, developed as an alternative to the Langmuir-Hinshelwood (L-H) model (Salvador, P. et al. Catalysis Today 2007, 129, 247), when chemisorption of dissolved substrate species is not favored and physisorption is the only existing adsorption mechanism, interfacial hole transfer takes place via an indirect transfer (IT) mechanism, the photo-oxidation rate exponentially depending on the incident photon flux (V ox = Vox IT ∞ ρn), with n = 1/2 under high enough photon flux (standard experimental conditions), whatever the dissolved substrate concentration, [(RH2)liq]. In contrast, under simultaneous physisorption and chemisorption of substrate species, hole capture takes place via a combination of an indirect transfer (IT) and a direct transfer (DT) mechanism (Vox = VoxIT + VoxDT), with VoxDT ∞ ρn and n = 1 for low enough ρ values, as long as adsorption-desorption equilibrium conditions existing in the dark are not broken under illumination, and monotonically decreasing toward n = 0 as ρ increases and adsorption-desorption equilibrium becomes broken. This behavior invalidates the frequently invoked axiom that the reaction order (exponent n) exclusively depends on the photon flux intensity, being in general n = 1 and n = 1/2 under low and high illumination intensity, respectively, independent of the nature of the sc-substrate electronic interaction. On the basis of a detailed analysis of the parameter defined as a = (Vox)2/2[(RH 2)liq]ρ, an experimental test able to determine the influence of both interfacial hole transfer mechanisms, DT and IT, in the photo-oxidation kinetics, is presented. A simple method allowing the estimation of the photon flux critical value where adsorption-desorption equilibrium of chemisorbed substrate species is broken and the reaction order starts to decreases from n = 1 toward n = 0, is described. © 2014 American Chemical Society.