Comprehensive kinetic and mechanistic analysis of TiO2 photocatalytic reactions according to the direct-indirect model: (II) experimental validation

Juan F. Montoya, Mohamed F. Atitar, Detlef W. Bahnemann, José Peral, Pedro Salvador

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As a continuation of the Direct-Indirect (D-I) model theoretical approach presented in Part I of this publication, concerning the photocatalytic oxidation of organic molecules in contact with TiO2 dispersions, a comparative photooxidation kinetic analysis of three model organic molecules, benzene (BZ) dissolved in acetonitrile (ACN), phenol (PhOH) dissolved in either water or acetonitrile, and formic acid (FA) dissolved in water, is presented to test the applicability of the D-I model under both equilibrium and nonequilibrium adsorption-desorption conditions. A previous analysis involving diffuse reflectance ultraviolet-visible (DRUVS) and Fourier transform infrared (FTIR) spectroscopy, combined with adsorption isotherm plots, shows that BZ chemisorption on the TiO2 surface is not allowed, physisorption being in this case the only possible adsorption mode. In line with D-I model predictions, BZ photooxidation is observed to take place via an adiabatic indirect transfer (IT) mechanism, with the participation of photogenerated terminal -Os•- radicals as oxidizing agents. In contrast, because of their strong chemisorption, FA species dissolved in water are found to be mainly photooxidized via inelastic direct transfer (DT) trapping of photogenerated valence-band free holes (hf+). Finally, when dissolved in water, PhOH chemisorption is not favored because of the strong electronic affinity of water molecules with the TiO2 surface, while chemisorption strength considerably increases when PhOH is dissolved in ACN, as far as the electronic interaction of solvent molecules with the TiO2 surface is negligible. Consequently, as predicted by the D-I model, PhOH dissolved in water is photooxidized via a combination of IT and DT mechanisms, the IT photooxidation rate (voxIT) being about 1 order of magnitude higher than DT photooxidation rate (voxDT). In contrast, when ACN is used as solvent, vox remains practically unchanged, while voxDT increases by about 2 orders of magnitude. These photooxidation results sustain the central D-I model hypothesis that the degree of substrate species interaction with the TiO2 surface is a decisive factor determining the kinetics of photocatalytic reactions. The effect of adsorption-desorption equilibrium rupture on the photooxidation kinetics of dissolved substrate species, predicted by the D-I model, is analyzed for the first time from experimental kinetic data concerning the photooxidation of PhOH dissolved in water under high enough illumination intensity (ρ ≈ 10 17 cm-2 s-1). © 2014 American Chemical Society.
Original languageEnglish
Pages (from-to)14276-14290
JournalJournal of Physical Chemistry C
Issue number26
Publication statusPublished - 3 Jul 2014


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