Abstract

GARCIA RODENAS, L.A. et al. A model for the dissolution of metal oxides mediated by heterogeneous charge transfer. An. Asoc. Quím. Argent. [online]. 2004, vol.92, n.1-3, pp.73-87. ISSN 0365-0375.

The experimental characterization of charge transfer transitions (LM and IV CTs) in surface complexes formed on iron oxides and TiO2, together with the measurements of the rate of magnetite dissolution in the presence of pentacyanoisonicotinatoferrate(II) are used to build and validate a general model for the dissolution of metal oxides mediated by charge transfer processes. A critical surface ensemble, denoted ≡S, is identified for the different cases. For reductive dissolution of iron(III) oxides, ≡S is often an iron(II) surface complex that dissolves in a first order kinetic process; the steady state concentration of ≡S, and hence its rate of dissolution is determined by the balance between its rate of formation and either the rate of oxo-bond breakage or the rate of diffusion/scavenging of the conjugate oxidised form of the reductant. In the case of IVCT within dimeric surface complexes, we postulate that thevibrational activation required for charge transfer also produces labilization; thus, charge transfer and dissolution are concerted processes. Charge trapping by the surface complex following the photochemical formation of an electron-hole pair in the semiconductor also generates a critical ensemble. In this case, however, successful dissolution requires of adequately high rates of removal of both charge carriers, and dissolution is arrested if there are no scavengers for the other charge carrier (the hole, in the case of reductive dissolution). This effect is realized experimentally in the photochemical dissolution of nickel ferrite, with and without oxalic acid as hole scavenger.

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