Photo-assisted dissolution of colloidal iron oxides by thiol-containing compounds: 2. Comparison of lepidocrocite (γ,-FeOOH) and hematite (α-Fe 2O 3) dissolution

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Comparison of the photo-assisted dissolution of lepidocrocite (γ-FeOOH) and hematite (α-Fe 2O 3) in the presence of a series of α-mercaptocarboxylic acids under a variety of conditions of reactant concentration, temperature, and light intensity provides considerable insight into the relative importance of reactions at the oxide—solution interface in determining the rate of oxide dissolution. For hematite, the rates of ligand adsorption to the oxide surface and electron transfer within surface-located precursor complexes appear comparable, with low steady-state surface coverage of the oxide surface by thiol ligand. Low apparent activation energies of dissolution and the dramatic effect of light in increasing dissolution rate suggest that for lepidocrocite the rate of ligand uptake is rapid followed by relatively slow rate-determining electron transfer. The use of photolysis to selectively alter the rate of the electron transfer step appears to be an effective way of examining the mechanism of complex photo-assisted surface reactions.