Photostimulated oxidation of magnetite: 2. Mechanism

It was demonstrated in an earlier paper that magnetite (Fe3O4) becomes oxidized upon exposure to UV radiation in O2-bearing atmospheres, and a set of kinetic constraints on the reaction mechanism was presented. In the present paper a model for the reaction mechanism has been derived that satisfies the constraints of the experimentally derived kinetic rate equation: (1) the protective oxide layer on the magnetite grain surfaces is disrupted by the action of absorbed H2O by promoting the migration of substrate cations to the surface; (2) the H2O desorbs from the magnetite grain surfaces, and atmospheric O2 molecules collisionally dissociate into adsorbed O atoms upon colliding with pairs of adjacent vacant adsorption sites; (3) upon illumination (λ ≤ 0.350 μ), electrons are photoejected from the magnetite, a portion of which attach to the physically adsorbed O atoms to form adsorbed O− ions; the capture of the electrons by the adsorbed O atoms results in the oxidation of Fe2+ ions to Fe3+ ions; (4) after S-O− formation a second photoejected electron attaches to form chemisorbed O2−; the chemisorbed O2− ions coordinate surface ferric ions to form ferric oxide; and (5) since the principal ferric oxide phase detected, i.e., hematite, has a more closely packed structure than magnetite, the ferric oxide layer fails mechanically (scale formation), exposing fresh magnetite grain surfaces for further photostimulated oxidation; as a result, photostimulated oxidation can proceed in the absence of additional adsorbed H2O.