Quantum-mechanical calculations allow quantifying important rate-controlling steps in redox reactions. The interaction of realgar with oxygen and co-adsorbed ions provides a model system for understanding the molecular-scale processes that underpin empirically-derived rate expressions, and provides clues to the oxidation mechanisms of other sulfides and oxides. Two activated processes are shown to dominate the kinetics of oxidation by molecular oxygen: i) a paramagnetic to diamagnetic spin transition and ii) oxygen dissociation on the surface. The activation energies for the spin transition and O2 dissociation step were determined to be 106 kJ/mol and 87 kJ/mol, respectively, for dry oxidation. 3O2 transfers its spin to As4S4 and forms a low-spin peroxo intermediate on the surface before dissociating. The adsorption of a hydroxide ion on the surface proximate to the 3O2 adsorption site changes the adsorption mechanism by lowering the activation energy barriers for both the spin transition (29 kJ/mol) and the O2 dissociation step (69 kJ/mol) in agreement with the experimental value.

First principles study of the oxidation energetics and kinetics of sulfides

241st National Meeting (2011)

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