Interaction of oxygen and water with the (100) surface of pyrite: Mechanism of sulfur oxidation

We present a density-functional study of the adsorption and reactions of oxygen and water with the (100) surface of pyrite. We find that dissociative adsorption is energetically favorable for oxygen, forming ferryl-oxo, Fe ⁴⁺=O ^2-, species. These transform easily to ferric-hydroxy, Fe ³⁺-OH ^-, in the presence of coadsorbed water, and the latter fully covers the surface under room conditions. A mechanism for surface oxidation is identified, which involves successive reactions with molecular oxygen and water, and leads to the complete oxidation of a surface sulfur to SO ₄ ^2-. The crucial recurring process is the surface O ^2- and OH ^- species acting as proton acceptors for incoming water molecules. Using a recently proposed method, we examine the oxidation state changes of the surface ions and the electron flow during the adsorption and oxidation processes. The oxidation mechanism is consistent with isotopic labeling experiments, suggesting that the oxygens in SO ₄ ^2- from gas-phase oxidation are derived from water.