Structure sensitivity of pyrite oxidation: Comparison of the (100) and (111) planes

The interaction of atomically clean (100) and (111) crystallographic planes of FeS2 with H2O vapor, O-2, and a H2O/O-2 mixture was investigated. A combined high pressure/ultra-high vacuum (UHV) apparatus allowed the surfaces to be reacted at environmentally relevant pressures and studied with X-ray photoelectron spectroscopy (XPS) without exposure to the atmosphere. Neither surface exhibited significant reaction in pure O-2. Exposure of FeS2(111) to H2O vapor resulted in significant oxidation, but under these same experimental conditions FeS2(100) exhibited a much smaller amount of oxidation. It is suspected that on FeS2(100), H2O only reacted on nonstoichiometric regions (i.e., defects). Both surfaces showed substantial reaction in H2O/O-2. The amount of FeS2(100) and FeS2(111) oxidation in the H2O/O-2 mixture was more than simply the sum of the reaction observed individually in purr O-2 and H2O. This result suggests that there is a synergy between H2O and O-2 in oxidizing pyrite. In all cases, the amount of oxidation that occurred on FeS2(111) was greater than on FeS2(100). We believe that this experimental observation is due to a higher concentration of under-coordinated Fe in the outermost surface of FeS2(111), relative to FeS2(100).