Comparative Pathways of SO2 Formation on the Pyrite (100) Surface: A DFT Study of O2 Activation and Surface Sulfur Oxidation

Abstract

The microscopic mechanism of SO2 formation from pyrite (the dominant inorganic sulfur species in coal) remains unclear, particularly the comparative energetics and relative competitiveness of pathways from O2 activation to SO2 formation and desorption. This knowledge gap limits the development of predictive models for pyritic sulfur conversion during coal combustion. In this work, density functional theory (DFT) calculations were used to examine a reaction network for pyrite oxidation on the FeS2 (100) surface. Multiple adsorption, dissociation, readsorption, precursor-formation, and desorption pathways were compared using LST/QST-derived transition-state candidates and consistent single-point energy calculations. Stage-wise comparison of the examined channels supports 3→A→V→V-(2) as a representative candidate sequence. For this sequence, the estimated local barrier for SO2 precursor formation is 119.28 kJ/mol, which is the lowest among the six precursor-formation routes examined. Compared with II-(2), the V-(2) transition-state candidate exhibits a smaller S displacement while retaining local Fe–O coordination.