Reactive Site Model of the Reduction of SO2 on Graphite

Abstract

Computational quantum chemistry calculations were carried out for the reduction of SO2 on graphite to produce elemental sulfur and CO2. Two models of the reactive site of graphite were used and a viable mechanism was proposed for the reaction pathways based on experimental results and known reactions. The SO2 OO approach to the zigzag edge of the model cluster yielded a sulfur-oxidized intermediate 1,3,2-dioxathiolane (1). Sulfur transfer step takes place from 1 to a neighbor benzyne site forming a reduced sulfur intermediate thiirene (2) along with a 1,3-dicarbonyl in equilibrium with the peroxide valence tautomer. The calculated barrier from 1 to thiirene 2 at 900 °C was 39.4, kcal mol–1. The peroxide tautomer isomerizes to a dioxirane intermediate that is eliminated as dioxicarbene to produce CO2. The total free energy of activation for the decarboxylation reaction at 900 °C was in the range 110.8–122.7 kcal mol–1 depending on the model (ΔG‡experimental, 114.3 kcal mol–1). The reduced intermediate thiirene 2 decomposes through a transport mechanism where polysulfane species with increasing number of sulfur atoms eleminate elemental Sx.