Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon Original Research Article

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO2 by O2/H2O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO2 adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO2 was first adsorbed on the bare zigzag sites, followed by reaction with H2O to form H2SO3, which was further oxidized by O2 to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO3 or H2SO3 as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO2 adsorption (and oxidation) on the amount of surface oxygen.