Ah initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO_2 by O_2/H_2O 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 SO_2 adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and a" 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, SO_2 was first adsorbed on the bare zigzag sites, followed by reaction with H_2O to form H_2SO_3, which was further oxidized by O_2 to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO_3 or H_2SO_3 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 SO_2 adsorption (and oxidation) on the amount of surface oxygen.
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