Role of the epoxy group in the heterogeneous CO2 evolution in carbon oxidation reactions

A density functional theory study was carried out to evaluate different possibilities of heterogeneous CO2 desorption in combustion/gasification reactions. First, we investigated the heterogeneous CO2 evolution induced as a secondary reaction caused by the CO readsorption on an oxidized surface, which produces a carbonatelike surface complex. This functional group was found to decompose as CO2 with an activation energy of 60 kcal/mol. Another possibility for heterogeneous CO2 formation during char combustion or gasification is through the molecular O-2 chemisorption on a carbon active site of an oxidized graphene layer. The peroxy complex thus formed undergoes rearrangement into a dioxiranyl complex, C(20). This complex can evolve as CO2 with an activation energy of 48 kcal/mol. The CO2 desorption is facilitated by the presence of an epoxy complex near the edge of the graphene layer. The epoxy complex undergoes transformation into a cyclic ether complex during the dioxiranyl decomposition. Transition states and energetic profiles of these decomposition reactions were determined. Variations of selected C-C, O-O, and C-O bonds were analyzed through the change in the bond orders calculated by natural bond orbital analysis. Overall, carbon oxidation reactions in the presence of epoxy functionalities are very important in the formation of heterogeneous CO2 and of cyclic ether complexes.