A quantum chemical study of nitric oxide reduction by ammonia (SCR reaction) on V2O5 catalyst surface

The reaction mechanism for the selective catalytic reduction (SCR) of nitric oxide by ammonia on (0 1 0) V2O5 surface represented by a V2O9H8 cluster was simulated by means of density functional theory (DFT) calculations performed at B3LYP/6-31G** level. The computations indicated that SCR reaction consisted of three main parts. For the first part, ammonia activation on V2O5 was investigated. Ammonia was adsorbed on Brønsted acidic V–OH site as NH4+ species by a non-activated process with an exothermic relative energy difference of 28.65 kcal/mol. Lewis acidic ammonia interactions were also considered and they were found to be energetically unfavorable. Therefore, it is concluded that the SCR reaction on (0 1 0) vanadium oxide surface is initiated favorably by the Brønsted acidic ammonia adsorption. The second part of the SCR reaction consists of the interaction of nitric oxide with the pre-adsorbed ammonia species to eventually form nitrosamide (NH2NO) species. The rate limiting step for this part as well as for the total SCR reaction can be identified as NH3NHO formation with a high activation barrier of 43.99 kcal/mol; however, it must be cautioned that only an approximate transition state was obtained for this step. For the last part, gas phase decomposition of NH2NO and decomposition of this species on catalyst surface were both considered. Gas phase decomposition of NH2NO was found to have high activation barriers when compared with the NH2NO decomposition on V2O9H8 cluster surface. NH2NO decomposition on this cluster was achieved by means of a push–pull hydrogen transfer mechanism between the active VO and V–OH groups.