Selective catalytic reduction of nitrogen oxide by ammonia over Cu/SAPO-11: a theoretical study

Quantum chemistry calculations were carried out using the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Cu/SAPO-11, NO–Cu/SAPO-11, NH₃–Cu/SAPO-11 and NO + NH₃–Cu/SAPO-11 to investigate the reaction pathway of the selective catalytic reduction of nitrogen oxides by ammonia in the presence of Cu/SAPO-11. NH₂NO is formed, and then is decomposed into N₂ and H₂O over Cu/SAPO-11 for the catalytic reduction of NO in two steps. The adsorption energy ΔE_ads of NO, NH₃ on Cu/SAPO-11 and the change of energy of the reduction reaction are presented. The energy profile of the formation of NH₂NO–Cu/SAPO-H key intermediate is shown. Some relevant charges and bond indexes were calculated. On adsorbing NH₃ or NO on Cu/SAPO-11 there is charge transfer to the aggregates. Vibrational frequencies of adsorbed NH₃, NO and of NH₂NO–Cu/SAPO-11cluster are reported. The importance of adsorbing NH₃ previously to the reduction of NO is emphasized.