Activation of oxygen on (NH3Cu NH3)+ in NH3-SCR over Cu-CHA

Cu-CHA materials are efficient catalysts for NH3–SCR of NOx in oxygen excess. A crucial step in the reaction is oxygen (O2) activation, which still is not well understood. Density functional theory calculations in combination with ab initio thermodynamics and molecular dynamics are here used to study O2 dissociation on Cu(NH3)2+ species, which are present under NH3–SCR conditions. Direct dissociation of O2 is found to be facile over a pair of Cu(NH3)2+ complexes whereas dissociation on a single Cu(NH3)2+ species is unlikely due to a high activation energy. The presence of NO promotes oxygen dissociation on both single and pairs of Cu(NH3)2+ complexes. Nitrites and nitrates are easily formed as O2 dissociates, and NO adsorption over nitrates leads to facile formation of NO2. The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH3–SCR.