Understanding the nature of NH3-coordinated active sites and the complete reaction schemes for NH3-SCR using Cu-SAPO-34 catalysts

Abstract

Cu-SAPO-34 zeolite catalysts show excellent NH₃-SCR performance at low temperature, which is due to the catalytic capacity of copper species. Isolated Cu^II ions and Cu^IIOH are active sites, but their nature and role are not fully understood. This paper reports the DFT calculations in combination with ab initio thermodynamics to investigate NH₃ and H₂O coordination to copper species under typical NH₃-SCR reaction conditions. In the reduction part of the NH₃-SCR reaction, NH₂NO and NH₄NO₂ intermediates will form on Cu^II-2NH₃/3NH₃ and Cu^IIOH-2NH₃ complexes, respectively. The Brønsted acid sites are crucial for the decomposition of these intermediates, rather than copper species. Furthermore, the decomposition of NH₂NO is more energetically favorable than NH₄NO₂ which are formed on the Brønsted acid sites. In the re-oxidation part of the NH₃-SCR reaction, O₂ dissociation and NO₂ formation occur on Cu^I-2NH₃ complexes in the presence of NO, and the regeneration of Cu^IIOH-2NH₃ requires the participation of H₂O. The proposed complete mechanisms highlight the importance of ligand coordinated copper species for intermediate formation and O₂ activation in NH₃-SCR.