(55g) Analysis of the Contributions of NO2 to N2o Production during NH3-SCR over Cu-CHA Catalysts

NH₃-SCR over Cu-CHA catalysts is a key technology for NO_x reduction, involving a Cu^II/Cu^I redox cycle. Formation of the greenhouse gas N₂O during NH₃-SCR remains a concern. Here, we use DFT calculations (CI-NEB and dimer methods) to examine the role of NO₂ in standard SCR reduction and oxidation half-cycles and its consequences for N₂O formation. In the reduction half-cycle, we compared N₂-forming and NO-consuming standard SCR reactions with parallel NO₂-consuming and N₂O-forming steps over various Cu^II active sites: framework associated Cu^II (Z₂Cu, ZCuOH), NH₃-solvated mononuclear (Z₂Cu(NH₃), ZCuOH(NH₃)), and binuclear Cu^II (ZCuO₂CuZ(NH₃)₄). Both NO and NO₂ reactions involve NO_x attack on Cu-bound NH₃ with proton transfer to the framework or Cu-bound O or OH^-. Activation energies for NO₂ reactions are generally higher than for NO, except for NH₃-solvated ZCuOH, where they are comparable. Microkinetic models predict highest N₂O selectivity on NH₃-solvated ZCuOH (1-10%) at a NO₂:NO_x ratio of 0.1 and 473 K, due to favorable proton transfer from NH₃ to OH^- and greater product stability. Other Cu^II sites show less than 1% selectivity. N₂O selectivity is greater on NH₃-solvated mononuclear Cu^II sites than their framework-associated counterparts. In the oxidation half-cycle, we examine parallel oxidation of mononuclear Cu^I (Cu^I(NH₃)₂) species with O₂ or NO₂ and their subsequent reactions with NO and NO₂. Computed Gibbs formation free energies suggest a strong thermodynamic driving force for the formation of Cu-nitrate or [CuNO₃]^I-like intermediates. DFT calculations indicate that reactions of Cu-nitrate species with NH₃ can lead to N₂O generation. The results are consistent with steady-state reactor experiments over Cu-SSZ-13 that showed N₂O formation rates increase with feed NO₂ pressure. The results suggest that suppressing NO₂-driven side reactions in both reduction and oxidation half-cycles can mitigate N₂O emissions during NH₃-SCR over Cu-CHA.