The Role of H+- and Cu+-Sites for N2O Formation during NH3-SCR over Cu-CHA

The mechanism for N2O formation over CHA and Cu-CHA zeolite catalysts during NH3-SCR is investigated using density functional theory calculations. Direct NH4NO3 decomposition, which is commonly regarded as the main source of N2O, is found to be associated with high barriers in the absence of Bronsted acid sites. Although Bronsted acid sites promote NH4NO3 decomposition, it is still a highly activated process. Low-temperature N2O formation is instead found to be connected with an NO + NH3 reaction over Cu-sites. In particular, N2O can be formed from H2NNO with a low barrier over Cu-OOH-Cu complexes, which are proposed intermediates in the catalytic cycle for NH3-SCR over Cu-CHA. This finding provides an explanation for the experimentally observed low-temperature N2O formation and the relation between Cu loading and N2O formation. The proposed mechanisms open up strategies to enhance the selectivity to N-2 during NH3-SCR.

Automated summary

The formation of N2O over CHA and Cu-CHA zeolite catalysts during NH3-SCR is found to be connected with NO + NH3 reaction over Cu-sites, rather than direct NH4NO3 decomposition. This finding provides an explanation for the experimentally observed low-temperature N2O formation and the relation between Cu loading and N2O formation. These proposed mechanisms offer strategies to enhance the selectivity to N-2 during NH3-SCR.