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

The mechanism for N 2 O formation over CHA and Cu-CHA zeolite catalysts during NH 3 -SCR is investigated using density functional theory calculations. Direct NH 4 NO 3 decomposition, which is commonly regarded as the main source of N 2 O, is found to be associated with high barriers in the absence of Brønsted acid sites. Although Brønsted acid sites promote NH 4 NO 3 decomposition, it is still a highly activated process. Low-temperature N 2 O formation is instead found to be connected with an NO + NH 3 reaction over Cu-sites. In particular, N 2 O can be formed from H 2 NNO with a low barrier over Cu-OOH-Cu complexes, which are proposed intermediates in the catalytic cycle for NH 3 -SCR over Cu-CHA. This finding provides an explanation for the experimentally observed low-temperature N 2 O formation and the relation between Cu loading and N 2 O formation. The proposed mechanisms open up strategies to enhance the selectivity to N 2 during NH 3 -SCR.