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a b s t r a c tA comparative study was carried out on a small-pore Cu-CHA and a large-pore Cu-BEA zeolite catalyst to understand the lower N 2 O formation on small-pore zeolite supported Cu catalysts in the selective catalytic reduction (SCR) of NOx with NH 3 . On both catalysts, the N 2 O yield increases with an increase in the NO 2 /NOx ratios of the feed gas, suggesting N 2 O formation via the decomposition of NH 4 NO 3 . Temperature-programmed desorption experiments reveal that NH 4 NO 3 is more stable on Cu-CHA than on Cu-BEA. In situ FTIR spectra following stepwise (NO 2 + O 2 ) and ( 15 NO + NH 3 + O 2 ) adsorption and reaction, and product distribution analysis using isotope-labeled reactants, unambiguously prove that surface nitrate groups are essential for the formation of NH 4 NO 3 . Furthermore, Cu-CHA is shown to be considerably less active than Cu-BEA in catalyzing NO oxidation and the subsequent formation of surface nitrate groups. Both factors, i.e., (1) the higher thermal stability of NH 4 NO 3 on Cu-CHA, and (2) the lower activity for this catalyst to catalyze NO oxidation and the subsequent formation of surface nitrates, likely contribute to the higher SCR selectivity with less N 2 O formation on this catalyst as compared to Cu-BEA. The latter is determined as the primary reason since surface nitrates are the source that leads to the formation of NH 4 NO 3 on the catalysts.

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