A comparative study was carried out on a small-pore Cu-CHA and a large-pore Cu-BEA zeolite catalyst to understand the lower N2O formation on small-pore zeolite supported Cu catalysts in the selective catalytic reduction (SCR) of NOx with NH3. On both catalysts, the N2O yield increases with an increase in the NO2/NOx ratios of the feed gas, suggesting N2O formation via the decomposition of NH4NO3. Temperature-programmed desorption experiments reveal that NH4NO3 is more stable on Cu-CHA than on Cu-BEA. In situ FTIR spectra following stepwise (NO2 + O-2) and ((NO)-N-15 + NH3 + 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 NH4NO3. 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 NH4NO3 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 N2O 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 NH4NO3 on the catalysts. (C) 2015 Elsevier Inc. All rights reserved. Agreements signed 2015.