Isotope labeled (NO)-N-15 was used to investigate the mechanism of the unusual overconsumption of NH3 during standard SCR over Fe-zeolite, under conditions during which NH3 oxidation with O-2 alone (without NO) was unfavorable. When the Fe-BEA catalyst was exposed to the (NO)-N-15 + (NH3)-N-14 + O-2 gas mixture at 250 and 300 degrees C, the resulting products included (NO)-N-14, a product of (NH3)-N-14 oxidation under SCR conditions. However, (NO)-N-14 was not detected when Fe-BEA was exposed to (NH3)-N-14 + O-2. Since the only source of N-14 derives from (NH3)-N-14, with the labeled gas mixture used, the (NO)-N-14 during SCR must have originated from oxidation of (NH3)-N-14. Furthermore, twice as much (NN)-N-14-N-14 was observed at 300 degrees C under SCR conditions in comparison with NH3 oxidation using O-2 alone. Under SCR conditions, the (NO)-N-14 formed through the unusual oxidation route further reacted with (NH3)-N-14 to produce N-14(2). Thus, for the first time, we have experimental evidence for the unusual overconsumption of ammonia during SCR over Fe-zeolites. (c) 2012 Elsevier Inc. All rights reserved.