In order to elucidate the contributions of standard (NO2/NOx = 0%) fast (=50%) and NO2 (=100%) SCR reactions under conditions where they occur simultaneously the NOx conversion rate and N2O production rate over Fe/zeolite were investigated while varying both NO2/NOx ratio and temperature in small increments The NO conversion rate decreased in the following order fast SCR > NO2 SCR > standard SCR although after excluding the contribution of N2O production the order was changed to fast SCR > standard SCR > NO2 SCR When the fast SCR conversion was subtracted from the total NO conversion the remaining apparent conversion of standard SCR (NO2/NOx <50%) or NO2 SCR (>50%) was independent of NO2/NOx ratio This indicates that standard or NO2 SCR progressed once fast SCR was completed N2O production reached a maximum at ca 250 C and was determined responsible for NO2 SCR At temperatures below 200 C the NOx conversion behaviors of fast and NO2 SCR changed drastically the fast SCR conversion dropped suddenly with decreasing temperature and the NO SCR conversion went through a minimum and then began to increase with no N2O production (C) 2010 Elsevier B V All rights reserved