A comparative study of “standard”, “fast” and “NO2” SCR reactions over Fe/zeolite catalyst

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 NOx 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 NOx conversion, the remaining “apparent” conversion of standard SCR (NO2/NOx  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 NO2 SCR conversion went through a minimum and then began to increase with no N2O production.