Effect of Oxygen Concentration in NH3-SCR Reaction over Fe- and Cu-loaded Beta Zeolites

The NH3-SCR of NO is a widely-used method for the removal of NOx from stationary sources and a variety of suitable transition metal ion-exchanged zeolite catalysts has been developed. The NH3-SCR has also been investigated in recent years as an important candidate for treatment of exhaust fumes from diesel engines. Of the reported transition metal ion-exchanged zeolite catalysts, Feand Cu-loaded beta zeolite catalysts have received significant attention due to their high catalytic activity, with differences in the catalytic behaviour of the two catalysts being discussed. Rahkamaa-Tolonen et al. have investigated the NH3-SCR of NO over a number of transition metal ion-exchanged beta zeolite catalysts and found that the catalytic activity at temperatures lower than 400 °C decreased as follows: Fe> Cu>Ag>H. The loaded metal species served as the active component for catalysing the oxidation of NO to NO2. Colombo et al. have reported notable differences in the NH3 effect on the NH3-SCR activity between Feand Cu-loaded beta zeolite catalysts, namely a negligible effect of NH3 on the Cu-loaded zeolite and a severe inhibitory effect of NH3 on the Fe-loaded zeolite2). Fedeyko et al. have reported an investigation into the adsorption behaviour of NOx and NH3 over Feand Cu-loaded beta zeolites by means of an FT-IR technique. In the case of the Fe zeolite catalyst, NH3 molecules adsorbed on the Fe active sites that would catalyse the oxidation of NO to NOx species, and severely inhibited the overall NH3-SCR activity. In the presence of NH3, the IR bands assigned to surface nitrate/nitrite groups, which exhibit the high reactivity with NH3, were not observed. By contrast, an existence of the nitrate/nitrite groups was apparent for the Cu zeolite catalyst3). It has also been reported in the literature that the oxidation of NO to NO2 is the rate-determining step for the NH3-SCR over Fe-loaded zeolite catalysts4)~8) and that the apparent reaction orders of NO and O2 are 1 and 0-0.5, respectively4)~6). The presence of NH3 suppressed the oxidation of NO to NO2, leading to reduction in the NH3-SCR activity, with the apparent reaction order of NH3 being zero or of negative order. The inhibition effect of NH3 strongly indicates the possibility of NH3 adsorption onto the active sites5),9), the reduction of a loaded metal10), or NH3 oxidation6). On the other hand, for Cu-loaded zeolite catalysts, several ratedetermining steps were proposed, such as NO oxidation to NO2, oxidative adsorption of NO12), or oxidation of Cu+ to Cu2+ 13). The inhibitory effect of NH3 was 57 Journal of the Japan Petroleum Institute, 55, (1), 57-66 (2012)