The kinetic study on the effect of O
2 concentration in the selective catalytic reduction (SCR) of nitrogen oxide (NO
x) by ammonia (NH
3) was systematically carried out at temperatures below 200 °C over Fe- and Cu-loaded beta zeolites. The large difference was observed in the effect of O
2 concentration over the two catalysts: for the Fe-loaded beta zeolite, the NO conversion increased considerably with the O
2 concentration. On the other hand, for the Cu-loaded beta zeolite, increasing the concentration of O
2 did not have a significant impact. In addition, the temperature dependence of the apparent reaction orders was investigated. The apparent reaction order of O
2 decreased with an increase in the reaction temperature, being 0.9 at 150 °C and 0.4 at 200 °C for the Fe-loaded beta zeolite, and 0.2 at 125 °C and 0.1 at 175 °C for the Cu-loaded beta zeolite. The fact that the degree of the reduction in the reaction order of O
2 was consisted with that of the increase in the reaction order of NH
3 when the reaction temperature was increased strongly suggests that adsorbed NH
3 inhibits the adsorption of O
2.
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