Kinetics and Mechanism of the N2O Reduction by NH3 on a Fe-Zeolite-Beta Catalyst

Abstract

In the context of decreasing the emissions of greenhouse gases, a Fe-exchanged zeolite-beta (Fe-BEA) catalyst is shown to be very active in the reduction of N2O by NH3 in the presence of O2. The temperature at which 50% N2O conversion is obtained is lower by ca. 80 K compared to its catalytic decomposition in the absence of NH3. TPR, TPO, and TPD experiments after treatments in various atmospheres provide evidence that the reaction involves the redox cycle FeIII↔FeII where the FeIII active species are Fe oxocations of low nuclearity. N2O decomposes into O* surface species on specific reduced Fe sites with the concurrent release of N2; these species do not compete with O* coming from O2 for their removal by NH3. In the absence of O2, catalytic experiments with 14N2O and 15NH3 show that: (1) N2 is mainly formed from 14N14N–O splitting to yield 14N2, the O* species being in turn removed by 15NH3 to give 15N2; (2) some 14N–14NO bond splitting occurs, which leads to 14N15N after reaction of 14NO* and 15NH3 through a classical SCR mechanism. The Fe active species in the N–NO splitting are inhibited in the presence of O2. The kinetics of N2O reduction by NH3 obeys a Mars and van Krevelen oxido–reduction mechanism modified with an inhibiting term of NH3.