The role of α-sites in N2O decomposition over FeZSM-5. Comparison with the oxidation of benzene to phenol

The kinetics and the mechanism of nitrous oxide decomposition (NOD) were studied using a set of FeZSM-5 samples with a wide variation in concentration of α-sites, which are special complexes of bivalent iron (FeII)α stabilized in the micropore space of the zeolite matrix. The results are compared with the N2O oxidation of benzene to phenol (OBP) studied earlier with the same set of samples. In spite of strong differences in turnover frequencies (ca. 100 times) and activation energies (92 kJ/mol), both reactions are catalyzed by the same active centers represented by α-sites. The rates of the reactions increase linearly with increasing α-sites concentration within two orders of magnitude.The NOD and OBP reactions are shown to have kindred mechanisms, which can be described by two main steps. The first step is a common one and includes oxidation of the α-site by the deposition of α-oxygen from N2O:(FeII)α + N2O → (FeIII–O−)α + N2The α-oxygen can be safely quantified, its properties being thoroughly studied in many previous works.The second step is the reduction of the site. It proceeds due to the removal of α-oxygen by the interaction of the latter with either N2O (NOD reaction) or benzene (OBP reaction). In both cases this step is the rate determining one.Mechanistically, the OBP can be considered as an N2O reduction by benzene. One may think that fundamental role of α-sites may hold also for N2O reduction by other substrates like CO, methane, propane, etc., which are widely used for this purpose.