Reduction of N 2O with CO over FeMFI zeolites: influence of the preparation method on the iron species and catalytic behavior

Abstract

The reduction of N 2O by CO was investigated over FeMFI zeolites prepared by different methods including sublimation, liquid ion exchange, and hydrothermal synthesis followed by steam activation. This leads to catalysts with different nature and distribution of iron species, as characterized by HRTEM, UV/vis, and EPR techniques. A common denominator in the FeZSM-5 samples is the heterogeneous iron constitution, with a significant degree of clustering in the form of iron oxide particles. Iron clustering was suppressed in steam-activated Fe-silicalite, presenting a remarkable uniform distribution of isolated iron species. In the presence of CO, the conversion of N 2O over the catalysts is strongly accelerated with respect to direct N 2O decomposition. The reaction rate increases linearly with the molar CO/N 2O feed ratio and strongly depends on the preparation method applied. A correlation was found between the fraction of isolated Fe(III) species in the as-prepared catalysts and the activity for N 2O reduction with CO. Steam-activated Fe-silicalite, containing mostly isolated iron ions in extraframework positions, shows the highest activity per mole of iron, while the highly clustered liquid-ion-exchanged catalyst presents the lowest activity. In situ UV/vis and EPR studies evidence the participation of mononuclear iron ions in the N 2O–CO reaction, and also support the involvement of oligonuclear Fe x O y species. The reaction mechanism is iron site dependent. Over isolated sites, the reduction of N 2O with CO occurs via coordinated CO species on Fe 3+ ions. The reaction over oligonuclear sites proceeds via a redox Fe 3+/Fe 2+ process with intermediate formation of O − radicals.