Catalytic activity of Fe ions in iron-based crystalline and amorphous systems: role of dispersion, coordinative unsaturation and Al content

Different Fe-based materials, where iron ions are anchored on crystalline and amorphous siliceous supports are studied by FTIR spectroscopy of adsorbed NO and in the N2O decomposition reaction test. The influence of Al concentration is also considered. We show that the coordination state of Fe dramatically changes when Fe is anchored on crystalline or amorphous matrices. In crystalline Fe-ZSM-5 and Fe-silicalite samples highly coordinatively unsaturated and isolated Fe sites are present, which form Fe2+(NO)2 and Fe2+(NO)3 complexes upon NO contact. The presence of oxidic clusters, forming Fe2+(NO) complexes, whose relative concentration strictly depends upon the presence of Al and the concentration of Fe, is also evidenced. Fe-ZSM-5 sample (Si/Fe = 1120) mainly contains isolated Fe2+ ions characterised by high coordinative unsaturation. This sample also shows the highest activity in N2O decomposition, indicating that isolated and coordinatively unsaturated Fe sites are the most active precursors for the catalytic reaction. It is thought that adsorbed oxygen is formed, upon N2O decomposition, on both isolated and oxidic clusters, forming ferryl groups in the former case and bridged structures in the latter.