Role of the Fe-zeolite structure and iron state in the N2O decomposition: Comparison of Fe-FER, Fe-BEA, and Fe-MFI catalysts

The decomposition of nitrous oxide was compared over Fe-FER, Fe-MFI, and Fe-BEA with well established iron distribution in cationic positions and low amounts of less well-established oxide species. It was evidenced that, despite a comparable content of Fe(II) in the cationic positions, the catalytic activity of Fe-FER greatly exceeds that of Fe-BEA and Fe-MFI. While about one half of the iron sites in Fe-FER (Fe/Al < 0.15) participate in the decomposition of nitrous oxide after activation at 450 °C, the number of active sites in Fe-BEA or Fe-MFI was much lower, and, accordingly, without acceleration of the reaction by the addition of NO, these samples exhibit much lower catalytic activity than Fe-FER. This could be likely correlated with the concentration of Fe(II) in positions with a specific spatial iron arrangement at optimal Fe⋯Fe distances. For that role we propose a local structure with two adjacent β sites, where the Fe⋯Fe distance would be 7 to 7.5 Å, i.e. comparable to the length of the N2O molecule, and provide potential for cooperation of the two iron cations on the N2O molecule splitting. Such arrangement is absent in both the Fe-BEA and Fe-MFI structures.