Role of the Fe-zeolite structure and iron state in the N 2O decomposition: Comparison of Fe-FER, Fe-BEA, and Fe-MFI catalysts

Abstract

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 N 2O molecule, and provide potential for cooperation of the two iron cations on the N 2O molecule splitting. Such arrangement is absent in both the Fe-BEA and Fe-MFI structures.