Reactivity of Binuclear Fe-Complexes in Over-Exhanged Fe/ZSM5 Studied by In Situ XAFS Spectroscopy : Part 1: Heat Treatment in He and O2

Abstract

The structure of the iron species in mildly calcined over-exchanged Fe/ZSM5, prepared by CVD of FeCl3, was studied during heat treatments in He or O2/He (50:50) by coupling in situ Fe K edge HR-XANES and EXAFS. The majority of iron appears to be present as Fe-binuclear complexes. EXAFS shows that the closest Fe---O shell in the complexes can be described with a [HO---Fe---O---Fe---OH]2+ core. Heating to a moderate temperature (up to ~150 °C) results in the desorption of water from the Fe-coordination sphere, in He as well as in a O2/He (50:50) mixture. The composition of the gas phase strongly influences the changes occurring to the binuclear complexes in the 150–350 °C temperature range. Above 250 °C in He a significant fraction of iron undergoes autoreduction. This is accompanied by the removal of approximately one oxygen atom from the closest Fe---O shell, ascribable to the Fe---O---Fe bridging oxygen atom. The presence of oxygen in the gas phase (PO2=0.5 bar), on the contrary, suppresses the auto-reduction capability of the binuclear complexes by inhibiting the removal of the Fe---O---Fe bridging oxygen.