Reactivity of Fe-binuclear complexes in over-exchanged Fe/ZSM5, studied by in situ XAFS spectroscopy: Part 1: Heat treatment in He and O2

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.