Reactivity of Fe-binuclear complexes in over-exchanged 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.