Probing A Mixed Fe2(MoO4)3-MoO3 Catalyst to Elucidate the Active Phase and Site for the Oxidative Dehydrogenation of Ethanol

It is known that mixed Fe₂(MoO₄)₃ – MoO₃  catalysts have a longer life and are more selective to acetaldehyde in the oxidative dehydrogenation of ethanol when synthesized to contain excess molybdenum.  Until now, the exact role of the excess Mo and location of the active phase and site of mixed Fe₂(MoO₄)₃ – MoO₃ catalysts have been debated.  This study presents work done to determine the active phase and site with a series of experiments probing the bulk and surface of the catalyst during both oxidative and non-oxidative reactions with ethanol, with the latter conditions enabling anaerobic titration of the active redox phase.  X-ray absorption near edge structure (XANES) was used to determine the extent of reduction in iron (Fe) and molybdenum (Mo) during anaerobic titration and showed that the active phase was associated synergistically with both species.  Extended X-ray absorption fine structure (EXAFS) identified the removable/active oxygen during anaerobic titration as a shared oxygen between Fe and Mo in the lattice of Fe₂(MoO₄)₃; thus Fe₂(MoO₄)₃ was determined to be the active phase of the catalyst.  X-ray photoelectron spectroscopy (XPS) identified surface oxidation states before and after anaerobic titration for Fe and Mo and showed that near surface Mo species stayed mostly oxidized throughout the anaerobic titration, while near surface Fe species were mostly reduced.  Based on XANES and EXAFS results, it was expected that Mo species would reduce similarly to Fe, however, there was much more Mo in the form of Mo oxide in the near surface region.  This suggests that the only phase reduced during this process is the Fe₂(MoO₄)₃ phase and that surface Mo species are kept oxidized by oxygen donation from the Fe₂(MoO₄)₃ phase.