A study to determine the active site in the oxidative dehydrogenation of ethanol over mixed iron molybdate catalysts

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 the active phase and site of mixed Fe2(MoO4)3 – MoO 3 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 active redox sites. 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 species before and after anaerobic titration and showed that surface Mo oxide species stayed almost completely oxidized throughout the anaerobic titration, confirming that bulk Fe2(MoO4) 3 was supplying its removable oxygen to keep the surface Mo oxide species oxidized and active for reaction