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Properties and structure of redox sites in two different catalyst systems were investigated.   Using in-situ anaerobic titration during X-ray absorption spectroscopy and high resolution TEM, atom-specific redox sites in both Fe₂(MoO₄)₃ – MoO₃ and dodecamolybdophosphoric acid (MPA) catalysts were identified.  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 – MoO₃ catalysts have been debated.  Through this work, the active phase and site was identified 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.

In a second study, the structural and electronic mechanism by which  redox sites in dodecamolybdophosphoric acid (MPA) activate and deactivate during reaction was investigated.  Such mechanistic insight is crucial to the intelligent design of more efficient chemical syntheses. Atomic resolution electron microscopy was used in conjunction with bulk characterization tools including as X-ray Diffraction (XRD), Ultraviolet–visible Diffuse Reflectance Spectroscopy (UV–vis DRS) and X-ray Absorption Spectroscopy (XAS) to understand the activation and deactivation mechanism of dodecamolybdophosphoric acid (MPA), a promising parent material for a class of polyoxometalate catalysts useful for the direct oxidation of isobutene to methacrylic acid. These techniques show that the thermal and reactive reconstruction of MPA arises from the migration of an oxomolybdate species from the cubic form of the anhydrous MPA structure. The reconstruction continues and results in complete degradation to MoO₃, which is inactive for isobutane oxidation. The mechanism by which reorganization occurs was investigated using High Resolution Transmission Electron Microscopy (HR-TEM) for the first time. These HR-TEM studies provided a picture of the atomic-scale rearrangement occurring in the catalyst. The initial structural reorganization in MPA is observed as the formation of annealing twins in the cubic form of the anhydrous polyoxomet- alate – this twinned structure is believed to be the active form of the catalyst. This twinning phenomenon is believed to originate from vacancies created in the MPA structure by the migration of atoms out of the primary structure. The twins then propagate across the MPA crystal and result in complete degradation of the MPA to MoO₃.