In-Situ Oxygen Isotopic Exchange Vibrational Spectroscopy of Rhenium Oxide Surface Structures on Cerium Oxide

The surface structures of ReO_x supported on CeO₂ at low loadings have been elucidated through ¹⁸O isotopic exchange Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The ReO_x is present in four distinct structures, a di-oxo structure with two ReO terminal bonds, a mono-oxo species that contains one ReO terminal bond, a mono-oxo species that contains a hydroxyl group, and a cross-linked ReO_x species. The isotopic exchange Raman spectroscopy shows a red shift resulting from the ¹⁸O exchange in the OReO, ReO, Re–OH, and Re–O–Re species, which allowed for the deconvolution of the various structures. Time-resolved DRIFTS showed significant exchange of ¹⁸O over time and reconfirmed the results from the Raman spectroscopy. The presence of multiple surface species supports the existence of competing reaction mechanisms for the simultaneous hydrodeoxygenation over the ReO_x–Pd/CeO₂ catalyst and deoxydehydration over the ReO_x/CeO₂ catalyst.