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

ReO_x/CeO₂ and ReO_x-Pd/CeO₂ are important catalysts for the deoxydehydration (DODH) and simultaneous hydrodeoxygenation (S-HDO) of biomass derived sugars. However, the surface structures of the ReO_x in these catalysts have only been theorized using Density Functional Theory (DFT) calculations in the literature^1,2. In our work³, 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). We found that 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 which contains a hydroxyl group, and a crosslinked 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 using the model for a simple rigid diatomic molecule. Time resolved DRIFTS showed significant exchange of ¹⁸O over time and further reconfirmed the results from the Raman spectroscopy. Our results show that there are multiple isolated monomeric structures and an oligomeric structure of ReO_x present when supported CeO₂.

References

(1) Ota, N.; Tamura, M.; Nakagawa, Y.; Okumura, K.; Tomishige, K. Performance, Structure, and Mechanism of ReO_x –Pd/CeO₂ Catalyst for Simultaneous Removal of Vicinal OH Groups with H₂. ACS Catal. 2016, 6 (5), 3213–3226.

(2) Xi, Y.; Yang, W.; Ammal, S. C.; Lauterbach, J.; Pagan-Torres, Y.; Heyden, A. Mechanistic Study of the Ceria Supported, Re-Catalyzed Deoxydehydration of Vicinal OH Groups. Catal. Sci. Technol. 2018, 8 (22), 5740–5752.

(3) MacQueen, B.; Ruiz-Yi, B.; Royko, M.; Heyden, A.; Pagan-Torres, Y. J.; Williams, C.; Lauterbach, J. In-Situ Oxygen Isotopic Exchange Vibrational Spectroscopy of Rhenium Oxide Surface Structures on Cerium Oxide. J. Phys. Chem. C 2020, 124 (13), 7174–7181.