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

Abstract

The surface structures of ReO x supported on CeO 2 at low loadings have been elucidated through 18 O 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 crosslinked ReO x species. The isotopic exchange Raman spectroscopy shows a red shift resulting from the 18 O 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 18 O 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/ CeO 2 catalyst and deoxydehydration over the ReO x /CeO 2 catalyst.

The Kinetic Model of Diffusion and Reactions in Powder Catalysts during Temperature Programmed Oxygen Isotopic Exchange Process

“…Complex Heteroexchange Reactions Not all of the simple hetero-exchange reactions listed in Table 1 result in a change in system composition. It may happen that an atom of the same type from the gas phase can be replaced with an atom of the same type from the surface (e.g., 16 O g with 16 O s ). In that case microscopically the oxygen exchange occurs, but the composition of system remains the same.…”

“…The change in composition in the gas phase and on the surface is calculated according to the law of mass action well known in chemistry. Using this law in the case of simple heteroexchange the variation of concentrations in gas phase n 32 , n 34 and n 36 of species 16…”

“…A powerful tool to study the various oxygen transport processes that may take place in a crystalline oxide is the 18 O- 16 O isotopic exchange technique [15][16][17]. The oxygen isotopic exchange is a suitable method to analyze the diffusivity and interaction of molecular oxygen with a metal oxide.…”

“…Information on the mechanism of exchange may be obtained from the kinetic curves of oxygen species partial pressures at the beginning of exchange reaction. The formation of 16 O 18 O species as a primary product from 18 O 2 indicates that exchange takes place via a simple hetero-exchange, if 16 O 2 species are formed firstly, it means multiple hetero-exchange mechanism [22]. Isotopic exchange is generally carried out in a recycle, close reactor coupled to a mass spectrometer [23,24].…”

“…Isotopic exchange is generally carried out in a recycle, close reactor coupled to a mass spectrometer [23,24]. The evolution of the partial pressures of 16 O 2 , 16 O 18 O and 18 O 2 during the exchange process are registered as a function of time in isothermal conditions. When the range of temperatures where the exchange process occurs is not well-known isothermal conditions could turn out to be tedious.…”