Cr/Al2O3 is one of the most commonly used industrial catalysts for propane dehydrogenation. Despite its ubiquitous use, there are still uncertainties surrounding the exact nature of the numerous Cr species in this system. In one of our recent studies of CrOx/Al2O3 catalysts, we observed that only a ppm level of Cr2O3 can be oxidized to Cr(VI) by air at 600°C forming chromium chromate, Cr3+2Cr6+3O12. However, when Cr2O3 was in direct contact with alumina oxide during the oxidation treatment, the rate of the CrO5O12 formation was increased by more than 100 times. Typically, Cr(VI) is readily reduced to Cr(III) in the CrOx/Al2O3 system, but the samples we studied were prepared in such a way to increase the levels of Cr(VI) so that the chromatic species could be studied Based on this result, it is suggested that the oxidation of Cr2O3 at the interface of Al2O3 should lead to formation of chromium-aluminum chromate, Al3+xCr3+2-xCr6+3O12. To prove or disprove this assumption, in this study a physical mixing of Cr2O3 and Al2O3 is investigated to identify the formation of Cr(VI) species and the local structure they occupy. Calcination of physically mixed Cr2O3 and Al2O3 results in samples containing 1.4 wt% Cr(VI), as determined by wet chemical analysis, whereas the uncalcined analogues contain less than 10 ppm Cr(VI), suggesting that the Cr2O3 and Al2O3 interface is critical for the formation of Cr(VI) species. X-ray absorption spectroscopy was used to determine the structure of the Cr(VI) species formed during calcination. Linear combination fitting results identified that approximately 1-2 wt% Cr(VI) was formed, matching the wet chemical analysis, though the structure and phase of the Cr(VI) could not be identified with certainty. Difference file analysis confirmed that Cr5O12 was the most probable structure in which the Cr(VI) species existed. FEFF XANES simulations are performed to show variations in the theoretical XANES spectra accounting for the potential formation of an aluminum-chromium- chromate species, Al3+xCr3+2-xCr6+3O12.

Identification of chromium oxide species in Cr/Al2O3 propane dehydrogenation catalysts

ACS Spring 2022

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