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 CrO_x/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.

Claus Lugmair

Clariant Corp

Appearances