Investigating the CrOx/Al2O3 dehydrogenation catalyst model: I. identification and stability evaluation of the Cr species on the fresh and equilibrated catalysts

In this study a series of the fresh and equilibrated CrOx/Al2O3 catalysts with the Cr loading from 0.58wt% to 13.2wt% were in detail studied using combined techniques, including Raman spectroscopy, TEM, XPS, O-2 chemisorption, TPR, XRD and chemical analysis, to identify surface chromium species and to evaluate their stability under the cyclic dehydrogenation-regeneration (redox) conditions. The study showed that the stable chromium species present in the equilibrated CrOx/Al2O3 catalyst can be achieved through complete surface restructuring of initial chromium species in the fresh one under the redox conditions. This study also showed that the fresh CrOx/Al2O3 catalyst contained not only previously known chromium species of 'isolated' mono/poly aluminum chromate(s), amorphous and crystalline Cr2O3 clusters with different sizes, but also chromium-chromate (Cr6+) species on the surface of Cr2O3 clusters and aluminum chromium-chromate (Cr6+) species coupled with Cr2O3 clusters. The 'isolated' aluminum chromate(s) and small Cr2O3 clusters on the catalyst surface were not stable under the cyclic redox conditions and disappeared after the break-in-period of the catalyst operation. The paper presents the mechanism of the surface restructuring of chromium species under the redox conditions and the revised model for the CrOx/Al2O3 catalyst with the consideration of the change of surface chromium species caused by the cyclic redox aging. The model can be applied to the commercial CrOx/Al2O3 catalysts. (C) 2016 Elsevier B.V. All rights reserved.