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

Abstract

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, O2 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.