Oxidative dehydrogenation of isobutane on Cr–Ce–O oxide: II. Physical characterizations and determination of the chromium active species Original Research Article

Cr–Ce–O catalysts with different Cr contents and prepared by different methods have been characterized by X-ray diffraction (XRD), XPS, ISS and UV–Vis spectroscopies, and the surface potential technique. On the basis of these studies, a model of catalysts has been proposed and the correlations between the structure of the catalysts and their performance in the oxidative dehydrogenation (ODH) of isobutane have been discussed. At low Cr content corresponding to the coverage of the ceria surface with chromium θ<35%, as determined by the ISS method, Cr6+Ox species are the only Cr species, irrespective of the preparation method. In the θ range 35–61%, well dispersed Cr3+ species appear as Cr2O3. The maximum coverage of 75% is obtained for the samples prepared by impregnation at 20 Cr atoms per nm2. From this Cr content, agglomeration of Cr2O3 is observed. The specific rate of isobutene formation increases with the increase in Cr content up to the coverage of about 61% and then remains constant. It is concluded that the activity is due to the well dispersed Cr6+Ox species on the ceria surface, whereas agglomerates of Cr2O3 do not contribute to the reaction.