Oxidative dehydrogenation of isobutane on Cr-Ce-O oxide II. Physical characterizations and determination of the chromium active species

Cr-Ce-O catalysts with different Cr contents and prepared by different meth ods 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 betwe en the structure of the catalysts and their performance in the oxidative de hydrogenation (ODH) of isobutane have been discussed. At low Cr content cor responding to the coverage of the ceria surface with chromium theta <35%, a s determined by the ISS method, Cr6+Ox, species are the only Cr species, ir respective of the preparation method. In the theta range 35-61%, well dispe rsed Cr3+ species appear as Cr2O3. The maximum coverage of 75% is obtained for the samples prepared by impregnation at 20 Cr atoms per nm(2). From thi s Cr content,agglomeration of Cr2O3 is observed. The specific rate of isobu tene 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. (C) 2000 Elsevier Science B.V. All rights reserved.