Chromia/zirconia catalysts with Cr content exceeding the monolayer. A comparison with chromia/alumina and chromia/silica for isobutane dehydrogenation Original Research Article

CrOx/ZrO2, CrOx/γ-Al2O3 and CrOx/SiO2 catalysts were prepared by wet impregnation in the loading range 0.05–14 atoms Cr nm−2, that is below and above the monolayer capacity of support. The samples were characterized by chemical analysis, including carbon monoxide reduction and CrVI extraction with sodium hydroxide or water, and by XRPD, UV-Vis DRS and ESR. Several Cr species were identified, namely CrVI, CrV (ESR γ-signal), CrIII (ESR δ-signal), small CrIII oxide clusters (ESR β-signal) and α-chromia. The relative abundance of these Cr species was found to be mainly controlled by the specific support employed and the Cr concentration. The catalytic activity in the isobutane dehydrogenation was studied at 723 and 773 K in a flow microreactor. It was found that the activity per total Cr atom was markedly higher for the zirconia-based catalysts compared with the other two systems in all the explored Cr concentration range. That is, the CrOx/ZrO2 interaction maintains its positive effect well beyond the monolayer capacity of this support. Moreover, the catalytic activity correlated with the amount of high-oxidation state chromium (CrVI and CrV), evaluated by the quantity of extractable Cr or by the consumption of carbon monoxide in the catalyst reduction. The larger concentration of mononuclear CrV species on zirconia, confirmed here, is thought to be responsible for the higher activity of the zirconia-based catalysts, through reduction to active mononuclear CrIII species under reaction conditions.