Dehydrogenation of i-butane on CrOx /Al2O3 catalysts prepared by ALE and impregnation techniques

Atomic layer epitaxy (ALE), a technique relying on saturating gas-solid reactions, was applied in the preparation of CrOx/Al2O3 catalysts using Cr(acac)(3) vapor and air as source materials for CrOx. Vaporized Cr(acac)(3) was reacted with preheated Al2O3, and the surface complex formed was treated with air to remove the ligand residues. The Cr loading increased from 1.3 to 12.5 wt.% as the number of saturating Cr(acac)(3) and air reactions was increased from one to 10. CrOx/Al2O3 catalysts were also prepared from solution by incipient wetness impregnation (0.3-21 wt.%). XPS and UV-VIS measurements of the catalysts revealed the presence of both Cr6+ and Cr3+. Although the oxidation state distribution was similar, H-2-temperature programmed reduction (TPR) and solubility measurements indicated that Cr6+ surface sites were in stronger interaction with Al2O3 and more uniformly distributed in the catalysts prepared by ALE than by impregnation. On the basis of the activity of the catalysts in the dehydrogenation of i-butane, we propose that the dehydrogenation reaction uses both reduced Cr6+, i.e. redox Cr3+, and exposed non-redox Cr3+ sites. Furthermore, the dehydrogenation reaction must be insensitive to the size of the CrOx ensembles since activities were similar for the catalysts: prepared by ALE and impregnation. The decay of the dehydrogenation activity in successive prereduction-reaction-regeneration cycles was attributed to a decrease in the number of redox Cr3+ sites. (C) 2000 Elsevier Science B.V. All rights reserved.