DOI: 10.1002/cctc.201200294 Support-dependent Performance of Size-selected Subnanometer Cobalt Cluster-based Catalysts in the Dehydrogenation of Cyclohexene Sungsik Lee, [a] Marcel Di Vece, [b] Byeongdu Lee, [a] Sçnke Seifert, [a] Randall E. Winans, [a] and Stefan Vajda* [b, c] Introduction Catalytic dehydrogenation of cyclic hydrocarbons is an impor- tant refining and reforming process. [1] Recently, cyclohexane has gained significant attention as a potential hydrogen stor- age and transportation source for fuel-cell applications. Cyclo- hexane has a higher hydrogen storage potential than the other candidates, e.g., compressed hydrogen, metal hydride and carbon materials. Its hydrogen storage density (7.2 wt %) is higher than the benchmark for the amount of reversible hy- drogen (6.5 wt %) targeted by the US Department of Energy (DOE). To develop a new catalyst for cyclohexane conversion, and to elucidate the complete reaction mechanism, it is necessary to understand the dehydrogenation step related to the cyclo- hexene intermediate. As the rate determining step in the dehy- drogenation of cyclohexane to benzene is the dehydrogena- tion of the cyclohexene intermediate including the adsorbed C 6 H 9 species, it consequently requires a specific orientation of the adsorbed cyclohexene relative to the catalyst surface. [2] Therefore, dehydrogenation of cyclohexene has been used for kinetic and structure–function studies in the studies of the de- hydrogenation of cyclohexane. [3] A reaction mechanism has been proposed that s-adsorption is followed by a stepwise elimination of hydrogen from cyclohexane to cyclohexene and cyclohexadiene intermediates. The p–s shift of the adsorbed cyclohexene has been identified as a rate determining step. [4] Adsorption and reaction studies carried out using cyclohexane, cyclohexene, 1,3-cyclohexadiene, and benzene on the Pt(111) surface also suggest that the other steps involved in the dehy- drogenation to benzene, that is, cyclohexane conversion to cy- clohexene and dehydrogenation of 1,3-cyclohexadiene to ben- zene, all proceed rapidly. [2a] Owing to the high activity and selectivity of platinum [5] and platinum alloys, [3b, 5a, 6] these catalysts are primarily used in these studies. However, non-precious metal catalysts have drawn more attention recently [1, 3a, 6, 7] with cobalt emerging as one of the promising candidates for the dehydrogenation of cyclohexane. [8] Also, cobalt is known as a good oxidation cata- lyst for example in the transformation of cyclohexane to KA oil (cyclohexanone/cyclohexanol mixture) [9] Few studies have been reported on cobalt based cyclohexene dehydrogenation catalysis. [10] Our developments of novel precious as well as non-precious metal catalysts by tuning catalyst performance through control of the size of the clusters and support properties have demon- strated the feasibility of this approach towards the discovery of new catalyst systems. [11] To develop tailored catalysts, it is The evolution of the chemical state and change in the mor- phology of subnanometer cobalt clusters during the dehydro- genation of cyclohexene was investigated in terms of metal- support interactions. The model catalyst systems were pre- pared by deposition of size selected subnanometer Co 27 4 clusters on various amorphous metal oxide supports (Al 2 O 3 , ZnO, and MgO), as well as on a carbon-based support (UNCD = ultrananocrystaline diamond). The reactivity, oxidation state, and sintering resistance of the clusters were monitored by temperature programmed reaction (TPRx), in situ grazing in- cidence X-ray absorption spectroscopy (GIXAS), and grazing in- cidence small angle X-ray scattering (GISAXS), respectively. The reactivity and selectivity of cobalt clusters show strong de- pendency on the support used, with clusters supported on UNCD possessing the highest activity at 300 8C. The evolution of the oxidation state of metal cluster during the reaction re- veals that metal-support interaction plays a key role in perfor- mance of the subnanometer catalyst. A reversible assembly of clusters into a nanostructure which evolves with reaction tem- perature was observed on the MgO support. [a] Dr. S. Lee, Dr. B. Lee, Dr. S. Seifert, Dr. R. E. Winans X-ray Science Division Argonne National Laboratory 9700 South Cass Avenue, Argonne, IL 60439 (USA) [b] Dr. M. Di Vece, Dr. S. Vajda Department of Chemical and Environmental Engineering School of Engineering Yale University 9 Hillhouse Avenue, New Haven, CT 06520 (USA) [c] Dr. S. Vajda Materials Science Division & Center for Nanoscale Materials Argonne National Laboratory 9700 South Cass Avenue, Argonne, IL 60439 (USA) Fax: (+ 1) 630-252-4954 E-mail : vajda@anl.gov Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200294. ChemCatChem 0000, 00, 1 – 7  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ