Support-dependent Performance of Size-selected Subnanometer Cobalt Cluster-based Catalysts in the Dehydrogenation of Cyclohexene

Abstract

The evolution of the chemical state and change in the morphology of subnanometer cobalt clusters during the dehydrogenation of cyclohexene was investigated in terms of metalsupport interactions. The model catalyst systems were prepared by deposition of size selected subnanometer Co27 4 clusters on various amorphous metal oxide supports (Al2O3, 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 incidence X-ray absorption spectroscopy (GIXAS), and grazing incidence small angle X-ray scattering (GISAXS), respectively. The reactivity and selectivity of cobalt clusters show strong dependency on the support used, with clusters supported on UNCD possessing the highest activity at 3008C. The evolution of the oxidation state of metal cluster during the reaction reveals that metal-support interaction plays a key role in performance of the subnanometer catalyst. A reversible assembly of clusters into a nanostructure which evolves with reaction temperature was observed on the MgO support.