Oxidative dehydrogenation of cyclohexene on size selected subnanometer cobalt clusters: improved catalytic performance via evolution of cluster-assembled nanostructures.

ABSTRACT

The catalytic activity of oxide-supported metal nanoclusters strongly depends on their size and support. In this study, the origin of morphology transformation and chemical state changes during the oxidative dehydrogenation of cyclohexene was investigated in terms of metal-support interactions. Model catalyst systems were prepared by deposition of size selected subnanometer Co(27±4) clusters on various metal oxide supports (Al(2)O(3), ZnO and TiO(2) and MgO). The oxidation state and reactivity of the supported cobalt clusters were investigated by temperature programmed reaction (TPRx) and in situ grazing incidence X-ray absorption (GIXAS) during oxidative dehydrogenation of cyclohexene, while the sintering resistance monitored with grazing incidence small angle X-ray scattering (GISAXS). The activity and selectivity of cobalt clusters shows strong dependence on the support. GIXAS reveals that metal-support interaction plays a key role in the reaction. The most pronounced support effect is observed for MgO, where during the course of the reaction in its activity, composition and size dynamically evolving nanoassembly is formed from subnanometer cobalt clusters.