Stable subnanometer cobalt oxide clusters on ultrananocrystalline diamond and alumina supports: Oxidation state and the origin of sintering resistance

The composition and stability of oxidized cobalt subnanometer clusters composed of four metal atoms supported on ultrananocrystalline diamond (UNCD) and alumina surfaces were studied using a combination of grazing-incidence X-ray absorption near-edge spectroscopy (GIXANES), grazing incidence small-angle X-ray scattering (GISAXS), and density functional calculations. GIXANES data revealed partially oxidized subnanometer cobalt clusters upon exposure to air, with similarity in the total degree of oxidation on both supports. The clusters were exposed to elevated temperatures of up to 300 °C under pure helium as well as oxygen and were found by GISAXS to be agglomeration resistant, whereas GIXANES showed the preservation of the composition of clusters during the heat treatment. Density functional calculations of cluster binding to model surfaces for UNCD and alumina were performed. The calculations indicate that the stability of the cobalt oxide clusters on UNCD is the result of electrostatic and dispersive interactions for the pristine hydrogen-terminated surfaces and covalent bonding between the cluster and defect sites on the surfaces. On alumina the origin of the stability is interactions between the cobalt and surface oxygens or the cluster oxygens with the surface aluminum atoms. These properties indicate that oxidized subnanometer cobalt clusters supported on UNCD and alumina are suitable candidate hybrid nanostructures for use as supported catalysts.