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Growth, sintering, and chemical states of Co supported on reducible CeO2(111) thin films: The effects of the metal coverage and the nature of the support.

The growth, sintering, and interaction of cobalt with ceria were studied under ultrahigh vacuum conditions by vapor-deposition of Co onto well-defined CeOx(111) (1.5 < x < 2) thin films grown on Ru(0001). Charge transfer from Co to ceria occurs upon deposition of Co on CeO1.96 and partially reduced CeO1.83 at 300 K. X-ray photoelectron spectroscopy studies show that Co is oxidized to Co2+ species at the cost of the reduction of Ce4+ to Ce3+, at a lesser extent on reduced ceria. Co2+ is the predominant species on CeO1.96 at low Co coverages (e.g., ≤0.20 ML). The ratio of metallic Co/Co2+ increases with the increase in the Co coverage. However, both metallic Co and Co2+ species are present on CeO1.83 even at low Co coverages with metallic Co as the major species. Scanning tunneling microscopy results demonstrate that Co tends to wet the CeO1.96 surface at very low Co coverages at room temperature forming one-atomic layer high structures of Co-O-Ce. The increase in the Co coverage can cause the particle growth into three-dimensional structures. The formation of slightly flatter Co particles was observed on reduced CeO1.83. In comparison with other transition metals including Ni, Rh, Pt, and Au, our studies demonstrate that Co on ceria exhibits a smaller particle size and higher thermal stability, likely arising from strong metal-support interactions. The formed particles upon Co deposition at 300 K are present on the ceria surface after heating to 1000 K. The Co-ceria interface can be tuned by varying the Co metal coverage, the annealing temperature, and the nature of the ceria surface.