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The Co 3 O 4 (110) single crystal surface has been characterized by low energy electron diffraction ͑LEED͒, Auger electron spectroscopy, and x-ray photoelectron spectroscopy ͑XPS͒. LEED analysis of the clean Co 3 O 4 (110) spinel surface shows a well-ordered pattern with sharp diffraction features. The XPS spectra are consistent with stoichiometric Co 3 O 4 as determined by the concentration ratio of oxygen to cobalt (C O /C Co ) and spectral peak shape. In particular, the cobalt 2p XPS spectra are characteristic of the spinel structure with Co 3ϩ occupying octahedral sites and Co 2ϩ in tetrahedral sites within the lattice. During prolonged heating at 630 K, bulk impurities of K, Ca, Na, and Cu segregated to the surface. Sodium desorbed from the surface as NaOH at 825 K, potassium and calcium were only removed by sputtering since no desorption from the surface was detected for temperatures up to 1000 K. Copper also disappeared upon heating above 700 K, most likely by desorbing although the possibility of diffusion back into the bulk could not be eliminated. The appearance of copper impurities correlated with Co 3 O 4 (110) surface reduction to CoO, and the surface could not be fully reoxidized even upon extended oxygen annealing as long as the copper impurity remained on the surface. Upon removal of the Cu from the near-surface region, the surface was easily reoxidized to Co 3 O 4 by O 2 .

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