Surface reduction of some transition-metal oxides. An X-ray photoelectron spectroscopic study of iron, cobalt, nickel and zinc oxides

Abstract
X-Ray photoelectron spectroscopy has been used to examine the effect of heating under high-vacuum conditions of some transition-metal oxides. Nickel and cobaltic oxides were found to undergo reduction to Ni metal and CoO below 300 °C. This reduction is shown to be caused by reaction with surface carbon-containing contaminants. Removal of this contamination by an in situ oxidation treatment resulted in a well defined surface which was stable to further heating. High-surface-area NiO underwent more extensive reduction than low-surface-area material. In contrast, ZnO was not reduced and only minor changes in the Fe2O3 spectra were observed. The results are explained by reference to bulk thermodynamic data. The findings are important particularly for characterisation of oxide surfaces used in catalytic studies.

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