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.