X-Ray Photoelectron Spectroscopic Studies of Catalysts—Chromia–Alumina Catalysts—

Abstract

The XPS studies of chromia–alumina catalysts were carried out to elucidate the surface of these catalysts. Photoreduction was found to occur during the XPS measurements. Therefore, all spectra were measured rapidly to prevent any photoreduction. It was found that the information about the spin-orbit splitting of the Cr2P level is very useful for determining the valence state of chromium. The surface state of chromium in chromia–alumina catalysts calcined at 500 °C for 5 h is predominantly Cr⁶⁺ at low chromia concentration catalysts. However, the Cr³⁺ phase appears in catalysts of higher chromium content, and the surface concentration of Cr³⁺ increases with increasing content of supported chromia. The XPS spectra due to Cr⁵⁺ were observed after mild reduction of the catalysts. Moreover, it was found that almost all surface chromium is present as Cr⁵⁺ at low chromia content catalysts after mild reductions. Qualitatively, the results obtained by the XPS technique, which gives information about the surface of catalysts, agree well with the results obtained by ESR and other techniques, which gives information about the bulk of catalysts, especially low chromium concentration catalysts. These findings indicate a high dispersion of chromium on γ-Al₂O₃ for low chromium content catalysts.