Characterization of chromia/alumina catalysts by X-ray photoelectron spectroscopy, proton induced X-ray emission and thermogravimetric analysis

Chromia/alumina catalysts with different metal loading were characterized using X-ray photoelectron spectroscopy (XPS), proton induced X-ray emission (PIXE) and thermogravimetric (TG) techniques to elucidate the surface structure of these catalysts. XPS studies on calcined samples show a sharp increase of the Cr/Al ratio at calcination temperatures up to 500°C while the ratio remains relatively unchanged at higher calcination temperature. The surface state of chromium shows predominantly Cr⁶⁺. At calcination temperatures higher than 500°C, calcination-induced reduction is observed of the Cr⁶⁺ to Cr³⁺, where the fraction of chromia in Cr³⁺ oxidation state increases with increasing temperature. A progressive increase of the intensity of the peak due to Cr 2p of the Cr³⁺ oxidation state is also observed with increasing amount of metal loading. The calcination-induced reduction of the alumina-supported chromia was found to be less than the corresponding reduction of bulk CrO₃. Also, the size of the spin-orbit splitting of the Cr 2p level of chromia catalysts which had undergone calcination-induced reduction was found to be smaller than would be expected for bulk Cr³⁺. The XPS spectra of chromium on the Cr/Al catalysts were found to be time dependent. Photoreduction of Cr⁶⁺ on Cr/Al samples was found for irradiation times longer than 4.0 min. It was found from PIXE analysis that at higher calcination temperature, the Cr/Al atomic ratio approaches the values obtained by XPS. For all samples, the chromium particles were found to be homogeneously distributed on the alumina support for calcination temperatures up to 800°C. Thermogravimetric results on uncalcined bulk CrO₃ agree well with the XPS observation as to the fact that the main phase transformation of Cr⁶⁺ compounds occurs at about 500°C, resulting in reduction to Cr³⁺.