Reduction of chromia/alumina catalyst monitored by DRIFTS-mass spectrometry and TPR-Raman spectroscopy

Abstract

The reduction of alumina-supported chromia with 13 wt.% chromium was investigated by X-ray photoelectron and X-ray absorption spectroscopies, in situ temperature-programmed Raman spectroscopy and in situ temperature-programmed diffuse reflectance infrared Fourier transform spectroscopy combined with mass spectrometry. For comparison, unsupported chromia was also studied. The oxidised chromia/alumina contained Cr³⁺ and Cr⁶⁺, whereas after reduction by carbon monoxide or hydrogen mainly Cr³⁺ was detected, although the formation of Cr²⁺ in carbon monoxide reduction was not ruled out. The polymeric chromates on chromia/alumina were more reducible than the monomeric ones due to the weaker interaction of the polychromates with the support. During reduction with hydrogen at high temperatures, hydroxyl species formed on chromia and chromia/alumina. In addition, surface restructuring to more crystalline chromia species took place on chromia/alumina. Measurements on the reduction by carbon monoxide suggested that the reduction could occur through formation of bicarbonate species. With increasing temperature, formate species—probably formed by reaction of Cr³⁺–CO species with surface hydroxyls—were observed, which reacted further to carbonates on chromia, and inorganic carboxylates and other carbonaceous species on chromia/alumina.