Combined DRS–RS–EXAFS–XANES–TPR study of supported chromium catalysts
The surface chemistry of supported chromium catalysts (Cr/SiO2· Al2O3) has been systematically investigated as a function of the support composition (Si:Al ratio) and the Cr oxide loading by a combination of diffuse reflectance spectroscopy (DRS), Raman spectroscopy (RS), X-ray Absorption spectroscopy (EXAFS–XANES) and temperature-programmed reduction (TPR). Combination of all the obtained results by these characterization techniques leads to a uniform interpretation and general picture of surface Cr. On hydrated surfaces, the molecular structure of the Cr oxide species depends on the isoelectric point of the oxide support and the Cr loading: more polymerized Cr oxide species correspond to higher Cr loading and silica content of the support. After calcination, the Cr oxide species are anchored onto the surface by reaction with surface hydroxy groups of the supports. On alumina the reaction starts with the most basic OH groups on alumina. This suggests that the anchoring process is an acid–base reaction. On calcined surfaces, the polymerization of the anchored Cr oxide species and the amount of Cr2O3 clusters increases with silica content and Cr loading. Reduced Cr samples possess both Cr2+ and Cr3+, the relative concentrations of which are support and loading dependent: lower Cr2+:Cr3+ ratios correspond to higher Cr loading and alumina content of the support.