Interactions between the chromium oxide phase and support surface; redispersion of alpha-chromia on silica, alumina and magnesia

The appearance of alpha-Cr2O3 on the support surface of Cr/SiO2 catalyst (ethene polymerization) suggests catalyst deactivation. This study evaluates (using TPR and XRD) the possible redispersion of the crystalline chromia on silica, alumina and magnesia during high-temperature oxygen treatment. Both wet impregnation and dry physical mixing preparations yielded active catalysts (Cr/SiO2) even when starting with alpha-Cr2O3. A chromium redispersion mechanism on oxide support surface is postulated. It involves the formation of mobile oxo species (CrO3) on crystalline chromia surface, followed by their migration on the support through chromia/support interface contacts, and finally their anchorage to suitable support adsorption sites resulting in stable monochromate-like species. The redispersion effects strongly depend on the adsorption capability of the support surface and the experimental conditions (temperature, atmosphere and traces of water).