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

Abstract

The appearance of α-Cr ₂ O ₃ on the support surface of Cr/SiO ₂ 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/SiO ₂ ) even when starting with α-Cr ₂ O ₃ . A chromium redispersion mechanism on oxide support surface is postulated. It involves the formation of mobile oxo species (CrO ₃ ) 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).