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 cataly st (ethene polymerization) suggests catalyst deactivation. This study evaluates (using TPR and XRD) the possible redispersion of the crystal line chromia on silica, alumina and magnesia during high-temperature o xygen treatment. Both wet impregnation and dry physical mixing prepara tions 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 suppor t through chromia/support interface contacts, and finally their anchor age to suitable support adsorption sites resulting in stable monochrom ate-like species. The redispersion effects strongly depend on the adso rption capability of the support surface and the experimental conditio ns (temperature, atmosphere and traces of water).