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Magnetic and optical studies were made on chromium trioxide supported at various concentrations on a porous alumina, the impregnated samples having been calcined at different temperatures over the range 50-1 150°C. The measurements of magnetic susceptibility and electron paramagnetic resonance spectra were carried out at controlled temperatures from -196 to 300°C. Optical reflectance spectra were obtained at room temperature on selected samples. The results suggest that clustering of the chromium oxide occurs, even at the lowest concentration studied (1.8 wt % Cr) ; heat treatment in air, or reaction with hydrogen, leads to the formation of Cr3+ ions, which are coupled by strong exchange interactions. Both processes of reduction occur more readily as the concentration of chromium oxide is increased.

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The Cr-silica catalyst system is commercially important in the polymerisation of ethene. The nature of the chromium species present on the support following activation, as well as the nature of the polymerisation active oxidation state, have been investigated extensively but results have not always been conclusive. This investigation attempts to identify the nature of the surface species and the oxidation state of the chromium on both the activated and hydrogen-reduced samples. Temperature-programmed reduction was employed to identify the number of surface species on the support. These species were identified with the aid of EPR and UV-VIS diffuse reflectance spectroscopy.The surfaces of the activated Cr-silica catalysts were generally covered by chromate-like surface species, accompanied by dichromate-like species and chromium aggregates, the proportions of which were dependent upon chromium loading and activation conditions. These samples were mostly oxidised to Crv' during activation with traces of Crlll remaining after the treatment. Samples which were reduced in hydrogen indicated that reduction of CrV' to Crlll had occurred, but traces of Crv' always remained. The reduction process was accompanied by an increase in the amount of aggregation of the chromium.