TPR STUDY OF THE MECHANISM OF RHENIUM PROMOTION OF ALUMINA-SUPPORTED COBALT FISCHER-TROPSCH CATALYSTS

The effect of small amounts of Re on the reduction properties of gamma -alumina supported cobalt catalysts has been studied by temperature-pr ogrammed reduction (TPR). An intimate mixture of Co/Al2O3 and Re/Al2O3 catalysts showed a promoting effect of Re similar to that for coimpre gnated Co-Re/Al2O3. A loose mixture of Co/Al2O3 + Re/Al2O3 did not sho w any effect of Re on the reduction of cobalt. However, when the loose mixture of Co/Al2O3 + Re/Al2O3 was pretreated with Ar saturated with water before the TPR, a promoting effect of Re on the reduction of Co was observed. It is suggested that Re promotes the reduction of cobalt oxide by hydrogen spillover, and that no direct contact between Re an d Co seems to be necessary in order to obtain the promoting effect as observed by TPR. It is also shown that the presence of a high temperat ure TPR peak at 1200 K assigned to cobalt aluminate is mainly a result of Co-ion diffusion during the TPR and not during calcination.