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 -alumina supported cobalt catalysts has been studied by temperature-programmed reduction (TPR). An intimate mixture of Co/Al₂O₃ and Re/Al₂O₃ catalysts showed a promoting effect of Re similar to that for coimpregnated Co-Re/Al₂O₃. A loose mixture of Co/Al₂O₃ + Re/Al₂O₃ did not show 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 and 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 temperature TPR peak at 1200 K assigned to cobalt aluminate is mainly a result of Co-ion diffusion during the TPR and not during calcination.