ALKALI PROMOTION OF ALUMINA-SUPPORTED COBALT FISCHER-TROPSCH CATALYSTS STUDIED BY TPR, TPD AND PULSE CHEMISORPTION

Supported cobalt catalysts are important in several hydrogenation reactions, e.g. hydrodesulfurization and the Fischer-Tropsch synthesis. This work deals with the effect of adding potassium to supported cobalt Fischer-Tropsch catalysts. The catalysts were studied by temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of hydrogen and by pulse chemisorption of hydrogen. The catalyst activity in the Fischer-Tropsch synthesis was also measured. The spectrum of unpromoted catalysts shows two peaks, at ca. 675 and 975 K. Low-level potassium addition does not change this much, whereas higher potassium levels give rise to additional TPR peaks. Owing to the activated nature of hydrogen chemisorption on supported cobalt, hydrogen chemisorption was studied by TPD and by pulse chemisorption at 298, 373 and 473 K. On all catalysts there was only a very small amount adsorbed at 298 K. The hydrogen uptake was highest at 373 K, but lower again at 473 K. This indicates a balance between the activation effect of adsorption and the rate of desorption from the surface. The catalyst activity and selectivity changed little with low levels of potassium, whereas the highest potassium level gave an almost inactive catalyst.