Heats of chemisorption on promoted iron surfaces and the role of alkali in Fischer-Tropsch synthesis

The heats of adsorption of CO, CO2 and H2 on unpromoted, on singly and on doubly promoted reduced magnetite were determined calorimetrically. The heats of CO adsorption are similar to those reported on evaporated iron films. The heats of hydrogen adsorption are lower than on iron films but nevertheless similar to those reported on reduced iron oxide samples. Promotion with only MgO does not markedly influence the heats of adsorption of any of the above gases. Promotion with K2O increases the heat of CO adsorption at low coverages while it decreases the initial heats of hydrogen adsorption. The K2O markedly increases the heat of adsorption of CO2 at all coverages. The CO2 appears to chemisorb not only on the surface alkali but also on the metallic iron. The known influence of K2O promotion on both the activity and selectivity of the Fischer-Tropsch synthesis is explained in terms of the observed influence of K2O on the heats of adsorption of CO and of hydrogen on iron catalysts. The postulates are also applied to the case of the ammonia synthesis.