Size and Promoter Effects in Supported Iron Fischer–Tropsch Catalysts: Insights from Experiment and Theory

The fundamentals of structure sensitivity and promoter effects in the Fischer–Tropsch synthesis of lower olefins have been studied. Steady state isotopic transient kinetic analysis, switching 12CO to 13CO and H2 to D2, was used to provide coverages and residence times for reactive species on supported iron carbide particles of 2–7 nm with and without promoters (Na + S). CO coverages appeared to be too low to be measured, suggesting dissociative adsorption of CO. Fitting of CH4 response curves revealed the presence of parallel side-pools of reacting carbon. CHx coverages decreased with increasing particle size, and this is rationalized by smaller particles having a higher number of highly active low coordination sites. It was also established that the turnover frequency increased with CHx coverage. To calculate H coverages, new equations were derived to fit HD response curves, again leading to a parallel side-pool model. The H coverages appeared to be lower for bigger particles. The H coverage was suppress...