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. CH_x 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 CH_x 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 suppressed upon addition of promoters in line with lower methane selectivity and higher lower olefin selectivity. Density functional theory (DFT) was applied on H adsorption for a fundamental understanding of this promoter effect on the selectivities, with a special focus on counterion effects. Na₂S is a better promoter than Na₂O due to both a larger negative charge donation and a more effective binding configuration. On the unpromoted Fe₅C₂ (111) surface, H atoms bind preferably on C after dissociation on Fe. On Na₂S-promoted Fe₅C₂ surfaces, adsorption on carbon sites weakens, and adsorption on iron sites strengthens, which fits with lower H coverage, less CH₄ formation, and more olefin formation.