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

Abstract

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 12 CO to 13 CO and H 2 to D 2, 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 CH 4 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. Na 2 S is a better promoter than Na 2 O due to both a larger negative charge donation and a more effective binding configuration. On the unpromoted Fe 5 C 2 (111) surface, H atoms bind preferably on C after dissociation on Fe. On Na 2 S-promoted Fe 5 C 2 surfaces, adsorption on carbon sites weakens, and adsorption on iron sites strengthens, which fits with lower H coverage, less CH 4 formation, and more olefin formation.