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 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.