Some Enduring Mechanistic Puzzles in the Fischer-Tropsch Synthesis Revisited

Density functional theory calculations and kinetic, spectroscopic, and isotopic experiments are combined here to address long-standing mechanistic puzzles in Fischer-Tropsch synthesis on Co and Ru-based catalysts: (i) the ability of low-index metal surfaces to activate CO at modest temperatures, high CO pressures, and thus high coverages of chemisorbed CO*; (ii) the increase in turnover rates and decrease in methane selectivity at higher water concentrations; (iii) the infrequent formation and rapid growth of hydrocarbon chains. Low-index surfaces exhibit higher reactivity as inferred from particle size studies despite the high barriers they exhibit for direct CO* activation because such routes are avoided in practice by more facile CO dissociation routes mediated by the addition of chemisorbed H-atoms before C–O cleavage to form *HCOH* species. These metal surfaces are covered by CO* and the densification of the CO* adlayer favors the dissociation of CO* because adlayer crowding preferentially destabilizes chemisorbed CO* relative to the *HCOH*-forming transition state as this reaction has a negative activation area. This H-assisted CO*-activation route, therefore, becomes facile at the moderate temperatures and large CO pressures typical of Fischer-Tropsch synthesis. Water, present at high concentrations in industrial reactors with large single-pass conversions, increases the rate of CO* activation by facilitating the H*-assisted CO* activation routes through proton-coupled electron transfer mechanisms for the addition of such H-atoms to chemisorbed CO. The rapid growth of few chains within dense CO adlayers reflect local disruptions of the CO adlayer around growing chains, which also facilitates CO* activation to form monomeric CH_x* species at sites vicinal to growing chains, thus avoiding high diffusional barriers of monomers through CO-saturated surfaces. These outstanding puzzles related to the ubiquitously observed effects of particle size, conversion, and chain growth patterns, therefore, are all resolved by a careful understanding of how CO* is activated on low-index surfaces through H*-assisted pathways that are facilitated by high CO* coverages, high H₂O concentrations, and CO*-adlayer disruption near growing hydrocarbon chains.