551221 Some Enduring Mechanistic Puzzles in the Fischer-Tropsch Synthesis Revisited

Wednesday, June 5, 2019: 10:54 AM
Texas Ballroom EF (Grand Hyatt San Antonio)
David Hibbitts1, Jianwei Liu2, Hale Ay2, Brett Loveless2, Matthew Neurock3 and Enrique Iglesia4, (1)Department of Chemical Engineering, University of Florida, Gainesville, FL, (2)Chemical and Biomolecular Engineering, University of California, Berkeley, CA, (3)Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, (4)Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, CA

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 CHx* 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.

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