We report here theoretical evidence for an enhancement in CO activation to form C1 monomers at locations near growing hydrocarbon chains as a result of their ability to disrupt the dense monolayers of chemisorbed CO∗ present during Fischer-Tropsch synthesis (FTS). These previously unrecognized routes become favored at the high CO∗ coverages that prevail on curved cluster surfaces at conditions of FTS practice and account for the rapid growth of chains, which requires a source of vicinal monomers. CO activation initially requires a vacant site (and consequently CO∗ desorption) and proceeds via CO∗ reactions with H∗ to form hydroxymethylene (CH∗OH∗), which then dissociates to form OH∗ and CH∗; CHx∗ species can subsequently act as monomers and insert into chains, a process denoted as the 'carbene' mechanism. These CH∗, and their larger alkylidyne (CnH2n-1∗) homologs, disrupt the dense CO∗ adlayers and in doing so allow the facile formation of vicinal CH∗OH∗ intermediates that mediate CO activation, without requiring, in this case, CO∗ desorption. This causes CO∗ activation effective enthalpy and free energy barriers to be ∼100 and ∼15 kJ mol-1 lower, respectively, near growing chains than within unperturbed monolayers. These effects are observed near alkylidyne (CnH2n-1∗) but not alkylidene (CnH2n∗) or alkyl (CnH2n+1∗) chains. These phenomena cause monomers to form preferentially near growing alkylidyne chains, instead of forming at undisrupted regions of CO∗ monolayers, causing chain growth (via CHx∗-insertion) to occur much more rapidly than chain initiation, a requirement to form long chains. Such routes resolve the seemingly contradictory proposals that CHx∗ species act as monomers (instead of CO∗) and chain initiators, but their formation and diffusion on dense CO∗ adlayers must occur much faster than chain initiation for such chains to grow fast and reach large average lengths. Chains disrupt surrounding molecules in the adlayer, causing faster monomer formation precisely at locations where they can readily react with growing chains. This work illustrates how interactions between transition states and co-adsorbates can dramatically affect predicted rates and selectivities at the high coverages relevant to practical catalysis.
Bibliographical noteFunding Information:
The authors acknowledge the financial support and computational resources provided by BP through the X-Conversion Consortium (BP-XC 2 ) . We thank Drs. Craig Plaisance, Corneliu Buda, Jay Labinger, John Shabaker, and George Huff for helpful technical discussions. Computational resources were also provided by the Molecular Science Computing Facility (MSCF) in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy, Office of Biological and Environmental Research at the Pacific Northwest National Laboratory.
- C-C bond formation
- CO activation
- Co-adsorbate interactions
- Coverage effects
- Fischer-Tropsch synthesis