Grand Canonical Monte Carlo Simulation of CO Adsorption on Cobalt(0001) in Near- and Super-critical CO/Hexane Mixtures: Relevance to Fischer–Tropsch Synthesis

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Grand canonical Monte Carlo (GCMC) simulations have been used to determine the adsorption thermodynamics of carbon monoxide (CO) and hexane on cobalt(0001) in the presence of near- and super-critical fluid phase CO/hexane mixtures at 523.15 K, a common supercritical fluid Fischer–Tropsch synthesis reaction temperature. GCMC results indicate that CO preferentially adsorbs on the catalyst surface relative to hexane, with surface coverages ranging from 0.9 to 1.5 monolayers, due to the strong chemisorption reaction energetics. Excess adsorption analysis indicates depletion of CO on the catalyst surface (relative to the bulk fluid mixture) at large values of bulk CO chemical potential, which correlates with more liquid-like fluid densities and larger fluid phase CO mole fractions. At smaller bulk CO chemical potentials within the fluid phase mixture, enhanced excess CO adsorption is largely observed. At smaller bulk CO and hexane chemical potentials, the bulk CO/hexane fluid mixture is more gas-like, resulting in excess CO adsorption values of 40–270%. For these same small bulk CO chemical potential values, there is a transition from gas-like to liquid-like adsorption near the pseudocritical point of the bulk fluid mixture, as one traverses from small to large bulk hexane chemical potential. At the more hexane-rich, liquid-like bulk fluid phase conditions, excess CO adsorption increases to almost 800%. Structurally, near monolayer CO coverage and beyond on the cobalt surface is observed at nearly all supercritical thermodynamic states explored. Overall, the magnitude of the energy of adsorption for CO decreased as the bulk hexane chemical potential increased. Moreover, the magnitude of the energy of CO adsorption decreased as the bulk CO chemical potential increased. This was attributed to both to the large number of repulsive, lateral interactions of CO within its monolayer coverage on the catalyst surface and the enhanced CO interactions in these denser, CO-rich, liquid-like bulk supercritical fluid environments. This enhanced CO fluid-phase solvation aligns with the observed depletion in CO loadings under these same conditions. The distinct gas- and liquid-like regions of adsorption energy behavior were marked by a transition at the bulk fluid pseudocritical pressure.

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