Catalytic Conversion of CO and H2 into Hydrocarbons on the Cobalt Co(111) Surface: Implications for the Fischer–Tropsch Process

David Santos-Carballal,Abdelaziz Cadi-Essadek,Nora H De Leeuw

doi:10.1021/acs.jpcc.1c00254

Abstract

The Fischer–Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H2), which are polymerized into liquid hydrocarbon chains, often using a cobalt catalyst, although the mechanistic pathway is not yet fully understood. Here, we have employed unrestricted density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections [DFT+U−D3−(BJ)] to investigate the reaction of syngas and the selectivity toward the hydrocarbons formed on the cobalt Co(111) surface. The single CO and dissociated H2 molecules prefer to adsorb at two different types of trigonal surface sites, and we discuss how the interatomic distances, fundamental vibrational modes, charge transfers, surface-free energies, and work functions are modified by the adsorbates. The coadsorption of the syngas molecules in close proximity provides enough energy for the system to cross the saddle points on the minimum energy pathway (MEP), leading to the catalytic hydrogenolysis of the C–O bond. The adsorbed CO, alongside the intermediates CH and OH, are further stabilized when the ratio of equilibrium coverage (C) is CH/CCO,CH,OH > 6:1 under the temperature conditions required for the FT process. We propose several mechanistic pathways to account for the formation of ethane (C2H6), as a model for long-chain hydrocarbons, as well as methane (CH4) which is an undesirable product. The MEPs for these processes show that the coupling of the C–C bond followed by hydrogenation is the most favorable process, which takes precedence over the production of CH4. The termination reaction suggests that water (H2O) remains weakly physisorbed to the surface, allowing the reutilization of its catalytic site. The simulated fundamental vibrational frequencies and scanning tunneling microscopy images of the surface-bound intermediates are in agreement with the available experimental data. Our findings are important in the interpretation of the elementary steps of the FT process on the Co(111) surface.

Highlights

IntroductionThe Fischer−Tropsch (FT) process is the most relevant method used industrially for the polymerization of one-carbon (C1) molecules into valuable products that can be used as chemicals or fuels.[1−8] FT is an indirect liquefaction process that uses a synthesis gas as a chemical feedstock[9−11] and a transition metal as the catalyst.[12−16] The so-called syngas comprises carbon monoxide (CO) and molecular hydrogen (H2) and produced by the gasification of coal,[17−20] organic waste, biomass,[17−19,21,22] or by reforming natural gas.[17,23,24]Straight-chain (oxygenated) hydrocarbons of high purity, containing between 10 and 20 carbon atoms, are produced which are commercially valuable as synthetic motor-gasoline (petrol)[25,26] as well as diesel[25,27] and high-performance jet fuels.[28−30] The combustion of these synfuels is very clean, causing negligible adverse health and environmental impacts compared to conventional fuels, which emit large amounts of harmful pollutants, including sulfur gen oxides (NOx),[32] and particulate omxaidtteesr.(30S−O33x)F,3T0−3s2ynnfuiterlos-, in particular those derived from biomass, have attracted great renewed attention as a sustainable route to achieve a net zerocarbon future and mitigate global warming by 2050.34−37It is generally accepted that transforming syngas into (oxygenated) hydrocarbons involves a series of concatenated elementary steps as well as surface-bound intermediates and byproducts that are difficult to detect.[38]
Spatial constrains favor terminal alkyl intermediates fixed to the catalyst surface site,[46,47] which controls the selectivity toward straight chain hydrocarbons.[48−50] the principle of selective inhibition developed by Schulz and co-workers posits that dissociative desorption of terminal alkyl species forming αolefins is more likely than addition of one H atom to form
These voids are responsible for the adsorption and catalytic properties of Co once they are exposed at the surfaces, as discussed in Sections 3.3 to 3.4

Summary

Introduction

The Fischer−Tropsch (FT) process is the most relevant method used industrially for the polymerization of one-carbon (C1) molecules into valuable products that can be used as chemicals or fuels.[1−8] FT is an indirect liquefaction process that uses a synthesis gas as a chemical feedstock[9−11] and a transition metal as the catalyst.[12−16] The so-called syngas comprises carbon monoxide (CO) and molecular hydrogen (H2) and produced by the gasification of coal,[17−20] organic waste, biomass,[17−19,21,22] or by reforming natural gas.[17,23,24]Straight-chain (oxygenated) hydrocarbons of high purity, containing between 10 and 20 carbon atoms, are produced which are commercially valuable as synthetic motor-gasoline (petrol)[25,26] as well as diesel[25,27] and high-performance jet fuels.[28−30] The combustion of these synfuels is very clean, causing negligible adverse health and environmental impacts compared to conventional fuels, which emit large amounts of harmful pollutants, including sulfur gen oxides (NOx),[32] and particulate omxaidtteesr.(30S−O33x)F,3T0−3s2ynnfuiterlos-, in particular those derived from biomass, have attracted great renewed attention as a sustainable route to achieve a net zerocarbon future and mitigate global warming by 2050.34−37It is generally accepted that transforming syngas into (oxygenated) hydrocarbons involves a series of concatenated elementary steps as well as surface-bound intermediates and byproducts that are difficult to detect.[38]. The Fischer−Tropsch (FT) process is the most relevant method used industrially for the polymerization of one-carbon (C1) molecules into valuable products that can be used as chemicals or fuels.[1−8] FT is an indirect liquefaction process that uses a synthesis gas as a chemical feedstock[9−11] and a transition metal as the catalyst.[12−16] The so-called syngas comprises carbon monoxide (CO) and molecular hydrogen (H2) and produced by the gasification of coal,[17−20] organic waste, biomass,[17−19,21,22] or by reforming natural gas.[17,23,24]. Bifunctional catalysts combining an FT-active Co phase embedded into the cavities of an acid mesoporous catalyst are used for the direct selective synthesis of hydrocarbons[56−58] as well as core−shell nanoreactors, characterized by hollow nanospheres based on an inorganic or organic matrix encapsulating an FT-active metal.[59−61]

Methods

Results

Conclusion