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

Abstract

The Fischer−Tropsch (FT) process consists of the reaction of a synthesis gas (syngas) mixture containing carbon monoxide (CO) and hydrogen (H 2 ), 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 H 2 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 C H /C CO,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 (C 2 H 6 ), as a model for long-chain hydrocarbons, as well as methane (CH 4 ) 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 CH 4 . The termination reaction suggests that water (H 2 O) 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.

Density Functional Theory Study of Monoclinic FeNbO4: Bulk Properties and Water Dissociation at the (010), (011), (110), and (111) Surfaces

“…After cleaving the surfaces, we relaxed the top half of the atom layers and kept the bottom half of layers fixed at their bulk positions. The surface energies of the relaxed (γ r ) and unrelaxed (γ u ) systems can be obtained through the following equations − where E slab,u and E slab,r refer to the total energy of the unrelaxed and half-relaxed slabs, respectively, E bulk refers to the energy of the bulk containing the same number of formula units as the surface, and A refers to the surface area of one side of the slab. In addition, 6 × 6 × 6, 4 × 4 × 1, and 1 × 1 × 1 centered Monkhorst Pack grids were used for the simulation of the bulk, surface slabs, and isolated water molecules, respectively.…”

Highly Efficient ZIF-67-Derived PtCo Alloy–CN Interface for Low-Temperature Aqueous-Phase Fischer–Tropsch Synthesis

“…The DFT-optimized geometries of Co (111) and Co 3 Pt (111) are shown in Figure S5. A similar Co (111) model has been used by Santos-Carballal et al to study the Fischer–Tropsch reaction of CO and H 2 to study the hydrogen-assisted CO dissociation to C + OH and the C–C coupling of CH x species over the Co(111) surface. In a different study, Yao et al performed a detailed ab initio microkinetic modeling for the Fischer–Tropsch reaction over a Co (0001) surface.…”

Catalytic formation of oxalic acid on the partially oxidised greigite Fe3S4(001) surface

“…We next analysed the relative stabilities of termination Fe A of the Fe 3 S 4 (001) surface containing different coverages of oxygen as a function of the ratio of the partial pressures of H 2 O and H 2 S and temperature. We have successfully used these ab initio thermodynamic techniques previously to model the oxidation 115 and hydrogenation 78 of cobalt, the hydration of platinum 85 and the redox behaviour of Fe 3 O 4 . 116 We have calculated several coverages, by replacing in turn each of the S atoms with dangling bonds by O atoms.…”

“…The D2 semiempirical method of Grimme was included in our calculations to correct the long-range dispersion interactions, 75 which is particularly important for an appropriate description of materials and interface properties. 59,68,[76][77][78][79][80][81][82][83][84][85] periodic plane-wave basis set with an upper kinetic energy threshold fixed at 400 eV was employed to expand the Kohn-Sham valence states. The electronic ground state was determined using a preconditioned conjugate gradients minimisation algorithm, which optimises completely and self-consistently the total (free) energy, which is the variational quantity within this methodology.…”

“…109,110 The cNEB algorithm used for modelling the MEPs has been employed successfully before to obtain the saddle point structures and energies of several processes in heterogeneous catalysis. 59,61,68,78,111 We have used five images to model the MEP, which were optimised globally by means of the limited-memory Broyden-Fletcher-Goldfarb-Shannon (LBFGS) method. 89,112 The saddle points were characterised by a single imaginary frequency along the reaction coordinate, which were determined using the central finite differences method.…”