Density functional theory based study of methanation in the presence of subsurface atomic hydrogen on Cobalt (0001) surface

Abstract

It has been shown experimentally that structural changes occur on the Cobalt catalyst surface under Fischer Tropsch (FT) conditions which lead to significantly enhanced reactivity. The reaction occurs at the length scale of an angstrom and time scale of 10^(-15) sec. To quantify this change in reactivity, properties such as bond lengths and energy of the molecules (in reactants, catalyst and products) need to be estimated. We have used Density Functional Theory as implemented in Quantum Espresso(QE) to calculate the optimized adsorption geometries, adsorption energies and to determine the activation barriers for various pathways leading to methanation.

Earlier theoretical works have shown that subsurface hydrogen is an important species for methanation at FT conditions in Heavy Paraffin Synthesis (HPS) section of the Shell-GTL process. Using DFT, it was reported that subsurface hydrogen enables a facile pathway from a chemisorbed surface methyl group to physisorbed methane. However, none of these studies were done at the actual CO coverages that might be present during FT conditions. Also, activation barriers for different pathways of methanation on a Co(0001) surface were not determined theoretically before. Similar studies on Ni(111) indicate that the reaction might involve two transition states corresponding to a resurfacing barrier for the subsurface hydrogen and a recombination barrier for the methyl group and surface hydrogen. Hence, we have focused on this particular aspect of the methanation process on a Co(0001) surface.

We began with an extensive testing of different pseudopotentials. This was followed with a method and model verification for a Cobalt vacuum slab that was used to represent the catalyst surface. Later, we continued with an estimation of the adsorption energies of subsurface hydrogen at different coverages of theta = 0.11, 0.22, 0.33 and 0.44 in the absence as well as the presence of surface CO at the realistic coverage of theta=1/3 and having a specific overlayer structure.

The next phase of the project involved doing a transition state search using the Climbing image-Nudged Elastic Band (CI-NEB) method to determine the barriers for hydrogen dissociation and subsurface diffusion. The CI-NEB was again used to determine the resurfacing and the recombination activation barriers for methanation. Density Functional Perturbation Theory (DFPT) as implemented in the Phonon package within QE was used to determine the normal modes of the initial, final as well as the transition state structures which was further used to make zero-point energy corrections to the activation barriers and also determine prefactors for the reaction rates as approximated within the Harmonic Transition State Theory (hTST).

From our calculations, it can be inferred that presence of CO (theta=1/3) destabilizes the surface hydrogen. However, it does not have any appreciable effect on the stability of hydrogen present in the first, second and third sublayers. Hence, under the Fischer-Tropsch conditions subsurface hydrogen would still be stable as shown by earlier theoretical works. Amongst the three different pathways considered for methanation, pathway-1 where a methyl group is adsorbed at a surface hcp site near a subsurface hydrogen resurfacing site had the overall lowest barrier. Continuing further, the prefactors and the rate constants for these pathways were estimated using the harmonic transition state theory (hTST) approximation at a temperature of 500 K. After making the adequate zero point energy corrections to the activation barriers, the estimated rate constants again confirmed that pathway-1 was statistically the most dominant for methanation on a Co(0001) surface close to FT reaction temperatures.