Effect of trace potassium on hydrogen adsorption and dissociation on hcp cobalt: A density functional theory study.

Highlights • H 2 adsorption energies are very similar on different facets and K preadsorbed on hcp Co. • H atom adsorption was site and facet dependent on hcp Co. • H 2 dissociation energy barriers were negligible (0–0.07 eV) on clean hcp Co. • K has a slight inhibiting effect on the H atom adsorption and H 2 dissociation. • Different K species (K and KOH) exhibit similar effect on H 2 dissociation on hcp Co. Abstract Trace amounts of potassium (K) have a significant influence on the activity and selectivity of cobalt-based catalysts in Fischer–Tropsch synthesis (FTS), in which hydrogen adsorption and dissociation is one of the initial and most important steps. In this work, hydrogen adsorption and dissociation behavior on typical facets ((0001), (10–11), (10–12), (10–15) and (11–20)) of hcp Co with and without adsorbed K were systematically studied. H 2 molecular adsorption results showed that H 2 mainly adsorbed in the perpendicular mode and close to the state of free H 2. Different facets and pre-adsorbed K did not show obvious effects on the H 2 adsorption energy. Atomic hydrogen adsorption was site and facet dependent, but the maximum hydrogen adsorption energy on the different facets of hcp Co were similar (-2.64 to -2.67 eV) with the exception on the (11–20) facet where the adsorption energy was significantly lower (-2.44 eV). K had a slight destabilizing effect on the H atom adsorption on the former Co surfaces due to a very weak repulsive interaction between K and H atoms. The initial H 2 dissociation had negligible energy barriers (0–0.07 eV) on the clean surface of hcp Co, suggesting the direct dissociative adsorption of H 2. The energy barriers for H 2 dissociation are mainly caused by the approach of molecular H 2 towards the Co surface and the rotation of the H 2 molecule from the perpendicular mode to the parallel mode. The H 2 dissociation energy barriers increase by 0.02–0.17 eV after the pre-adsorption of K, indicating a slight inhibition of H 2 dissociation by K. However, the energy barriers for H 2 dissociation in the presence of K were also small (0.05–0.21 eV). This indicates that H 2 dissociates readily at typical Co-based FTS reaction temperatures (210–240 °C), both in the absence and presence of K. Different K species (K and KOH) exhibit similar effects on H 2 dissociation on hcp Co. The B 5 sites on the stepped facets, the preferred sites for K adsorption are not the most favorable site for H 2 dissociation, and K slightly hinders H 2 dissociation at the B 5 site of hcp Co. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]