Improving the coking resistance of Ni-based catalysts by promotion with subsurface boron

Carbon deposition on Ni catalysts was analyzed using first-principles density functional theory calculations. Based on the analysis, we propose boron as a promoter to improve the coking resistance of Ni-based catalysts. Three types of chemisorbed carbon were identified: on-surface carbon atoms, bulk carbon atoms, and extended graphene islands. Extended graphene islands were calculated to be the thermodynamically most stable form of deposited carbon. However, the formation of graphene islands requires high carbon surface coverage and might be kinetically limited at lower carbon coverage. Bulk carbon is found to be more stable than on-surface carbon and can form readily, even at low carbon coverage. Both bulk carbon and graphene islands might lead to catalyst deactivation and should be prevented. Addition of small amounts of boron to the Ni catalyst was found to inhibit the formation of bulk carbide and weaken the on-surface carbon binding energies, possibly slowing the formation of graphene islands. According to our calculations, boron prefers to adsorb in the octahedral sites just below the surface, rather than in the Ni bulk. A small amount of boron corresponding to a single monolayer was found to be sufficient to reduce coking of Ni-based catalysts.