First principles study of the stability and the formation kinetics of subsurface and bulk carbon on a Ni catalyst

Abstract

Ni-based catalysts are widely used in hydrocarbon conversion processes. First principles based analysis of the stability of subsurface and bulk carbon on Ni catalysts indicates that they might be formed under typical reaction conditions. It was shown that carbon diffusion to the octahedral sites of the first subsurface layer is thermodynamically favorable until about 75% of the subsurface sites are occupied. Activation barriers for diffusion to the subsurface octahedral sites were found to decrease with on-surface carbon coverage, and typical barriers of less than 110 kJ/mol were calculated for moderate to high surface coverages. Diffusion from the subsurface sites to the Ni bulk was found to be thermodynamically favorable if more than 50% of the subsurface octahedral sites are occupied. Such diffusion is, however, kinetically limited, with typical barriers above 150 kJ/mol. For carbon concentrations above 50% in the first subsurface layer, calculations indicate that the Ni lattice is likely to expand. This expansion decreases the diffusion barriers to below 100 kJ/mol and opens up a diffusion path. Hence, carbon diffusion into Ni catalysts is proposed to be a gradual process, first partially filling the subsurface octahedral sites before diffusing to the bulk. © 2008 American Chemical Society.