First Principles Study of the Effect of Carbon and Boron on the Activity of a Ni Catalyst

Abstract

Ni catalysts are commonly used for hydrocarbon processes such as steam reforming. First principles calculations indicate that carbon atoms are relatively unstable on the terraces of a Ni catalyst, and diffusion to subsurface octahedral sites and to step sites is thermodynamically favorable. The presence of subsurface carbon significantly increases the barrier for methane activation from 91 to 143 kJ/mol and is hence expected to reduce catalyst activity. The presence of carbon atoms at the step sites also increase methane activation barriers, from 57 to 93 kJ/mol. On the basis of first principles simulations, boron has been proposed to enhance the stability of Ni catalysts by selectively blocking both the subsurface and the step sites. Boron has a strong preference for the subsurface octahedral sites, and subsurface boron causes a reconstruction of the Ni(111) surface. This reconstruction lowers methane activation barriers to 64 kJ/mol, and hence subsurface boron atoms do not reduce catalyst activity. Boron adsorption at the step sites increases the methane activation barrier from 57 to 70 kJ/mol. The activity variations are well described by the d-band model.