Vacancy-mediated diffusion of carbon in cobalt and its influence on CO activation

We present a theoretical understanding of carbon deposition and diffusion in FCC and HCP cobalt from first principles. We found that the deposited carbon atom can readily penetrate into the first sub-layer of Co substrates, while further diffusion into deeper interstices seems unfeasible. In the presence of cobalt vacancy, the carbon diffusion can be greatly promoted and possibly leads to the carburization of cobalt catalysts. The infiltrated carbon atoms have a pronounced influence on the catalytic activity toward CO adsorption and dissociation. Compared to the clean cobalt surfaces, the C-O bond is less weakened on the carburized cobalt due to the depletion of Co-d electrons. As a result, the activation barrier for CO dissociation is substantially increased. We suggest that the carburization is another important cause to the deactivation of Co-based catalysts in addition to the site blockage by the surface carbon deposition.