Theoretical study of carbon species on Pd(111): Competition between migration of C atoms to the subsurface interlayer and formation of C_n clusters on the surface

Subsurface carbon species of Pd catalysts recently attracted considerable attention because they affect the selectivity of hydrogenation reactions. We calculated the migration of C atoms from the Pd(111) surface to interstitial subsurface sites to be energetically favorable. Yet, thermodynamically more stable is a graphene-like phase on the Pd surface. Applying a density functional method on periodic models, we explored the formation of C_n (n = 2-4) clusters on Pd(111). At low coverage, carbon monomers on the surface and at octahedral subsurface sites were calculated to be more stable than dimer species, C₂, on the surface. However, at a C coverage of about half a monolayer, the formation of C₂ and C₃ species, precursors of a graphene phase, becomes competitive with migration of C monomers to octahedral subsurface sites. While discussing these findings, we also addressed the problem of C₁ formation on Pd catalysts from simple organics.