Carbon dissolution and segregation in platinum

Abstract

Recent experimental studies showed evidence for C dissolution in Pt nanoparticles after CH4 decomposition, and the posterior low temperature segregation to form surface graphene, highlighting graphene growth from below. There are indications of an easier C transfer between surface and subsurface regions at Pt grain boundaries, although the ultimate atomistic mechanism remains unclear. A plausible explanation is provided here by exploring and comparing C incorporation in Ni, Pd, and Pt(111) surfaces by density functional (DF) calculations on slab models under a low coverage regime, evaluating the energetic stability and subsurface sinking kinetic feasibility. Four DF functionals have been used, avoiding possible biased results. All functionals showed that C atoms occupy octahedral subsurface (oss) sites in Ni(111), with high sinking energy barriers of 80-90 kJ mol(-1), whereas both oss and tetrahedral subsurface (tss) sites can be occupied in Pd(111), with low sinking energy barriers of 20-50 kJ mol(-1). The oss sites are strongly disfavoured on Pt(111), whereas the tss sites are found to be isoenergetic to surface sites, with low subsurface sinking energy barriers of 27-41 kJ mol(-1). Calculations on Pt-79 and Pt-140 nanoparticle models reveal how tss sites are more stabilized at low-coordinated sites, where subsurface sinking energy barriers drop to values of similar to 17 kJ mol(-1). These results explain the experimentally observed C dissolution and segregation in Pt systems, more favoured at grain boundaries, as well as the graphene growth from below and the formation of double layer models. In addition, the present results open a gate for profiting from the small quantities of C placed at the subsurface region in order to tune the surface catalytic activity of Pt nanoparticle based catalysts.