Cobalt on nickel surfaces and the role of carbide on its stability

Transition metal atoms are commonly used in catalysis and photo- catalysis, but their potential reactivity reduces with aggregation and alloying. We investigate in particular whether Cobalt adatoms float on Ni surfaces or dissolve into the metal. Density functional theory calculations have been performed in or- der to evaluate the stability of different Cobalt adsorption configurations on Nickel surfaces, mainly at (100) terraces and steps, and the relevant energy barriers for dif- fusion on terraces and across steps, segregation and dissolution into the substrate. The simulations have been compared with variable temperature scanning tunneling microscopy and low energy electron diffraction. The results show that the Cobalt adatoms and small aggregates are unstable with respect to the formation of Co-Ni alloys, but the presence of a carbide monolayer on Ni surface improve their stability.