Theoretical study of surface segregation and ordering in Ni-based bimetallic surface alloys

Ni-based bimetallic materials are promising for a series of important heterogeneous catalytic reactions because of their low cost and potential high activity. In order to understand their catalytic performances in catalytic processes, it is important to know the structural properties of these bimetallic surfaces, including, in particular, how the guest metal is distributed in the nickle host at finite temperature. By using the cluster expansion model built on density-functional theory calculations, combined with Monte Carlo simulation, we study the segregation and ordering behaviors in several frequently studied Ni-based bimetallic catalysts NiX (X = Fe, Co, and Cu). We found that Ni tends to segregate to the top most layer of the surface in NiFe and NiCo, while Cu tends to segregate to the topmost layer of NiCu surfaces. NiCo and NiCu lose short-range order quickly as the temperature increases. Under low temperature, NiFe forms an ordered Ni₃Fe structure, which, however, disappears above 550 K because of the order-disorder transition. These findings can provide important information for the understanding of the stability and activity of Ni-based bimetallic catalysts at high temperatures.