ABSTRACT

This review summarizes recent results in atomic-level simulations and calculations on the structural properties of nanometer scale binary clusters of transition metals. Such bimetallic clusters, primarily via the functions of their exposed surface atoms, are widely used as heterogeneous catalysts in the industry. Theoretical techniques employed include molecular dynamics and Monte Carlo simulations and their hybridization, multiple-state statistical-mechanical modeling, and numerical solutions for the minimization of the free energy. The focus is on the surface segregation profiles of small  (201-atom) bimetallic catalysts. Also reviewed is the structural stability of very small (13-atom) pure metal clusters and much larger bimetallic catalysts of sizes up to 10,000 atoms and 6 nanometers in diameter, existing in noncrystalline and different (truncated) crystallographic phases. Among the bimetallic combinations of Pt, Rh, Ni, Pd, Cu, and Ag, a system (Cu-Pd) that exhibits very weak surface segregation is chosen to illustrate the reverse surface segregation phenomenon: the surface depletion of an element (Cu), which normally enriches on the surface, at low concentrations. Simulations verify that the reverse surface segregation, recently observed in experiments, can be driven by the exothermic formation of alloys at the core. The issues of structural metastability and the influence of the environment are discussed in the context of recent infrared (IR), extended  X-ray absorption fine structure (EXAFS), nuclear magnetic resonance (NMR), and X-ray diffraction experiments on real catalysts.