Monte Carlo modeling of UBI-QEP coverage-dependent atomic chemisorption

The unity bond index-quadratic exponential potential (UBI-QEP) method is used to determine the equilibrium coverage-dependent atomic binding energies and overlayer structures for fcc(1 1 1) and (1 0 0) surfaces by Monte Carlo simulations. We modify the current UBI-QEP formalism to include changes in the total energy of the overlayer under adsorption and desorption of atoms, which gives a more accurate description of the reaction enthalpy and desorption activation barrier. Preferred coordination modes are found to be coverage-dependent and typically change with coverage as: (a) only hollow site occupancy; (b) hollow and bridge site occupancy with metal coordination remaining equal to unity; (c) bridge site occupancy with mono- and di-coordinated metal atoms. Although hops to the atop sites are allowed, their population is very rarely seen in the equilibrium state at any coverage. The results of Monte Carlo simulations are compared with the earlier projections made for various local coverage situations. Within the scope of comparability, the agreement is good. Some ad hoc modifications are mentioned, and possible applications of the results are discussed.