Monte Carlo modeling of O2 adsorption kinetics on unreconstructed fcc(1 0 0) surfaces of metals using UBI–QEP coverage-dependent energetics

We have developed a Monte Carlo (MC) model of dissociative molecular oxygen adsorption on unreconstructed fcc(1 0 0) metal surfaces. The model explicitly and uniformly considers elementary steps/events including adsorption (trapping), dissociation, diffusion (hopping), recombination, and desorption. The unity bond index–quadratic exponential potential (UBI–QEP) formalism was used to calculate the energetics (atomic O and molecular O2 binding energies, enthalpies and activation barriers), and the kinetic formalism of the MC modeling makes use of Arrhenius-type reaction rates and the Metropolis-type algorithm. The coverage-dependent UBI–QEP formalism was substantially refined, which included the use of (a) hollow sites for atomic oxygen as the only stable ones, (b) rescaling the atomic binding energies due to metal sharing A–M–A, (c) the spatial constraint k, and (d) hot atom traveling distance v. In the kinetic simulations, the preexponential factors were assumed to be proportional to the attempt frequency of the events. The explored coverage range was from 0 to 0.30 ML (monolayer), and the temperature range was from 100 to 700 K. The model makes it possible to calculate a multitude of coverage-dependent parameters of the process, in particular binding energies of molecular and atomic oxygen, the normalized O2 sticking probability, and the amount and mutual arrangement of neighbors in an overlayer. With inclusion of very weak (<1 kcal/mol) lateral attractive nnn A–A interactions, the model also enables simulations of the development of p(2 × 2)-O and c(2 × 2)-O domains from disordered phases, generating the snapshots of the overlayer structures. The Ni(1 0 0) surface, for which the detailed experimental data on O2 dissociative adsorption are available, was used as a model system. Practically all model projections, including those concerning the O2 and O coverages and their binding energies, normalized sticking probabilities, and the overlayer structures, are in good agreement with experimental data. Moreover, the simulated sticking probabilities may be more relevant to the intrinsic Ni(1 0 0) surface behavior because the deviation from the experimental curve at high coverages (>0.25 ML) appears to be due to the formation of the NiO phase. The trends for the binding energies were also modeled for unreconstructed fcc(1 0 0) surfaces of Cu, Ag, and Pd. Because relevant experimental data are scarce, the model results are mostly projections to be verified by experiment.