Lattice-gas modeling of the formation and ordering of oxygen adlayers on Pd(100)

We construct a lattice-gas (LG) model to describe not just the ordering of equilibrated adlayers of chemisorbed oxygen on Pd(1 0 0), but also the non-equilibrium ordering observed during dissociative adsorption of molecular oxygen on Pd(1 0 0). First, by combining transfer matrix analysis and Monte Carlo simulations, we determine the equilibrium phase diagrams for candidate LG models with pairwise-additive interactions including nearest-neighbor (NN) exclusion, second NN repulsion, third NN attraction, and possibly fourth NN repulsion. These interactions are selected so as to recover the p(2 x 2) and c(2 x 2) ordering observed in experiment. Interaction strengths are assessed by matching simulated and experimental phase diagrams. Second, a kinetic model is constructed to describe dissociative adsorption of molecular oxygen on second NN empty sites (consistent with NN exclusion), as well as subsequent thermally activated surface diffusion, and adlayer formation. Hopping rates for the latter reflect the selected adspecies interactions through detailed-balance constraints. By comparing simulated diffracted intensities for non-equilibrium adlayer structure during adsorption with experimental data for various temperatures and pressures, we demonstrate the existence of transient mobility for dissociative adsorption in this system, and also determine the magnitude of the thermal diffusion barrier. (C) 2004 Elsevier B.V. All rights reserved.