Equilibrium Au-Pd(100) Surface Structures under CO Pressure: Energetic Stabilities and Phase Diagrams

In this work first-principles density-functional-theory (DFT) calculations and Monte Carlo (MC) simulations are used to study the stability of the new surface phases formed under CO gas pressure on Au-Pd (100). The segregation isotherms reveal the apparition at theta(Co) = 0.5 monolayer (ML) of two exotic ordered phase structures over the (100) facet, depending on the CO adsorption configurations. When CO is adsorbed on bridge sites, the ordered phase is formed by Pd-CO ensembles organized in linear chains, which are separated by Au atoms forming also linear chains free of adsorbed CO molecules. When CO is adsorbed on the on-top site, a checkerboard-like stable structure appears with CO adsorbed on Pd. The phase diagrams (temperature and CO pressure) of these large surface Pd ensembles for different bulk Pd concentrations [Pd-bulk are calculated. The effect of local density approximation (LDA) and generalized gradient corrected (GGA) exchange-correlation (XC) functionals are compared. The phase diagram of the configuration with linear chains, predicted by GGA calculations, shows excellent agreement with experimental results. In order to understand the origin of the formation and the stability of the two ordered phases, the energetic stability and the electronic structure properties of different large Pd ensembles are compared and deeply analyzed.