Configurations of different Pd-containing Au(111) bimetallic surfaces with Pd substituents varying from one to three atoms have been studied using density functional theory. The stability of the so-formed Pd monomers, dimers or trimers in the surface and subsurface layers of a Au(111)-(3 x 3) unit cell and their influence on the adsorption of hydrogen have been investigated. We find that before hydrogen adsorption the surface prefers to form Pd monomers over dimers or trimers located in subsurface positions, which is in agreement with experimental observations. However, adsorption of atomic hydrogen reverses this trend, leading to a stabilization of Pd trimers over dimers or monomers all located in the surface layer. Also, the binding energies of two or three hydrogen atoms on clean Au(111) and selected PdAu(111) ensembles have been determined. While pure Au(111) is not able to promote hydrogen splitting, after substitution of surface atoms with Pd the adsorption energy becomes sufficiently high for this activation.

The interface of experiment and theory: A combination of distance tunneling spectroscopy (left; Au gray, H white, O red, S green) and density functional theory calculations has been employed to derive a detailed model of the electric double layer for Au(111) in H2SO4 at positive potential. Evidence for a double layer structure normal to the surface is presented, and the absolute width of the tunnel gap was determined through the DFT calculations.

Density Functional Theory (DFT) studies were performed on the adsorption of sulfate on Au(111). Focusing on the well-known (√3 × √7)R19.1° structure reported by various surface sensitive techniques, the coadsorption of H3O+ and/or H2O has been considered in different combinations. The calculated binding energies show that the coadsorption of a single H3O+ per sulfate is the most stable configuration, which is in agreement with experimental observations. Further, we find that in the case of coadsorption of both H3O+ and H2O along with sulfate, one of the protons of hydronium moves to sulfate, finally leading to bisulfate with two coadsorbed water molecules. Besides the morphology and energetics of the different configurations, we also discuss the nature of the surface bonds by analyzing the charge density distribution.

The potential-induced surface reconstruction of Au(100) has been studied by a combination of density functional theory and thermodynamic considerations. Surface free energies of reconstructed-(5×1) and unreconstructed-(1×1) surfaces were calculated as function of an external electric field using the extended ab-initio atomistic thermodynamics approach. After relating electric field and electrode potential by using capacitance measurements, we calculate lifting of the reconstruction to occur at 0.58 V in 0.01 M HClO 4 and 0.27 V in 0.01 M H 2 SO 4 , being in agreement with the experimental values of 0.60 V and 0.27 V (vs. SCE). Finally, the consequences of using experimental capacitance measurements for calculating surface free energies are discussed.

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