The local reactivity of Pd overlayers supported by Au has been studied by calculating atomic hydrogen and CO adsorption energies as a microscopic probe. The calculations are based on density functional theory within the generalized gradient approximation. The binding energies show a maximum on two Pd layers on Au, both for the ͑100͒ and ͑111͒ surfaces. We have furthermore analyzed local trends by considering different adsorption sites on the Pd overlayers. The results can be rationalized within the d-band model if also second-nearestneighbor interactions and bond-length effects are taken into account.

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We utilize ab initio quantum mechanical calculations in order to explore structural conformations and cooperative mechanisms at a minimally hydrated 2D array of flexible acidic surface groups. This system serves as a model for rationalizing interactions and correlations of protons and water with ionized side chains that are affixed to hydrophobic polymer aggregates in polymer electrolyte membranes (PEMs). The model exhibits two basic minimum energy configurations upon varying the separation of surface groups from 5 to 12 A. In the "upright" structure at small separation, surface groups are fully dissociated and oriented perpendicular to the basal plane. Together with hydronium ions (H3O+) they form a highly ordered network with long-range correlations. At larger separations we found the transition to a "tilted" structure with cluster-like conformation of surface groups. This structure retains only short-range correlations. Moreover, we investigated the strength of water binding to the minimally hydrated structures. At small separations between surface groups, an additional water molecule interacts only weakly with the minimally hydrated array (binding energy < 0.1 eV) while the energy needed to remove one water molecule exceeds 1 eV. This shows that the minimally hydrated systems are very stable. Ideally, these studies would expedite the design of cheap, highly performing PEMs for fuel cells, with a major focus on membranes that could operate stably at minimal hydration and elevated temperatures (>120 degrees C).

We have employed ab initio calculations based on density functional theory in order to study stability and oxygen adsorption energies of Pt nanoparticles. For particles with sizes up to 200 atoms and various geometric shapes, we have explored the dependence of cohesive energies on atomic coordination number and on lattice strain effects. A simple empirical relation, which is consistent with the well-known Gibbs-Thomson relation, represents the cohesive energy over the range of considered sizes and shapes. For hemispherical cuboctahedral particles with 37 and 92 atoms, we have generated contour plots of the adsorption energy of atomic oxygen on all nanofacets. These plots furnish the known trend of strongly enhanced oxygen adsorption energies in comparison to extended surfaces. We found that the interplay of geometric effects, involving the periodic arrangement of surface atoms and edge effects on nanofacets, causes the high site-selectivity of Pt-oxygen interaction energies, with the largest adsorption energies found at the edges. Particle relaxation upon oxygen adsorption exhibits a significant influence on adsorption energies. The presented results provide a map of the peculiar site-selectivity of adsorption at Pt nanoparticles, which should be accounted for in building detailed models of reaction mechanisms and reactivity.

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We used density functional theory to investigate the reaction pathway of oxygen reduction/water splitting at a tetrahedral Pt(4) cluster. Four extra water molecules were included to account for the effect of water in mediating elementary surface processes. We propose a 6-step reaction sequence that includes a proton transfer between neighbouring active sites. Thermochemical considerations and the nudged elastic band method were employed to calculate reaction and activation energies for the elementary reaction steps. We generated the free energy diagram along the reaction path for various applied potentials. This plot provides vital information on the stability of intermediates and the rate determining processes in oxygen reduction and water splitting. Results suggest that removal of the reaction product, viz. molecular oxygen or water, is an energetically strongly hindered step in either direction.