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The adsorption of atomic and molecular oxygen on bimetallic Pt/Ru overlayer systems has been studied by periodic density functional theory calculations. A strong interaction between the Pt and the Ru layers is found resulting in a hcp stacking for the first Pt layer on Ru. The chemical properties of the Pt/Ru overlayers are beyond those of the single constituents Pt and Ru. Both the compression of the Pt overlayers as well as the strong direct interaction between Pt and Ru, which reaches up to the second Pt layer, reduce the atomic and molecular oxygen adsorption energies compared to pure Pt and Ru surfaces. Although the influence of the electrolyte and any external electric field are neglected, the results should still be relevant in the electrochemical context. The consequences of the theoretical findings for the oxygen reduction reaction in electrocatalysis are discussed.

We have studied the adsorption of CO on Pd(2 1 0) by performing density functional theory (DFT) calculations within the generalized gradient approximation. We find a relatively small corrugation in the CO adsorption energies with the two bridge sites being energetically almost degenerate. CO is furthermore known as a strong poison in heterogeneous catalysis. We have therefore also addressed the coadsorption of CO with atomic hydrogen. There is a significant inhibition of the hydrogen adsorption due to the presence of CO which is analysed in terms of the electronic structure of the adsorbate system.

By studying the adsorption of CO on up to 30 layers of Pt deposited on Ru(0001) the influence of surface strain on the adsorption energy has been disentangled from the residual chemical interaction with the substrate. While the electronic influence of the substrate has largely vanished for three Pt layers, the effect of surface strain due to the 2.5% lattice mismatch of Pt and Ru remains initially intact and is only gradually released for $n\ensuremath{\ge}5$ Pt layers. Electronic structure calculations confirm the experimental observations, in particular, the dramatic decrease of the CO adsorption energy on a single Pt layer which is caused by the strong Pt-Ru interlayer coupling.

We present self-consistent density-functional calculations of the adsorption of atomic and molecular hydrogen on the (210) surface of palladium using a plane-wave basis set with optimized ultrasoft pseudopotentials. The layer relaxations of the (210) surface and the preferred adsorption sites for atomic hydrogen adsorption are determined. Furthermore, we show that on the rather open Pd(210) surface a molecular ${\mathrm{H}}_{2}$ adsorption state becomes stabilized by the presence of atomic hydrogen on the surface. This provides a consistent explanation of recent experiments. An analysis of the bonding situation in terms of the local density of states is also presented.

A novel H{sub 2} molecular adsorption state on metal surfaces has been detected by temperature-programmed desorption and electron energy loss spectroscopy experiments of the H{sub 2}/Pd (210) system. The molecular nature of this state has been verified by isotope exchange experiments. This molecular state leads to a decrease of the surface work function while atomic hydrogen on Pd(210) causes an increase. Ab initio total-energy calculations have confirmed all experimental findings. Through these calculations the microscopic nature of this novel molecular adsorption state could be identified; it turns out that this state is stabilized by the presence of atomic hydrogen on the Pd(210) surface.

We study the conductance properties of a straight two-dimensional quantum wire with impurities modeled by $s$-like scatterers. Their presence can lead to strong inter-channel coupling. It was shown that such systems depend sensitively on the number of transverse modes included. Based on a poor man's scaling technique we include the effect of higher modes in a renormalized coupling constant. We therefore show that the low-energy behavior of the wire is dominated by only a few modes, which hence is a way to reduce the necessary computing power. The technique is successfully applied to the case of one and two $s$-like scatterers., 7 pages, 7 figures included; to be published in Phys. Rev. B

We study the conductance properties of a straight two-dimensional electron waveguide with an s-like scatterer modeled by a single delta-function potential with a finite number of modes. Even such a simple system exhibits interesting resonance phenomena. These resonances are explained in terms of quasi-bound states both by using a direct solution of the Schroedinger equation and by studying the Green's function of the system. Using the Green's function we calculate the survival probability as well as the power absorption and show the influence of the quasi-bound states on these two quantities., 5 pages, 6 figures, to be published in Physical Review B