Energetics driving the short-range order in CuxPd1-x/Ru(0001) monolayer surface alloys

Vacancy assisted diffusion on single-atom surface alloys

Bimetallic surfaces can exhibit an improved catalytic activity through tailoring the concentration and/or the arrangement of the two metallic components. However, in order to be catalytically active, the active bimetallic surface structure has to be stable under operating conditions. Typically, structural changes in metals occur via vacancy diffusion. Based on the first-principles determination of formation energies and diffusion barriers we have performed kinetic Monte-Carlo (kMC) simulations to analyse the (meta-)stability of PtRu/Ru(0001), AgPd/Pd(111), PtAu/Au(111) and InCu/Cu(100) surface alloys. In a first step, here we consider single-atom alloys together with one vacancy per simulation cell. We will present results of the time evolution of these structures and analyse them in terms of the interaction between the constituents of the bimetallic surface.

Formation, atomic distribution and mixing energy in two-dimensional Pd(x)Ag(1-x) surface alloys on Pd(111)

The formation and atom distribution in two-dimensional Pd(x)Ag(1-x)/Pd(111) monolayer surface alloys were studied by high resolution scanning tunnelling microscopy (STM) with chemical contrast. From short-range order (SRO) parameters, we calculate preferences for like or unlike nearest neighbours to elucidate the mixing behaviour of the two components for various sub monolayer Ag surface contents. In the regime of low Ag surface contents (<40% Ag), the system shows a weak tendency towards phase separation, high Ag coverages (>60% Ag) result in a disperse distribution of the atoms in the surface. Effective pair interactions (EPIs) were derived by comparing the measured distribution with distributions obtained using Monte Carlo (MC) simulations. From the EPIs, we derived a function for the mixing energy, which can describe the change from clustering to a disperse distribution. The effects of the resulting surface atom distributions and of the Ag coverage dependent surface mixing/demixing on catalytic reactions are discussed.

Patterning of heteroepitaxial overlayers from nano to micron scales

Thin heteroepitaxial overlayers have been proposed as templates to generate stable, self-organized nanostructures at large length scales, with a variety of important technological applications. However, modeling strain-driven self-organization is a formidable challenge due to different length scales involved. In this Letter, we present a method for predicting the patterning of ultrathin films on micron length scales with atomic resolution. We make quantitative predictions for the type of superstructures (stripes, honeycomb, triangular) and length scale of pattern formation of two metal-metal systems, Cu on Ru(0001) and Cu on Pd(111). Our findings are in excellent agreement with previous experiments and call for future experimental investigations of such systems.

Entropy effects in atom distribution and electrochemical properties of Au(x)Pt(1-x)/Pt(111) surface alloys

We report on the structural and electrochemical properties of Au(x)Pt(1-x) surface alloys prepared by Au vapour deposition onto Pt(111) followed by annealing to 1000 K. Driven by configurational entropy, Pt and Au atoms are distributed homogeneously over the surface. On the nm scale, however, atomically resolved scanning tunnelling microscopy images with chemical contrast reveal the formation of nm-sized Pt-rich and Au-rich aggregates, similar to the behaviour recently reported for Pd(x)Ru(1-x)/Ru(0001) [H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H. E. Hoster, R. J. Behm, Surf. Sci. 2009, 603, 1439]. Based on the STM data, we determine the abundance of specific adsorption sites for different Au contents, and we derive effective pair interaction parameters that allow reproducing the lateral distribution in Monte Carlo simulations. Cyclic voltammograms of the surface alloys have many similarities with Pt(111). H(ad) and OH(ad) related features both decrease with increasing amount of Au. Both seem to adsorb only on Pt sites, but H(ad) requires smaller ensembles of Pt atoms than OH(ad). The onset potential for H(ad)-formation decreases with increasing Au content. This is can be explained by an effect of the Au atoms on the entropy of adsorption.

Interaction of Hydrogen with Bimetallic Surfaces

The interaction of hydrogen with bimetallic surfaces is assessed and discussed using structurally well defined PdAu and PtRu single crystal surfaces as model systems. The description is based on a correlation between the hydrogen adsorption behavior and the presence and abundance of specific bimetallic nanostructures. The latter are determined quantitatively by high resolution scanning tunneling microscopy. Following the changes in the hydrogen adsorption behavior on surfaces with systematically varied composition and thus varying concentrations of specific structural elements and adsorption ensembles allows a direct correlation between structural and chemical properties and thus to identify the adsorption characteristics of individual adsorption ensembles. This provides a structural basis for parallel theoretical studies, which are essential for a systematic understanding of the complex changes in the adsorption properties.Based on results from experimental studies and calculations, the role of electronic strain and ligand effects, including lateral and vertical ligands, as well the influence of geometric ensemble effects on the hydrogen adsorption characteristics are discussed. The distinct differences between surfaces modified by a monolayer or monolayer islands of a second metal and mixed surfaces such as surface alloys or alloy surfaces are discussed in these terms.

Tailoring the reactivity of bimetallic overlayer and surface alloy systems

Changing the composition and structure of a bimetallic surface system modifies its electronic properties and thus its catalytic activity. On the basis of density functional theory calculations, the electronic factors underlying the modified properties of bimetallic surfaces such as overlayer systems and, in particular, surface alloys will be discussed. It will be demonstrated that by mixing two metals a new metallic compound can result whose properties are not intermediate but beyond those of both constituents, so that for example by adding a relatively inert metal a more reactive surface can result. Besides the reactivity, the stability of the bimetallic systems will also be briefly discussed.