Interaction of Hydrogen with Bimetallic Surfaces

Probing the Surface Chemistry of Nanoporous Gold via Electrochemical Characterization and Atom Probe Tomography

Surface chemistry information is crucial in understanding catalytic and sensing mechanisms. However, resolving the outermost monolayer composition of metallic nanoporous materials is challenging due to the high tortuosity of their morphology. In this study, we first elaborate on the capabilities and limitations of atom probe tomography (APT) in resolving interfaces. Subsequently, an electrochemical approach is designed to characterize the surface composition of nanoporous gold (NPG), developed from dealloying an inexpensive precursor (95 at. % Ag, 5 at. % Au), by the means of aqueous electrochemical measurements of the selective electrosorption of sulfide ions, which react strongly with Ag, but to a significantly lesser extent with Au. Accordingly, cyclic voltammetry was performed at various scan rates on NPG in alkaline aqueous solutions (0.2 M NaOH; pH 13) in the presence and absence of 1 mM Na2S. Calibrations via similar voltammetric measurements on pure polycrystalline Ag and Au surfaces allowed for a quantitative estimation for the Ag surface coverage of NPG. The sensitivity threshold for the detection of the adsorbate-Ag interaction was assessed to be approximately 2% Ag surface coverage. As curves measured on NPG only showed featureless capacitive currents, no faradaic charge density associated with sulfide electrosorption could be detected. This study opens a new avenue to gain further insight into the monolayer surface coverage of metallic nanoporous materials and assists in enhancement of the interpretation of APT reconstructions.

Quantum electrocatalysts: theoretical picture, electrochemical kinetic isotope effect analysis, and conjecture to understand microscopic mechanisms

The fundamental aspects of quantum electrocatalysts are discussed together with the newly developed electrochemical kinetic isotope effect (EC-KIE) approach.

Challenges in bimetallic multilayer structure formation: Pt growth on Cu monolayers on Ru(0001)

In a joint experimental and theoretical study, we investigated the formation and morphology of PtCu/Ru(0001) bimetallic surfaces grown at room and higher temperatures under UHV conditions. We obtained the PtCu/Ru(0001) surfaces by deposition of Pt atoms on a previously created Cu/Ru(0001) structure which includes only one Cu monolayer. Bimetallic surfaces prepared at different Pt coverages are investigated using STM imaging, revealing the existence of reconstruction lines and Cu islands. Although primarily created Cu islands continue growing in size by increasing Pt coverage, a continuous formation of new Cu islands is observed. This leads to an atypical exponential increase of the island density as well as to an atypical behavior of the average number of atoms per island for low Pt coverages. Although coalescence of the islands is observed for high Pt coverages, the island density remains almost constant in that regime. In order to understand the trends observed in the experiments, we study the stability of these surfaces, atom adsorption, and adatom diffusion using periodic density functional theory calculations. On the basis of the experimental observations and the first-principles calculations, we suggest a model that includes exchange of Pt adatoms with Cu surface atoms, Pt and Cu adatom diffusion, and attractive (repulsive) interactions between Cu (Pt) adatoms with substitutional Pt surface atoms, which explains the main trends in island formation and growth observed in the experiment.

Characterisation of gold catalysts

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions.

Hydrogen adsorption on bimetallic PdAu(111) surface alloys: minimum adsorption ensemble, ligand and ensemble effects, and ensemble confinement

Microscopy and spectroscopy measurements together with periodic DFT calculations provide detailed insight into the adsorption behavior of hydrogen on disordered, but structurally well defined PdAu–Pd(111) surface alloys.

Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au

The catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure-activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.

Interaction of CO and deuterium with bimetallic, monolayer Pt-island/film covered Ru(0001) surfaces

The adsorption properties of structurally well defined bimetallic Pt/Ru(0001) surfaces, consisting of a Ru(0001) substrate partly or fully covered by monolayer Pt islands or a monolayer Pt film, were studied by temperature programmed desorption (TPD) using CO and deuterium as probe molecules. Additionally, the adsorption of CO was investigated by infrared reflection absorption spectroscopy (IRAS). The presence of the pseudomorphic platinum islands or monolayer film leads to considerable modifications of the adsorption properties for both adsorbates, both on the Pt covered and, to a smaller extent, on the bare Ru part of the surfaces. In addition to distinct weakly bound adspecies, which are adsorbed on the monolayer Pt islands, we find unique contributions from island edge desorption, from spill-over processes during the desorption run, and a general down-shift of the peak related to desorption from Pt-free Ru(0001) areas with increasing Pt coverage. These effects, which we consider as characteristic for adsorption on bimetallic surfaces with large contiguous areas of the respective types, are discussed in detail.

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.

Self-assembly of metal nanostructures on binary alloy surfaces

Deposition of metals on binary alloy surfaces offers new possibilities for guiding the formation of functional metal nanostructures. This idea is explored with scanning tunneling microscopy studies and atomistic-level analysis and modeling of nonequilibrium island formation. For Au/NiAl(110), complex monolayer structures are found and compared with the simple fcc(110) bilayer structure recently observed for Ag/NiAl(110). We also consider a more complex codeposition system, (Ni + Al)/NiAl(110), which offers the opportunity for fundamental studies of self-growth of alloys including deviations for equilibrium ordering. A general multisite lattice-gas model framework enables analysis of structure selection and morphological evolution in these systems.

Tuning adsorption via strain and vertical ligand effects

We report on the structure and electrochemical adsorption properties of well-defined pseudomorphic Pt mono- and multilayers on Ru(0001). These act as model surfaces for Pt(111) with slightly decreased affinity to adsorbed hydrogen (H(ad)) and hydroxyl (OH(ad)). In cyclic voltammograms, this is reflected in more negative/positive potential regions for the reversible adsorption of upd-H(ad)/OH(ad), respectively, compared to Pt(111). For upd-H(ad), we show that the corresponding trends can be predicted with high accuracy by density functional theory (DFT). In particular, the upd-H(ad) onset regions can be precisely simulated using the H(ad) adsorption energies from DFT, the layer thickness distribution from STM, and the base voltammogram of Pt(111) as reference.

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.