Au Alloy Surfaces Research Articles

Orientation-Dependent Hindrance to the Oxidation of Pd-Au Alloy Surfaces.

Oxidation of monometallic Pd and bimetallic Pd3Au alloy surfaces are observed by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at an elevated pressure (100 mTorr O2 ambient). It is directly evidenced that the alloying with Au hinders the surface oxidation of Pd3Au surfaces compared with monometallic Pd surfaces. Remarkably, the oxidation behavior is clearly different between Pd3Au(111) and (100) surfaces. The (100) surface has a relatively Pd-rich surface composition, and the surface oxide layer is formed, whereas the (111) surface has a Au-rich composition, and the surface oxidation is quite limited. A combined approach of experimental and theoretical techniques reveals that Pd/Au surface composition and atomic arrangement are key factors determining the oxidation behavior.

H–D Exchange Mechanism of Butene on a D-Absorbed Pd–Au Alloy Surface

Complete H–D exchange of butene was reported on a D-absorbed Pd–Au alloy surface. However, its reaction mechanism has not been clarified yet. Here we investigate the reaction of cis-2- and 1-butene on Pd70Au30(110) by means of high-resolution electron energy loss spectroscopy. We demonstrate that both butene isomers are in the π-bonded state and that 1-butene irreversibly converts to cis-2-butene around 250 K. This isomerization accounts for previously observed similar product distributions in the H–D exchange reaction of cis-2- and 1-butene on the D-absorbed Pd70Au30(110) surface. Since the reverse change from cis-2- to 1-butene is not observed at any temperature, we conclude that the H–D exchange proceeds by the dissociative mechanism via vinylic and π-allylic intermediates rather than the Horiuti–Polanyi mechanism.

Surface Alloy Composition Controlled O2 Activation on Pd–Au Bimetallic Model Catalysts

Oxygen is an important reactant in several catalytic conversions and partial oxidation reactions on Pd–Au alloy surfaces; however, adsorption and dissociation are not fully understood, especially as a function of the surface alloy composition. In this study, we probe the influence of the atomic makeup of the surface of Pd–Au catalysts regarding control of the catalytic activity toward O2 dissociation and the reactivity of the resulting oxygen adatoms. To experimentally investigate this, we prepared various bimetallic surfaces under ultrahigh vacuum via evaporation of Pd onto a Au(111) surface. Hydrogen molecules were used to characterize the composition of the Pd–Au surfaces, which we simplistically group into two categories: (i) Pd–Au interface sites and (ii) Pd(111)-like island sites. When the Pd coverage is 1.0 ML, which predominantly indicates Pd–Au interface sites, no dissociative adsorption of O2 at 300 K is observed, but dissociation begins to be measurable on the surfaces with larger Pd loadings (...

TiO2 layers on Ti-Au alloy formed by two-step thermal oxidation and their photocatalytic activity in visible-light

Titania (TiO2) layers consisting of anatase and rutile phases were formed on Ti-4at%Au alloy surface, using a two-step thermal oxidation method of carburization followed by air oxidation. Au from the alloy was found in the formed TiO2 layer as metallic Au nanoparticles with diameter <10nm and dissolved Au3+. The visible-light photocatalytic activity of the TiO2 layer was evaluated by the degradation of stearic acid coating upon irradiation. Marked degradation of stearic acid was observed through the measured decrease in the water contact angle. Possible mechanisms of the visible-light photocatalytic activity are discussed.

CO Adsorption on Pd–Au Alloy Surface: Reversible Adsorption Site Switching Induced by High-Pressure CO

The interaction between carbon monoxide (CO) and a Pd70Au30(111) alloy surface was investigated under CO pressures with a wide range from ultrahigh vacuum (UHV) to sub-Torr at room temperature by a combination of near-ambient pressure (NAP) X-ray photoelectron spectroscopy and density functional theory calculations. The adsorption site and surface coverage of CO are reversibly controlled by the CO pressure. Under UHV conditions, the CO molecules occupy bridge and hollow sites on contiguous Pd clusters in the Au-rich surface layer. Exposure to sub-Torr CO gas induces site switching of the adsorbed CO on the contiguous Pd clusters from multiple-coordination (hollow and bridge) sites to single-coordination (top) sites, even though the latter sites are energetically less favorable. This behavior is explained by a pressure-induced entropic effect on gas-phase CO, which is in equilibrium with the adsorbed CO. This site switching highlights an important aspect of high-pressure-induced adsorption behavior.

Localized dealloying corrosion mediated by self-assembled monolayers used as an inhibitor system.

The structure and chemistry of thiol or selenol self-assembled organic monolayers have been frequently addressed due to the unique opportunities in functionalization of materials. Such organic films can also act as effective inhibition layers to mitigate oxidation or corrosion. Cu-Au alloy substrates covered by self-assembled monolayers show a different dealloying mechanism compared to bare surfaces. The organic surface layer inhibits dealloying of noble metal alloys by a suppression of surface diffusion at lower potentials but at higher applied potentials dealloying proceeds in localized regions due to passivity breakdown. We present an in situ atomic force microscopy study of a patterned thiol layer applied on Cu-Au alloy surfaces and further explore approaches to change the local composition of the surface layers by exchange of molecules. The pattern for the in situ experiment has been applied by micro-contact printing. This allows the study of corrosion protection with its dependence on different molecule densities at different sites. Low-density thiol areas surrounding the high-density patterns are completely protected and initiation of dealloying proceeds only along the areas with the lowest inhibitor concentration. Dealloying patterns are highly influenced and controlled by molecular thiol to selenol exchange and are also affected by introducing structural defects such as scratches or polishing defects.

煮色着色したCu-1 mass%Au合金の電気化学特性

The electrochemical characteristics and corrosion behavior of the Niiro-colored Cu-1 mass%Au alloy have been investigated. The oxidation potential of the Niiro-colored Cu-1 mass%Au alloy is higher than that of a raw Cu-1 mass%Au alloy. The reaction electron number of the Cu-1 mass%Au alloy and the Niiro-colored Cu-1 mass%Au alloy is approximately 1 in 5∼14 pH solutions. The electron number of the Niiro-colored Cu-1 mass%Au alloy is similar to that of Niiro-colored Cu. The Niiro-colored Cu-1 mass%Au alloy surface consists of two layers chemically. The thin upper-layer is a copper (II) oxide such as CuO and the underlayer is a copper (I) oxide such as Cu2O. This structure is the same as that of Niiro-colored Cu. The Niiro-colored Cu-1 mass%Au alloy shows good corrosion resistance when the specimens are exposed at 353 K in 90%RH atmosphere. The corrosion current of the raw Cu-1 mass%Au alloy in a solution containing Cl− of 8.3 pH is higher than that of the Niiro-colored Cu-1 mass%Au alloy. These results indicate that the corrosion resistance of the Niiro-colored Cu-1 mass%Au alloy is higher than that of the raw Cu-1 mass%Au alloy without Cl−. A copper oxide layer formed on the surface of the Cu-1 mass%Au alloy protects the alloy from corrosion.

Photoemission linewidth studies of d-like Tamm states at the Cu(100) and [formula omitted] alloy surfaces

We report a high-resolution study of the well-known Tamm surface state existing on Cu(100) around the M̄ point of the surface Brillouin zone. From temperature-dependent spectra its intrinsic width (full width at half maximum) is extrapolated to be < 13 meV at T → 0. Special attention is given to the linewidth during formation of the Cu(100) Au surface alloy at room temperature, when going from clean Cu(100) to the fully developed Cu(100)-c(2 × 2)Au alloy surface. The observed initial increase in linewidth (coverages ≤0.3 ML) is explained by scattering due to the statistically distributed Au-“defects” incorporated in the surface layer. At coverages between 0.3 and 0.5 ML additional umklapp processes are induced by surface reciprocal lattice vectors supplied by the c(2 × 2) geometry. These umklapps may scatter the photohole to regions where no projected bulk band gaps exist, which results in additional decay channels into bulk states and a correspondingly shortened hole-lifetime.

Tight-binding study of the adsorption of hydrogen on Pt, Au and Pt–Au (111) surfaces

The results of a tight-binding study of the adsorption of hydrogen on Pt, Au and Pt–Au (111) surfaces using the Extended Huckel approximation are presented and discussed. It is shown that, in general, the bond between the adsorbed H atom and the Pt–Au alloy surface is weaker than that observed for pure metals. Therefore, the Pt-rich alloys can be catalytically more active than pure Pt because the weakening of the H–surface bond, due to the presence of inert Au atoms, makes the H atoms on the surface more mobile and more reactive with other adsorbed species. In pure Pt and pure Au, the top site is more stable than the bridge and hollow sites, while for Pt-rich and Au-rich alloys, the different sites show the same stability.

Electrochemically induced surface modifications of Pt-Au alloy

Pt—Au alloy surfaces have been modified electrochemically, by thermal treatment and by sputtering with Ar + ions. The resulting surface compositions were investigated by cyclic voltammetry, XPS and Auger-spectroscopy. On the basis of the different mean escape depths of electrons with different kinetic energies the attempt was made to get additional information about the composition of sub-surface layers. Formic acid oxidation was used as a model reaction for the investigation of the electrocatalytic activity of the different surfaces. The results are best explained by the third-body-mechanism.

Electron spectroscopic study of surface segregation and oxidation of Cu-In and Cu-Au alloys

XPS and AES techniques were employed to study the surface segregation and oxidation of Cu-1at%In alloy. Surface segregation of In has been observed with an enthalpy of segregation of about −34 kJ/mol. The surface oxidation of the Cu-In alloy at 480 K showed first a formation of Cu 2O on the alloy surface. The displacement reaction 3Cu 2O + 2In → 6Cu + In 2O 3 occurred on the alloy surface, on further heating of the oxidized surface in vacuum at 700 K. XPS and UPS techniques were employed to study the oxygen interaction with Cu-5at%Au and Cu-11at%Au alloys. At 300 K, oxygen was dissociatively chemisorbed on the Cu sites of the Cu-11% Au alloy surface, and the oxygen desorbed on heating. TDS study showed that desorption follows second order kinetics with an activation energy of desorption of about 75 kJ/mol.

Auger study of preferred sputtering on Ag–Au alloy surfaces

Auger electron spectroscopy has been used to investigate the preferred sputtering behavior on homogeneous Ag‐Au alloy surfaces. Measurements were made on a range of alloy compositions with Ar+ sputter ions of 0.5–2 keV energy. The time variation of the surface composition during sputtering was analyzed according to a kinetic model which specifically accounts for the composition change caused by sputtering in the altered layer. Based on this analysis, we were able to determine the individual sputter yields for Ag and Au atoms in the alloy and the depth of the altered layer. The sputter yields were found to increase almost linearly with energy but remained almost constant with the alloy composition. The depth of the altered layer was found to be about 15–30 Å.

Segregation at CuAu alloy surfaces

Experimental results for Auger measurements on clean evaporated CuAu alloy films having (111)-orientation are presented. Signals from Auger transitions at 72 eV, 239 eV, and 2024 eV from Au in the alloys were normalized to signals from pure Au references. The experimental data were converted to atomic layer compositions using a model which allowed the first two atomic layers to differ in composition from the bulk and using estimates of the energy dependence of the electron attenuation length derived from published results. Significant enrichment of the first layer with Au was found over the entire range of composition studied.

Abstract: Segregation at Cu−Au alloy surfaces

Theoretical considerations indicate that surface segregation in alloys should be confined to the outer few atomic layers. A model in which the compositions of only the first two atomic layers can differ from the bulk composition was used to interpret in situ Auger measurements on clean Cu−Au(111) evaporated alloy films. Exponential attenuation lengths independent of alloy composition were assumed to be 4, 8, and 30 Å for Au peaks at 72, 239, and 2024 eV. After normalization of alloy Au Auger intensities to those from pure Au, the data were analyzed to determine the compositions of the first two layers and the bulk, including a correction for the change in lattice parameter with bulk composition. The results indicate that segregation in Cu−Au is confined essentially to the top layer, which is gold rich for the large range of compositions studied. Uncertainties associated with backscattering effects, limitations in knowledge of escape depths, and possible lattice parameter changes near the surface are discussed. The observed gold enrichment agrees qualitatively with predictions based on the lower surface energy of gold.