Surface Metal Atoms Research Articles

Electrical characterization of Cu Schottky contacts to n-type GaAsN grown on (311)A/B GaAs substrates

Abstract The contact behavior of Cu on n-type GaAsN, grown on (100) and (311)A/B GaAs substrates by chemical beam epitaxy, has been investigated by current–voltage (I–V) and capacitance–voltage (C–V) measurements. Both N incorporation and growth orientation are found to influence the electrical properties of Cu/GaAsN Schottky diodes. The increasing N composition leads to an increase in the ideality factor and Schottky barrier height (C–V), whereas a decrease in the carrier concentration in samples with all adopted growth orientations. Compared with (100) and (311)B samples, the Schottky diodes constructed on (311)A GaAsN epilayer exhibit better performance with ideality factor close to 1 and higher barrier height. The interfacial chemical reaction between metal layer and surface atoms on the growth surface is believed to account for the observations.

Electrospun Ni nanofibres as Pt supports for PEMFC electrodes

Nickel nanofibres were prepared by electrospinning followed by specific thermal treatments. Pt/Ni fibrous electrocatalysts were obtained by galvanic displacement of the metal surface atoms with platinum. A novel, very fast Ni displacement assisted by microwaves is described. The Pt/Ni nanofibres were characterised for their activity towards oxygen reduction for their application in PEMFC electrodes.

Lattice-Directed Formation of Covalent and Organometallic Molecular Wires by Terminal Alkynes on Ag Surfaces.

Surface reactions of 2,5-diethynyl-1,4-bis(phenylethynyl)benzene on Ag(111), Ag(110), and Ag(100) were systematically explored and scrutinized by scanning tunneling microscopy, molecular mechanics simulations, and density functional theory calculations. On Ag(111), Glaser coupling reaction became dominant, yielding one-dimensional molecular wires formed by covalent bonds. On Ag(110) and Ag(100), however, the terminal alkynes reacted with surface metal atoms, leading to one-dimensional organometallic nanostructures. Detailed experimental and theoretical analyses revealed that such a lattice dependence of the terminal alkyne reaction at surfaces originated from the matching degree between the periodicities of the produced molecular wires and the substrate lattice structures.

Stability of surface and subsurface hydrogen on and in Au/Ni near-surface alloys

Periodic, self-consistent DFT-GGA (PW91) calculations were used to study the interaction of hydrogen atoms with the (111) surfaces of substitutional near-surface alloys (NSAs) of Au and Ni with different surface layer compositions and different arrangements of Au atoms in the surface layer. The effect of hydrogen adsorption on the surface and in the first and second subsurface layers of the NSAs was studied. Increasing the Au content in the surface layer weakens hydrogen binding on the surface, but strengthens subsurface binding, suggesting that the distribution of surface and subsurface hydrogen will be different than that on pure Ni(111). While the metal composition of the surface layer has an effect on the binding energy of hydrogen on NSA surfaces, the local composition of the binding site has a stronger effect. For example, fcc hollow sites consisting of three Ni atoms bind H nearly as strongly as on Ni(111), and fcc sites consisting of three Au atoms bind H nearly as weakly as on Au(111). Sites with one or two Au atoms show intermediate binding energies. The preference of hydrogen for three-fold Ni hollow sites alters the relative stabilities of different surface metal atom arrangements, and may provide a driving force for adsorbate-induced surface rearrangement.

Electrochemical and Quantum Chemical Assessment of 2-Aminothiazole as Inhibitor for Carton Steel in Sulfuric Acid Solution

Corrosion is the destructive attack of metal or alloy chemically or electrochemically against its environment which leads to the loss of useful properties of materials. Several approaches have been proposed to minimize or prevent metal dissolution; one of these approaches is the use of substances called corrosion inhibitors. Inhibitors can adhere to a metal surface to form a protective barrier against corrosive agents in contact with metal. The effectiveness of an inhibitor to provide corrosion protection depends to a large extent on the interaction between the inhibitor and the metal surface. Most well-known acid inhibitors are organic compounds with π-electrons and functional groups containing heteroatoms which can donate lone-pair electrons. Clarification of the interaction between inhibitor molecules and surface metal atoms at the molecular level is important in terms of finding new and effective inhibitors. Quantum chemical calculations are proved to be a very powerful tool for studying the inhibition mechanism. Adsorption-related electronic properties, adsorption geometries and energies, charge transferred from or toward the surface or a full description of the bonding electronic structure, are useful and can be achieved by means of density function theory methodology. 2-Aminothiazole is a heterocyclic organic compound with different anchoring sites suitable for surface bonding: nitrogen atom with its lone sp electron pair, the sulfur atom, the amino Electrochemical and Quantum Chemical Assessment of 2-Aminothiazole as Inhibitor for Carton Steel in Sulfuric Acid Solution

Corrosion resistance and adsorption behavior of bis-(γ-triethoxysilylpropyl)-tetrasulfide self-assembled membrane on 6061 aluminum alloy

The model before (left) and after (right) the dynamics simulation. It can be deduced that the formation of the self-assembled membrane was mainly dependent on interaction between the silicon oxygen bond and the metal surface atom.

Ligand- and oxygen-isotope-exchange pathways of geochemical interest

Environmental context Most chemical processes in water are either ligand- or electron-exchange reactions. Here the general reactivity trends for ligand-exchange reactions in aqueous solutions are reviewed and it is shown that simple rules dominate the chemistry. These simple rules shed light on most molecular processes in water, including the uptake and degradation of pesticides, the sequestration of toxic metals and the corrosion of minerals. Abstract It is through ligand-exchange kinetics that environmental geochemists establish an understanding of molecular processes, particularly for insulating oxides where there are not explicit electron exchanges. The substitution of ligands for terminal functional groups is relatively insensitive to small changes in structure but are sensitive to bond strengths and acid–base chemistry. Ligand exchanges involving chelating organic molecules are separable into two classes: (i) ligand substitutions that are enhanced by the presence of the chelating ligand, called a ‘spectator’ ligand and (ii) chelation reactions themselves, which are controlled by the Lewis basicity of the attacking functional group and the rates of ring closure. In contrast to this relatively simple chemistry at terminal functional groups, substitutions at bridging oxygens are exquisitely sensitive to details of structure. Included in this class are oxygen-isotope exchange and mineral-dissolution reactions. In large nanometer-sized ions, metastable structures form as intermediates by detachment of a surface metal atom, often from a underlying, highly coordinated oxygen, such as μ4-oxo, by solvation forces. A metastable equilibrium is then established by concerted motion of many atoms in the structure. The newly undercoordinated metal in the intermediate adds a water or ligand from solution, and protons transfer to other oxygens in the metastable structure, giving rise to a characteristic broad amphoteric chemistry. These metastable structures have an appreciable lifetime and require charge separation, which is why counterions affect the rates. The number and character of these intermediate structures reflect the symmetry of the starting structure.

Influence of Adsorbed Water on the Oxygen Evolution Reaction on Oxides

We study the interface between adsorbed water and stoichiometric, defect-free (110) rutile oxide surfaces of TiO2, RuO2, and IrO2 in order to understand how water influences the stabilities of the intermediates of the oxygen evolution reaction (OER). In our model the water is treated as explicitly adsorbed H2O molecules, which are found to form two-dimensional water chains (layers) on all investigated oxide surfaces. The first chain formed by the most strongly bound H2O molecules is adsorbed on the 5-fold coordinated surface metal atoms. The second chain is composed of less strongly bound H2O molecules binding to bridging oxygens. The third chain interacts weakly and predominantly with the H2O molecules of the second layer, resembling bulk water. We find that the stability of the water layer close to the oxide surface is almost the same as the one found on flat metal surfaces, such as the Pt(111) surface, despite the highly different adsorption pattern of the water molecules. We show that the presence of a water network has some effect on the interaction of individual intermediates of the OER with the oxide surface. However, the theoretical OER overpotential remains almost unchanged in the case of RuO2 and IrO2, while it is increased by ∼0.4 eV for TiO2.

Carbon-Induced Surface Transformations of Cobalt

A reactive force field has been developed that is used in molecular dynamics (MD) studies of the surface transformation of the cobalt (0001) surface induced by an overlayer of adsorbed carbon atoms. Significant surface reconstruction is observed with movement of the Co atoms upward and part of the C atoms to positions below the surface. In a particular C ad atom coverage regime step edge type surface sites are formed, which can dissociate adsorbed CO with a low activation energy barrier. A driving force for the surface transformation is the preference of C adatoms to adsorb in 5- or 6-fold coordinated sites and the increasing strain in the surface because of the changes in surface metal atom–metal atom bond distances with the increasing surface overlayer concentration. The process is found to depend on the nanosize dimension of the surface covered with carbon. When this surface is an overlayer on top of a vacant Co surface, it can reduce stress by displacement of the Co atoms to unoccupied surface positio...

Selenium adsorption at different coverages on Fe(1 0 0) and Fe(1 1 1): A DFT study

Se adsorption on Fe(1 0 0) and (1 1 1) surfaces is examined using the density functional theory (DFT). Selenium is adsorbed in a distorted bridge on the (1 1 1) surface while in the (1 0 0) surface it prefers a 4-fold hollow site, with energies of −10.36 and −5.25 eV, respectively. Se adsorption results in surface reconstruction. There is some contraction in the case of the (1 0 0) plane for 1/4 and 1/2 ML coverage and some relaxation at 1 ML (4.5%). Contraction increases to 15% for the (1 1 1) plane at ¼ ML. At a higher coverage, there is a non-regular movement of surface metal atoms, and there is almost no change at 1 ML. The magnetic moment for surface Fe atoms decreases with coverage. The most important changes are in the (1 0 0) plane, followed by the (1 1 0) and then the (1 1 1) planes with a reduction of 52%, 24% and 7% respectively. The density of states presents a contribution of Se states at −5.0 and −13.1 eV, when stabilized after adsorption. The Fe–Fe bond weakening is higher in the (1 0 0) plane. Fe–Se bonds are formed at the expense of the metallic bond.

Selective hydrogen production from formic acid decomposition on Pd-Au bimetallic surfaces.

Pd-Au catalysts have shown exceptional performance for selective hydrogen production via HCOOH decomposition, a promising alternative to solve issues associated with hydrogen storage and distribution. In this study, we utilized temperature-programmed desorption (TPD) and reactive molecular beam scattering (RMBS) in an attempt to unravel the factors governing the catalytic properties of Pd-Au bimetallic surfaces for HCOOH decomposition. Our results show that Pd atoms at the Pd-Au surface are responsible for activating HCOOH molecules; however, the selectivity of the reaction is dictated by the identity of the surface metal atoms adjacent to the Pd atoms. Pd atoms that reside at Pd-Au interface sites tend to favor dehydrogenation of HCOOH, whereas Pd atoms in Pd(111)-like sites, which lack neighboring Au atoms, favor dehydration of HCOOH. These observations suggest that the reactivity and selectivity of HCOOH decomposition on Pd-Au catalysts can be tailored by controlling the arrangement of surface Pd and Au atoms. The findings in this study may prove informative for rational design of Pd-Au catalysts for associated reactions including selective HCOOH decomposition for hydrogen production and electro-oxidation of HCOOH in the direct formic acid fuel cell.

Erosion of Electrochemically Generated Metastable Metal Surfaces (MMS) on Gold and Platinum Electrodes

Erosion of Electrochemically Generated Metastable Metal Surfaces (MMS) on Gold and Platinum Electrodes

Dissociative adsorption of 2,3,7,8-TCDD on the surfaces of typical metal oxides: a first-principles density functional theory study

The initial dissociative adsorption step of the 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) molecule on the surfaces of MgO, CaO, and CuO has been studied by density functional theory (DFT) using periodic slab models. It is found that the 2,3,7,8-TCDD molecule undergoes a similar dissociative adsorption step during the decomposition over the three metal oxide surfaces. The adsorption configuration of 2,3,7,8-TCDD first converts from a parallel mode into a vertical one, then a nucleophilic substitution process takes place, where the surface oxygen atom attacks the aromatic carbon to form a surface phenolate with the chlorine atom moving to the top of the nearest surface metal atom. The calculated apparent activation energy of the dissociation increases in the order of CuO < CaO < MgO. The reaction heat is -0.67 eV, -0.75 eV, and 0.45 eV for CuO, CaO, and MgO, respectively, suggesting the thermodynamic tendency of MgO < CuO < CaO, which parallels the trend of the nucleophilicity of surface oxygen atoms. This study suggests that metal oxides with more nucleophilic and less tightly-bonded surface oxygen atoms might be more promising for the decomposition of polychlorinated dibenzo-p-dioxins and dibenzofurans.

ChemInform Abstract: Anchored Metal Nanoparticles: Effects of Support and Size on Their Energy, Sintering Resistance and Reactivity

AbstractReview: 160 refs.

Éruptions cutanées psoriasiformes sous-abatacept

The effect of Ru addition upon catalyst characteristics and performance in Fischer–Tropsch synthesis was investigated using a series of differently prepared, Ru-promoted Co/Al2O3catalysts. Ru promotion facilitated reduction of Co/Al2O3leading to catalysts with higher reducibilities. Parallel to this effect, the number of reduced metal atoms exposed on the surface increased for the Ru-promoted catalysts. Ru-promoted catalysts showed an increase in CO hydrogenation activity; however, constant turnover frequencies for promoted and unpromoted catalysts suggest that the increase in the number of metal surface atoms caused the activity enhancement. Isotopic transient kinetic analysis of CO hydrogenation under methanation conditions gave identical results. Catalysts prepared by a co impregnation technique, avoiding chloride salts, appear to be optimal for the production of high-molecular-weight hydrocarbons.

Initial adsorption of water molecule on HfC and TaC (0 0 1) surfaces from density-functional study

We investigated the initial adsorption of molecular water on HfC and TaC (001) surfaces using first-principles density-functional theory. In our investigation, the revised Perdew–Burke–Ernzerhof functional (RPBE) generalized gradient approach (GGA) was used to treat the exchange and correlation potential. We modeled the (001) surfaces with (√2×√2) R45° supercell. Every supercell contains five atomic planes. Our calculations show that the adsorption energies of H2O are low. The energies are −0.44eV/molecule and −0.65eV/molecule on HfC and TaC (001) surfaces, respectively. On both the (001) surfaces, molecular water prefers to be adsorbed on top of surface metal atom. However, only the oxygen atom of water molecule is bound with surface metal atom. And the plane of water molecule tends to be parallel to the surfaces. Compared with the oxygen and hydrogen atoms of an isolated water molecule, the oxygen and hydrogen atoms in H2O/HfC (001) and H2O/TaC (001) have less negative and positive charges, respectively. Our results show that water molecule is adsorbed molecularly on HfC and TaC (001) surfaces at initial adsorption stage. Moreover, the interaction of water molecule with the (001) surfaces is weak. Charge density differences and density of states show that OTa bond is slightly stronger than OHf bond. Due to the strong OTa bond, the oxidation resistance of TaC may stronger than HfC in water vapor atmosphere.

Adsorption of S2− and HS− ions on the (111) face of coinage metals: A quantum-chemical study

The interactions of S2− and HS− ions with gold, silver, and copper were studied by density functional theory using the cluster model of the metal surface. The geometrical and energy characteristics of the interactions of these ions with the surface metal atoms were evaluated. The S2− ions form stronger chemical bonds with the surface metal atoms than HS− ions. A significant charge transfer from anion to metal occurs during the adsorption. The adsorbability increased in the series Ag < Cu < Au for both anions. The HS− ion showed greater ability to be transferred to the surface during the electrochemical adsorption due to the strong hydration of the S2− ion. In alkaline media, however, the dissociation of the adsorbed HS− leads to its conversion to S2−.

On the composition of bimetallic near-surface alloys in the presence of oxygen and carbon monoxide

Periodic, self-consistent density functional theory calculations (GGA-PW91) are used to examine surface segregation in close-packed bimetallic Pt-overlayer alloy surfaces (Pt*/M, M=Au, Ag, Cu, Pd, Ir, Rh, Os, Ru, and Re) in different environments. In particular, we find that the thermodynamically stable surface termination in these Pt*/M alloys can be inverted from Pt-terminated in vacuum to M-terminated under exposure to oxygen (for an M that is more oxophilic than Pt). Interestingly, in many of these alloys, Pt is not driven into the bulk; rather it remains in the first subsurface layer where it enhances oxygen binding through a ligand interaction with the surface metal atoms. On the other hand, exposure to CO provides a much milder driving force for the surface composition inversion. To quantify segregation under catalytically relevant conditions, we constructed approximate phase diagrams for the PtRu system as a function of O2 and CO chemical potential (temperature, pressure). The results show that the surface termination inverts with many orders of magnitude higher CO pressure than with O2.

Oxygen induced transformations of the δ-Pu(111) surface

The oxides that form on metal surfaces are critical to the preservation of the material under oxidizing conditions. For the case of plutonium, oxide film properties may affect a variety of kinetic phenomena related to the corrosion of the material. To investigate the behavior of the metal–oxygen interactions critical to the formation of such passive films, electronic structure calculations were performed on the interactions between O and the δ-Pu(111) surface. By probing the relationship between structure (coordination), charge transfer (valence) and thermodynamics (reactivity), it was found that oxidation is initially a local phenomenon, but, at 1.0ML oxygen coverage, significant reconstructions that alter the phase structure of the near-surface region are induced. At this high-surface coverage the surface metal atoms are disbonded and the PuO coordination becomes four-fold as opposed to three fold. Even at this high coverage, however, the charge state more closely resembles Pu2O3 rather than PuO2. The ability for surface Pu atoms to adapt to a variety of changes in charge state and coordination provides a fundamental basis for understanding the multilayer structure of passive films formed on clean Pu surfaces. An analysis using first-principles thermodynamics also suggests that these films should develop uniformly, and not via an island or columnar growth mechanism.

Glycerol Hydrogenolysis to Propylene Glycol and Ethylene Glycol on Zirconia Supported Noble Metal Catalysts

Monoclinic zirconia (m-ZrO2) supported Ru, Rh, Pt, and Pd nanoparticles with controlled sizes were prepared and examined in glycerol hydrogenolysis to propylene glycol and ethylene glycol at similar conversions in the kinetic regime. Their activity (normalized per exposed surface metal atom, i.e., turnover rate) and selectivity depend sensitively on the nature of the noble metals and their particle size. At a similar size (ca. 2 nm), Ru exhibited a greater turnover rate than Rh, Pt, and Pd, and the rate decreased in the sequence Ru ≫ Rh > Pt > Pd by a factor of about 25 (from 0.035 to 0.0014 mol glycerol (mol surface metal·s)−1) at 473 K and 6.0 MPa H2. Following such activity sequence, Ru was more prone to catalyze excessive cleavage of C–C bonds, leading to the formation of ethylene glycol and methane, while Pd exhibited the highest selectivity to cleavage of C–O bonds to propylene glycol. Similarly, larger Ru particles possessed higher glycerol hydrogenolysis activity concurrently with higher selectivi...