A new class of electrocatalysts for hydrogen production from water electrolysis: metal monolayers supported on low-cost transition metal carbides

This work explores the opportunity to substantially reduce the cost of hydrogen evolution reaction (HER) catalysts by supporting monolayer (ML) amounts of precious metals on transition metal carbide substrates. The metal component includes platinum (Pt), palladium (Pd), and gold (Au); the low-cost carbide substrate includes tungsten carbides (WC and W(2)C) and molybdenum carbide (Mo(2)C). As a platform for these studies, single-phase carbide thin films with well-characterized surfaces have been synthesized, allowing for a direct comparison of the intrinsic HER activity of bare and Pt-modified carbide surfaces. It is found that WC and W(2)C are both excellent cathode support materials for ML Pt, exhibiting HER activities that are comparable to bulk Pt while displaying stable HER activity during chronopotentiometric HER measurements. The findings of excellent stability and HER activity of the ML Pt-WC and Pt-W(2)C surfaces may be explained by the similar bulk electronic properties of tungsten carbides to Pt, as is supported by density functional theory calculations. These results are further extended to other metal overlayers (Pd and Au) and supports (Mo(2)C), which demonstrate that the metal ML-supported transition metal carbide surfaces exhibit HER activity that is consistent with the well-known volcano relationship between activity and hydrogen binding energy. This work highlights the potential of using carbide materials to reduce the costs of hydrogen production from water electrolysis by serving as stable, low-cost supports for ML amounts of precious metals.     

Electrochemistry of metal nanoparticles: the effect of substrate

It is shown that nanoparticles localized on a foreign solid electrode may display two opposite shifts of dissolution potential, namely, a shift toward either more positive or more negative values as compared with the equilibrium potential of M ⁿ⁺/M ⁰ or the potential of bulk metal electrooxidation. The observed phenomena are interpreted in view of three energy states of substance, which are realized depending on contribution of the surface Gibbs free energy (ΔG°) to the energy of the system. Literature data concerning different metal-substrate pairs and specially conducted experimental investigations of electrooxidation of gold nanoparticles (radius equal to 10 and 150?nm), which are localized on the surface of glassy carbon, bulk gold, and platinum electrodes are presented and discussed. A shift of maximum current potential of small nanoparticles oxidation toward more positive values is observed in this series. The oxidation potential of large nanoparticles is not affected by the nature of the substrate. In all cases, electrooxidation of gold nanoparticles occurs at the more negative potentials than those of the bulk gold electrooxidation. It is shown that depending on the nature of the substrate and nanoparticle size, the dominating effect is either interaction of nanoparticles with the substrate (ΔG°?<?0) and electrochemical potential shifts toward positive values or impact of surface Gibbs free energy of nanoparticles (ΔG°?>?0) into energy of the system and electrochemical potential shifts toward negative values. The validity of the proposed assumptions is confirmed by good correlation of literature and our experimental data with calculated ones.     

An Au clusters related spill-over sensitization mechanism in SnO(2)-based gas sensors identified by operando HERFD-XAS, work function changes, DC resistance and catalytic conversion studies

The role of Au additives in SnO(2)-based thick film gas sensors was investigated by a combination of operando investigation techniques, namely spectroscopic high energy resolved fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and simultaneous DC resistance and work function change measurements. The results have shown that the Au is present in the form of small metallic particles at the surface of the host metal oxide without changing its bulk or surface electronic properties. The sensitization effect of Au can therefore be attributed to the "spill-over effect", meaning that the Au particles enrich the surface of the active metal oxide with oxygen species which consequently react with reducing gases such as CO and H(2). This is in contrast to the effect of Pd and Pt promoters which were found to be distributed at an atomic level on the surface and in the bulk of the supporting sensing material and therefore have a tremendous effect on its bulk and surface electronic properties.     

Embedded cluster model studies of impurities at metal surfaces

A knowledge of the electronic properties of impurities at metal surfaces is of great value in the understanding of such important phenomena as chemisorption and surface segregation in alloys. We have adopted here a unified approach based on an Embedded Cluster model to study the properties of surface impurities. We have mainly concentrated on hydrogen impurities either adsorbed above the surface or incorporated into the bulk of metals. We have also considered the case of substitutional metal impurities at the surface of host metals.For hydrogen chemisorption we have considered such substrates as free-electron, transition and noble metals as well as bimetallic substrates composed of a single metal impurity in a host matrix or a metallic overlayer on a metal support. The electronic structure of the chemisorbed system is compared to photoemission data when available, from which interpretation of the details of the experimental spectra may be made. It is found that hydrogen adsorption on transition and noble metals results in the formation of a pair of bonding/antibonding resonances on either side of the metal d-band, while for hydrogen on free-electron metals a single hydrogen induced resonance is observed. One-electron energy differences between the H on jellium and H on metal systems are estimated and trends in such energies across the 3d and 4d transition series are compared to the trends in experimental chemisorption energies for H on these metals. The change in hydrogen chemisorption capacity of an inert substrate due to the introduction of chemically active impurities is investigated. The different properties of Pd overlayers with respect to Pd surfaces are also investigated. Interaction energies between adatoms on surfaces are estimated in order to predict the geometry of ordered structures on surfaces.One-electron heats of segregation for binary alloys are calculated. These show a strong solute surface segregation for noble metal impurities in group VIII metals, which is due to the higher d-band occupancy of the noble metal.     

过渡金属碳化物(111)面电子结构的理论研究

采用第一性原理的密度泛函方法对过渡金属碳化物MC(111)清洁表面构型和电子结构进行系统研究.结果表明,与理想表面相比,表面弛豫导致表层金属原子和次表层碳原子分别朝体相和真空方向位移,从而导致层间距的收缩.由能带计算结果得知,紧邻或被EF穿越的活性表面态成分均为表面金属原子的dxz/dyz轨道.进一步考察了弛豫对表面态组成的影响,并对表面芯能级位移和功函进行了探讨.     

Diamondoids approach to electronic,structural, and vibrational properties of GeSi superlattice nanocrystals: a first-principles study

Germanium silicide diamondoids are used to determine electronic, structural, and vibrational properties of GeSi superlattice nanocrystals and bulk as their building block limit. Density functional theory at the generalized gradient approximation level of Perdew, Burke, and Ernzerhof (PBE) with 6-31G(d) basis including polarization functions is used to investigate the electronic structure of these diamondoids. The investigated molecules and diamondoids range from GeSiH₆ to Ge₆₃Si₆₃H₉₂. The variation of the energy gap is shown from nearly 7 eV toward bulk value which is slightly higher than the average of Si and Ge energy gaps. Variations of bond lengths, tetrahedral, and dihedral angles as the number of atoms increases are shown taking into account the effect of shape fluctuations. Localized and delocalized electronic charge distribution and bonds for these molecules are discussed. Vibrational radial breathing mode (RBM) converges from its initial molecular value at 332 cm^?1 to its bulk limit at 0 cm^?1 (blue shift). Longitudinal optical-highest reduced mass mode (HRMM) converges from its initial molecular value 332 cm^?1 to experimental bulk limit at 420.7 cm^?1 (red shift). Hydrogen vibrational modes are nearly constant in their frequencies as the size of diamondoids increases in contrast with lower frequency Ge–Si vibrational modes. GeSi diamondoids can be identified from surface hydrogen vibrational modes fingerprint, while the size of these diamondoids can be identified from Ge–Si vibrational modes.     

Recent developements in charge-transfer theory

In this paper we discuss some recent theoretical developments of importance in the area of charge transfer between atoms and surfaces. Using the complex scaling method we have calculated the energy shift and broadening of atomic levels near metal surfaces. Two novel applications will be discussed. The first concerns the interaction of atomic Rydberg levels with clean metal surfaces. It is shown that as Rydberg atoms approach a surface, strong hybridization occurs that depends sensitively on both the atom-surface separation and the details of the surface potential. The widths of the hybridized states can differ by several orders of magnitude depending on their orientation with respect to the surface. The second application is an investigation of how dielectric overlayers adsorbed on metal surfaces can influence the energy shift and broadening of atomic levels. The calculations show that the energies and widths of atomic levels near metal surfaces can be influenced strongly by thin dielectric films adsorbed on the surface.     

Surface Tension Calculations for Liquid Metals

Abstract

We present a class of models for the surface of a liquid metal, which may be part of an electrochemical interface. The particles of the system, for the purpose of derivation of thermodynamic properties, are the charged ion cores, while the energy of the electrons is evaluated using the electron density functional formalism, previously principally applied to solids. An expression for the surface energy U^s , defined as the energy required to create unit area of surface by separation of a volume of homogeneous metal into two parts, is derived (Eqs. 18–20). The surface tension γ is obtained by differentiating the Helmholtz free energy with respect to the area of the system, keeping volume and particle number constant (Eqs. 27–37). The surface tension is also equal to the difference between the free energy of the system containing a surface and the free energy of a reference system. It thus defines a surface energy through the Gibbs-Helmholtz equation, and this surface energy is shown to be identical to U^s .

The expressions for U^s and γ are made explicit (Eqs. 45–57) by insertion of particular assumptions for the ion-density profile, the electron-density profile, the interionic interaction and pair distribution function, and the electronic energy. Only information about bulk liquid metal is used. The parameter in the electron-density profile is obtained by minimizing the surface energy. The simplest assumption for the interionic interaction, hard-sphere and Coulombic repulsions, requires a choice for the hard-sphere diameter, which is made such that the pressure of bulk metals is given correctly (52–55). For the alkali metals, the surface tension calculated from this model is about half the experimental value in each case, while calculated surface energies are too high (1/5 too high for Cs, but three times too high for Li). For the electrical potential difference between the inside and the outside of a metal, and for the electrochemical potential, agreement with experiment is good. The main reason for the disagreements in the other properties is traced to the simple form used for the ion pair distribution function.     

Communication: Systematic shifts of the lowest unoccupied molecular orbital peak in x-ray absorption for a series of 3d metal porphyrins

Porphyrins are widely used as dye molecules in solar cells. Knowing the energies of their frontier orbitals is crucial for optimizing the energy level structure of solar cells. We use near edge x-ray absorption fine structure (NEXAFS) spectroscopy to obtain the energy of the lowest unoccupied molecular orbital (LUMO) with respect to the N(1s) core level of the molecule. A systematic energy shift of the N(1s) to LUMO transition is found along a series of 3d metal octaethylporphyrins and explained by density functional theory. It is mainly due to a shift of the N(1s) level rather than a shift of the LUMO or a change in the electron-hole interaction of the core exciton.     

Effects of carbon on the stability and chemical performance of transition metal carbides: a density functional study

Density functional theory was employed to study the stabilities and chemical activities of transition metal carbides. Here we take the well-known Mo carbides and Ti carbides as an example. Different kinds of structures including the bulk surfaces [Mo(2)C(001), MoC(001), and TiC(001)] and metcars [Mo(8)C(12) and Ti(8)C(12)] are taken into consideration. Systematic studies show that by raising the C coordination number of the metal atoms in the carbides, in general the stability of the carbides increases (metcars are an exception since they include both high-coordinated and low-coordinated metal atoms.); at the same time, the chemical activities of the carbides decrease due to a downshift of the metal d-band center (ligand effect). Considering the better catalysts those that combine high stability and moderate chemical activity, our results suggest that the catalytic potential of Mo carbide systems should decrease in the following sequence: Mo(8)C(12)>Mo(2)C(001) or MoC(001)>pure Mo(110). In spite of having the largest C/Mo ratio, the metcar appears as the most attractive system. Our studies also indicate that the "magic" behavior of metcars is not unique for Mo carbides. Similar behavior is also observed for Ti carbides. This implies that nanoparticles like metcar species could exhibit better performances than the corresponding bulk metal carbides as catalysts.     

Tuning of redox properties of iron and iron oxides via encapsulation within carbon nanotubes

We report the tuning of the redox properties of iron and iron oxide nanoparticles by encapsulation within carbon nanotubes (CNTs) with varying inner diameters. Raman spectroscopy was employed to investigate the interaction of the encapsulated nanoparticles with the CNTs. A red shift of the Fe-O mode is observed in the nanoparticles deposited on the outer CNT surfaces with respect to bulk Fe2O3. However, this mode is found to be stepwise blue-shifted with decreasing inner diameter in the CNT-encapsulated Fe2O3 nanoparticles, suggesting an enhanced interaction of Fe2O3 with the inner CNT surface as its curvature increases. The autoreduction of the encapsulated Fe2O3 is significantly facilitated inside CNTs with respect to the outside nanoparticles. Interestingly, it becomes more facile with decreasing CNT channel diameter as evidenced by temperature programmed reaction, in situ XRD, and Raman spectroscopy. The oxidation of encapsulated metallic Fe nanoparticles on the other hand is retarded in comparison to that of the outside Fe particles as shown by in situ XRD and gravimetrical measurements with an online microbalance. We attribute this tunable redox behavior of transition metal nanoparticles inside CNTs to a particular electronic interaction of the encapsulates with the interior CNT surface, which stabilizes the metallic state of Fe.     

Origin of Extraordinary Stability of Square‐Planar Carbon Atoms in Surface Carbides of Cobalt and Nickel

Surface carbides of cobalt and nickel are exceptionally stable, having stabilities competitive with those of graphitic C on these surfaces. The unusual structure of these carbides has attracted much attention: C assumes a tetracoordinate square‐planar arrangement, in‐plane with the metal surface, and its binding favors a spontaneous p4g clock surface reconstruction. A chemical bonding model for these systems is presented and explains the unusual structure, special stability, and the reconstruction. C promotes local two‐dimensional aromaticity on the surface and the aromatic arrangement is so powerful that the required number of electrons is taken from the void M₄ squares, thus leading to Peierls instability. Moreover, this model predicts a series of new transition‐metal and main‐group‐element surface alloys: carbides, borides, and nitrides, which feature high stability, square‐planar coordination, aromaticity, and a predictable degree of surface reconstruction.     

A discussion on the energies involved in specific adsorption of ions

The change in specific adsorption of I^? ions on the series of metals Au, Hg, Bi, Pb, Cd, and Ga is analyzed using data of specifically adsorbed charge and shift in potential of zero charge. Factors determining the change in adsorbability are discussed in the light of previous formulations. It is shown that the work connected with water desorption as an ion becomes adsorbed, usually neglected or underestimated in previous discussions, is very likely to be the main factor determining the change in adsorbability along the series of metals. A rough estimation of energies involved in water desorption suggests that metal—water surface bonds are probably weak on sp-metals so that they are unable to affect the reactivity of metal surfaces with respect to the gas phase as strong covalent surface bonds are involved, for instance in the hydrogen evolution reaction. Conversely, the strong effect of water desorption on the specific adsorption of ions may be an indication of ion—metal interactions to be substantially independent of the nature of the metal. This suggests that covalent contributions to the surface bond are apparently minor for metals more electropositive than Au.     

Inverse photoemission from metal surfaces

Ultraviolet inverse photoemission spectroscopy (IPES) is a technique for exploring unoccupied electronic states, particularly in the energy range between the Fermi level and vacuum level, a range inaccessible in ordinary photoemission. Theories of inverse photoemission and its special instrumentation requirements are outlined. IPES measurements on clean metal surfaces have revealed an abundance of new Shockley surface states and the Rydberg series of image states converging on the vacuum level. Empty surface states of d-like character have also been seen. The systematics of the occurrence of surface states (including image states) associated with s,p bulk band gaps are well described by a simple adaptation of multiple-reflection theory. This model is propounded and its implications discussed with regard to effective masses, surface corrugation, and determination of the surface barrier potential. IPES measurements on adsorbate-covered metal surfaces have revealed antibonding levels (O on Ni surfaces being the prime example) and valuable information on empty molecular levels (the 2π state in adsorbed CO and NO being the prime example). A straightforward Blyholder interpretation is modified by considerations of electronic relaxation and screening in the emission process. We compare and contrast the role of these effects in photoemission and inverse photoemission. Polarization selection rules and molecular shape resonances are also discussed.     

Aggregates of rod-coil diblock copolymers adsorbed at a surface

The behavior of rod-coil diblock copolymers close to a surface is discussed by using extended scaling methods. The copolymers are immersed in selective solvent such that the rods are likely to aggregate to gain energy. The rods are assumed to align only parallel to each other, such that they gain a maximum energy by forming liquid crystalline structures. If an aggregate of these copolymers adsorbs with the rods parallel to the surface the rods shift with respect to each other to allow for the chains to gain entropy. It is shown that this shift decays with increasing distance from the surface. The profile of this decay away from the surface is calculated by minimization of the total free energy of the system. The stability of such an adsorbed aggregate and other possible configurations are discussed as well.     

An in situ FTIR study of the electrochemical oxidation of methanol at small platinum particles

It is shown for the first time that it is possible to employ electrochemical in situ Fourier transform infrared (FTIR) spectroscopy to study the electro-oxidation of methanol at small platinum particles. A number of differences with respect to the behaviour at a bulk metal electrode are observed; these observations demonstrate beyond doubt the very significant difference between bulk and particle electrode mechanisms for the oxidation of methanol, and reveal the dangers of extrapolating from the former to the latter.     

原子数精确的金属纳米团簇在电催化领域的应用研究进展

金属纳米团簇(MNCs)是由几个到数百个金属原子组成,其尺寸一般小于2nm.金属纳米团簇在许多催化反应中表现出高的催化活性和选择性,这与金属纳米团簇具有高的比表面积、较多暴露的活性原子,以及与金属纳米粒子(MNPs)不同的电子结构有关.金属纳米团簇确定的组成和结构使其成为一种新型模型催化剂,对纳米团簇的催化性能研究有利...     

SnO2(110)弛豫表面构型与电子结构的第一性原理研究

采用基于第一性原理的密度泛函方法对SnO₂(110)表面的构型和电子结构进行了系统研究. 结果表明, 与理想表面相比, 表面弛豫导致表层五配位Sn原子向体相方向位移, 六配位Sn原子以及表面氧原子往真空方向移动, 而桥氧原子位置基本保持不变. 当表面厚度小于3 nm时, 表面能和表层原子的弛豫大小随着层数的增加出现振荡现象. 由能带计算结果得知, 以桥氧的2p_y/2i>p_z轨道为主要成分的能带出现在体相的带隙中. 进一步考察了弛豫对表面电子结构的影响.     

Carbon on transition metal surfaces

The review surveys the conditions of formation and properties of four forms of surface carbon on transition metals, to wit, adsorbed atoms and clusters, surface carbide and graphite, and their role in the physical and chemical processes on the surface. The first-order phase transition in the adlayer, when graphite islands coexist with carbon gas, are considered. The effect of intercalation, when atoms (Cs, K, Na, Ba, Pt, Si) penetrate spontaneously under the graphite islands physisorbed on the metal, and its mechanism are discussed. An analysis is made of the poisoning of platinum-group metal catalysts in the reaction of dissociation, when graphite islands characterized by extreme adsorption and catalytic passivity form in the adlayer. The method of CsCl dissociation to probe the surface carbon is treated. Attention is drawn to the adsorption of a number of atoms (Cs, K, Ba, Pt) on a graphite monolayer on metals, and the properties of such systems are discussed. The effects observed in coadsorption of CsCl molecules with K, Na, Ba, Tm atoms on a graphite monolayer on metals are covered. By analogy with the bulk carbides, surface carbides of fixed stoichiometry and very strong metal-carbon bonding have been revealed to form on the surface of transition metals (W, Re, Mo). The effect of displacement of surface carbon into the bulk of the metal stimulated by the adsorption of some atoms (Si, S, O) is discussed. The carbon clusters adsorbed on metals are considered. The transport of surface carbon, its desorption and diffusion between the surface and the bulk of the metal with a single- and double-phase adlayer are reviewed.     

Factor analysis and XPS-data preprocessing for non-conducting samples

The use of factor analysis for automatic interpretation of electron spectra needs in some cases a special data preprocessing. It was shown for x-ray photoelectron spectroscopy that in the case of non-conducting samples in addition to experimental methods (low energy electron flooding) a shift of the measured spectra with respect to a reference peak was useful. Thus, the residual energy shift due to sample charging, especially if the surface conductivity changes during measurement, could be removed. Several shift methods and the influence of different reference peaks were discussed.