吸附氢气后的钯团簇在氧化亚铜表面上的稳定性研究

为了探究吸附H2后的Pdn团簇在Cu2O（111）完整表面和铜缺陷表面上的稳定性,计算了负载在Cu2O（111）完整表面和铜缺陷表面上的Pdn（n=1-4）对H2分子的最稳定吸附结构;利用在给定H2压力和温度下Pdn / Cu2O表面吸附H2的相图揭示了Pdn团簇在Cu2O（111）两个表面的变化情况。结果表明,在吸附了H2分子以后,Pdn团簇更倾向于保持原有的结构,且随着Pd团簇的增大,吸附H2的数量也逐渐增长。     

Equilibrium properties of Ag clusters on Pt(111)

Molecular dynamics simulations (MD) and Embedded Atom Method (EAM) potentials are employed to investigate the properties of small Ag clusters deposited on the (111) surface of Pt. The computation of the dissociation energies reveals the most stable structures up to 19 atoms, while MD simulations allow to extract the dynamical diffusion barrier for adatom, dimer and trimer. We obtain dissociation energies in the range of 0.3 eV for the trimer and 0.45 eV for the heptamer and a migration energy as low as 0.05 eV for the adatom.     

Adsorption mode of cinchonidine on Cu(111) surface

The adsorption mode of cinchonidine on Cu(111) was directly obtained by in situ STM. The molecules were found to adsorb on the substrate surface and form a long-range ordered adlayer with (4 x 4) symmetry. While the quinoline rings lie parallel to Cu(111), the chiral quinuclidine moiety extends out of the surface. The enantioselectivity of catalysts may relate to this special adsorption conformation of cinchonidine on the surface.     

Structures and vibrational frequencies of CO adlayers on Rh(111) surface

The adsorption of carbon monoxide on single-crystal transition metal surfaces has been the subject of numerous studies, because it has served as a model system for the adsorption of small molecules on transition metal surfaces, and its industrial importance is obvious in such areas as catalytic reaction. The bonding of carbon monoxide to rhodium is of special interest since this metal catalyzes the hydrogenation of CO to produce hydrocarbons in both heterogeneous and homogeneous media, and it …     

Adsorption of linear alkanes on Cu(111): temperature and chain-length dependence of the softened vibrational mode

The vibrational spectra of linear alkanes, with lengths ranging from n-propane to n-octane, were examined on a copper surface by reflection-absorption infrared spectroscopy. The appearance and frequency of the "soft mode," a feature routinely seen in studies of saturated hydrocarbons adsorbed on metals, were examined and compared between the different adsorbates. The frequency of the mode was found to be dependent on both the number of methylene units of each alkane as well as specific aspects of the order of the monolayer phase. Studies of monolayer coverages at different temperatures provide insights into the nature of the two-dimensional (2D) melting transitions of these adlayer structures, ones that can be inferred from observed shifts in the soft vibrational modes appearing in the C-H stretching region of the infrared spectrum. These studies support recently reported hypotheses as to the origins of such soft modes: the metal-hydrogen interactions that mediate them and the dynamics that underlay their pronounced temperature dependencies. The present data strongly support a model for the 2D to one-dimensional order-order phase transition arising via a continuous rather than discrete first-order process.     

Theoretical study on AlnO2 (n = 1–10) clusters and O2 adsorption on the Al(111) surface

The structural properties of neutral and ionic Al_nO₂ (n = 1–10) clusters have been systematically investigated using the density functional method B3LYP with a standard 6‐311+G(d) basis set. The calculated results show that in the Al_nO, Al_nO₂, and Al_nO (n ≥ 3) clusters, O atoms tend to penetrate into the aluminum clusters with some Al atoms moving outward. The binding energies and natural charges populations indicate that the oxygen‐etching is generally stronger in the order Al < Al_n < Al for n < 3, and Al > Al_n > Al for n ≥ 3. To further understand the mechanism of interaction between Al and O₂, the adsorption of O₂ on the Al(111) surface was studied using the density functional theory with plane wave pseudopotential method. The calculated results are consistent with the experimental observation that the O₂ molecule would dissociate on the Al(111) surface and be adsorbed in adjacent hollow sites, forming a local structure of Al₃O–Al₃O. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

原子H在Cu(100)(111)(110)上的吸附扩散研究

采用5-MP势方法,对原子氢在金属Cu的3个低指数面上的吸附特性,如吸附几何、吸附能、振动频率等以及吸附扩散势能面结构进行了比较系统的研究,计算结果显示低温低覆盖条件下,氢原子在Cu(110)表面上只存在赝式三重位和长桥位吸附态,没有短桥位吸附态,并且获得了实验和理论的支持.     

Electron-induced isomerisation of dichlorobenzene on Cu(111) and Ag(111)

The constitutional isomerisation of single dichlorobenzene molecules adsorbed on the surfaces of Ag(111) and Cu(111) between their meta- and para-isomers is induced and investigated by means of a low temperature scanning tunneling microscope. On both substrates similar isomerisation thresholds are found indicating that the excitation mechanism of this reaction does not depend significantly on the underlying substrate. The isomerisation threshold voltage of (170 +/- 7) meV most likely corresponds to excitation of a C-C stretch mode whose gas-phase energies we calculated ab initio to lie at 174 and 172 meV for meta- and para-isomers respectively. Though the reaction is found to be localized on the submolecular scale, it depends heavily on the second substituent both in terms of excitation energy and reaction outcome.     

Adsorption properties and vibrational spectra of propyne adsorbed on Rh(111). Comparison with other (111) metal surfaces

We have studied the adsorption properties of propyne on the Rh(111) surface by means of the generalized gradient approach of density functional theory using periodic slab models. The simulation of the vibrational spectra has permitted us to corroborate and complete the experimental band assignment and to confirm the adsorption site preference. Propyne prefers to sit on a 3-fold hollow site, with the C[triple bond]C axis parallel to a Rh-Rh bond and the molecular plane tilted away from the surface normal. The comparison between the adsorption behaviour of propyne on Rh(111) and on other (111) metal surfaces allows one to provide an explanation for the different reactivity observed experimentally.     

Pd/Cu(111)双金属表面催化糠醛脱碳及加氢的反应机理

采用密度泛函理论(DFT)研究糠醛在最稳定Pd/Cu(111)双金属表面上的吸附构型和糠醛脱碳及加氢的反应机理。结果表明,当糠醛初始吸附于O_3-Pd-top、O_7-Cu-hcp位时,吸附构型最稳定,其吸附能为73.4 kJ/mol。糠醛在Pd/Cu(111)双金属表面上更易发生脱碳反应。对于糠醛脱碳反应,所需活化能较低,各个基元反应均为放热反应,糠醛更易先失去支链上的H形成(C_4H_3O)CO,然后中间体脱碳加氢得到呋喃,其中,C_4H_3O加氢生成呋喃所需活化能(72.6 kJ/mol)最高,是反应的控速步骤。对于加氢反应,糠醛与首个氢原子的反应需要最大的活化能(290.4 kJ/mol),是反应的限速步骤。     

Ab initio study on adsorption of hydrated Na+ and Cu+ cations on the Cu(111) surface

The interactions of Na+ and Cu+ cations with a Cu(111) surface in the presence and absence of water molecules were investigated using cluster models and ab initio methods. Adsorption in aqueous solution was modeled with one to five water molecules around the adsorbing cation. The Cu surface was described with Cu10 and Cu18 cluster models and the computational method was MP2/RECP/6-31+G. The effect of the basis set superposition error (BSSE) was taken into account with counterpoise (CP) correction, and the accuracy of HF-level results was examined. The interactions between Na+ and the Cu surface were found to be primarily electrostatic, and the energy differences among the different adsorption sites were small. The largest CP-corrected MP2 adsorption energy for the Cu18 cluster was -188 kJ/mol. When water molecules were added around it, Na+ receded from the Cu surface and finally was surrounded totally by the water molecules. The interactions between Cu+ and the Cu surface were dominated by orbital interactions, and Cu+ preferred to adsorb on sites where it could bind to more than one surface atom. The largest CP-corrected MP2 adsorption energy for the Cu18 cluster was -447 kJ/mol. Adding water molecules around it did not cause Cu+ to draw away from the surface, but instead the water molecules began to form hydrogen bonds with one another. The magnitude of BSSE was substantial in most cases. CP corrections did not, however, have a significant impact on the relative trends among the interaction energies.     

A first-principles potential energy surface and vibrational states for hydrogen on Cu(100)

Density-functional theory calculations based on plane-wave expansion and pseudopotential treatment were carried out for atomic hydrogen on a rigid Cu(100) surface. A global potential energy surface was then obtained by using a three-dimensional spline interpolation. It is found that the minimum of the potential is located at the fourfold hollow site with a diffusion barrier of 88 meV at the bridge site. The vibrational states of atomic hydrogen and deuterium on the Cu(100) surface were calculated on the potential surface. Our calculations show that the vibrational states A(1) (0), A(1) (1), E(1), and B(2) (1) of H/Cu(100) exhibit strong localized character and very narrow band widths, whereas other excited vibrational states have considerable delocalized character and broad band widths. The vibrational frequency of 71.2 (51.5) meV for H(D) in the perpendicular direction obtained in this study is in good agreement with the experimentally observed value of 70 (52) meV.     

Static surface temperature effects on the dissociation of H(2) and D(2) on Cu(111)

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H(2) and D(2) on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (T(s) = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found.     

Photoassisted adsorption of allylamine and 1-butene on H:Si(111) studied by surface vibrational spectroscopies

Ultraviolet photoassisted adsorption of terminally double-bonded molecules, allylamine (CH2=CH-CH2-NH2) and 1-butene (CH2=CH-CH2-CH3), on hydrogen-terminated silicon (111) surface was attempted to obtain adsorbates covalently terminating the surface Si atoms. The adsorption process was monitored by high-resolution electron energy loss spectroscopy, multiple internal infrared reflection-absorption spectroscopy, and Auger electron spectroscopy. Allylamine adsorbates emerged upon delivery of allylamine gas under ultraviolet irradiation. The N-H bonds in allylamine were evidenced to survive over the photoadsorption process by vibrational analysis and by the reaction with ketene. CH3- groups were detected at low coverage, indicating anchoring of the organic moieties by the secondary (sec-) type carbon atoms, which were taken over by the primary (n-) type with increasing coverage. C-D bonds were detected after deposition on deuterium-terminated Si(111) upon incorporation of Si-terminating H into the hydrocarbon part of adsorbates. In the case of 1-butene, not only the C=C end but also the CH3- end of a molecule might attach on Si, resulting in emergence of adsorbates composed of CH2 groups. The newly obtained adsorbates are prospective as a material applied for nanolithography, fine electrochemistry, and nano-biotechnology.     

氧化锌有序结构在Au(111)和Cu(111)上的生长

利用NO₂或O₂作为氧化剂,研究了氧化锌在Au(111)和Cu(111)上的生长和结构。NO₂表现了更好的氧化性能,有利于有序氧化锌纳米结构或薄膜的生长。在Au(111)和Cu(111)这两个表面上,化学计量比氧化锌都形成非极性的平面化ZnO(0001)的表面结构。在Au(111)上,NO₂气氛下室温沉积锌倾向于形成双层氧化锌纳米结构;而在更高的沉积温度下,在NO₂气氛中沉积锌则可同时观测到单层和双层氧化锌纳米结构。O₂作为氧化剂时可导致形成亚化学计量比的ZnO_x结构。由于铜和锌之间的强相互作用会促进锌的体相扩散,并且铜表面可以被氧化形成表面氧化物,整层氧化锌在Cu(111)上的生长相当困难。我们通过使用NO₂作为氧化剂解决了这个问题,生长出了覆盖Cu(111)表面的满层有序氧化锌薄膜。这些有序氧化锌薄膜表面显示出莫尔条纹,表明存在一个ZnO和Cu(111)之间的莫尔超晶格。实验上观察到的超晶格结构与最近理论计算提出的Cu(111)上的氧化锌薄膜结构相符,具有最小应力。我们的研究表明,氧化锌薄膜的表界面结构可能会随氧化程度或氧化剂的不同而变化,而Cu(111)的表面氧化也可能影响氧化锌的生长。当Cu(111)表面被预氧化成铜表面氧化物时,ZnO_x的生长模式会发生变化,锌原子会受到铜氧化物晶格的限域形成单位点锌。我们的研究表明了氧化锌的生长需要抑制锌向金属基底的扩散,并阻止亚化学计量比ZnO_x的形成。因此,使用原子氧源有利于在Au(111)和Cu(111)表面上生长有序氧化锌薄膜。     

Plasma-assisted oxidation of Cu(100) and Cu(111)

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO₂. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO₂RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O₂-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu₂O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate''s orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu₂O is formed during the initial growth phase, both Cu₂O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu₂O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

A low-pressure oxygen plasma oxidized Cu(100) and Cu(111) surfaces at room temperature. The time-dependent evolution of surface structure and chemical composition is reported in detail for a range of exposure times up to 30 min.     

Theoretical study of the diffusion of alkali metals on a Cu(111) surface

We present a theoretical study of the diffusion of Li, Na and K on a Cu(111) surface. Various diffusional paths are identified and characterized in terms of kinetic parameters such as diffusion constants and activation energies. We use a model potential parametrized from DFT calculations to determine adsorption energies, surface corrugation and diffusional behaviors. Two representations of the copper surface (2D and 3D) are used to investigate its effect on the adsorption patterns, diffusion constants and activation energies. An interesting result is that the adsorption pattern for Na and K changes when adding layers of substrate (2D → 3D) favouring unusual adsorption sites, which is in agreement with recent theoretical evidence.     

单原子分散的Au/Cu(111)表面合金的表面结构与吸附性质

Au-Cu双金属合金纳米颗粒对包括CO氧化和CO2还原等在内的多个反应有较好的催化活性,然而关于其表面性质的研究却相当匮乏。在此工作中,我们通过对低覆盖度的Au/Cu(111)和Cu/Au(111)双金属薄膜退火,制备出了单原子级分散的Au/Cu(111)和Cu/Au(111)合金化表面,并利用高分辨扫描隧道显微镜(STM)和扫描隧道谱(STS)进一步研究了掺杂原子的电子性质及其对CO吸附行为的影响。研究发现,分散在Cu(111)表面的表层和次表层Au单原子在STM上表现出不同衬度。在-0.5 e V附近,前者表现出相较于Cu(111)明显增强的电子态密度,而后者则明显减弱。吸附实验表明表层Au单原子对CO的吸附能力并没有得到增强,甚至会减弱其周围Cu原子的吸附能力。与Au在Cu(111)表面较好的分散相反,Cu原子倾向于钻入Au(111)的次表层,并且形成多原子聚集体。且Cu原子受Au(111)衬底吸电子作用的影响,其对CO的吸附能力明显减弱。这个研究结果揭示了合金表面的微观结构与性质的关联,为进一步阐明Au-Cu双金属催化剂的表面反应机理提供参考。     

Imaging the Bonds of Dehalogenated Benzene Radicals on Cu(111) and Au(111)

Dissociative adsorption of doubly substituted benzene molecules leads to formation of benzyne radicals. In this study, co‐adsorbed hydrogen molecules are used in scanning tunneling hydrogen microscopy to enhance the contrast of the meta‐ and the para‐isomers of these radicals on Cu(111) and Au(111). Up to three hydrogen molecules are attached to one radical. One hydrogen molecule reveals the orientation of the carbon ring and its adsorption site, allowing discrimination between the two radicals. Two hydrogen molecules reflect the bond picture of the carbon skeleton and reveals that adsorption on Cu(111) distorts the meta‐ isomer differently from its gas‐phase distortion. Three hydrogen molecules allow us to determine the bond picture of a minor species.     

Structure and bonding of alkanethiols on Cu(111) and Cu(100)

The local structure of the sulfur atom of methanethiolate and ethanethiolate on the Cu(111) and Cu(100) surfaces was investigated from first principles employing the periodic supercell approach in the framework of density functional theory. On the 111 surface, we investigated the (square root 3 x square root 3)R30 degrees and (2 x 2) structures, whereas on the 100 surface, we investigated the p(2 x 2) and c(2 x 2) structures. The landscape of the potential energy surface on each metal surface presents distinctive features that explain the local adsorption structure of thiolates found experimentally. On the Cu(111) surface, the energy difference between the hollow and bridge sites is only 3 kcal/mol, and consequently, adsorption sites ranging from the hollow to the bridge site were observed for increasing surface coverages. On the Cu(100) surface, there is a large energy difference of 12 kcal/mol between the hollow and bridge sites, and therefore, only the 4-fold coordination was observed. The high stabilization of thiolates on the hollow site of Cu(100) may be the driving force for the pseudosquare reconstruction observed experimentally on Cu(111). Density of states analysis and density difference plots were employed to characterize the bonding on different surface sites. Upon interaction with the metal d bands, the pi* orbital of methanethiolate splits into several peaks. The two most prominent peaks are located on either edge of the metal d band. They correspond to bonding and antibonding S-Cu interactions. In the case of ethanethiolate, all the back-bonds are affected by the surface bonding, leading to alternating regions of depletion and accumulation of charge in the successive bonds.