吡啶与Rh(111)面相互作用的EHMO研究

吡啶作为加氢脱氮的模型化合物,与过渡金属Pt(111),Ni(100),Pd(110,111),Mo(110),Rh(111)面的吸附作用已有大量的实验研究,所采用的技术基本上是LEED,TDS,XPS,HREELS等,然而吡啶与Rh(111)面作用的理论研究尚未见报道.本文用EHMO法研究了吡啶与Rh(111)面的吸附作用,得到了最优吸附构型、结合能、集居数以及电荷分布和转移等,为新的脱氮催化剂开发提供了理论依据. 计算采用EHMO法,其中非对角矩阵元采用MWH近似:     

Molecular dynamics simulations of the adsorption of bone morphogenetic protein-2 on surfaces with medical relevance

Bone morphogenetic protein-2 (BMP-2) plays a crucial role in osteoblast differentiation and proliferation. Its effective therapeutic use for ectopic bone and cartilage regeneration depends, among other factors, on the interaction with the carrier at the implant site. In this study, we used classical molecular dynamics (MD) and a hybrid approach of steered molecular dynamics (SMD) combined with MD simulations to investigate the initial stages of the adsorption of BMP-2 when approaching two implant surfaces, hydrophobic graphite and hydrophilic titanium dioxide rutile. Surface adsorption was evaluated for six different orientations of the protein, two end-on and four side-on, in explicit water environment. On graphite, we observed a weak but stable adsorption. Depending on the initial orientation, hydrophobic patches as well as flexible loops of the protein were involved in the interaction with graphite. On the contrary, BMP-2 adsorbed only loosely to hydrophilic titanium dioxide. Despite a favorable interaction energy between protein and the TiO(2) surface, the rapid formation of a two-layer water structure prevented the direct interaction between protein and titanium dioxide. The first water adlayer had a strong repulsive effect on the protein, while the second attracted the protein toward the surface. For both surfaces, hydrophobic graphite and hydrophilic titanium dioxide, denaturation of BMP-2 induced by adsorption was not observed on the nanosecond time scale.     

Structure and dynamics of water at water|Pt interface as seen by molecular dynamics computer simulation

Five molecular dynamics computer simulations were performed to study the structural and dynamical properties of water next to uncharged and charged Pt surfaces. The results show that the structure of a water layer adsorbed on the metal surface is very sensitive to the details of the water–metal potential. While patches of short-living hexagonal ice-like structure are observed in the adsorbed water layer next to the uncharged Pt(111) surface, a square lattice solid-like structure is seen for the layer on top of the uncharged Pt(100) surface. The orientational ordering for the following two layers of water next to uncharged Pt is displaying a preference towards the orientations that are characteristic of hexagonal ice-I, while water is liquid-like in these layers. In the presence of a high value external electric field water reorients and undergoes a layering transition.     

Hydrogen-induced reconstruction of transition metal surfaces

This study compares the results of a number of recent papers on hydrogen adsorption on Rh(110), Rh(311) and Fe(211) as well as on Ni(111) and Fe(110) surfaces. It particularly deals with the structural aspect of these low energy electron diffraction (LEED) investigations and correlates them, if available, with respective thermodesorption data. Upon dissociative adsorption by a non activated process hydrogen induces local displacements of the atoms about the adsorption sites. With increasing coverage these displacements order to form a sequence of weakly reconstructed phases and gradually lift the surface layer relaxation of the formerly clean surface. Along close packed rows of metal surface atoms hydrogen atoms tend to occupy threefold coordinated adsorption sites which, in turn, arrange in single or double chains. The coverage dependent periodicity of these adlayer structure elements together with the respective shift buckling of the substrate surface generates the observed superstructures. Since not only open but also close packed surfaces show this weak (and sometimes strong) reconstruction upon hydrogen adsorption it should be generally considered in all adsorption systems.     

Dual-scale modeling of benzene adsorption onto Ni(111) and Au(111) surfaces in explicit water.

We present a multiscale modeling approach for studying interactions of organic molecules with metal surfaces in explicit water. The approach is based on combining adsorption energies of isolated molecules on transition metal surfaces calculated by ab initio density functional methods and classical molecular dynamics simulations using atomistically detailed force fields. The interaction of benzene with Ni(111) and Au(111) surfaces was studied. It is shown that a strong affinity of water for the hydrophilic surfaces makes benzene adsorption on Au thermodynamically unfavorable, while on Ni there is no preference. The work presented here serves as a first step in modeling the interactions of larger organic molecules with metal surfaces.     

2-氯噻吩在 Rh(111) 表面吸附的密度泛函理论研究

 采用密度泛函理论探讨了 2-氯噻吩分子在 Rh(111) 表面上吸附行为. 结果表明, 平行的 hol 位及 bridge 位上的吸附最稳定. 吸附后, 2-氯噻吩键长发生明显变化, 分子平面被扭曲, 分子中 C–H(Cl, S) 相对于金属表面倾斜上翘. 垂直吸附模式不如平行吸附模式稳定, 但吸附后噻吩环未发生变形. hol 及 bridge 吸附模式下 2-氯噻吩的芳香性已遭破坏, 噻吩环上的碳原子呈现准 sp3 杂化. 在平行的 hol 位吸附后, 2-氯噻吩环累计得到 0.77 个电子, 而 Rh(111) 表面累计失去 1.19 个电子.     

Voltammetric and radioactive labeling studies of single crystal and polycrystalline rhodium electrodes in sulfate-containing electrolytes

Adsorption of sulfate/bisulfate anions on single crystal Rh(111) (ordered/disordered) and polycrystalline rhodium electrodes in perchloric acid solution was studied by the use of cyclic voltammetry and the radioactive labeling method. The ordered Rh(111) surface was characterized by LEED and Auger spectroscopy to verify its well-defined character. Details of the surface chemistry of the anions interacting with the three rhodium substrates are different. This is considered as evidence that a long-range order of the metal electrodes alters the structure of chemisorbed hydrogen and oxygen significantly and that this structure is affected by the reversibly interacting species. Likewise, the adsorbed layer of sulfate anions on Rh(111) is more stable than that on other rhodium surfaces due to a favorable spatial configuration of the anions and surface water molecule network, which are proposed to be cross linked by hydrogen bonding. Overall, the adsorption reversibility of sulfate was confirmed with respect to the bulk concentration of the adsorbate and the electrode potential (provided that the surface oxidation is avoided). Radiochemical data reveal that, in the studied bulk concentration range (up to about 10⁻³ M), the uptake of anions by Rh(111) is limited to ca. 40% of the theoretical maximum coverage. The interaction of sulfate with Pt(111), and corresponding voltammetry in perchloric acid electrolyte are also brought into focus. It is concluded that only a part of the Pt(111) “butterfly” can be accounted for by adsorption of high energy hydrogen. No high energy hydrogen has been found to exist on Rh(111), as inferred earlier in our laboratory.     

Potential‐Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular‐Scale In Situ Video‐STM Study

Room‐temperature ionic liquids are of great current interest for electrochemical applications in material and energy science. Essential for understanding the electrochemical reactivity of these systems are detailed data on the structure and dynamics of the interfaces between these compounds and metal electrodes, which distinctly differ from those in traditional electrolytes. In situ studies are presented of Au(111) electrodes in 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSA]) by high‐speed scanning tunneling microscopy (video‐STM). [BMP][TFSA] is one of the best‐understood air and water stable ionic liquids. The measurements provide direct insights into the potential‐dependent molecular arrangement and surface dynamics of adsorbed [BMP]⁺ cations in the innermost layer on the negatively charged Au electrode surface. In particular, two distinct subsequent transitions in the adlayer structure and lateral mobility are observed with decreasing potential.     

Acetonitrile as Adsorbate or Solvent to Probe the Crystal Face Specificity of Metal–Water Interaction at Silver Electrode/Solution Interfaces

The crystal face specificity of metal–water interaction at Ag electrode/solution interfaces is investigated by using acetonitrile (ACN) as a probe molecule of the water interfacial structure or as a solvent in which water is a solute. Capacitance and voltammetric curves suggest that ACN is weakly adsorbed from aqueous solution on Ag in the order (111) > (100) > (110). Apparent inconsistencies of adsorption parameters are explained by the occurrence of two ACN adsorption modes: (i) directly on the metal surface and (ii) on the water layer adsorbed on the metal surface. Ag surface oxidation in ACN in the presence of variable amounts of water suggests that water has an inhibiting effect on Ag oxidation, the diminution of the water content in ACN leading to free anodic dissolution of the metal surface.     

Adsorption of Naphthalene and Quinoline on Pt,Pd and Rh: A DFT Study

The adsorption of naphthalene and quinoline on Pt(111), Pd(111) and Rh(111) surfaces is studied using density functional theory. The metal surfaces are simulated by means of large confined clusters and for Pt by means of a slab with periodic boundary conditions (PBC). Calculation parameters such as basis set convergence, basis set superposition error and effects of cluster relaxation and size are analyzed in order to assess the aptness of the cluster model. For all the metals, the preferred sites of adsorption are analyzed, thus revealing their different behaviors concerning structure and stability of adsorption modes. On Pt, the molecules have the richest theoretical configurational variety. Naphthalene and quinoline are found to adsorb preferentially on di‐bridge[7] sites on the three metals, and Rh exhibits higher adsorption energies than Pt and Pd. Structural features of the adsorbed molecules are correlated to the calculated adsorption energies. The di‐bridge[7] adsorption modes are studied in deeper detail decomposing the adsorption energies in two terms arising from molecular distortion and binding interaction to the metal. Molecular distortion is correlated to the HOMO–LUMO energy gap. The larger adsorption energies found for interactions with Rh result from the lower contribution of the distortion term. Binding interactions are described by analyzing the wave functions of naphthalene and quinoline adsorbed on a subunit of the large clusters in order to reduce the complexity of the analysis. Molecular orbitals are studied using concepts of Frontier Molecular Orbitals theory. This approach reveals that in the adsorption of naphthalene and quinoline on Pt and Pd, an antibonding state lies below the Fermi energy, while on Rh all antibonding states are empty, in agreement with the larger interaction energies. In addition, further insight is gained by projecting the density of states on the d band of the clean surfaces and of the adsorbed systems. This results in the rationalization of the structural features in terms of the concepts of electronic structure theory. The distributions of electronic density are described by means of Hirshfeld charges and isosurfaces of differential electron density. The net electron transfer from the metals to the molecules for most of the sites correlates with the trends of the adsorption energies.     

Multilayer adsorption of water at a rutile TiO2)(110) surface: towards a realistic modeling by molecular dynamics

We present a model combining ab initio concepts and molecular dynamics simulations for a more realistic treatment of complex adsorption processes. The energy, distance, and orientation of water molecules adsorbed on stoichiometric and reduced rutile TiO(2)(110) surfaces at 140 K are studied via constant temperature molecular dynamics simulations. From ab initio calculations relaxed atomic geometries for the surface and the most probable adsorption sites were derived. The study comprises (i) large two-dimensional surface supercells, providing a realistically low concentration of surface oxygen defects, and (ii) a water coverage sufficiently large to model the onset of the growth of a bulk phase of water on the surface. By our combined approach the influence of both, the metal oxide surface, below, and the bulk water phase, above, on the water molecules forming the interface between the TiO(2) surface and the water bulk layer is taken into account. The good agreement of calculated adsorption energies with experimental temperature programmed desorption spectra demonstrates the validity of our model.     

Adsorption, vibration, and diffusion of O atoms on Rh low-index and (711) stepped defective surfaces

The adsorption, vibration, and diffusion of O atoms on Rh(100), Rh(111), Rh(110), and Rh(711) surfaces were studied using the 5-parameter Morse potential (5-MP) of interaction between an adatom and a metal surface cluster. Our theoretical calculations provide information about adsorption sites, adsorption geometry, binding energy, and eigenvibration. Our results agreed very well with experimental results. Four major results follow. First, the theoretical calculation showed that on the Rh(100) surface the 4-fold hollow site is the only adsorption site. Second, on the O-Rh(111) system, the 3-fold hollow site is the stable adsorption site. Third, on the Rh(110) surface at low coverage, the O atom is adsorbed preferably on the pseudo-3-fold site, while with increasing coverage, the O atom is adsorbed not only on the pseudo-3-fold site but also on the long bridge site. Last, as for the Rh(711) stepped surface, the 3-fold site on the (111) step is metastable, whereas the 4-fold sites on the (100) terrace are stable, which enables the O atoms to diffuse easily from the 3-fold to the 4-fold site at low coverage. Therefore, the O atoms are adsorbed preferrably on the stable 4-fold sites of the (100) terrace and then later as coverage increases on the metastable 3-fold site of the (110) step.     

Ab initio thermodynamics examination of sulfur species present on Rh, Ni, and binary Rh-Ni surfaces under steam reforming reaction conditions

The stable form of adsorbed sulfur species and their coverage were investigated on Rh, Ni, and Rh-Ni binary metal surfaces using density functional theory calculations and the ab initio thermodynamics framework. S adsorption, SO(x) (x = 1-4) adsorption, and metal sulfide formation were examined on Rh(111) and Ni(111) pure metals. Both Rh and Ni metals showed a preference for S surface adsorption rather than SO(x) adsorption under steam reforming conditions. The transition temperature from a clean surface (<(1)/(9) ML) to S adsorption was identified on Rh(111), Ni(111), Rh(1)Ni(2)(111), and Rh(2)Ni(1)(111) metals at various P(H(2))/P(H(2)S) ratios. Bimetallic Rh-Ni metals transition to a clean surface at lower temperatures than does the pure Rh metal. Whereas Rh is covered with (1)/(3) ML of sulfur under the reforming conditions of 4-100 ppm S and 800 °C, Rh(1)Ni(2) is covered with (1)/(9) ML of sulfur at the lower end of this range (4-33 ppm S). The possibility of sulfate formation on Rh catalysts was examined by considering higher oxygen pressures, a Rh(221) stepped surface, and the interface between a Rh(4) cluster and CeO(2)(111) surface. SO(x) surface species are stable only at high oxygen pressure or low temperatures outside those relevant to the steam reforming of hydrocarbons.     

Fibronectin and bovine serum albumin adsorption and conformational dynamics on inherently conducting polymers: a QCM-D study

Quartz crystal microbalance with dissipation monitoring (QCM-D) was employed to characterize the adsorption of the model proteins, bovine serum albumin (BSA) and fibronectin (FN), to polypyrrole doped with dextran sulfate (PPy-DS) as a function of DS loading and surface roughness. BSA adsorption was greater on surfaces of increased roughness and was above what could be explained by the increase in surface area alone. Furthermore, the additional mass adsorbed on the rough films was concomitant with an increase in the rigidity of the protein layer. Analysis of the dynamic viscoelastic properties of the protein adlayer reveal BSA adsorption on the rough films occurs in two phases: (1) arrival and initial adsorption of protein to the polymer surface and (2) postadsorption molecular rearrangement to a more dehydrated and compact conformation that facilitates further recruitment of protein to the polymer interface, likely forming a multilayer. In contrast, FN adsorption was independent of surface roughness. However, films prepared from solutions containing the highest concentration of DS (20 mg/mL) demonstrated both an increase in adsorbed mass and adlayer viscoelasticity. This is attributed to the higher DS loading in the conducting polymer film resulting in presentation of a more hydrated molecular structure indicative of a more unfolded and bioactive conformation. Modulating the redox state of the PPy-DS polymers was shown to modify both the adsorbed mass and viscoelastic nature of FN adlayers. An oxidizing potential increased both the total adsorbed mass and the adlayer viscoelasticity. Our findings demonstrate that modification of polymer physicochemical and redox condition alters the nature of protein-polymer interaction, a process that may be exploited to tailor the bioactivity of protein through which interactions with cells and tissues may be controlled.     

Kinetically forbidden transformations of water molecular assemblies in hydrophobic micropores

Water adsorption hysteresis is one of the most important phenomena observed during the interaction of water with hydrophobic surfaces. Adsorption hysteresis in micropores has strong relevance to the structure of adsorbed water. We used three typical models (cluster, monolayer, and uniform distribution structure models) to determine the structure of the water molecules adsorbed in hydrophobic slit-shaped carbon micropores. In each model, stabilization energy profiles were calculated for various fractional fillings by using the interaction potential theory. Simultaneously, molecular dynamics (MD) simulations of water adsorbed in the micropore of 1.1 nm pore width, which shows significant adsorption hysteresis, were performed to determine the kinetics of the observed structural transformations. The transformations between monolayer and cluster were slow, that is, kinetically forbidden at the fractional filling of 0.2 and 0.6, whereas the cluster-uniform distribution structure and uniform distribution structure-monolayer transformations were kinetically allowed. The kinetically forbidden transformation resulted in the occurrence of metastable structure of adsorbed water and was responsible for the observed adsorption hysteresis.     

Energetically driven reorganization of a modified catalytic surface under reaction conditions

The compositional and structural rearrangements at the catalyst surface during chemical reactions are issues of great importance for understanding and modeling the catalytic processes. Low-energy electron microscopy and photoelectron spectromicroscopy studies of the real-space structure and composition of a Au-modified Rh(110) surface during water formation reveal reorganization processes due to Au mass transport triggered by the propagating reaction fronts. The temporal evolution of the surface reaction results in a 'patterned' surface consisting of separated Au-rich and Au-poor phases with different oxygen coverage, Rh surface structure, and reactivity. The experimental results are complemented by ab initio electronic-structure calculations of the O and Au adsorption phases, which demonstrate that the reorganization of the Au adlayer by the propagating reaction fronts is an energetically driven process. Our findings suggest that reaction-induced spatial inhomogeneity in the surface composition and structure is a common feature of metal catalysts modified with adatoms which become mobile under reaction conditions.     

The first layer of water on Rh(111): microscopic structure and desorption kinetics

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (square root 3 x square root 3)R30 degrees commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8 x 10(16) ML(1/2)/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3 x 10(14) ML(1/2)/s, respectively.     

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.     

Interface between platinum(111) and liquid isopropanol (2-propanol): a model for molecular dynamics studies

A molecular dynamics model and its parametrization procedure are devised and used to study adsorption of isopropanol on platinum(111) (Pt(111)) surface in unsaturated and oversaturated coverages regimes. Static and dynamic properties of the interface between Pt(111) and liquid isopropanol are also investigated. The magnitude of the adsorption energy at unsaturated level increases at higher coverages. At the oversaturated coverage (multilayer adsorption) the adsorption energy reduces, which coincides with findings by Panja et al. in their temperature-programed desorption experiment [Surf. Sci. 395, 248 (1998)]. The density analysis showed a strong packing of molecules at the interface followed by a depletion layer and then by an oscillating density profile up to 3 nm. The distribution of individual atom types showed that the first adsorbed layer forms a hydrophobic methyl "brush." This brush then determines the distributions further from the surface. In the second layer methyl and methine groups are closer to the surface and followed by the hydroxyl groups; the third layer has exactly the inverted distribution. The alternating pattern extends up to about 2 nm from the surface. The orientational structure of molecules as a function of distance of molecules is determined by the atom distribution and surprisingly does not depend on the electrostatic or chemical interactions of isopropanol with the metal surface. However, possible formation of hydrogen bonds in the first layer is notably influenced by these interactions. The surface-adsorbate interactions influence the mobility of isopropanol molecules only in the first layer. Mobility in the higher layers is independent of these interactions.     

Surface plasmon fluorescence investigation of energy-transfer-controllable organic thin films

Thin functional organic films on a gold substrate were fabricated by adsorbing tetrakis(carboxyphenyl)porphyrin (TCPP) on a spacer layer, which was prepared by the layer-by-layer adsorption of a dendrimer and a linear polymer. The thickness and photoluminescence of the films were investigated by surface plasmon resonance and surface plasmon fluorescence techniques, respectively. TCPP adsorbed on the spacer layer in aqueous solutions of different ionic strengths resulted in a thick TCPP adlayer at high ionic strength and a shrunk spacer layer at low ionic strength. The fluorescence was quenched at high ionic strength but could be observed at low ionic strength. The effects are explained by the states of dye aggregation. This study shows the control of energy transfer from a metal surface to a dye layer by changing the dye adlayer. It can contribute to the development of molecular devices involving energy-transfer systems.