Experimental and theoretical studies of reaction pathways of direct propylene epoxidation on model catalyst surfaces

The direct epoxidation of propylene to propylene oxide (PO) using molecular oxygen is an attractive alternative to current production methods using chlorohydrin or hydroperoxide-mediated processes, which are environmentally harmful and expensive. Although direct ethylene epoxidation using Ag-based catalysts has been practiced industrially for decades, due to the presence of allylic hydrogen in propylene the selectivity toward epoxide is generally much lower for propylene than for ethylene. Mechanistic understanding on well-characterized surfaces of model catalysts can potentially provide guidance to effectively alter the electronic properties of the catalyst in order to increase PO selectivity. This review summarizes both experimental and theoretical studies on model catalysts for propylene epoxidation and their contributions to elucidating the reaction mechanism, intermediates, and active sites. We first show examples of experimental studies on Cu, Ag, and Au surfaces, and compare the reaction pathways and intermediates on these surfaces. Novel approaches including plasmon-mediated catalysis and utilization of shape-controlled crystal facets that open new opportunities for improving PO selectivity will also be discussed. We then describe how density functional theory (DFT) calculations have provided important insights into the reaction mechanism and active sites on Cu, Ag, and Au surfaces and clusters. Propylene oxidation pathways on other relevant metal surfaces will also be discussed. The combined experimental and computational studies elucidate the nature of surface oxygen species and the role of the oxametallacycle intermediate. We conclude by highlighting design principles and insights for guiding further development of active and selective propylene epoxidation catalysts.     

Adsorption-desorption properties and surface structural chemistry of chlorine on Cu(111) and Ag(111)

The adsorption, desorption, and structural properties of chlorine adlayers on Cu(111) and Ag(111) have been studied by LEED, Auger, Δ?, and thermal desorption measurements. Ancillary experiments were also carried out on cuprous chloride for purposes of comparison with the Cu(111)-Cl data. Chlorine adsorption is rapid on both metals and follows precursor kinetics, the absolute initial sticking probabilities being ~1.0 (Cu) and ~0.5 (Ag). Δ? results suggest that significant depolarisation of the chemisorption bond occurs at high coverages, the maximum values being + 1.2 eV (Cu) and + 1.8 eV (Ag). On Cu(111), adsorption leads to the formation of a sequence of well-ordered phases; in order of increasing coverage, these are as follows: (√3 × √3)R30°, (12√3 × 12√3)R30°, (4√7 × 4√7)R19.2°, and (6√3 × 6√3)R30°. On Ag(111) (√3 × √3)R30°, and (10 × 10) structures are observed. All six structures are susceptible to a straightforward interpretation in terms of coincidence lattices resulting from the progressive uniform compression of a hexagonal layer of Cl atoms. This interpretation is consistent with all the experimental results, and gives values for the nearest-neighbour ClCl spacing on both Cu(111) and Ag(111) which are in good agreement with other work on other surfaces. Chlorine desorbs exclusively as atoms from both metals with first-order desorption kinetics, and apparent desorption energies of 236 (Cu) and 209 (Ag) kJ mol^?1. These values, which depend on an assumed pre-exponential factor of 10¹³ s^?1, are shown to be inconsistent with the thermochemical constraints on the system necessitated by the complete absence of Cl₂ desorption. Lower limits for the pre-exponential factors are then deduced, and the values are found to be consistent with the differences between the CuCl and AgCl systems.     

直接置换法制备包覆型纳米铜-银双金属粉末

 以AgNO₃为主盐，采用直接置换法初步制备了包覆型纳米铜-银双金属粉末，分析了工艺条件对包覆效果的影响，并用透射电子显微镜、X射线能量色散谱仪和X射线衍射仪进行了表征。实验结果表明：纳米铜-银双金属粉末为包覆型结构，平均粒径约70 nm，分散性较好，表面银的原子分数达到74.28%。在洗涤过程中加入一定量的保护剂，有效解决了纳米铜粉的氧化问题。     

理论预测晶体表面吸附二维原子岛的形状

提出了凝结势概念用以建立一种预测晶体表面吸附二维原子岛几何结构的理论方法.基于半经验相互作用势（SEAM势和OJ势）的计算表明,同相外延生长的二维原子岛在Cu和Ag的（111）面呈现正六边形结构, 而在Pt（111）面呈现截角三角形状;Cu和Ag的（100）面二维岛则形成正方形.这些理论预测均与实验观测结果一致.由于凝结势的计算不受原子数量的限制,该模型可普遍应用于预测各种表面二维原子岛形状. 关键词： 表面吸附 二维原子岛 量子点     

Energy barriers of single-adatoms diffusion on unreconstructed and reconstructed (110) surfaces

The present paper is aimed mainly to investigate theoretically the diffusion of Ag, Cu, Au and Pt adatoms on the (1 × 1) unreconstructed geometry for Ag, Cu and Pt (110), and reconstructed geometries ((1 × 2), (1 × 3) and (1 × 4)) for Pt and Au (110) surfaces. We consider the single adatom diffusion when additional atoms are deposited in adjacent row. For this study, we have used the molecular statics simulations combined with the embedded atom method. For several systems, we have calculated the activation barriers for hopping mechanism. For the diffusion on the unreconstructed surfaces, the trends for the activation barriers are the same for all considered systems except for Cu/Ag (110) system, where the activation barrier do not change. Further, our results indicate that additional atoms lead to a small decreasing of activation barriers for diffusion on reconstructed surfaces for some systems, while for other systems; the activation barrier remains practically unchanged.     

F+ laser performance and interaction of Cu,Ag and Au at the reduced oxygen coordination of MgO surface: first principles calculations

F⁺ laser performance and interaction of the title group IB transition metals at the reduced oxygen coordination of MgO surface were investigated using the TD and DFT methods of ab initio molecular electronic structure calculations. The considered ion clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces and the nearest neighbor ions to the F⁺ site were allowed to relax to equilibrium in each case. The F⁺ laser performance fades quickly as the reduced oxygen coordination decreases from 5 to 4 to 3. The relaxed excited states (RESs) of the defect containing surfaces are compact and deep below the conduction bands of the perfect MgO surface. The probability of orientational destruction of the center in laser experiment is expected to follow the order flat>corner>edge. The excited state at the edge has higher energy than that at the flat or at the corner. F⁺ is easily formed at the lower oxygen coordination and the disappearance of anisotropy and 2p splitting observed in absorption of F⁺ at the surface follow the order corner>flat>edge. The Glasner–Tompkins relation is generalized to include the F⁺ bands at the reduced oxygen coordination of a metal oxide surface. As far as the adsorbate–substrate interactions are concerned, the F⁺ center enhances the adsorptivity of Ag, Cu and Au by ca. 1.91–3.33 eV and changes the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies follow the order Cu>Au>Ag and are explainable in terms of electrostatic potential curves, energy gaps and spin pairing. Cu and Ag act as electron donors while Au acts as electron acceptor and the MgO surface cannot be made semiconducting by F⁺ imperfection.     

Superhydrophobic surfaces via electroless displacement of nanometric Cu layers by Ag

This paper explores the possibility of making hydrophobic and superhydrophobic surfaces from electroless displacement of Cu by Ag⁺, in the case where Cu oxidation is limited owing to Cu layers of nanometric thicknesses. The morphology of the Ag layers is studied by scanning electron microscopy for Cu thicknesses between 10 and 80 nm. The mapping of the elemental content of the layers by electron dispersive X-ray analysis also has been used to clarify the particle growing by diffusion limited aggregation. It is shown that the average size and the shape complexity of the Ag particles increase with the Cu thickness. The addition of dimethyl sulfoxide in the Ag⁺ aqueous solution improves the surface homogeneity, increases the particle density and decreases their sizes. The wetting behaviour of the surfaces, after grafting with octadecanethiol, has been studied from measurements of the contact angles of a drop of water. According to the thickness of the initial Cu layer and the morphology of the Ag layer, contact angles range between 110° and 154°. Superhydrophobic surfaces are obtained from 80 nm thick Cu layers.     

Thermo-physical parameters and characteristics of self-organized nanogratings induced by femtosecond laser

Self-organized nanoripples are induced on bulk metal Cu and Ag by femtosecond laser, and the influence of number of shots on nanostructure formation has been investigated. The AFM images show that obtained grooves on Cu are about 50 nm deep, and have an average spacing of 481.41 nm, which is smaller compared to the incident radiation wavelength (800 nm). Arrays of ablated craters are machined on Cu and Ag surfaces by femtosecond laser in order to determine the optical characteristics of laser irradiated surface. Compared with that of untreated sample, the locations of maximum absorption wavelength of laser treated samples are not shifted, while average absorbance intensities are enhanced both for modified Ag and Cu surfaces. Finally, the effects of thermal conductivity, dielectric function as well as electron–phonon coupling coefficient on nanograting morphology induced by femtosecond laser are discussed qualitatively.     

Scattering of surface states at step edges in nanostripe arrays

Surface states of noble metal surfaces split into Ag-like and Cu-like subbands in stepped Ag/Cu nanostripe arrays. The latter self-assemble by depositing Ag on vicinal Cu(111). Ag-like states scatter at nude step edges in Ag stripes, leading to umklapp bands, quantum size effects, and peak broadening. By contrast, Ag stripe boundaries become transparent to Cu-like states, which display band dispersion as in flat Cu(111). We find a linear relationship between the quantum size shift and peak broadening that applies in a variety of stepped systems, revealing the complex nature of step barrier potentials.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

Ag induced restructuring of the oxygen precovered Cu(1 1 0) surface

The interplay between chemisorbed oxygen and deposited Ag on the Cu(1 1 0) surface has been studied by scanning tunneling microscopy (STM) and photoelectron emission microscopy (PEEM). The Cu-CuO stripe phase formed on the clean Cu(1 1 0) surface upon oxygen chemisorption at 660 K is partly dissolved by Ag deposition at 300 K. Upon annealing, however, a phase separation is observed, where the Cu-O compounds agglomerate into large CuO islands and the Ag is located in between. Also a strong preference for the Ag to attach to step bunches is observed. Especially on the fully (2×1)O reconstructed Cu(1 1 0) surface, all the deposited Ag is found at the step bunches giving rise to a contrast in PEEM.     

NEXAFS and SEXAFS studies of chemisorbed molecules

Near-edge x-ray absorption fine structure (NEXAFS) and surface extended x-ray absorption fine structure (SEXAFS) spectroscopies and their application to the determination of the adsorption geometry and bonding of low-Z molecules on surfaces are discussed. NEXAFS is characterized by intramolecular resonances and probes the internal structure of the molecule (intramolecular bond lengths and possibly bond angles) as well as its orientation relative to the surface. SEXAFS provides information about the adsorption site and the molecule-substrate distances. After demonstrating the full power of SEXAFS in the analysis of oxygen adsorption on Cu(110) and on Ag(110) an example is given of a complete structure determination for the formate species (HCO₂) on Cu(110) using NEXAFS and SEXAFS.     

Lateral manipulation of small clusters on the Cu and Ag(1 1 1) surfaces with the single-atom and trimer-apex tips: Reliability study

We study the reliability of the lateral manipulation of small Cu clusters (dimer and trimer) on the flat Cu(1 1 1) surface with both the single-atom and trimer-apex tips and that for the Ag/Ag(1 1 1) system, and compare the results between the two systems as well as with the single-atom manipulation on these surfaces. Manipulations are simulated using molecular statics method with semi-empirical potentials. The dependence of the manipulation reliability on the tip height and tip orientation are investigated. Overall, the manipulation reliability increases with decreasing tip height although it depends obviously on the tip orientation. For the Cu/Cu(1 1 1) system, the manipulation of the dimmer and trimer can be successful with both tips. The manipulation reliability can be improved by the trimer-apex tip, and the tip-height range for the successful manipulation is also broader, as compared to the single-atom apex tip. Differently from the single-atom manipulation, the tip orientation has a noticeable influence on the manipulation reliability even for the single-atom tip due to the stronger tip-cluster and surface-adatom interactions in cluster manipulation. For the Ag/Ag(1 1 1) system, successful manipulations only be achieved with the trimer-apex tip, and the manipulation reliability is worse than that of the Cu/Cu(1 1 1) system, indicating the difference in mechanic properties between the two surfaces at the atomic level.     

Spectroscopy of image-potential states by two-photon photoemission

Unoccupied electronic states in solids and at solid surfaces are usually studied by inverse photoemission. An alternative method is two-photon photoemission. It is superior in resolution but limited to states of sufficiently long lifetime below the vacuum level. So far this method has mainly been applied to image-potential states on metal surfaces. On Ag(111) and Cu(111) a narrow surface state below the Fermi level serves as the initial state, which results in a pronounced resonance in the two-photon photoemission. Ni(111) shows similar results. In the resonance the image-potential state is so highly populated that electron-electron interaction leads to an Auger-type process. Nevertheless, the system is not so greatly disturbed as to show deviations from the one-photon photoemission results concerning the occupied states. Ag(100) and Cu(100) have a smooth continuum of initial states. Consequently, no resonance occurs. The binding energy does not depend on the material but changes with surface orientation: it is about 0.80 eV at the (111) surfaces and about 0.55 eV at the (100) surfaces. The effective mass is free electron like except on Ag(111), where it is 30% heavier. The lifetime on Ag(100) is about 20 fs. The agreement with theory is excellent in some cases and only fair in others.     

Coadsorption of gold with chlorine on CeO2 (111) surfaces: A first principles study

To investigate the effects of chlorine on the Au/ceria catalysts,the adsorption of gold or chlorine and their coadsorpiton on the stoichiometric and partially reduced CeO₂（111） surfaces are studied from the first principles.It is found that the adsorption of Au is significantly enhanced by the chlorine preadsorption on the stoichiometric CeO₂（111） surface;while on the partially reduced CeO₂（111） surface,the preadsorbed chlorine inhabits the oxygen vacancy（which is the preferred adsorption site for gold）,leading to a CeOCl phase and the dramatical weakening of the Au adsorption.Therefore,chlorine on the CeO₂（111） surface can affect the Au adsorption thus the activity of the Au/CeO₂ catalyst.     

Quantitative analysis of the surface reconstruction induced band-gap in the Shockley state on monolayer systems on noble metals

One monolayer of Ag deposited on Cu(1 1 1) shows two kinds of characteristic reconstruction, depending on the conditions of the preparation: the incommensurate moiré structure appears for one monolayer prepared at 200 K whereas a monolayer deposited at room temperature (or higher) exhibits a quasi-commensurate triangular structure. By high-resolution ARUPS measurements on the triangular structure we find an opening of a gap in the Shockley state band, which is a signature of the super-lattice. On the other hand, no gap opening is observed on the moiré structure. In addition, we show that the Shockley state plays an important role in the adsorption process of rare gas atoms on these surfaces. ARUPS experiments on adsorbed Xe on 0.6 ML Ag/Cu(1 1 1) show clearly that the Xe atoms favor the adsorption on the Ag islands, before the Cu terraces will be covered at higher Xe exposure.     

Influence of applied potential on bradykinin adsorption onto Ag,Au, and Cu electrodes

Surface‐enhanced Raman scattering, electrochemistry, and generalized two‐dimensional correlation analysis (G2DCA) methods were used to characterize bradykinin (BK), a hormone which is known to be involved in small‐cell and non‐small‐cell lung carcinoma and prostate cancer. BK was deposited onto Ag, Au, and Cu electrode surfaces under different applied electrode potentials (−1.000 V to 0.200 V) in aqueous solutions. Based on the analysis of the enhancement, the broadening, and the shifts in the wavenumbers of individual bands, specific conclusions were drawn regarding the peptide geometry and changes in this geometry that occurred when the electrode type and applied electrode potential were varied. Briefly, BK deposited onto the Ag, Au, and Cu electrode surfaces showed bands that were due to the vibrations of moieties in contact with or in close proximity to the electrode surfaces and were thus located on the same side of the polypeptide backbone. These moieties included the Phe, Arg, and Pro residues. The findings for adsorbed BK were fully supported by G2DCA, which also allowed us to determine the order in which changes occurred when the electrode potential was changed. In addition, it was found that at negative electrode potentials, the Phe rings and methylene groups interact with Ag electrode surface. No such interaction was observed for Au and Cu electrodes. Copyright © 2013 John Wiley & Sons, Ltd.     

DFT calculations of adenine adsorption on coin metal (1 1 0) surfaces

Density functional theory is used to analyze in detail the adsorption of the adenine molecule on the (1 1 0) surfaces of Cu, Ag, and Au. While the adsorption configurations are similar in all three cases – the molecule bonds via two nitrogen atoms to the substrate – the details like charge transfer or local strain a rather different. The molecule–substrate interaction in case of Cu is stronger than for the more noble metals Ag and Au. Longe-range dispersion forces stabilize the adsorption configuration in dependence on the specific adsorption geometry. In case of Ag and Au, relativistic effects are found to be important.     

An ab initio analysis of adsorption and diffusion of silver atoms on alumina surfaces

The adsorption and diffusion of silver adatoms on a variety of different surface terminations of alumina was studied with density functional theory. Both Al and O terminated surfaces of α-Al₂O₃(0 0 0 1), as well as hydroxylated versions of these surfaces, were examined. The results indicate that Ag bonds weakly to the Al terminated surface and has low barriers for diffusion. This suggests that the diffusion of Ag on the alumina terminated surface is very rapid. Ag bonds much more strongly, however, to the O-terminated surface which results in higher diffusion barriers. Calculations for Ag on hydroxylated surfaces indicate that the presence of water serves to lower barriers to diffusion for Ag atoms as compared to the oxygen terminated surface but decreases diffusion as compared to the aluminum terminated surface. The results described herein help to provide a more detailed understanding of the observed surface wetting of other transition metals upon hydroxylated alumina surfaces.     

"Magic" heteroepitaxial growth on vicinal surfaces

The step period (Lambda) of vicinal surfaces can be used as a new parameter for the control of metallic heteroepitaxial growth. This is evidenced here in the case of Ag/Cu(211). The deposition of 1 monolayer (ML) exhibits a c(2 x 10) superstructure leading to the formation of [111] steps in the Ag adlayer in contrast with the original [100] steps for the Cu substrate. This wetting layer can be viewed as a (133) Ag plane and it will be the starting point for the epitaxial growth. The deposition of 4 ML shows that the thin Ag film results homogeneous and no twins or stacking faults are detected. Moreover, the film grows along the [133] axis which is the orientation that minimizes the misfit between Cu(211) and the Ag film. Thus, the use of a regular stepped substrate allows one to select the crystallographic orientation of the growth and seems to be a way to avoid the creation of stacking faults.