Molecular surface chemistry by metal single crystals and nanoparticles from vacuum to high pressure

Model systems for studying molecular surface chemistry have evolved from single crystal surfaces at low pressure to colloidal nanoparticles at high pressure. Low pressure surface structure studies of platinum single crystals using molecular beam surface scattering and low energy electron diffraction techniques probe the unique activity of defects, steps and kinks at the surface for dissociation reactions (H-H, C-H, C-C, O=O bonds). High-pressure investigations of platinum single crystals using sum frequency generation vibrational spectroscopy have revealed the presence and the nature of reaction intermediates. High pressure scanning tunneling microscopy of platinum single crystal surfaces showed adsorbate mobility during a catalytic reaction. Nanoparticle systems are used to determine the role of metal-oxide interfaces, site blocking and the role of surface structures in reactive surface chemistry. The size, shape and composition of nanoparticles play important roles in determining reaction activity and selectivity and is covered in this tutorial review.     

Photoelectron studies of adsorption and catalysis on the surface of metal single crystals and polycrystals

The potential of photoelectron spectroscopy (XPS) and its use in studies of the state of the surface and adsorption processes on bulk metal samples (disk single crystals, polycrystalline foil and plates) are discussed. Methods for determining the electronic state of the surface layer and geometrical localization of adsorbed atoms on the metal surface using angle-resolved photoelectron spectroscopy are described. The spectrokinetic use of the dynamic mode of XPS with high accuracy and time resolution for kinetic studies of fast processes on the surface (adsorption-desorption, reconstruction, reaction, diffusion-segregation) is considered in detail.     

The preparation of thin ordered transition metal oxide films on metal single crystals for surface science studies

In this paper the preparation, characterization and properties of metal oxide overlayers on dissimilar metal substrates is reviewed. It is shown that using a general recipe metal oxide surfaces can be produced, which are easily accessible with modern surface science techniques. Many different stoichiometries and structures of the oxides can be prepared by variation of the preparation conditions, that do or do not have a counterpart in bulk oxide surfaces. In addition information about the metal oxide surfaces is obtained without experimental problems such as sample mounting, sample heating and sample purity.     

揭秘单原子催化:表面科学方法

理解单原子催化的基本机理对于设计高性能和高稳定性的催化剂体系至关重要.然而,这是个有待解决的问题,因为用现有的实验技术来表征单原子催化活性位极端困难.在过去的40年里,表面科学为理解多相催化提供了基础,但是有关反应温度下、已知结构金属氧化物上稳定的金属原子的模型体系罕见报道.本视角讨论了已知的、吸附在模型金属氧化物表面上的、孤立的金属原子,并探讨了如何利用这些信息去理解单原子催化.一个关键的问题是,尽管在表面科学研究中的高度理想化的模型体系可能无法代表真实反应条件下的催化剂,但是它们与采用理论模拟计算得出的模型非常相似.因此,表面科学有望成为评估单原子催化模型的方法.更令人兴奋的是,几个研究组已经发展出在升温条件下金属吸附原子仍保持稳定的模型体系.但到目前为止,还不能清楚地解释催化活性.最后,本文简要地讨论了在真实反应条件下扫描隧道显微镜的实验前景.     

Bridging the pressure and material gap in heterogeneous catalysis: cobalt Fischer-Tropsch catalysts from surface science to industrial application

The Fischer-Tropsch (FT) process is the heart of many natural gas conversion processes as it enables the conversion of a mixture of CO and H(2) into valuable long-chain hydrocarbons. Here we report on the use of state-of-the-art surface science techniques to obtain information on the relationship between the surface atomic structure of model catalysts and their performance in the Fischer-Tropsch reaction. Cobalt single crystals and polycrystals were modified with non-reducible oxides as to resemble industrial catalysts. Reflection absorption infrared spectroscopy was used for examining the CO adsorption behaviour at room temperature as well as at 493 K at CO pressures spanning 10(-7) to 300 mbar on both (modified) Co single/polycrystals and an industrial catalyst. Polarization modulation was applied to cancel the CO gas phase absorption. Subsequently, they were subjected to reaction tests in the same apparatus at 1 bar and 493 K. This allowed us to close the material, pressure and instrument gap in the field of Fischer-Tropsch synthesis on cobalt-based catalysts.     

Evolution of single crystalline dendrites from nanoparticles through oriented attachment

Single crystalline PbMoO4 dendrites were prepared by a simple hydrothermal method in the presence of surfactants. The formation and evolution of these dendrites was investigated by transmission electron microscopy, and the results clearly showed that the dendritic structure was achieved through oriented attachment of nanoparticles along crystallographically specific direction while the traditional Ostwald ripening mechanism also acted to form the initial particles before attachment and smooth the morphology of the dendrites after attachment.     

Lamellar thickening in polyethylene single crystals annealed under low and high pressure

Single crystals of linear polyethylene, prepared from a dilute xylene solution, were annealed below their melting temperature under atmospheric and 6 kbar pressure. In order to preserve the identity of the single crystals, they were suspended in an inert solvent medium, silicone oil and ethanol, during annealing. Examination of the annealed crystals under an electron microscope revealed development of numerous reorganization centers consisting of a central, elongated hole surrounded by a raised edge. Characteristics of these holes, especially their location and orientation, were interpreted in terms of the molecular packing that existed prior to the annealing and the mechanism of molecular reorganization that occurred during the annealing. The effect of high pressure was primarily to flatten out the crystals and to increase the number of reorganization centers, but the height of the raised edges remained about the same irrespective of the applied pressure. The present study also showed examples pointing to the importance of differentiating the annealing behavior of monolayer crystals from that of multilayer crystals.     

Monolithic systems: from separation science to heterogeneous catalysis

Recent results that have been obtained in the ring-opening metathesis polymerization (ROMP)-based synthesis of monolithic supports are summarized. We have elaborated a synthetic concept that allows modifying monolithic supports in a way that they can be used both for applications in separation science, for SEC and as supports for catalytically active systems. In all cases, a tailor-made microstructure was accessible due to the controlled character of the transition-metal catalyzed polymerization. Taking advantage of the “living” catalytic sites, an “in situ” functionalization was accomplished by subsequently grafting a variety of functional monomers and catalyst precursors onto the rod. Their design and use as supports for high-performance separation devices (e.g. for ds-DNA) and catalytic supports (e.g. supported Grubbs-type catalysts) is summarized.     

Organometallic approach to the synthesis and surface reactivity of noble metal nanoparticles

The use of organometallic precursors allows the synthesis in mild conditions of nanoparticles of uniform small size (1–3 nm) and of clean surface, which can be stabilized by polymers or ligands. These nano-objects display an interesting surface chemistry comparable to that of molecular species. This involves classical elementary steps of organometallic chemistry such as substitution and oxidative addition as well as ligand fluxionality. An overview of recent work in this field from our group will be presented including stabilization of metal nanoparticles by asymmetric ligands and preliminary catalytic attempts. To cite this article: K. Philippot, B. Chaudret, C. R. Chimie 6 (2003).     

Absorption and scattering microscopy of single metal nanoparticles

Several recently developed detection techniques opened studies of individual metal nanoparticles (1-100 nm in diameter) in the optical far field. Eliminating averaging over the broad size and shape distributions produced by even the best of current synthesis methods, these studies hold great promise for gaining a deeper insight into many of the properties of metal nanoparticles, notably electronic and vibrational relaxation. All methods are based on detection of a scattered wave emitted either by the particle itself, or by its close environment. Direct absorption and interference techniques rely on the particle's scattering and have similar limits in signal-to-noise ratio. The photothermal method uses a photo-induced change in the refractive index of the environment as an additional step to scatter a wave with a different wavelength. This leads to a considerable improvement in signal-to-background ratio, and thus to a much higher sensitivity. We briefly discuss and compare these various techniques, review the new results they generated so far, and conclude on their great potential for nanoscience and for single-molecule labelling in biological assays and live cells.     

Topochemical synthesis and catalysis of metal nanoparticles exposed on crystalline cellulose nanofibers

Topochemical synthesis of gold nanoparticles (AuNPs) was achieved on crystalline cellulose single nanofibers (CSNFs), which were tailored from native cellulose. Exposed AuNPs@CSNFs composite exhibited an excellent catalytic efficiency: the turnover frequency of the AuNPs@CSNFs was up to 840 times that of conventional polymer-supported AuNPs, for a model aqueous reduction reaction. Our novel strategy provides a promising solution to realize efficient use of limited noble metals using natural bioresources.     

Fold surface of polyethylene single crystals as assessed by selective degradation studies. III. Application of the improved techniques to single crystals

Previous studies on selective degradation of polyethylene single crystals with fuming nitric acid have been extended, both by using acid of lower concentration which gave better control over the degradation, and by resorting to ozone as an oxidizing agent which among others enabled the degradation temperature to be conveniently lowered. The molecular weight distribution was followed by gel-permeation chromatography in the course of degradation. Complete consistency between these different methods has been established, modifying some of the previous conclusions reached by this method. The principal feature which emerges is that we have a distribution of fold lengths. The largest straight fold stems can stretch across nearly the entire layer defined by the low-angle x-ray period, while there is a continuous distribution of shorter folds terminating deeper down in the crystals. The limiting depth at which the number of terminating folds becomes negligible can be identified and quantitatively assessed. The method of analysis is described, and individual data are discussed in detail. This picture of a fold surface layer containing essentially adjacently reentrant folds of uneven length agrees with quite recent results on other related chain-folded systems (annealed crystals, short chains, bulk structures) obtained in these laboratories and thus appears to be of general validity. The consequences of the model for our picture on polymer crystals in particular on the nature of the “amorphous” component, are discussed.     

Interference of surface plasmon resonances causes enhanced depolarized light scattering from metal nanoparticles

We show that the strongly depolarized light scattering from noble metal particles is a result of interference of two surface plasmon resonances on the same particle. The maximum depolarization occurs between two resonances. Under favorable conditions the anisotropy of the scattering light can be much lower than what is possible for dielectric particles. This explanation is discussed in relation to earlier published experimental measurements. Comparison of experimental results with theoretical calculations provides information on the shape distribution of metallic particles in the suspension.     

过渡金属氧化物催化剂上甲烷催化燃烧的研究

与传统燃烧方式相比 ,催化燃烧因具有能量利用率高 ,且基本无污染等突出优点而受到人们的重视。以Ｐｄ为代表的贵金属燃烧催化剂[1] 的研究发展得较为完善 ,但价格昂贵使这类催化剂难以被推向实用。而过渡金属氧化物则价格便宜 ,并且也被认为是很有潜力的燃烧催化剂[2 ] 。但这类催化剂的高温稳定性差 ,活性也远不及贵金属催化剂。为此 ,人们试图使用Ｌａ ,Ｓｒ等作为助剂 ,使其与活性组分生成具有钙钛矿结构的化合物 ;或使用六铝酸盐催化剂[3] 来克服过渡金属氧化物催化剂的缺点。这些方法对过渡金属氧化物催化剂的性能有所改善 ,但催化剂…     

Electrochemical assembling of methionine-gold nanoparticles and catalysis on the surface of glassy carbon electrode

In this paper cyclic voltammetry was used for the synthesis of linear array spherical gold nanoparticles on the surface of glassy carbon electrode using methionine as a stable reagent. The methionine-gold nanoparticles on the surface of glassy electrode were obtained. The methionine-gold nanoparticles were characterized by cyclic voltammetry, scanning electron microscopy, energy dispersive spectrometry and powder X-ray diffraction. Electrochemical behavior of methionine at methionine-gold nanoparticle modified electrode was investigated. It was demonstrated that the methionine-gold nanoparticles can catalyze electrochemical transformations of methionine.     

From Pd nanoparticles to single crystals: 1,3-butadiene hydrogenation on well-defined model catalysts

Although 1,3-butadiene hydrogenation is known to be a structure-sensitive reaction, correlation of the catalytic activity with the exact Pd particle surface structure shows that the reaction is in fact particle size independent.     

The surface functionalisation of gold nanoparticles with metal complexes

The use of nanoparticles is now expanding well beyond the field of pure materials science. In particular, the modification of the surface functionality is playing a key role in catalysis, sensor applications and immobilisation of biomolecules. The use of coordinated metals on the outer surface of gold nanoparticles is the focus of this short article.     

Tailoring surface plasmons through the morphology and assembly of metal nanoparticles

Metal nanoparticles can be used as building blocks for the formation of nanostructured materials. For the design of materials with specific (optical) properties, several approaches can be followed, even when starting from the very same basic units. In this article, a survey is provided of the optical properties of noble metal nanoparticles, specifically gold, silver, and their combinations, prepared in solution through colloid chemical methods. The optical properties are shown to be mainly influenced by the surface plasmon resonance of conduction electrons, the frequency of which is not only determined by the nature of the metal but also by a number of other parameters, such as particle size and shape, the presence of a capping shell on the particle surface, or the dielectric properties of the surrounding medium. Recent results showing how these various parameters affect the optical properties are reviewed. The results highlight the high degree of control that can now be achieved through colloid chemical synthesis.     

NQR of hydrogen bonded crystals under high pressure

The work presents the results of NQR under high hydrostatic pressure for the following ferroelectrics: KDA, KD^xA, LHA, LDA and AHCA. Most of the below presented discussions concerns the behaviour of these ferroelectrics and the explanation of this behaviour on the basis of pseudo-spin-lattice coupling model.