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1.
The high catalytic activity of gold clusters on oxides has been attributed to structural effects (including particle thickness and shape and metal oxidation state), as well as to support effects. We have created well-ordered gold mono-layers and bilayers that completely wet (cover) the oxide support, thus eliminating particle shape and direct support effects. High-resolution electron energy loss spectroscopy and carbon monoxide adsorption confirm that the gold atoms are bonded to titanium atoms. Kinetic measurements for the catalytic oxidation of carbon monoxide show that the gold bilayer structure is significantly more active (by more than an order of magnitude) than the monolayer. 相似文献
2.
Burgert R Schnöckel H Grubisic A Li X Stokes ST Bowen KH Ganteför GF Kiran B Jena P 《Science (New York, N.Y.)》2008,319(5862):438-442
The reactivity pattern of small (approximately 10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH-, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity. 相似文献
3.
Gas-phase ion chromatography can separate ions that have the same mass but differ in isomeric structure or electronic configuration. The main features of this technique are briefly outlined, and applications to a series of problems in transition metal chemistry and carbon cluster chemistry are described. Examples in transition metal chemistry include state-selective reactivity, excited state deactivation, and state-selective ligand binding energies. For clusters, ion chromatography was used to determine the structure of pure carbon cluster ions as a function of size from C(4) to C(84). The results indicate that carbon grows first in linear chains, transforms to monocyclic planar rings at about C(10), and forms new families of planar bi-, tri-, and tetracyclic rings at C(20), C(30), and C(40), respectively. Fullerenes, which mysteriously appear at C(30) and dominate by C(50), are generated by heating the planar ring systems above an isomerization barrier rather than by growth of graphite precursors. 相似文献
4.
Experimental and theoretical studies of small clusters of metal atoms are aimed at revealing how properties change in the ultrafinely divided state. Thermodynamic, electronic, and structural features can be size-dependent and may be involved in determining the activity of small-particle catalysts. Correlations between these properties give indications of the features important in catalysis. 相似文献
5.
Herzing AA Kiely CJ Carley AF Landon P Hutchings GJ 《Science (New York, N.Y.)》2008,321(5894):1331-1335
Gold nanocrystals absorbed on metal oxides have exceptional properties in oxidation catalysis, including the oxidation of carbon monoxide at ambient temperatures, but the identification of the active catalytic gold species among the many present on real catalysts is challenging. We have used aberration-corrected scanning transmission electron microscopy to analyze several iron oxide-supported catalyst samples, ranging from those with little or no activity to others with high activities. High catalytic activity for carbon monoxide oxidation is correlated with the presence of bilayer clusters that are approximately 0.5 nanometer in diameter and contain only approximately 10 gold atoms. The activity of these bilayer clusters is consistent with that demonstrated previously with the use of model catalyst systems. 相似文献
6.
High-resolution infrared laser spectroscopy was used to obtain rotationally resolved infrared spectra of adsorbate-metal complexes. The method involves forming the bare metal clusters in helium nanodroplets and then adding a molecular adsorbate (HCN) and recording the infrared spectrum associated with the C-H stretching vibration. Rotationally resolved spectra were obtained for HCN-Mg(n) (n = 1 to 4). The results suggest a qualitative change in the adsorbate-metal cluster bonding with cluster size. 相似文献
7.
Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures between 80 and 1000 K. Ferromagnetism occurs even for the smallest sizes: for clusters with fewer than about 30 atoms the magnetic moments are atomlike; as the size is increased up to 700 atoms, the magnetic moments approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. A crystallographic phase transition from high moment to low moment in iron clusters has also been identified. 相似文献
8.
The termination of a solid induces changes in the locations of the outermost atoms of the solid. The changes can be minor or as dramatic as the rearrangement of the atoms into a different crystallographic group. Surface crystallography studies have determined that all surfaces are altered by forces induced at the solid-vacuum interface. At the least, the outermost atomic layers are displaced away from positions that they would have had in the bulk environment. Results from experimental and theoretical investigations for the Al(110) surface are discussed to illustrate present understanding of the surface atomic displacements. Some effects that the truncation- induced forces have on the surfaces of binary metal alloys are also discussed. 相似文献
9.
Bimetallic electrodes are used in a number of electrochemical processes, but the role of particular arrangements of surface metal atoms (ensembles) has not been studied directly. We have evaluated the electrochemical/catalytic properties of defined atomic ensembles in atomically flat PdAu(111) electrodes with variable surface stoichiometry that were prepared by controlled electrodeposition on Au(111). These properties are derived from infrared spectroscopic and voltammetric data obtained for electrode surfaces for which the concentration and distribution of the respective metal atoms are determined in situ by atomic resolution scanning tunneling microscopy with chemical contrast. Palladium monomers are identified as the smallest ensemble ("critical ensemble") for carbon monoxide adsorption and oxidation, whereas hydrogen adsorption requires at least palladium dimers. 相似文献
10.
Calorimetric measurements of metal adsorption energies directly provide the energies of metal atoms in supported metal nanoparticles. As the metal coverage increases, the particles grow, revealing the dependence of this energy on particle size, which is found to be much stronger than predicted with the usual Gibbs-Thompson relation. It is shown that this knowledge is crucial to accurately model long-term sintering rates of metal nanoparticles in catalysts. 相似文献
11.
Shin JW Hammer NI Diken EG Johnson MA Walters RS Jaeger TD Duncan MA Christie RA Jordan KD 《Science (New York, N.Y.)》2004,304(5674):1137-1140
We report the OH stretching vibrational spectra of size-selected H+(H2O)n clusters through the region of the pronounced "magic number" at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6 相似文献
12.
The selective oxidation of alcohols in aqueous phase over supported metal catalysts is facilitated by high-pH conditions. We have studied the mechanism of ethanol and glycerol oxidation to acids over various supported gold and platinum catalysts. Labeling experiments with (18)O(2) and H(2)(18)O demonstrate that oxygen atoms originating from hydroxide ions instead of molecular oxygen are incorporated into the alcohol during the oxidation reaction. Density functional theory calculations suggest that the reaction path involves both solution-mediated and metal-catalyzed elementary steps. Molecular oxygen is proposed to participate in the catalytic cycle not by dissociation to atomic oxygen but by regenerating hydroxide ions formed via the catalytic decomposition of a peroxide intermediate. 相似文献
13.
Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C(n)(+) clusters, which tend to fragment with a characteristic loss of a neutral C(3), are found to be different from the modes for Si(n)(+) and Ge(n)(+) clusters, which tend to fragment to "magic" clusters such as Si(4)(+), Si(6)(+) and Si(10)(+). These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences. 相似文献
14.
Ebert LB 《Science (New York, N.Y.)》1990,247(4949):1468-1471
Soot generated from diesel fuel in a combustion tube is characterized by microanalysis, x-ray diffraction, chemical reactivity, and nuclear magnetic resonance to address the recent proposal of the significance of carbon clusters in soot. The data support a traditional model of soot as polynuclear aromatic compounds rather than as clusters of carbon atoms with minimal edge site density. The amounts of noncarbon atoms in the soot (hydrogen, oxygen, nitrogen, and sulfur) are commensurate with the edge density of the crystallites (2 by 2 nanometers) inferred from diffraction. The chemistry of soot, in being reduced by potassium metal and alkylated by alkyl iodides, is that known for aromatic compounds and not that anticipated for materials such as graphite, with a small fraction of carbon atoms on edges. 相似文献
15.
农业产业集群高效率生产的竞争优势需物流系统的支撑,物流对农业产业集群能够辐射的市场区域大小有很大影响,进行物流对农业产业集群发展的影响研究具有积极意义.对农业产业集群及其物流进行分析,并采用层次分析法分析物流各因素对农业产业集群发展影响的相对重要程度,结果表明物流的专业化程度是影响农业产业集群发展的首要因素.提出农业产业集群与第三方物流从战略适应和功能整合两个维度合作,从而加大第三方物流对农业产业集群发展的推动作用. 相似文献
16.
Xu RX Hassell AM Vanderwall D Lambert MH Holmes WD Luther MA Rocque WJ Milburn MV Zhao Y Ke H Nolte RT 《Science (New York, N.Y.)》2000,288(5472):1822-1825
Cyclic nucleotides are second messengers that are essential in vision, muscle contraction, neurotransmission, exocytosis, cell growth, and differentiation. These molecules are degraded by a family of enzymes known as phosphodiesterases, which serve a critical function by regulating the intracellular concentration of cyclic nucleotides. We have determined the three-dimensional structure of the catalytic domain of phosphodiesterase 4B2B to 1.77 angstrom resolution. The active site has been identified and contains a cluster of two metal atoms. The structure suggests the mechanism of action and basis for specificity and will provide a framework for structure-assisted drug design for members of the phosphodiesterase family. 相似文献
17.
Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles. 相似文献
18.
Wang PK Ansermet JP Rudaz SL Wang Z Shore S Slichter CP Sinfelt JH 《Science (New York, N.Y.)》1986,234(4772):35-41
In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possible to detect progressively smaller numbers of nuclei. Experiments and studies previously thought to be impractical can now be undertaken, for example, the study of phenomena at surfaces. Nuclear magnetic resonance has been applied to study simple molecules (carbon monoxide, acetylene, and ethylene) adsorbed on metal surfaces (ruthenium, rhodium, palladium, osmium, iridium, and platinum). The metals, in the form of clusters 10 to 50 angstroms in diameter, supported on alumina, are typical of real catalysts. The experiments provide information about the bonding of the molecules to the metal, the structures the molecules assume after adsorption, the motion of molecules on the surface, the breakup of molecules induced by heating, and the products of such breakup. 相似文献
19.
Overlayer structures can be formed on surfaces by the deposition of clusters containing hundreds or thousands of atoms. Cluster assembly alters the reaction pathway at the surface so that novel structures with unique chemical and physical properties can be stabilized. This article discusses the process of cluster assembly. Cluster-assembled interfaces are compared to those obtained by conventional techniques, and examples are given for metal-semiconductor and semiconductor-high temperature superconductor systems. 相似文献
20.
采用分子动力学方法对液态Ca50Zn50合金的快速凝固过程进行了模拟研究.并采用双体分布函数、HA键型指数法、原子团类型指数法(CTIM)和可视化等方法对凝固过程中微观结构的演变特性进行了分析.结果表明：由于Ca,Zn原子的半径差异较大,导致总双体分布函数的第1峰明显分裂成为3个次峰,随着温度下降,半径较小的Zn原子互为近邻的几率明显增加.系统中1551键型所构成的二十面体基本原子团(12 0 12 0)的数量占绝对优势,对非晶态结构的形成起主导作用.系统中基本原子团的中心原子主要是原子半径较小的Zn原子.具有短程序的基本原子团之间大多以共顶点(VS),共棱线(ES),共面(FS)和相互交叉(IS)的方式相结合,在凝固形成非晶态的过程中,基本原子团大多以相互交叉(IS)的方式相结合,最后形成各种不同尺寸的、原子结合比较紧密、结构比较稳定的较大团簇. 相似文献