Metallic clusters fundamentals

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

The foregoing results characterizing structurally simple supported metal clusters can be generalized, at least qualitatively, to provide fundamental understanding that pertains to industrial supported metal catalysts, with their larger, nonuniform particles of metal. 

The mobility of metal atoms in bare metal clusters and small metallic nanoparticles (NPs) is of fundamental importance to cluster science and nanochemistry. Atomic mobility also has significant implications in the reactivity of catalysts in heterogeneous transformation [6]. Surface restmcturing in bimetallic NP and cluster catalysts is particularly relevant because changes in the local environment of a metal atom can alter its chemical activity [7, 8]. 

Rudolph RWT, Young RC (1979) The nature of naked metal clusters in solution. In Tsutsui M (ed) Fundamental research in homogeneous catalysis, vol 3. Plenum, New York, pp 997-1005... 

Dialkylzinc derivatives are inert towards conjugated enones (e.g. 181) in hydrocarbon or ethereal solvents. The discovery that a conjugate addition can be promoted by Cu(I) salts in the presence of suitable ligands L (e.g. sulphonamide 182) opened a new route to zinc enolates (e.g. 183), and hence to the development of three-component protocols, such as the tandem 1,4-addition/aldol addition process outlined in equation 92186. If the addition of the aldehyde is carried out at —78 °C, the single adduct 184 is formed, among four possible diastereomeric products. The presence of sulphonamide is fundamental in terms of reaction kinetics its role is supposed to be in binding both Cu(I) and Zn(II) and forming a mixed metal cluster compound which acts as the true 1,4-addition catalyst. 

Significant new insight has been gained into the formation of small clusters and nanosized metallic particles [501,502]. This fundamental information is not only inherently fascinating, but it is vitally important for the construction of new generations of advanced nanostructured materials. Evolution of nanosized metallic particles from non-metallic clusters and the chemistries of these species will, therefore, be discussed in the following sections. 

Clusters and alloys are molecular species that may show different catalytic activity, selectivity and stability than bulk metals and alloys. Small metal clusters and alloy clusters have been studied reeendy for potential use as catalysts, ceramic precursors, and as thin films. Several fundamental questions regarding such clusters are apparent. How many atoms are needed before metallic properties are observed How are steric and electronic properties related to the number, type and structure of such clusters Do mixed metal clusters behave like bulk alloy phases ... 

Molecular electron transfer is the basis for many important natural and commercial processes. During the past decade photochemists have relied upon supramolecular arrays of molecules to facilitate their understanding of the chemical and physical basis for this fundamentally important process. It therefore seems appropriate that several chapters in this volume examine thermally and photo-chemically induced electron transfer in supramolecular assemblies consisting of inorganic molecular building blocks such as covalently linked donor-acceptor dyads, transition metal clusters, and nanocrystalline semiconductor particles. 

Small metal clusters have received considerable attention because of their possible involvement as "active sites" in a variety of catalyzed reactions. Although not particularly noted for their catalytic activity, alkali clusters have a simple chemical composition and may, therefore, model the more complicated systems in a manner analogous to the role played by the hydrogen atom in atomic structure. Less emphasized is the fundamental nature of alkali clusters per se. Since the ground state of Hj is not chemically bound, alkali trimers are the most elementary species which can exhibit a Jahn-Teller interaction. 

Because of their probable Importance to the understanding of the fundamental mechanisms of catalysis and numerous chemical conversions, the basic properties (geometry, bond strength, reactivity) of small metallic clusters Mji (2 < n < 6) have become the subject of intense theoretical and experimental study (1-36). Because experimental characterization is complicated, theory abounds and experimental studies are much less prevalent. Ligand-... 

In the last few decades, metal clusters and nanomaterials have attracted increasing attention due to their unique properties which differ from those of the bulkd Metal nano-objects are of great interest in the field of nanosciences. They are the ideal structures for fundamental research and applications. Indeed, from the confinement of the charge carriers in such limited objects, one expects a shift of the plasmon resonance absorption, non-linear optical effects, non-metallic conductivity, and nanocatalytic effects. Nanomaterials can have important applications in several fields such as catalysis, electrocatalysis, electronics, optical limitation, biology, etc. 

Heiz U, Bullock EL (2004) Fundamental aspects of catalysis on supported metal clusters. J Mater Chem 14 564... 

Basset J-M, Lefebvre F, Santini C (1998) Surface organometallic chemistry Some fundamental features including the coordination effects of the support. Coord Chem Rev 178-180 1703 Gates BC (2000) Supported metal cluster catalysts. J Mol Catal A Chem 163 55 Fierro-Gonzalez JC, Kuba S, Hao Y, Gates BC (2006) Oxide- and zeoUte-supported molecular metal complexes and clusters Physical characterization and determination of structure, bonding, and metal oxidation state. J Phys Chem B 110 13326... 

The topic of defect sites at oxide surfaces therefore becomes crucial in order to fully understand the metal-oxide bonding. This subject has been addressed theoretically only recently. In this review we have shown how defect sites at both MgO and Si02 surfaces play a fundamental role in both stabilization and nucleation, but also that they modify the cluster electronic properties. In particular, some defect centers that act as electron traps like the oxygen vacancies at the MgO surface are extremely efficient in increasing the electron density on the deposited metal atoms or clusters, thus augmenting their chemical activity toward other adsorbed molecules. Understanding the metal-oxide interface and the properties of deposited metal clusters also needs a deeper knowledge of nature, concentration and mechanisms of formation, and conversion of the defect sites of the oxide surface. 

Electronic Size Effects. Understanding the size-dependent electronic structure of the Au /MgO(Fsc) model catalysts, which is fundamental for elucidation of their atom-by-atom controlled reactivity, is facilitated by analysis of the spectra of the LDOS projected on the oxygen molecule and on the metal cluster (see also Electronic Size Effects ). Figure 1.78a shows the LDOS projected on the O2 molecule which is adsorbed at the peripheral site (Fig. 1.77f) of the more reactive isomer of the Aug/MgO(F5c) model catalyst. As shown above, bonding and activation of O2 on the octamer is enabled by resonances formed between the cluster s electronic states and the 2jt molecular states... 

Where VS can contribute to the synthesis of new materials, it can clearly also contribute, by implication, to fundamental studies of matter. For example, through the synthesis of well-defined colloids and metal clusters, research in adsorption, adhesion, mineral processing, corrosion, pollution and the physics of spatially confined structures is enhanced. 

In this chapter, we have discussed recent theoretical and experimental studies that provide evidence for the important role of surface defects, such as oxygen vacancies, in the metal-oxide bonding. The cases of defect sites, in both MgO and SiC>2 surfaces, clearly show not only the fundamental role played by these sites in both stabilisation and nucleation but also their ability to change the electronic and magnetic properties of the metal atoms. The understanding of the metal-oxide interface and of the properties of deposited metal clusters also requires a deeper knowledge of the nature, concentration and mechanisms of formation and conversion of the defect sites of the oxide surface. 

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