Characterization of metal clusters

Support preparation Nucleation and growth Reactivity Structure, morphology and characterization of metal clusters of the metal clusters... 

One of the challenging aims of cluster chemistry is to elucidate the factors controlling the formation of cluster molecules and small metal crystallites. Despite remarkable achievements in the synthesis and structural characterization of metal clusters, the pathways to the assembly of large numbers of metal atoms in the course of the synthesis of high nuclearity metal clusters remains rather mysterious. Some insight into this problem has been gained by recent studies of so-called giant clusters of palladium and platinum. 

Characterization is relatively simple since the dusters contain only metal atoms, and usually of only a single element. However, there is one problem asso-dated with the characterization of metal clusters in cages. In contrast to the situation for metal carbonyl dusters, there is no base data set for these metal clusters themselves in a pure state to be used for comparison. This means that spectra of encaged metal dusters cannot be compared with those of their analogues in the liquid or solid state because they simply are not known. Thus the basis for structure determination is in a sense weaker than that for metal carbonyl dusters. 

Rapid progress has been made in the preceding five years in the production and characterization of metal cluster beams. Details of the technology of production and analysis can be found in review articles [1-3]. Pulsed beams of metal clusters are obtained by expanding and condensing in a high-pressure helium stream metal vapors produced by pulsed laser vaporization. Reactant molecules can be introduced into the gas phase along the pathway of metal cluster beams. The nuclearity of the clusters and the products of the reaction between clusters and added molecules can be identified by mass spectroscopy. 

Duncan M A 1997 Synthesis and characterization of metal-carbide clusters in the gas phase J. duster Soi. 8 239... 

Characterization of these clusters indicate an unusual 2n framework electron count having geometries reminiscent of stricdy metallic clusters (11,164). 

With the development of intense synchrotron X-ray sources, it has become possible to use X-ray absorption spectroscopy to structurally characterize the metal clusters in metalloproteins. A wide variety of systems have been studied in this manner in the last ten years. The present article reviews the nature of X-ray absorption spwtroscopy and the information which can be obtain fix)m its study. The strengths and Kmitations of the technique are discussed with reference to the recent literature. 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

A number of novel products have been isolated from the reaction of [B Hg]- [31426-87-6] and CoCl and [C HJ in THF (162,163). The predominant product is /< -2-(CpCo)-B4H8 [43061-99-0]. Also obtained are isomeric clusters containing up to four cobalt atoms, eg, (t]5-C5H5Co)4B4H8 [59370-82-0]. Characterization of these clusters indicate an unusual 2n framework electron count having geometries reminiscent of stricdy metallic clusters (11,164). 

Mossbauer spectroscopy has thus far only been used to characterize mixed-metal clusters that contain iron, but it has been useful in deducing structures in several instances. The principal utility of the technique has been in determining whether or not the several iron atoms in a cluster are in equivalent environments. An application to the characterization of... 

EXAFS Results Characterizing Supported Metal Clusters During Catalytic Hydrogenation of Propene Fed in an Equimolar Mixture to a Flow Reactor Operated at Steady State at 25°C and 1 atm (Panjabi, Argo, and Gates, 1999)a... 

Further studies indicate that the phenomena characterized here can be extended to the study of other bound-free transitions, furnishing a means of mapping the repulsive states which characterize small metal clusters (51). This may have significant importance for the characterization of photocatalytic processes. Further it appears that one can make use of the shallow nature of the bonding potentials characterizing many metal clusters and supersonic expansions in general to obtain "hot band" structure... 

The presence of the distinct XRD patterns provides us new opportunities for precisely monitoring various physico-chemical phenomena that take place inside the well-defined carbon pores or at the pore walls such as adsorption, impregnation, framework changes, formation of metal clusters and grafted frmctional groups. The materials constructed with such well defined and controllable pore diameters are suitable as standards or references for the characterization of porosity of the carbons, similar to the already well-known case of the MCM-41 silica. 

Low-temperature MCD spectroscopy has been used to characterize the metal clusters present in the VFe protein of A. vinelandii (42). The temperature dependence of the MCD transitions of dithionite-reduced protein is similar to those arising from the S = 3/2 spin system of FeMoco of the MoFe proteins. The spectra of the VFe proteins have slightly different electronic and magnetic properties but provide strong spectroscopic evidence for the presence of a V-Fe-S cluster. In this oxidation state, only small additional spectral contributions from the two other paramagnetic species (S = 1/2 signal and the species detected in the dispersion mode) are observed. 

Chapter 4, by Batzill and his coworkers, describes modern surface characterization techniques that include photoelectron diffraction and ion scattering as well as scanning probe microscopies. The chapter by Hayden discusses model hydrogen fuel cell electrocatalysts, and the chapter by Ertl and Schuster addresses the electrochemical nano structuring of surfaces. Henry discusses adsorption and reactions on supported model catalysts, and Goodman and Santra describe size-dependent electronic structure and catalytic properties of metal clusters supported on ultra-thin oxide films. In Chapter 9, Markovic and his coworkers discuss modern physical and electrochemical characterization of bimetallic nanoparticle electrocatalysts. 

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