Metal clusters, experimental techniques

In this paper, the photofragmentation of transition metal cluster complexes is discussed. The experimental information presented concerning the gas phase photodissociation of transition metal cluster complexes comes from laser photolysis followed by detection of fragments by ionization (5.). Ion counting techniques are used for detection because they are extremely sensitive and therefore suitable for the study of molecules with very low vapor pressures (6.26.27). In addition, ionization techniques allow the use of mass spectrometry for unambiguous identification of signal carriers. 

Although STM is not a particularly fast observation technique, the growth of a metal cluster can still be monitored during its initial stages, provided the growth rate is slowed down by appropriate experimental parameters, such as overpotential and electrolyte composition. In addition, the interference of the STM tip with electro-... 

A study of the electronic nature of metal clusters is a difficult theoretical and experimental problem. Information on small isolated clusters has been obtained with the field-ion microscope, mass spectrometer, or electron microscope, but other techniques are needed for such studies. The semiempirical MO procedure is applicable to this problem and can provide a great deal of detailed information to compare with experiment concerning the electronic structure of the metal cluster. 

The preliminary studies described above indicate the power of combining molecular-beam techniques for synthesizing metal clusters of known size and composition and techniques for studying individual supported clusters. It is to be expected that this fusion of experimental methods will lead to increased understanding of the complex world of supported metal catalysts. 

One of the key issues of supported model catalysts is to prepare collection of metal particles having a well-defined morphology. Indeed, if a catalytic reaction is structure-sensitive [54], it will depend on the nature of the facets present on the particles. Moreover, the presence of edges, the proportion of which is increasing rapidly below about 5 nm, can affect the reactivity by their intrinsic low coordination and also by their role as boundary between the different facets. In this section I first discuss the theoretical predictions of the shape of small particles and clusters, then I briefly describe the available experimental techniques to study the morphology, and finally I discuss from selected examples how it is possible to understand and control the morphology of supported model catalysts. 

Several earlier review articles are relevant to our subject. Slichter reviews the work done in his laboratory [16], most of it concerned with atoms or molecules adsorbed on the metal clusters, and the experimental techniques used in such studies [17]. Duncan s review [9] pays special attention to the C NMR of adsorbed CO. Most recently, one of us has given a rather detailed review of the held, in particular on metal NMR of supported metal catalysts [18]. While the topics and examples discussed in this chapter will inevitably have some overlap with these previous reviews, particular emphasis is directed toward highlighting the ability of metal NMR to access the iff-LDOS at both metal surfaces and molecular adsorbates. The iff-LDOS is an attractive concept, in that it contains information on both a spatial (local) and energy (electronic excitations) scale. It can bridge the conceptual gap between localized chemical descriptors (e.g., the active site or the surface bond) and the delocalized descriptors of condensed matter physics (e.g., the band structure of the metal surfaces). 

The focus of this paper is experimental work on the chemical properties of neutral transition metal clusters. The outline is as follows. We first discuss in some detail the techniques used to generate neutral gas-phase clusters. Next the known physical properties of metal clusters are summarized. This is followed by a discussion of the definition of chemical reactivity in the context of the cluster experiments. Finally, several examples of specific reactions are presented and an electronic model is proposed which can explain many of the more striking observations. Results from recent cluster ion reaction studies... 

Our understanding of the behavior of isolated metal atoms and small metal clusters adsorbed on oxide surfaces has increased enormously in the last few years, thanks to spectacular advances on the experimental side but also to the high level of reliability reached by first principle calculations. Not surprisingly, this is a complex problem which cannot be solved from scratch by using one or another technique. Rather, from the combination of several results and different approaches one can get a deeper insight at an atomistic level of the interaction of a metal atom or cluster with the support. 

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