Gas phase metal clusters

Gas-Phase Metal Cluster Cations of Transition Metals, Production, and Gas-Phase Reactions... 

MOLECULAR SURFACE CHEMISTRY REACTIONS OF GAS-PHASE METAL CLUSTERS... 

Vibrational spectroscopy is an important probe used to determine the bonding and structural properties of molecules. Powerful techniques such as electron energy loss spectroscopy (EELS) have been developed, which allow one to obtain the vibrational properties of molecules chemisorbed upon surfaces. Due to low concentration, the highly reactive nature of the clusters, and the large number of possible species which are typically present in the cluster beams used to date, unconventional techniques are required in order to obtain spectroscopic information. One unconventional but powerful technique, infrared multiple photon dissociation (IRMPD), has recently been applied to the study of the vibrational properties of gas-phase metal clusters upon which one or more molecules have been chemisorbed. This same technique, IRMPD, has previously been used to obtain the vibrational spectra of ions, species for which it is difficult to apply conventional absorption techniques. 

In this chapter, we focus on recent experimental and theoretical studies of chemical and catalytic properties of gas-phase metal clusters and metal clusters deposited onto support materials with sizes in the nonscalable regime. 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

J2.13 Molecular surface chemistry reactions of gas-phase metal clusters... 

In conclusion, the optical response of gas phase metal clusters in conjunction with their theoretical interpretation represents a powerful method for characterizing these systems. In the low nudearity regimes, accurate ab initio Cl calculations can be applied, at least to clusters of simple metals having small numbers of valence electrons. They have provided a basis for the characterization of the geometric structures of smaller aggregates. On the other hand, the RPA-jellium approach yields a simple and suffidently accurate picture for the evolution of the optical properties of metal clusters towards the bulk. Hie prindple underlying... 

Thus, recent experimental evidence seems to support the idea that growing small particles have maximum chemical reactivities, and certain sized/shaped small particles may have the highest reactivities. What size and/or shape varies with the metal in question and the reaction in question This information strongly supports three ideas (1) structure sensitivity in chemical reactions on metal surfaces is very important, (2) more than one atom is necessary to carry out at least some bond breaking processes, and (3) defect sites on growing small particles are extremely reactive (see Fig. 9). It has also been possible by pulsed laser vaporization to produce many types of gas phase metal clusters. Particularly interesting have been reactivity studies of niobium clusters Nb where X = 5-20. A definite cluster size dependence on reactivity was observed. Exposure... 

Reactivity of much larger gas phase metal clusters have also been examined in recent years. An example is the determination of binding sites for NH3 gas on Ni c and Fe where x = 50-147. The adsorption of ammonia on gas phase metal clusters can lend information, since the number of binding sites would vary with cluster structure. For Ni clusters, the number of ammonia molecules adsorbed showed pronounced minima in the 50-116 atom range for those specific clusters that are particularly stable (those that are a magic number are in larger amounts than statistically predicted (see Fig. 10)). 

In another example of the interesting behavior of small metal clusters, the ability to produce small gas phase metal clusters, and to slow them down so they can be soft-landed on a surface, has been utilized in an ingenious... 

A comprehensive comparison of the catalytic properties of nanopartides obtained via chemical and physical routes is difficult, because of the scarce information on nanoparticles prepared via physical methods. Catalytic applications were described for Pd nanopartides obtained via electrochemical route, and metal clusters obtained in a SMAD reactor.P3 l The differences in the catalytic behaviour relative to analogous conventional catalysts obtained via chemical route h concerning activity and selectivity in, e.g., the hydrogenation of 1-heptene or 1,4-butadiene lie not only in the size and shape of the partides, but also in specific interactions with the solvent. Thus, it is daimed that Pd nanopartides, e.g., react with polar solvent molecules and acquire a negative charge. The absence of other chemical species, which can interact with the metal and the small size of the nanopartides make them in some degree comparable with gas phase metal clusters and their unique reactivity. 4]... 

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