Size-selected clusters

Gas-phase metal oxide cluster cations have become increasingly interesting because of the possibility that a controlled reaction of size selected clusters might shed some light on condensed-phase catalysts. The possibility of producing small clusters in the condensed phase from deposition of gas-phase clusters has also been a driving force in the study of the reactivity of gas-phase ions. 

Tada, M. and Iwasawa, Y. (2009) Model Systems in Catalysis From Single Crystals and Size Selected Clusters to Supported Enzyme Mimics (ed. R.M. Rioux), Springer, in press. 

Using photoelectron detection in a femtochemistry arrangement, we studied size-selected clusters of ionic systems, covering the transition from gas phase to condensed phase dynamics [6]. We investigated the solvent effect on Oj dissociation dynamics, and observed... 

We have discussed recent computational and spectroscopic results on the photoinduced hydrogen transfer and proton transfer chemistry in hydrogen-bonded chromophore-solvent clusters. The interplay of electronic spectroscopy of size-selected clusters and computational studies has led to a remarkably detailed and complete mechanistic picture... 

The Buck method is most useful for medium sized clusters, and in experiments that do not attempt detailed photochemical studies of the size selected clusters. High resolution spectroscopy of the selected clusters is impractical because the number per quantum state is small, due to the many occupied ro-vibrational... 

Size selected clusters from molecular beams deposited on planar 2-dimensional (2-D) supports [ 1 ]... 

This material has been reviewed by Heiz and Schneider [7, 8], by Heiz and Bullock [9], and more recently by Heiz and coworkers [10]. This review is not intended to be comprehensive but will rather focus on some of the most notable research highlights. The synthesis and characterization of size-selected model catalysts will be discussed, but the focus will be on the catalytic chemistry of these supported clusters. This review will concentrate on recent results, and in the conclusion, we suggest new experiments based on supported size-selected cluster catalysts. 

An example of a size-selecting cluster source is depicted in Fig. 16.1 [11]. A laser ablates a metal target, generating gas-phase metal atoms. Clusters then form in an aggregation tnbe that allows rough control of cluster size due to the inert gas flow rate, pressure and length of the channel. 

Fig. 16.1 A typical experimental set-up for a size selection cluster source. Reproduced from [11]. Copyright 2006 Springer...

Future Experiments Involving Size-Selected Clusters... 

Clearly, size-selected cluster catalysts will play a key role in the future of model catalysis and will be an important tool in developing a detailed understanding of size effects in catalysis. Improvements in characterization under reaction conditions are needed to study the stability of these systems. In addition, exciting new possibilities for examining the effects of surface loading (number density) and alloy composition exist which will drive the field forward in the next decade. 

Heiz U, Schneider WD (2001) Size-selected clusters on solid surfaces. Crit Rev Solid State 26 251... 

Harbich W (1999) Soft landing of size-selected clusters in chemically inert substrates. Phil Mag B 79 1307... 

Lee S, Fan CY, Wu TP, Anderson SL (2005) Hydrazine decomposition over Ir /Al O model catalysts prepared by size-selected cluster deposition. J Phys Chem B 109 381... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

The examples presented above showed that size-selected clusters on surfaces reveal remarkable size-effects, however, they have been studied so far only by one-cycle experiments. Thus an experimental proof that these systems are active for catalytic processes, e.g. several cycles of a catalytic reaction are promoted without destruction of the catalyst, is still missing. In the following we present an experimental scheme to study catalytic processes of clusters on surfaces with high sensitivity and we report turn-over frequencies TOFs for the oxidation of CO on size-distributed supported Pd clusters. 

The term nanocatalysis was introduced by Somorjai in 1994 when he used confined electrons of an STM tip to induce an electrochemical process. Earlier experiments on free clusters pointed towards the possibility of using small clusters with intrinsically confined valence electrons as catalysts to tune the properties atom-by-atom. These two completely different pioneering ideas have become further sophisticated during the last few years. It has become possible to use size-selected clusters on surfaces to catalyze simple chemical reactions and to tune the catalytic properties with size as well as using the tip of an STM to control every step of a chemical reaction on a local scale. With these examples a deeper understanding of nanocatalytic factors is now emerging and such studies will have profound impact on the catalysis of systems at the ultimate size limit. 

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