Size-selected clusters results

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... 

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. 

Recently, STS and STM have been applied to study the onset of the catalytic activity of Au particles grown on titania [238], which appeared to be correlated with the layer thickness of the particles on the surface and, as shown above, the bonding of CO with small gold clusters could be characterized in STM/STS experiments (Fig. 1.39 see Morphological Properties of Supported Clusters ) [190]. The above presentation of the spectroscopic results on deposited, size-selected clusters clearly shows that valuable information on these nanosystems can only be obtained by the application of an arsenal of local and nonlocal surface science analysis methods. Therefore, in the near future a much more intense employment of scanning probe techniques such as STM, STS, AFM, and others will beyond any doubt improve considerably assembly, characterization, and functionalization of size-selected clusters on solid surfaces. 

The problem in these experiments is to get cluster specific information. The cluster beam is usually generated as a distribution of different cluster sizes and a liquid-He-cooled bolometer is not able to discriminate between the different masses. But even a mass spectrometer is only of limiting help because of the extensive fragmentation which occurs when these weakly bound clusters are ionized. A way out of this problem is either to use very dilute mixtures in the expansion and to take the high resolution spectrum itself for identification which works mainly for dimers and a few trimers or to carry out the experiments with size selected clusters. We have recently developed a method which selects smaller clusters by a scattering process with a He beam. This technique has been successfully applied to infrared photodissodation experiments. After the first experimental results on C2H4 dimers... 

The second results part, dealing with ambient cluster catalysis is divided into three sections. First, the outcome of the characterization of the cluster catalysts (STEM/XPS) is shown. Second, first results on the stability (ETEM/INPS) are presented. Last, two examples for reactivity of size-selected clusters are presented. As an... 

In the following paragraphs, preliminary results on unselected clusters, obtained by additional methods are described and discussed. These results of currently ongoing experiments are meant to show the potential of these methods to give supplementary and complementary information for the understanding of ethene hydrogenation on size-selected clusters. 

First preliminary results of ongoing studies with respect to the stability of size-selected clusters are discussed in this section. By means of ETEM (in collaboration with CEN/DTU) as well as INPS (in collaboration with Chalmers) in combination with STEM (at CEN/DTU), two conceptional different approaches are chosen to probe the temperature stability. The data serve as a basis for future experiments when probing the stability at elevated pressures under a reactive atmosphere. 

All together the results from the /u-reactors as a platform to test the reactivity of size-selected cluster materials under applied conditions has a high potential to shed light onto the open question of size dependent reactivity. The ongoing measurements of the CO oxidation reaction have the potential to contribute to the understanding of the controversial discussion of the structure in/sensitivity of the reaction for small particles. 

These impressive results are underlaid by a thorough characterization by means of high resolution STEM and XPS, showing that the photocatalysts do indeed consist of size-selected clusters and are surprisingly stable in aqueous solution. 

This section summarizes the results of size-selective synthesis of the Au SR clusters formed in reactions (l)-(3) of Figure 1. 

U. Buck and I. Ettischer, Vibrational predissociation spectra of size selected methanol clusters New experimental results../. Chem. Phys. 108, 33 38 (1998). 

A size-selective synthesis of nanostructured transition metal clusters (Pd, Ni) has been reported166, as has the preparation of colloidal palladium in organic solvents167, the latter of which is an active and stable catalyst for selective hydrogenation. The use of microwaves in the preparation of palladium catalysts on alumina and silica resulted in hydrogenation catalysts with improved crystallite size and activity168. 

Our work on hydrated clusters manifests the value of gas phase experiments. Condensed phase studies reveal the properties of the bulk system. However, it is difficult to distinguish intrinsic vs. collective properties of a system. Gas phase studies, on the other hand, directly provide information on bare molecules. Moreover, the investigation of size selected water complexes can mimic the transition from an isolated molecule to the bulk. The comparison of gas phase experimental results with theoretical calculations can also provide a direct test of theoretical models. This test is in urgent need if theoretical modeling is to evolve into calculations of solvated systems with credibility. 

Ag2S was also effectively stabilized by cysteinyl ligands. These clusters are synthesized using a molar ratio of 2 1 cysteine silver ions upon which stoichiometric amounts of inorganic sulfide were added to nucleate the nanoparticle, with subsequent size-selective precipitation. The resultant nanoparticles had an absorbance shoulder at 300 nm. Further analysis using high-resolution transmission electron microscopy (HRTEM) revealed a particle size of approximately 9.00 2.25 run in diameter. Selected area electron diffraction (SAED) analysis also demonstrates the highly crystalline natme of the product. ... 

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