Size-selected clusters experiment

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... 

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. 

Future Experiments Involving Size-Selected Clusters... 

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. 

A further less commonly employed but very classical method for mass-selection is the Wien-filter which combines an electrostatic and a magnetic field for mass-selective cluster deflection. This device is applied in particular with the purpose to obtain large cluster currents for cluster-surface interaction and deposition experiments at moderate resolution [75, 76]. Another approach for size-selection in combination with a sputter source ]47] or a laser vaporization source [61,77,78] has been chosen recently. In both cases, a magnetic dipole field has achieved the size-selection. Subsequent deceleration of the size-selected clusters to less than 1 eV per atom enables soft-landing under UHV conditions [47]. Monodispersed chromium cluster beam densities range from 0.1 to 5nA mm, depending on the cluster size ]47]. 

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. 

FTIR has been successfully applied for studying chemical properties of size-selected clusters on oxide surfaces [39,192,244,252]. In these experiments, the clusters were deposited onto thin oxide films grown on a metal single crystal. In this approach, the metal is used as the mirror in the optical setup the oxide films ought to be thinner than the wavelength of the IR radiation. In this way, vibrations of adsorbates on size-selected clusters with densities as low as 0.2% ML could be detected. Mechanistic information of chemical reactions... 

Fig. 1.52. Typical experimental setup for a pulsed molecular beam experiment for studying the catalytic properties of size-selected clusters on surfaces. It mainly consists of a pulsed valve for the generation of a pulsed molecular beam and a differentially pumped, absolutely calibrated quadrupole mass spectrometer. The length of the valve extension tube is adjusted to obtain a beam profile of similar dimensions as the sample under investigation. A typical time profile is also shown. It can be adjusted up to continuous operation. The pulse-to-pulse stability is better than 1%...

We focused on the size-selected cluster ions with small sizes in the above. On the other hand, the magnetic moments of neutral transition metal clusters in the size range between several and several hundreds are successfully measured by deflection in an inhomogeneous magnetic field. This method is basically the same as the Stern-Gerlach experiment known by the discovery of the electron spin by using the deflection of an atomic beam of silver. 

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... 

With respect to experiments under ambient conditions, the previous approach [20] of cluster transfer for ex situ studies (see Sect. 2.3) is followed up, however with size-selected clusters. As a first step, characterization of the prepared materials is performed. Further, for reactivity studies a knowledge on the stability under reactive conditions is a prerequisite and thus the clusters are tested towards their temperature stability. Last, the reactivity of cluster catalysts is probed under different conditions, towards two different reactions. Various new experimental means are applied to probe and analyze Pt) clusters and serve as a perspective for future testing and application of cluster based materials. 

Photocatalysis experiments were conducted using an established semiconductor system and for decoration with noble metals, combining it with size-selected clusters. The general synthesis and measurement approach is depicted in Fig. 3.17. Colloidal semiconductor CdS nano rods are spin coated onto conducting ITO glass substrates... 

Second, the well known chemisorption behavior of ethene is characterized with the same combination of EES and TPD on surfaces and serves as a future comparison for the study of the chemisorption behavior of ethene on size-selected Pt clusters by means of EES. The reactivity of ethene towards the hydrogenation reaction is probed by TPR and also further preliminary experiments (AES and IRRAS) are shown in order to investigate the mechanism of the ethene hydrogenation reaction on size-selected clusters. 

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. 

Experiments on Rh clusters5,106 reveal an oscillatory pattern of the average magnetic moment, with large values for N = 15, 16, and 19, and local minima for N = 13-14, 17-18, and 20. DFT calculations have been performed for selected clusters in that size range, usually assuming symmetric structures except for the smallest clusters.108-112 The conclusion reached by the various researchers is that the Rh clusters are magnetic. However, different experiments for the same cluster size show a lot of dispersion. 

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