Clusters on Surfaces

Palmer, R.E. and C. Leung (2007) Immobilisation of proteins by atomic clusters on surfaces. Trends Biotechnol. 25, 48-55. 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

Me90b G. Meijer and D. S. Bethune, Laser Depositions of Carbon Clusters on Surfaces A We89 New Approach to the Study of Fullerenes, ... 

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. 

To study catalytic processes of clusters on surfaces with high sensitivity, a newly designed pulsed valve with excellent pulse-to-pulse stability, in combination with absolutely calibrated mass spectrometry, was used to determine turn-over frequencies [75]. 

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. 

Clusters on Surfaces. - In many experimental studies clusters deposited or grown on some surface are studied instead of free, isolated clusters in the gas phase. Of course, the presence of a substrate may modify the structure of the clusters in unknown ways and, therefore, theoretical studies of such systems are highly relevant. 

In Sect. 1.2.1 of the present chapter, we describe the most important cluster sources successfully used today. Section 1.2.2 introduces experimental techniques for mass-selecting single cluster sizes from the distribution generated by the cluster sources. In the gas phase as well as for clusters on surfaces, the densities are extremely low, thus only highly sensitive methods can be used for the characterization of the chemical and catalytic properties of the model systems. Some of the most commonly used techniques employed in gas-phase experiments are presented and discussed in Sect. 1.2.3, surface analysis techniques for cluster studies are presented in Sect. 1.2.4. 

The microscopic structure of small clusters on surfaces is of primordial interest as it influences most of their physical and chemical properties. Acquiring... 

An indirect method to image small clusters on surfaces was obtained for size-selected Agj g clusters on a Pt(fff) surface where the cluster was decorated with rare gas atoms [197]. 

The most widely used technique to get information on the electronic structure of clean surfaces, nanostructures on surfaces, or even molecules adsorbed on surfaces is ultraviolet photoelectron spectroscopy (UPS). The difficulty of this method, when applying it to clusters on surfaces, is to obtain sufficient spectral contrast between the low number of adsorbed clusters and the substrate [45]. Thus, electron energy loss spectroscopy (EELS) is more successfully used as a tool for the investigation of the electronic structure of supported clusters. An interesting test case for its suitability is the characterization of supported monomers, i.e., single Cu atoms on an MgO support material [200], as this system has been studied in detail before with various surface science techniques [201-204]. The adsorption site of Cu on MgO(lOO) is predicted... 

There are several experimental techniques available to study the chemical properties of clusters on surfaces and when used in combination, a detailed picture can be obtained. A classical approach is the use of temperature programmed desorption (TPD) and temperature programmed reaction (TPR) spectroscopy [39, 239,240] in combination with temperature-dependent and eventually time-dependent Fourier transformed infrared spectroscopy (FTIR) [39,192,241-244]. This combination allows for obtaining information on estimates of binding energies of reactant molecules (TPD), for characterizing... 

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%...

Support materials most adapted for stabilizing clusters on surfaces are oxide materials. They reveal a relatively large band gap and thus the characteristic, discrete electronic levels of the clusters are maintained to a certain extent. Furthermore, the interaction of the clusters with oxide surfaces, especially with their defects may be substantial and trapping of the clusters is feasible. Oxide supported metal particles are also relevant in industrial catalysis. 

The Reactivity of Pure and Mixed Gold Clusters on Surfaces... 

J. M. van Ruitenbeek, Metal clusters on Surface Structure, Quantum properties, Phys. Chem. Series, Springer Verlag, Berlin, 2000. 

Meijer G and Bethune D S 1990 Laser deposition of carbon clusters on surfaces-a new approach to the study of fullerenes J. Chem. Rhys. 93 7800-2... 

Energy (10) plays an essential role governing the process of formation of cluster on surface of nanofiber, because its maximal value represents the so-called nucleation barrier which has to be overcome to obtain stable, growable cluster. It may be also readily proved that... 

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