Cluster-based materials

Approaches to the formation of three new types of micro-porous materials that complement zeolites will be discussed. In each case, whether metal coordination polymers, metal-linked ceramic oxide clusters, or new hybrids containing both coordination and ceramic components, engineering of the Secondary Building Unit (SBU) is of critical importance. Successful examples of these approaches include the first thermally stable 3-D micro-porous coordination polymer with chemical functionalizability [Cu3(TMA)2(H20)3] , as well as a 3-D micro-porous cluster based material [V,2B18O60H8(Cd(en)(H2O) 3]". ... 

As an example, Fig. 5.6 depicts a typical diffraction spectrum. It is evident that long range order does not exist in our chalcogenide samples. However, the broad difffactrogram peak centered at 20 = 42.5° has the characteristic of a nanodivided ruthenium metal [22]. This points out that the active center in this chalcogenide materials is essentially of metallic nature. The material, either in powder or colloidal form, was analyzed by the EXAFS technique [11]. The local range order of this technique allowed for some structural determination of our samples. Thus, for example, the co-ordination distances for ruthenium-selenium and ruthenium-ruthenium are R(RU-se) = 2.43 A y R(ru.rU) = 2.64 A, respectively. The metal-metal co-ordination distance is of the same order of magnitude as that of well known cluster based materials such as the Chevrel phase [35, 37], cf. Fig. 5.2b. This testifies that the used chemical route leads to the formation of cluster-like materials. 

Status of Cluster-Based Materials for Multi-Electron Charge Transfer... 

STATUS OF CLUSTER-BASED MATERIALS FOR MULTI-ELECTRON CHARGE TRANSFER... 

The field of clusters and fullerenes represents areas of modern science where the properties are determined by the reduced coordination. This will modify the functional properties when clusters are used in disperse forms or as units in cluster assembled materials. Examples of applications can be catalysts, sensor materials, units in nanophase/nanocrystalline materials with improved mechanical, electrical, magnetic or optical properties, of cluster based materials for sun protection, solar energy conversion, as an alternative to quantum dots produced with traditional techniques, fabrication of mesoscopic systems etc. The hope is to tune the properties with cluster size, making cluster based materials with characteristics more advanced than those of conventional materials. Production of these types of cluster and exploration of their properties of free as well as deposited clusters are a challenging task of basic and applied science which will be covered in the following sections of this article. 

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. 

Beside the interest to probe supported size-selected clusters under different conditions and with the general challenges as introduced above, the use of cluster based materials in this work is motivated by a variety of individual scientific questions. The motivation for the particular choice of the different systems and reactions studied are briefly presented, a literature survey covering some theoretical and mechanistic details is subject of a later section (Sect. 2.1). 

In the second part, first results on the stability of the cluster based materials is presented. A data set from ETEM measurements at different temperatures shows the influence of temperature on a selected cluster sample. Further, by monitoring changes in the plasmon signal by means of INPS the stability of size-selected and unselected cluster samples is investigated as a function of temperature and correlated with (conventional) STEM results. 

In order to discern the feasibility of EES to probe the electronic structure of cluster based materials, the sensitivity of the MIES/UPS setup was determined. Using adsorbed TCE as a probe molecule, and correlating EES with TPD measurements, the results prove a superior submonolayer sensitivity for MIES compared to UPS, and NEXAFS data from the literature. On Mo 12) the achieved detection limit was quantified to be as low as 0.02 /SA, (13 % of a ML). 

The POM clusters tend to be anionic in nature, being based upon metal oxide building blocks with a general formula of MO (where M is Mo, W, V and sometimes Nb and x can be 4, 5, 6 or 7). POM-based materials have a large range of interesting physical properties [6-10] which result from their many structures, the ability to delocalize electrons over the surface of the clusters, and the ability... 

A gold-based material has been formulated for use as a three-way catalyst in gasoline and diesel applications.28 This catalyst, developed at Anglo American Research Laboratories in South Africa, consisted of 1% Au supported on zirconia-stabilized-Ce02, ZrC>2 and TiC>2, and contained 1% CoOx, 0.1% Rh, 2% ZnO, and 2% BaO as promoters. The catalytically active gold-cobalt oxide clusters were 40-140 nm in size. This catalyst was tested under conditions that simulated the exhaust gases of gasoline and diesel automobiles and survived 773 K for 157 h, with some deactivation (see Section 11.2.7). 

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