Metal cluster size

Meier DC, Goodman DW. 2004. The influence of metal cluster size on adsorption energies CO adsorbed on Au clusters supported on Ti02. J Am Chem Soc 126 1892-1899. 

The cobalt metal cluster size of the catalyst is believed to be related to the catalyst deactivation,21-47 as the number of surface active sites varies with cobalt... 

Deactivation is due primarily to two mechanisms formation of carbon-containing deposits and sulfur poisoning. Carbon deposition may be minimized by the addition of alkali metals, optimization of metal cluster size, and use of oxygen ion-conducting supports. Sulfur poisoning is usually irreversible and there are few reports of catalysts that are tolerant of sulfur levels typical of commercial fuels. 

XAS can be used in several different ways to determine local structural information about catalysts in reactive atmospheres. This structural information may be static or dynamic it may be geometric or electronic. The depth of information that can be ascertained is often dependent upon the type of catalyst, for example, supported metal nanoclusters versus bulk or surface oxides. It may also be controlled by some property of the catalyst, for example, the concentration of the element in the catalyst that is being investigated. In this section a few examples are provided to highlight the importance and relevance of XAFS in catalyst characterization. The examples are focused on (1) structural information characterizing samples in reactive atmospheres, (2) transformation of one species to another, (3) oxidation state determination, (4) determination of supported metal cluster size and shape, and (5) electronic structure. These examples illustrate the type of information that can be learned about the catalyst from XAFS spectroscopy. 

Many of the TEM studies of catalysts reported in the literature use model catalysts with visible metal crystallites supported on a metal oxide support, which usually is selected because it offers minimal electron scattering interference. These model catalysts are studied because direct measurements regarding crystallite size and structure can be made. Extrapolation of this information to smaller metal cluster sizes usually present in commercial catalysts is anticipated to provide insight into the relationship between structure and chemical reactivity (10). 

In the bulk form, Au is known to be chemically inert compared to the other Pt group metals. However, it has recently been shown that Au clusters, deposited as finely dispersed, small clusters (<5-nm diameter) on reducible metal oxides like Ti02, Fe20s, and C03O4, exhibit catalytic activity with respect to a number of industrially important reactions i.e., CO oxidation, hydrogenation, partial oxidation of hydrocarbons, and selective oxidation of higher alkenes [13,19,25-28,43 5]. This unusual catalytic activity has been shown to be a function of metal cluster size. 

These samples offer a long-envisioned opportunity to vary metal cluster size and to determine how the catalytic properties depend on it. Results of some of the first experiments with zeolite-supported metal cluster catalysts are summarized below. 

In general, our calculations showed that the interaction of small Rh clusters with support OH groups leads to oxidation of metal atoms in close contact with the support. Such redox processes of spillover may be quite general for small supported transition metal clusters, but it remains to be determined how these effects depend quantitatively on the metal, support, and metal cluster size. 

It should be noted here that the results of cluster calculations can at present only be qualitative in character. Energies and partial charges are not converged with respect to metal cluster size and level of approximation. The value of the calculations lies more in the opportunity to compare different ions with each other (in a given group) and the relative stability of different adsorption sites. The prediction of absolute adsorption energies is hardly possible. More promising for the future are calculations of adsorbates on periodic surfaces within the framework of the local density approximation of density functional theory (e.g.. Ref. 123). 

XANES metal clusters change of oxidation state of metal Qualitative information only influence of metal cluster size on the white line intensity needs to be accounted for. 

TEM metal clusters size and location of cluster on zeolite Metal cluster size and location can be modified by the eledron beam, due to high local temperature rises lack of resolution prevents detection of smallest metal clusters. 

There are some important limitations to TEM, 1) Electron beams are liable to produce high local temperature rises, and hence the metal cluster size and the location can be modified by the powerful electron beam, [146] 2) hundreds of metal clusters from many different micrographs should be measured in order to obtain a statistical representation of the size distribution, and 3) electron beam absorption by the zeolite crystal can hamper the detection of the particles. The best images seem to be those obtained from thin slabs of the zeolite crystals which have been cut at different heights and at different places with an ultramicrotome equipped with a diamond knife. The best images are from end-on views looking down into the pores with stacks of clusters in the cages. 

Entry Catalyst Metal dispersion (%) Metal cluster size (mn) BET (m"/gpd) pHinitiai... 

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