Metallic phase, dispersion

The generation of monometallic catalysts with suitable characteristics is the first key step for obtaining supported bi- and organobimetallic catalysts via SOMC/M techniques. Ion-exchange techniques are generally used in the preparation of these catalysts, which lead to solids with good metallic phase dispersion and homo-... 

The TEM analysis of the samples obtained from the reactor for the (Ni-I) catalyst, indicates the presence of scarce carbon filaments and of modifications in the distribution of particle size. If the histogram of the fresh catalyst is compared to that of the activated one, it can be observed that the larger crystallites disappear and the mean size Ni crystal undergoes a 30% reduction. Thus, it would support the experimental observation of an increase not only of the catalytic activity by carbon elimination but also in the metallic phase dispersion. There are no morphological changes in the samples taken from the reactor. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

For a supported metal catalyst, the BET method yields the total surface area of support and metal. If we perform our measurements in the chemisorption domain, for example with H2 or CO at room temperature, adsorption is limited to the metallic phase, providing a way to determine the dispersion of the supported phase. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Supported ruthenium catalysts prepared from Ru3(CO),2 have been used in CO hydrogenation because of the highly dispersed metallic phase achieved when this carbonyl-precursor is used [70,107-109]. However, under catalytic reaction conditions the loss of ruthenium from the support could take place, ft has been reported that at low temperatures it takes place through the formation of Ru(CO)s species, whereas at high temperature dodecarbonyl formation occurs [110]. Decarbonylation of the initial deposited carbonyl precursor under hydrogen could minimize this problem [107]. 

In the colloidal technique, the size and distribution of a dispersed thoria (Th02) phase is controlled to produce dispersion-strengthened alloys, primarily with nickel as the metallic phase. The so-called TD (thoria-dispersed) nickel has modest strength at room temperature, but retains this strength nearly to its melting point. TD nickel is 3 to 4 times stronger than pure nickel in the 870-1315°C range, and oxidation resistance of the alloy is better than that of nickel at 1100°C. 

When a material used for the dispersion of the active agents is bonded to a support, it is called washcoat. A characteristic example is the case of automotive monolithic catalysts, where the monolith is die support and a thin film of alumina attached to the monolith constitutes the washcoat, the phase where the catalytically active metals are dispersed. In contrast to these supported catalysts, there are some catalytic materials that... 

One type of colloidal system has been chosen for discussion, a system in which the solid metal phase has been shrank in three dimensions to give small solid particles in Brownian motion in a solution. Such a colloidal suspension consisting of discrete, separate particles immersed in a continuous phase is known as a sol. One can also have a case where only two dimensions (e.g., the height z and breadth y of a cube) are shrank to colloidal dimensions. The result is long spaghettihke particles dispersed in solution—macromolecular solutions. 

AB5 metal hydride particles have been dispersed in a polymer matrix in order to entrap the micro and nanoparticles produced by repeated fragmentation processes of the metal phase during the... 

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