Metal dispersion mechanism

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

For optimal performance of dual function isomerization catalysts based on zeolite Y or mordenite, extensive removal of sodium is necessary. The finished catalyst must be highly crystalline, and the finely dispersed metallic hydrogenation function should be well distributed throughout the catalyst particles. The proposed mechanism explains the stabilizing influence on conversion and the suppression of cracking reactions by addition of the metallic hydrogenation function to the active acidic catalyst base. 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

One major reason for the great interest in the processes of thin metal-containing films is that reactions on the surface of small metal clusters can be studied. Indeed, prior to the development of thin-film chemistry, reactions of similar particles were studied only in the gas phase at rather high temperatures. Under these conditions, most of the primary products are unstable and decompose in the course of further reaction, which is non-selective. As a result, the information obtained on the routes and mechanisms of reactions of disperse metals appears to be scarce, while the use of such reactions in synthesis is inexpedient. Conversely, low-temperature reactions in the films of co-condensates are very promising from the standpoint of determining the detailed reaction mechanism, as well as for synthesis of previously unknown complexes and organometallic compounds. It is important that atoms of only a few metals react with organic compounds immediately at the instant of their contact on the cooled substrate. Rather often, atoms and/or small (molecular) clusters are first stabilized in the film, and then their transformations are observed. 

Catalytic reactions can be run over large, massive metal particles as well as the much smaller, dispersed metal crystallites. The massive metal catalysts can be the single crystal catalysts such as those shown in Fig. 3.2 or polycrystalline forms of bulk metal such as wires, foils or ribbons. These latter materials were used somewhat routinely in the early catalytic research efforts that were involved with developing the mechanisms of vapor phase catalytic processes. These materials were considered to be analogs of the supported catalysts in which the effect of the support, if any, was eliminated. 

Most theoretical and experimental research on sintering has centered around highly dispersed metals, such as platinum on alumina. This is understandable since these systems are close to ideal, are easily studied, and suffer the greatest deactivation when sintered.Figure. 7 illustrates two accepted mechanisms for crystallite growth. The first is crystallite migration. Small crystallites have a large fraction of surface atoms, which become mobile at lower temperatures than the bulk. A shape displacement... 

The catalytic decomposition of hydrogen peroxide on solid and colloidally dispersed metals has been studied particularly by Bredig et al. (20). These reactions have been treated by Weiss (21) from the point of view of the radical mechanism. On the basis of the electron theory the metal can be considered both as a source and as a sink of electrons. In conformity with the electron transfer processes outlined above, the reactions at the surface of the metal catalyst can be considered to be represented by processes of the type ... 

In spite of the great number of investigations on CP-driven chemical deposition of metal particles there have been almost no attempts to address the mechanism of this process, either in homogeneous or heterogeneous cases. In a homogeneous system with dispersed metal ions and CP species, the process most probably includes continuous formation of metal nuclei that provide a catalytic surface for further reduction of metal ions and give rise to autocatalytic growth [254]. 

The formation of disperse metal electrodeposits is discussed by the consideration of corresponding physical and mathematical models. It is shown that the mechanisms of formation of all different forms can be elucidated by the use of conclusions of a few classic works in the field of metal electrodeposition making a general theory of disperse metal electrodeposits formation. 

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