Catalyst dispersed metal

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

Investigations utilizing EXAFS have the very important feature of yielding information in an environment of the kind actually encountered in catalysis. We have recently demonstrated the feasibility of making measurements while a catalytic reaction is actually occurring. One can anticipate that measurements of this type will receive Increased emphasis in the future. For studies of the structures of highly dispersed metal catalysts, EXAFS may well be the most generally applicable physical probe currently available. 

Most often, these disperse metal catalysts are supported by an electronically conducting substrate or carrier that should provide for uniform supply or withdrawal of electrons (current) to or from all catalyst crystallites. The substrate should also serve to stabilize the disperse state of the catalyst and retard any spontaneous coarsening of the catalyst crystallites. Two situations are to be distinguished (1) the disperse metal catalyst is applied to a substrate consisting of the same metal, and (2) it is applied to a chemically different substrate (a foreign substrate). Platinized platinum is a typical example of the former situation. 

Different ways exist to prepare eiectrodes with highiy disperse metal catalysts and lead to the corresponding electrode varieties. 

Experience has shown that the specific or intrinsic catalytic activity of electrodes with disperse metal catalysts when referred to the true working surface area often remains below that of smooth (compact) electrodes consisting of the same metal. Of course, owing to the large increase in total working surface area, the overall reachon rate is larger, but as a rule it is not larger by the expected factor of y. 

For the present purpose, we take the term ultrathin to refer to an evaporated metal film where the concentration of metal on the substrate is low enough for the film to consist of small isolated metal crystals. If the average concentration of metal atoms on the substrate is of the order of a monolayer or less, the metal crystals are small enough for ultrathin films to serve as models for highly dispersed metal catalysts, but where surface cleanliness and catalyst structure can be better controlled. 

In a more general context, metal carbonyls on zeolites can be a unique way to prepare highly dispersed metal catalysts. In the present work, this is especially the case for iron as no other mild methods are operative. It is expected that the method could be applied to the preparation of bi- and polymetallic catalysts even though the starting material are not bi- or polymetallic clusters, but more conveniently homometallic clusters. 

For this reason, these catalysts are also known under the name of supported hydrogen-bonded (SHB) catalysts and, in conjunction with Pd° particles on the same support material, have contributed to generate active heterogeneous systems for the hydrogenation of benzenes in aprotic solvents. Irrespective of the substrate, the combined single-site/dispersed-metal catalyst RhI-Pd0/SiO2 shown in Figure 16.5a was from four- to six-fold more active than supported palladium... 

An efficient, low temperature oxidation catalyst was developed based on highly disperse metal catalyst on nanostructured Ti02 support. Addition of dopants inhibits metal sintering and prevents catalyst deactivation. The nanostructured catalyst was formulated to tolerate common poisons found in environments such as halogen- and sulfur-containing compounds. The nanocatalyst is capable of oxidizing carbon monoxide and common VOCs to carbon dioxide and water at near ambient temperatures (25-50 °C). 

Metal catalysed skeletal reactions of hydrocarbons on metal catalysts. Dispersed metal catalysts. 

In supported metallic catalysts, the metals are usually from Groups VIII and VB of the Periodic Table. For highly dispersed metallic catalysts, the support or the carrier is usually a ceramic oxide (silica or alumina) or carbon with a high surface area, as described in chapter 2. Supported metallic catalysts can be prepared in a number of ways as described by Anderson (1975). A description of some of the methods used to prepare representative model (thin film) and practical (technological) powder systems follows. 

They have been used to obtain well dispersed metal catalysts. Early experiments dealt with platinum-loaded zeolites (1,2). 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

It appears that supported metal catalysts can be used to promote synthetically useful organometallic reactions. The utilization of such reactions can be of practical, economic, and environmental importance to the fine chemical industry. Frontier Molecular Orbital and mechanistic considerations indicate that these reactions, along with hydrogenations and, presumably, oxygenations, take place on the coordinately unsaturated comer atoms present on the surface of these dispersed metal catalysts. 

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20-1000 A) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured both are prerequisites for the mechanistic study of the catalysis on the atomic scale. 

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 ... 

Real dispersed metal catalysts are prepared by precipitation from solution. Investigating processes involving oxide surfaces in solution is a challenging topic. Figure 4.4.1B schematically shows the type of system investigated [10]. Particles are deposited from an aqueous solution of a metal chloride as a function the pH... 

While this work did show that a number of different types of single atom sites can be present on the surface of dispersed metal catalysts, the possibility was... 

Being able to measure the amounts of the saturation sites on the surface of a dispersed metal catalyst provides a means of further defining the specific activity... 

As discussed previously, this reaction was also run under these same conditions over the series of specifically cleaved platinum single erystals shown in Fig. 3.2. 3 The results of these experiments show that it was the corner atoms on these crystals that promoted C-H bond breaking. Thus, the saturation sites on the dispersed metal catalysts are also comer atoms. Since this saturation site description agrees with that proposed on the basis of the butene deuteration described previously,5 -62 it is likely that the isomerization sites, M, are edge atoms and the hydrogenation inactive sites, M, are face atoms. A similar approach can be used to determine the nature of the active sites responsible for promoting almost any type of reaction. 5.70... 

The presence on the surface of a dispersed metal catalyst of at least three distinct corner sites having different activities is, however, not compatible with the octahedral models of the 3M sites shown in Fig, 3.4 and used in Schemes 3.2 and 3.4 to develop analogies with specific homogeneous catalysts. A more detailed description of these corner atom sites and others present on the surface of metal catalysts is presented in the next chapter in conjunction with a discussion of the surface electronic orbitals of such species. 

If the metal catalyst particles were present only in the form of these idealized crystals, then the number of active comer atoms present would be very low. However, STO evaluations of dispersed metal catalysts have shown that these active atoms are present in rather large amounts, at times as high as 30%-35% of the total metal atoms present. Such high surface concentrations of the highly unsaturated atoms can only be accounted for by the presence of the irregular particle shapes that were observed using dark field TEM imaging techniques. Additional active sites are probably present as adatoms on the 111 (M) and 100 (K) planes as shown in Fig. 4.4. 

Heterogeneous catalysts are defined as solids or mixture of solids that are used to accelerate a chemical reaction without undergoing change themselves. The types of solids used in industry as heterogeneous catalysts include simple oxides, mixed oxides, metal salts, solid acids and bases, metals, and dispersed metals. Catalysts are used in a wide variety of chemical and environmental processes worldwide. The global value of fuels and chemicals produced by catalytic routes is about US 2.4-3 trillion per year. About 20% of all products produced in the United States are derived from a catalytic process of some form. As important as catalysis is to the world economy, the number of various chemicals used as a catalyst as well as the form and shape of the material vary as much as the number of processes that use catalysts. Fig. 1 is a picture of a number of various types of catalysts and illustrates the numerous possibilities of shapes and sizes. Naturally, the preparation processes of such a wide variety of products is also numerous. 

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