Supported metals catalysts

The high surface area of clays also makes them particularly attractive as catalyst and reagent supports. Discussion in this chapter will be largely confined to clay-based catalysts in which metal complexes or other ions are specifically incorporated in the clay matrix. Other clay-based catalytic materials will be discussed in Chapter 4. [Pg.39]

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. [Pg.40]

A wide range of techniques may be employed for the incorporation of a catalytically active component into clay supports. An outline of the two most important techniques is given below as an introduction to later sections in this chapter, which describe the more important chemical and physical factors involved in the dispersion of metal salts onto clays and their influence on the activity and selectivity of the catalyst system. Methods for supporting species onto high surface area materials are described in some detail in Chapter 4. [Pg.40]

Impregnation—Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt from which the solvent is subsequently evaporated. The catalyst is prepared either by spraying the support with a solution of the metal compound or by adding the support material to a solution of a suitable metal salt, so that the required amount of active component is incorporated into the support without the use of excess solution. This is then followed by drying and subsequent decomposition of the metal salt at an elevated temperature, either by thermal decomposition or reduction. [Pg.40]

Adsorption from solution—Adsorption is defined as the selective removal of metal salts or metal ion species from their solution by a process of either physisorption or chemical bonding with active sites on the support. Depending upon the strength of adsorption of the adsorbing species, the concentration of the active material through the support may be varied and controlled. [Pg.40]

X being the average displacement of the particles in the time t. The obvious difference between these colloidal dispersions aird the catalyst particles on the surface of a support, is that the above model would require that the particles [Pg.128]

Chemistry without catalysis would be a sword without a handle, a light without brilliance, a bell without sound. [Pg.75]

Among the various types of composite systems, that of the metal-support ranks as one of the most important, because of its crucial role in catalysis. The situation under consideration is that of chemisorption on a thin metal him (the catalyst), which sits on the surface of a semiconductor (the support). The fundamental question concerns the thickness of the film needed to accurately mimic the chemisorption properties of the bulk metal, because metallization of inexpensive semiconductor materials provides a means of fabricating catalysts economically, even from such precious metals as Pt, Au and Ag. [Pg.75]

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... [Pg.1869]

Heterogeneous vapor-phase fluorination of a chlorocarbon or chlorohydrocarbon with HP over a supported metal catalyst is an alternative to the hquid phase process. Salts of chromium, nickel, cobalt or iron on an A1P. support are considered viable catalysts in pellet or fluidized powder form. This process can be used to manufacture CPC-11 and CPC-12, but is hampered by the formation of over-fluorinated by-products with Httle to no commercial value. The most effective appHcation for vapor-phase fluorination is where all the halogens are to be replaced by fluorine, as in manufacture of 3,3,3-trifluoropropene [677-21 ] (14) for use in polyfluorosiHcones. [Pg.268]

Initially, aluminum chloride was the catalyst used to isomerize butane, pentane, and hexane. Siace then, supported metal catalysts have been developed for use ia high temperature processes that operate at 370—480°C and 2070—5170 kPa (300—750 psi), whereas aluminum chloride and hydrogen chloride are universally used for the low temperature processes. [Pg.207]

Supported metal catalysts are reduced, for example, by treatment in hydrogen at temperatures in the range of 300—500°C. The reduction temperature may influence the stabiUty of the metal dispersion. [Pg.174]

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. [Pg.737]

The relationship between metal carbonyl clusters and supported metal catalysts. J. Evans, Chem. Soc. Rev., 1981,10,159-180 (94). [Pg.40]

For reasons which will become apparent in Chapters 4,8 and 11 of this book it is very likely that the increasing commercial importance of Zr02 and Ce02 supports for conventional supported metal catalyst is due to the ability of these supports to continuously provide backspillover anionic oxygen on the surface of the supported metal catalyst. [Pg.104]

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... [Pg.500]

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. [Pg.187]

Determination of Metal Precursor Mobilities During Pretreatment. Relative precursor mobilities were obtained by premixing the sllica-or alumina-supported metal catalysts with pure silica (Cab-O-Sll, grade M-5, Cabot Corp.) or pure alumina (Alon C, Cabot Corp.) In a 1 2 ratio prior to pretreatment. The catalyst and silica were ground together using a mortar and pestle for at least 0.5 hr. before they were placed in the Pyrex microreactor for pretreatment. [Pg.296]

Table III. Effect of Dilution on the Dispersion of Supported Metal Catalysts...

In principle, it is possible to fully automate the procedure (2) and software can be written to obtain the results to an operator spemFied precision, as the error equations are available. Unfortunately, this online procedure is sometimes difficult with catalysts because most supported metal catalysts contain only the order of one percent or less of metal, the peaks are broad due to the small size of the crystallites, and the large amount of support gives strong background scattering which has features of its own. Visual inspection of the data is often necessary prior to processing. [Pg.386]

Figure 3. Principle of modifying supported metal catalysts by co-adsorbed auxiliaries... [Pg.55]

The interactions between metals and supports in conventional supported metal catalysts have been the focus of extensive research [12,30]. The subject is complex, and much attention has been focused on so-called strong metal-support interactions, which may involve reactions of the support with the metal particles, for example, leading to the formation of fragments of an oxide (e.g., Ti02) that creep onto the metal and partially cover it [31]. Such species on a metal may inhibit catalysis by covering sites, but they may also improve catalytic performance, perhaps playing a promoter-like role. [Pg.219]

The literature of metal-support interactions includes httle about the possible chemical bonding of metal clusters or particles to supports. Supported molecular metal clusters with carbonyl ligands removed have afforded opportunities to understand the metal-support interface in some detail, and the results provide insights into the bonding of clusters to supports that appear to be generalizable beyond the small clusters to the larger particles of conventional supported metal catalysts [6]. [Pg.219]

The foregoing results characterizing structurally simple supported metal clusters can be generalized, at least qualitatively, to provide fundamental understanding that pertains to industrial supported metal catalysts, with their larger, nonuniform particles of metal. [Pg.228]

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. [Pg.326]

This reaction serves for removal of carbon monoxide from gas mixtures and is usually carried out over supported metal catalysts. In reforming techniques, carbon monoxide, poisonous for the catalyst in fuel cells, is removed in such a way. It is also applied in automobiles for reducing the exhaust gas carbon monoxide to an environmentally acceptable level. [Pg.327]

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