Supported metal

Since catalyzed reactions take place on the surface of the catalytically active material, the most efficient catalysts are those in which a high percentage of the active species is exposed to the reaction medium. Some metal catalysts, such as those discussed in Chapter 12, are composed of very finely divided metal particles that have the high ratio of surface to bulk atoms needed for good catalyst activity. These fine particles, however, sinter on heating. As discussed in Chapter 9, the most common way of minimizing metal catalyst sintering is to distribute the active component over a porous, thermostable, support. 

In most cases, however, the supported catalyst precursors are prepared by a procedure that can be described in general terms as an impregnation, contacting the support with a solution of the active element. The word, impregnation, though, has been used to describe a number of different preparation procedures 

There is an extensive literature concerned with the preparation of supported catalysts but, unfortunately, it is sometimes difficult to determine precisely what procedures were used because of the ambiguity of many of the experimental descriptions. While an lUPAC manual standardizing the terms used in catalysis has been published the proposed usage is still not widely applied in the literature. The procedures described below are defined in these standard terms with some of the more commonly used descriptors also mentioned where appropriate. 

The initial step in the preparation of a coprecipitated catalyst is the reaction between a solution of two or more metal salts and a base, generally a hydroxide, alkali carbonate or bicarbonate. The resulting precipitate may contain not only the insoluble hydroxides and/or carbonates but also a mixed metal compound if the solubility equilibria are favorable. Even if the formation of a mixed metal compound is not favorable, some of the support material is usually trapped in the active metal precipitate. This dilutes the precipitate and inhibits the formation of large crystals of the active metal compound. Smaller crystals are easier to reduce and give more finely divided metal particles.  

The second phase in catalyst preparation by this process involves heating the precipitate. This takes place in two stages. In the first, the water of crystallization is lost and in the second, there is a loss of carbonate and other cinions as well as the dehydration of the metal hydroxides. The first occurs on heating to about 250°C, while the second stage requires calcination usually at temperatures between 300° and 600°C with 350°-400°C reported to be optimal. At these temperatures metal carbonates and hydroxides are converted to an intimate mixture of the component oxides (Eqn. 13.1). The oxides of the active metals are generally more easily reduced than are other metal compounds.3-5 

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As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

Fig. XVin-24. TVimover frequencies for methanation using silica-supported metals. (From Ref. 270.)...

M. Boudart, Supported Metals as Heterogeneous Catalysts, the Science of Precious Metals Applications, International Precious Metals Institute, Allentown, PA, 1989. 

Vuurman M A and Waohs I E 1992 In situ Raman speotrosoopy of alumina-supported metal oxide oatalysts J. Rhys. Chem. 96 5008-16... 

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... 

Transition metals absorbed onto a solid support metal Pd,Pt, Ni, Rh support Carbon, alumina, silica solvent EtOH, EtOAc, Et20, hexanes, etc. 

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. 

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. 

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. 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Ethylene Oxidation to Ethylene Oxide. A thoroughly investigated reaction catalyzed by a supported metal is the commercially appHed partial oxidation of ethylene to give ethylene oxide (90). The desired reaction is the formation of ethylene oxide, ie, epoxidation the following reaction scheme is a good approximation ... 

The appHcations of supported metal sulfides are unique with respect to catalyst deactivation phenomena. The catalysts used for processing of petroleum residua accumulate massive amounts of deposits consisting of sulfides formed from the organometaHic constituents of the oil, principally nickel and vanadium (102). These, with coke, cover the catalyst surface and plug the pores. The catalysts are unusual in that they can function with masses of these deposits that are sometimes even more than the mass of the original fresh catalyst. Mass transport is important, as the deposits are typically formed... 

Research in catalysts for ammonia manufacture is stiU going on, and though the use of supported metals such as mthenium may be two to three times as active as promoted iron oxide, catalysts 50—100 times more active than promoted iron oxide are required to affect process economics significantly. 

In laboratory preparations, sulfuric acid and hydrochloric acid have classically been used as esterification catalysts. However, formation of alkyl chlorides or dehydration, isomerization, or polymerization side reactions may result. Sulfonic acids, such as benzenesulfonic acid, toluenesulfonic acid, or methanesulfonic acid, are widely used in plant operations because of their less corrosive nature. Phosphoric acid is sometimes employed, but it leads to rather slow reactions. Soluble or supported metal salts minimize side reactions but usually require higher temperatures than strong acids. 

The reaction is carried out over a supported metallic silver catalyst at 250—300°C and 1—2 MPa (10—20 bar). A few parts per million (ppm) of 1,2-dichloroethane are added to the ethylene to inhibit further oxidation to carbon dioxide and water. This results ia chlorine generation, which deactivates the surface of the catalyst. Chem Systems of the United States has developed a process that produces ethylene glycol monoacetate as an iatermediate, which on thermal decomposition yields ethylene oxide [75-21-8]. 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

Oxide-supported metals constitute one of the most important classes of heterogeneous catalysts, and for this reason they have been investigated by many techniques adsorption isotherms, IR of chemisorbed molecules, electron microscopy, EXAFS, etc. Flowever, the fact that they have been studied by so many methods proves that no one technique is totally satisfactory. 

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

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. 

This is a process that takes place via specific chemical forces, and the process is unique to the adsorbent or adsorbate used. In general, it is studied at temperatures much higher than those of the boiling point of the adsorbate consequently, if supported metals are studied, little or no physical adsorption of the chemisorbing gas takes place on the high surface area support. 

To give an idea of the wide rai e of catalytic systems that have been investigated where chemisorption data were essential to interpret the results, some of the author s papers will be discussed. Measurements were reported on the surface areas of a very wide range of metals that catalyze the hydrogenation of ethane. In the earliest paper, on nickel, the specific catalytic activity of a supported metal was accurately measured for the first time it was shown also that the reaction rate was direcdy proportional to the nickel surface area. Studies on the same reaction... 

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