Catalyst active component

Applying the Broensted Polanyi correlations is sometimes useful for describing the dependence of the reaction rate on the size of the catalyti caUy active component. A huge amount of experimental data have been compiled to date regarding the effect of the particle size of the catalyst active components on the specific catalytic activity, SCA, as well as on the turnover frequency, TOP, of the active center. Both parameters do not relate to the total surface area of the catalyticaUy active phase or to the total number of active centers and, therefore, characterize directly the properties of the active center. There are also some experimental data on the size dependence of the adsorption properties of small metal parti cles, as well as on the selectivity of a few catalytic processes. 

A large variety of very different crystal structures and morphologies may be characteristic of the growing nanofiber depending on the nature of the catalyst active component and pyrolyzed hydrocarbon, as well as on the pyrolysis temperature (Figure 5.3). 

Structural promotion can take two main forms, although both are concerned with maintaining the effective surface area of the catalyst active component. The use of alumina to generate the pore structure of iron catalysts has been investigated extensively and is discussed fully in an earlier chapter. In the absence of alumina, iron sinters on reduction, giving low surface areas. In the presence of the optimum level of alumina (approximately 2%) surface areas as high as 25 m g" can be obtained although, in the presence of potassium, this is reduced to 10-15 g ... 

Keywords hydrotreating catalyst, textural properties of carrier, catalyst active component localization... 

Bosch and co-workers devised laboratory reactors to operate at high pressure and temperature in a recycle mode. These test reactors had the essential characteristics of potential industrial reactors and were used by Mittasch and co-workers to screen some 20,000 samples as candidate catalysts. The results led to the identification of an iron-containing mineral that is similar to today s industrial catalysts. The researchers recognized the need for porous catalytic materials and materials with more than one component, today identified as the support, the catalyticaHy active component, and the promoter. Today s technology for catalyst testing has become more efficient because much of the test equipment is automated, and the analysis of products and catalysts is much faster and more accurate. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

CatalyticaHy Active Species. The most common catalyticaHy active materials are metals, metal oxides, and metal sulfides. OccasionaHy, these are used in pure form examples are Raney nickel, used for fat hydrogenation, and y-Al O, used for ethanol dehydration. More often the catalyticaHy active component is highly dispersed on the surface of a support and may constitute no more than about 1% of the total catalyst. The main reason for dispersing the catalytic species is the expense. The expensive material must be accessible to reactants, and this requires that most of the catalytic material be present at a surface. This is possible only if the material is dispersed as minute particles, as smaH as 1 nm in diameter and even less. It is not practical to use minute... 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

Since catalyst activity is dependent on how much catalytically active surface is available, it is usually desirable to maximi2e both the total surface area of the catalyst and the active fraction of the catalytic material. It is often easier to enlarge the total surface area of the catalyst than to increase the active component s surface area. With proper catalyst design, however, it is possible to obtain a much larger total active surface area for a given amount of metal or other active material in a supported catalyst than can be achieved in the absence of a support. 

The performance of a catalyst often depends as much on the care and method of preparation as on the identity of the active components. This fact has been learned by many who have failed to obtain reproducibiUty among catalyst preparations ia the laboratory or have been responsible for quaUty assurance ia catalyst manufacture. Also, there are many examples of strong effects of trace impurities ia raw material or catalyst support on catalyst performance. 

Emission Control Catalysts. An appHcation of growing importance for cerium is as one of the catalyticaHy active components used to remove pollutants from vehicle (autoexhaust) emissions (36). The active form of cerium is the oxide that can be formed in situ by calciaation of a soluble salt such as nitrate or by deposition of slurried oxide (see Exhaust control, automotive). 

With phosphoric acid-based catalysts, in which the active component is Hquid acid absorbed in the pores of the support, the reaction probably follows the path (119) for the hydration of olefins in aqueous solution ... 

The regenerator has two main functions It restores catalyst activity and supplies heat to crack the feed. The spent catalyst entering the regenerator contains between 0.8-2.5 wt% coke, depending on the quality of the feedstocks. Components of coke are carbon, hydrogen, and trace amounts of sulfur and nitrogen, which burn according to the reactions shown in Table 4-3. 

In addition to having the required spedfidty, lipases employed as catalysts for modification of triglycerides must be stable and active under the reaction conditions used. Lipases are usually attached to supports (ie they are immobilised). Catalyst activity and stability depend, therefore, not only on the lipase, but also the support used for its immobilisation. Interesterification reactions are generally run at temperatures up to 70°C with low water availability. Fortunately many immobilised lipases are active and resistant to heat inactivation under conditions of low water availability, but they can be susceptible to inactivation by minor components in oils and fats. If possible, lipases resistant to this type of poisoning should be selected for commercial operations. 

Two other components, methanol and benzene, were included in this study. Methanol is important in processes using Rectisol Systems for C02 removal prior to methanation. Benzene was considered in order to determine the effect of aromatics on catalyst activity and potential carbon formation. 

Most catalysts consist of active components dispersed as small crystallites on a thermally stable, chemically inactive support such as alumina, ceramics, or metallic wires and screens. The supports are shaped into spheroids, cylinders, monolithic honeycombs, and metallic mesh or saddles. 

The active component of the chromium oxide catalyst is a surface compound of Cr(VI). In the case of silica as a support this stage may be presented by the schemes ... 

In the cases of Cr03/Si02 and Cr(7r-C3H6)3/Si02 systems a considerable part of the chromium contained in the catalyst is involved in the propagation center formation. In these catalysts all the ions of the transition metals are on the surface and the active component seems to be the main type of compounds present on the catalyst surface. 

Chromium compounds as catalysts, 188 Chromium oxide in catalytic converter, 62 Chromium oxide catalysts, 175-184 formation of active component, 176,177 of Cr-C bonds, 177, 178 propagation centers formation of, 175-178 number of, 197, 198 change in, 183, 184 reduction of active component, 177 Clear Air Act of 1970, 59, 62 Cobalt oxide in catalytic converter, 62 Cocatalysts, 138-141, 152-154 Competitive reactions, 37-43 Copper chromite, oxidation of CO over, 86-88... 

A catalytic system may contain active components other than H30+, H2O, and OH-. Weak acids and bases may also be efficient catalysts. These include, of course, both components of the buffer. Their contributions are in addition to the three terms seen before. If they are designated as BH+ and B, the rate constant is... 

By coprecipitating the catalytically active component and the support to give a mixture that is subsequently dried, calcined (heated in air), and reduced to yield a porous material with a high surface area. This procedure is followed when materials are cheap and obtaining the optimum catalytic activity per unit volume of catalyst is the main consideration. 

Filling the pores of the support with a solution of the catalytically active element, after which the solvent is removed by drying, is a straightforward way to load a support with active material. However, in this process various interactions are possible between the dissolved catalyst precursor and the surface of the support, which can be used to obtain a good dispersion of the active component over the support. To appreciate the importance of such interactions we need to take a closer look at the surface chemistry of hydroxylated oxides in solution. 

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