Metal charcoal-supported

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

Hydrogenation with homogeneous catalysis involves a soluble catalyst rather than the more common heterogeneous catalysis with, say, Pd metal dispersed on an insoluble charcoal support as in Chapter 24. In general terms homogeneous catalysts are those that are soluble in the reaction mixture. 

Granulate catalyst—c.g., ion-exchange resins, precious metals on alumina, or activated charcoal supports—may be embedded between two corrugated permeable screens to form a sandwich, and the sandwiches in turn stacked such as to give the characteristic OCFS structure—Fig. 5. This variant is of particular interest for heterogeneous gas-liquid systems such as trickle beds and bubble columns—see Section IV—and catalytic distillation—see Section V. 

Pd/C means palladium metal dispersed on a charcoal support—usually 5-10% by mass Pd and 90-95% C. It is made by suspending charcoal powder in a PdCl2 solution, and then reducing the PdCb to Pd metal, usually with H2 gas, but sometimes with fomnaldehyde, HCHO (which becomes oxidized to formic acid, HCO2H). The palladium metal precipitates on to the charcoal, which can be filtered off and dried. The fine Pd particles present maximum surface area to the reaction they catalyse and, while Pd is an expensive metal, it is recyclable since the Pd/C is insoluble and can be recovered by filtration. 

One of the most fundamental examples of a surface-mediated reaction is the heterogeneous catalytic hydrogenation of olefins and acetylenes on charcoal-supported transition metals like Pd, Pt, Ni,... 

This behavior depended on the surface atomic M/Cu ratio and resulted from an electron transfer from Cu to the second metal. Bimetallic Cu-Pt and Cu-Au nanoparticles prepared in the same way but on a charcoal support... 

Catalytic hydrogenation takes place on the surface of the metal. The metal must therefore be finely divided, and is usually dispersed on the surface of an inert support. This is what Pd/C means—finely divided palladium carried on a charcoal support. The first step is chemical absorption of hydrogen onto the metal surface, a process that results in breakage of the H—H bonds and distributes hydrogen atoms where they can react with the organic substrate. Now the alkene can also bond to the metal, and hydrogen can be transferred from the metal to the alkene. 

Transition metals are used as catalysts for a wide range of chemical reactions, such as hydrogenations, oxidations, and Fischer-Tropsch reactions. So r, these catalysts have been usually used in the form of metal powders or metal particles supported on inorganic materials, such as silica, alumina, and activated charcoal Recently, the use of synthetic polymers as protective matrices has become increasingly interesting, since such systems show several advantages in conoparison to traditional catalysts (7, 2) ... 

The optimization of Pt-Fe catalysts supported on charcoal was made by means of an empirical method. The carbon support was washed, oxydized in a liquid phase and thermally desorbed before its impregnation. The metals precursors were reduced by hydrogen. The reduced metals were characterized by electron microscopy, magnetization measurements and X Ray diffraction. The two metals are alloyed under the form of finely divided particles. The charcoal supported Pt-Fe catalysts are very active and selective in hydrogenation of cinnamaldehyde to clnnamyl alcohol. 

Some data are also available (5) on the use of metallic cobalt and nickel supported on charcoal for high polymerization of ethylene. However, the application and investigation of these catalysts were not subsequently developed. 

Since carbon is a supplier of electrons, it acquires a positive charge in the process. In order to maintain electrical neutrality, carbon attracts the hydroxyl (OH-) ions. When the aqueous solution contains metal anions that have a greater affinity towards carbon, the hydroxyl ions are exchanged. In support of this theory, mention may be made of the observation that no adsorption takes place in the absence of oxygen and that hydrogen peroxide is liberated when oxygen is bubbled through an aqueous slurry of charcoal. 

X-Ray studies confirm that platinum crystallites exist on carbon supports at least down to a metal content of about 0.03% (2). On the other hand, it has been claimed that nickel crystallites do not exist in nickel/carbon catalysts (50). This requires verification, but it does draw attention to the fact that carbon is not inert toward many metals which can form carbides or intercalation compounds with graphite. In general, it is only with the noble group VIII metals that one can feel reasonably confident that a substantial amount of the metal will be retained on the carbon surface in its elemental form. Judging from Moss s (35) electron micrographs of a reduced 5% platinum charcoal catalyst, the platinum crystallites appear to be at least as finely dispersed on charcoal as on silica or alumina, or possibly more so, but both platinum and palladium (51) supported on carbon appear to be very sensitive to sintering. 

Scattering from Thin Crystals. The simple arguments given above hold only if samples are amorphous. Although some catalyst supports are non-crystalline, such as charcoal and silica, others such as alumina are not. Furthermore, the metal catalyst clusters themselves are generally crystalline and thus the above arguments must be modified to account for Bragg reflections from crystalline areas. 

Nicolau et al. (187) have reported EPR absorption of platinum and palladium supported on charcoal. These workers attribute a sharp resonance, centered at p = 2.0034, to a species resulting from the interaction of the metal and the charcoal. The intensity of the EPR signal increases... 

In order to increase the contact of a catalyst with hydrogen and the compounds to be hydrogenated platinum (or other metals) is (are) precipitated on materials having large surface areas such as activated charcoal, silica gel, alumina, calcium carbonate, barium sulfate and others. Such supported catalysts are prepared by hydrogenation of solutions of the metal salts, e.g. chloroplatinic acid, in aqueous suspensions of activated charcoal or other solid substrates [28. Supported catalysts which usually contain 5, 10 or 30 weight percent of platinum are very active, and frequently pyrophoric. 

Supported electrodes. The mixture of catalyst and charcoal is poured into the space between two mechanically rigid walls, with asbestos paper as support and a graphite felt or metal sheet as current collector. No binder is necessary. With such electrodes, both liquid and gaseous working materials can be studied. For the experiments with dissolved fuels described in Section 4.2, we used modified electrodes of this type 6 mg chelate was mixed with 6 mg soot and poured between two graphite felt discs. 

In contrast, stepwise substitution reactions on M(CO)6 (M = Cr, Mo, W) have been achieved with a series of heterogeneous catalysts including co-balt(ll) chloride (27), activated charcoal (159), and platinum metals dispersed on oxide or carbon supports (31), to give mono-, di-, tri-, and complete substitution (124) in yields > 90%. Representative reaction times are given in Table II (159). The efficiency of the method was further demonstrated by the stepwise synthesis of the mixed isocyanide complexes m-Mo(CO)4(CNMe)(CNBu ) and /ac-Mo(CO)3(CNMeXCNBu )2 from Mo(CO)6 in <25 min in 85 and 95% yields, respectively (159). 

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