Preparation of Carbon-Supported Catalysts

Another interesting example of the exploitation of the relative inertness of the carbon surface is provided by the use of Fe/C catalysts in the hydrogenation of carbon monoxide. In this case, the inertness of the carbon surface facilitates the presence of zero-valent iron in the catalyst [32,33], which is more difficult for other supports, such as alumina, on which the reduction of the oxidized iron species is hindered. Vannice et al. carried out an extensive study of carbon-supported iron catalysts using different carbons and preparation methods and concluded that highly dispersed Fe/C catalysts could be prepared on high-surface-area carbons, due to the weak chemical interactions between oxidized iron precursors and the carbon surface [32-34]. 

The methods used for the preparation of carbon-supported catalysts are similar to those used with other supports. However, some particularities of carbon materials make it necessary, in some cases, to adapt the preparation recipes to be used with these supports. A fundamental point in obtaining reproducible results is to have a comprehensive knowledge of the characteristics of the support material, primarily with regard to its porous texture and surface chemical properties. 

Catalyst preparation by impregnation is carried out by contacting the support with a solution containing the precursors of the active phases. Two different approaches can be followed incipient-wemess impregnation and excess-solution impregnation. In the former, the carbon support is wetted with a solution of the precursor, drop by drop, in the proper amount to just fill its pores. A slurry is formed with the pore volume filled wifh fhe solution, which is finally dried to remove the solvent, leaving the precursor of the active phase deposited on the 

Electrostatic interactions between the carbon surface and the active-phase precursors have also to be taken into account in the preparation of carbon-supported catalysts. The presence of oxygen functionalities on the carbon surface, which can be produced upon the activation process (for activated carbons) and/or by subsequent oxidation treatments, renders it amphoteric. This implies that it can be more or less charged, positively or negatively, depending on the pH of the surrounding solution. Preparation variables such as the polarity of the solvent, the pH of the solution, the anionic or cationic nature of the metal precursor, and the isoelectric point (lEP) of the carbon support determine the extent of precursor-support interaction and, in this way, the total uptake and dispersion of the active phase in the final catalyst [17,20,37]. Thus, for carbons containing acidic surface groups and, as a consequence, a low isoelectric point, best results in the preparation of supported catalysts are achieved when a cationic precursor is used in basic media. Under these conditions, the acidic complexes (-COOH, -OH) are deprotonated (-COO , -0 ) in such a way that 

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This is a topic of great practical interest because of water treatment and metal recovery applications. Its fundamental aspects are also important for the preparation of carbon-supported catalysts [22], where the catalyst precursor is typically dissolved in water prior to its loading onto the porous support. 

Carbonaceous materials are widely used as supports for the preparation of heterogeneous catalysts. The use of carbon as a catalyst support presents many obvious advantages, mainly robustness, ease of recovery and cheapness. But, in spite of these very interesting aspects, carbon supports and materials derived from them are generally difficult to characterize spectroscopically. Consequently, the preparation of carbon-supported catalysts still lacks control at each synthetic step. 

The carbon surface contains a given number of heteroatoms (O, N, H) in the form of functional groups, similar to the way that heteroatoms appear in organic compounds. The presence of these groups can affect the preparation of carbon-supported catalysts, as they confer the carbon surface acid-base and hydrophilic character. 

Several other methods have been employed for the preparation of carbon-supported catalysts, although to a lesser extent that impregnation methods. Nakamura et al. [38] prepared molybdenum catalysts for ethene homologation by physical deposition of gaseous [Mo(CO)6]. Their supports were commercial activated carbons that were subjected to different treatments to modify then-surface. The authors compared these supports with oxidic supports and concluded that the interaction between the metal carbonyl and the carbon supports were weaker. Furthermore, they observed that oxidation of the carbon surface was effective in enhancing the catalytic activity of Mo/C, and they ascribed this effect to the contribution of the surface oxygen groups to the partial oxidation of decomposed [Mo(CO)6]. 

After reviewing the literature we conclude that in all three methods—wet impregnation, incipient-wetness impregnation, and ion adsorption—two major factors govern the final dispersion of the active phase precursor-support interaction and pore structure. These two issues are discussed in the next two subsections. Note that we discuss primarily that literature which adds fundamental insights in the preparation of carbon-supported catalysts. 

Although the primary focus of this review was carbon and carbon-supported catalysts, attempts have been made to identify the difference in the effect of carbon supports compared with the oxidic supports, particularly that of y-Al203. It has been noted that many studies had the same objective. For this purpose, the difference in catalyst activity and stability was estimated using both model compounds and real feeds under variable conditions. The conditions applied during the preparation of carbon-supported catalysts have... 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

Simonov PA, Likholobov VA. 2003. Physicochemical aspects of preparation of carbon-supported noble metal catalysts. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Decker. 

There is increased interest in the use of Ru-based systems as catalysts for oxygen reduction in acidic media, because these systems have potential applications in practicable direct methanol fuel cell systems. The thermolysis of Ru3(CO)i2 has been studied to tailor the preparation of such materials [123-125]. The decarbon-ylation of carbon-supported catalysts prepared from Ru3(CO)i2 and W(CO)6, Mo(CO)is or Rh(CO)is in the presence of selenium has allowed the preparation of catalysts with enhanced activity towards oxygen reduction, when compared with the monometallic ruthenium-based catalyst [126],... 

Physicochemical Aspects of Preparation of Carbon-Supported Noble Metal Catalysts... 

Preparation of carbon-supported noble metal catalysts (Me/C) is usually based on supporting metal precursors on carbon followed by their transformation into the metal particles. Design of an appropriate catalyst implies an optimal selection of both the support and the method of synthesis of the active component, which requires understanding the following issues ... 

Cobalt porphyrins and phthalocyanines were used as precursors for preparation of carbon supported cobalt catalysts that displayed high activity in reduction of oxygen in fuel cells [9-12], These samples were prepared by deposition of cobalt phtalocyanine on active carbon following by heat treatment at 650-700°C in an inert atmosphere providing formation of Co-N structure deposited on active carbon. 

In a previous work [13], we reported on the preparation of carbon-supported bimetallic Bi-Pd catalysts by the thermal degradation of Bi and Pd acetate-type precursors under nitrogen at 773 K and described their catalytic properties in glucose oxidation. The formation of various BixPdy alloys (BiPd, BiPds, Bi2Pds) or, at least, associations on the surface of these catalysts during the activation step was heavily suspected. Alloy formation in supported bimetallic Pd-based catalysts has been mentioned several times in the literature in die presence of other promoting elements, like Pb or Te [14-16] and is sometimes assumed as responsible for the deactivation of the catalysts. 

Preparation of carbon-supported Pd and Au-Pd catalysts via optimized adsorption of metallic complexes... 

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