Carbon xerogels

Figure 2. Pore size distributions of (a) carbon cryogels, (b) carbon xerogels and (c) carbon MW gels synthesized under the condition of R/C = 200.

Metal catalysts supported on texture-tailored carbon xerogels... 

The dilution ratio, i.e. the solvenfr(resorcinol+fonnaldehyde) molar ratio, and the resorcinol/formaldehyde molar ratio were fixed at 5.7 and 0.5, respectively. The pH of the precursors solution was adjusted at the chosen value with NaOH the pH range for the gel series was fixed between 4.0 and 6.5. After gelation at 70°C, the samples were dried and pyrolyzed at 800°C imder nitrogen flow for 3 h in order to obtain carbon xerogels. 

The pore texture of carbon xerogels is controllable at the meso- or macroporous level through the choice of the pH of the precursors solution (Fig. la). The final carbon material is composed of intercoimected microporous carbon nodules. 

Fig. 1. Carbon xerogels. (a) Pore texture total pore volume and mean pore size (mictopores excluded) as a function of the pH of the precursors solution, (b) Monolidis black lines and circles represent the size of the gel before shrinkage.

Fig. 3. Pt/C catalysts (4% wt.). (a) Support commercial activated charcoal (b) support mesoporous carbon xerogel.

Fig. 4. Effectiveness factor, t], as a function of the pellet size. Carbon xerogel supported catalysts ( ) small mesopores ( 10 nm), (A) large mesopores ( 30 run), (O) macropores ( 70 nm). Catalyst supported on activated charcoal Ceca Acticarb NC 45 (O).

Diffusional limitations are often analysed through the use of the Weisz modulus, 0, which compares the observed reaction rate to the difiusion rate [18]. When 0 1, the diffusion phenomenon is not significant and the observed reaction rate is equal to the intrinsic reaction rate. When 1, diffusional limitations modify the apparent kinetics, and the observed reaction rate can be very different fi om the intrinsic reaction rate. Since carbon xerogels are composed of two distinct levels, i.e. the pellet level and the microporous nodules level, both with their own pore size and length scale, 0 must be calculated at both levels. This was the object of a complete study [19]. 

This chapter begins with a general description of the several strategies to het-erogenize transition-metal complexes onto solid supports, with a special emphasis on those methodologies that have been used for complex grafting onto carbon materials. It will include sections that will focus on the various transition-metal complexes that have been immobilized onto several carbon materials activated carbons, black carbons, carbons xerogels, and carbon nanotubes the specific catalytic reactions with these carbon-based systems are also discussed in some detail. 

Subcritical drying can sometimes yield porous xerogels when the gel structure is strong enough to withstand capillary pressures [11,31], although it is not possible to produce carbon xerogels with both small pores and high pore volumes... 

Thus, Cu- and Co-doped carbon xerogels were used by Liu and co-workers... 

NO conversion to N2 at 773 K was about 90% over the Cu-doped carbon xerogel obtained by dissolving Cu nitrate in the initial RF mixture, and this rate was maintained for 1000 minutes onstream. Conversely, NO conversion over the same catalyst prepared by impregnation was about 85% and decreased gradually with time onstream. CO2 formation was also observed, together with NO reduction ... 

The carbon xerogel with no metal reached about 10% NO conversion, which indicates the catalytic effect of the metal. Co-doped carbon xerogels showed the same behavior as that of Cu-doped carbon xerogels. 

The influence of the mesopore size of carbon aerogels on ORR using Pt-doped carbon aerogels has also been reported by other authors [86]. They found practically no influence of pore texture on Pt dispersion. However, they indicate that the ORR activity increased when the mean mesopore size increased, reaching the best ORR performance for a mesopore size of 18.5 nm. Pt-based catalysts have also been used as anodic catalysts in DMFC systems, since Pt is able to activate the C-H bond cleavage in the temperature range of fuel cell operation (298 to 403 K). Thus, different Pt, Pt-Ni, and Pt-Ru catalysts supported on carbon xerogels have been used as catalysts in DMFC systems [87-90]. 

A Pt-doped carbon xerogel containing 10 wt% Pt was prepared by an incipient wetness technique [88]. The mean particle size was around 2 nm. The activity of the catalyst for methanol electrooxidation was higher in alkaline than in acid solution. In addition, the Pt surface slowly developed more favorable sites during cycling in alkaline solution. Pt supported on carbon xerogel showed better performance in the methanol electrooxidation than another Pt catalyst supported on Vulcan XC-72R with the same metal content. The large mesoporous surface... 

Another carbon xerogel was nsed as snpport for Pt and Pt-Ni catalysts used in methanol electrooxidation in an alkaline medinm [87], The performance of Pt-Ni was shown to be snperior to that of Pt-snpported catalysts by electrocatalytic tests. Different shapes in the cyclic voltammograms of these catalysts indicated different electrooxidation mechanisms. 

Sipos and co-workers studied enantioselective hydrogenation of isophorone and 2-benzylidene-l-benzosuberone using Pd catalysts supported on mesoporous carbon xerogels [94]. Enantioselective hydrogenation reactions can be strongly affected by the type of support and catalyst [97,98]. A carbon xerogel and its... 

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