Mesoporous carbon aerogels

Mesoporous carbon aerogels [138-142] were produced by Pekala and co-workas by pyrotysis of organic aerogels of resorcinol and formaldehyde. Recently, their porosity characteristics [143] and applications in catalysis [144] have tsrai revkwed. 

Long D, Chen Q, Qiao W, Zhan L, Liang X, Ling L (2009) Three-dimensional mesoporous carbon aerogels ideal catalyst supports for enhanced H2S oxidation. Chem Commun... 

Figure 21.14. Left Typical scattering pattern of a mesoporous carbon aerogel in the small-angle scattering regime and the range of wide-angle scattering (large q values). The dashed extrapolation of the decay of the intensity is...

Tamon, H., Ishizaka, H., Araki, T. and Okazaki, M., Control of mesoporous structure of organic and carbon aerogels, Carbon, 36 (1998) pp. 1257-1262. 

First results indicate a dependence of the surface capacitance of untreated carbon aerogels on their microstructure. Micro- and mesopores exhibit different storage capacitances (6.6 and 19.4 pF/cm in 1 M sulfuric acid, respectively).. An optimized thermal activation procedure of low density aerogels at 950°C in controlled CO2- atmosphere leads to an increase of the specific surface area and capacitance. On the other hand, the increase of the capacitive current after anodic oxidation in sulfuric acid is caused by electroactive surface groups, while the BET-surface area remains almost constant. 

In table 1 the densities and surface capacitances of various carbon aerogels are shown. The data reveal that the surface capacitance varies from one species to another.. According to Shi et al. the micro- and mesopore surface areas were separated and the total capacitance of the samples was split up in contibutions from both pore species [3]. The correlation can be expressed as... 

High surface areas are normally obtained by using porous materials, and the pore sizes may condition the accessibility of the reactants to the active sites, especially in the case of microporous materials such as activated carbons. Pore diffusion limitations become more important as the pore sizes decrease in addition, the smaller pores may be more easily blocked (e.g., by coke deposition). Therefore, deactivation and diffusion phenomena will in general affect more strongly the performance of microporous carbons. As a result, there has been a drive to develop mesoporous carbon catalysts (such as aerogels, xerogels, and templated carbons) for some applications, especially in the liquid phase. 

Similarly, Pt catalysts supported on carbon aerogels were used in the combustion reaction of toluene, o-xylene, and m-xylene [41,67]. Carbon aerogels were obtained by carbonization of an organic aerogel at 773 and 1273 K. Both samples were mesoporous, and their microporosity was equally accessible to N2 and CO2 at 77 and 273 K, respectively. Pt was deposited on both carbon aerogels by an incipient wetness technique using an aqueous solution of [Pt(NH3)4]Cl2. The supported catalysts thus obtained were pretreated in different atmospheres to obtain different mean Pt particle sizes. 

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]. 

Finally, the nse of Pt-Rn catalysts snpported on a carbon aerogel as an anode for DMFC has been reported by Dn and co-workers [90]. The total metal loading was fixed to 20 wt%, and the Pt/Rn atomic ratio varied from 3 1 to 1 1. Metal particles were dispersed on the snpport nniformly, with a mean size of 3 nm. These anthors fonnd that with mnch less metal loading on the carbon aerogel, the membrane electrode assemblies had the same power density as that of commercial catalysts. This was attributed to the mesopore texture of the carbon aerogel, which facilitated methanol transportation in the electrode. 

To increase the micropore volume, the above carbon aerogels were activated with CO2 at 1173 K for 1-7 h [146]. Activation for 5 h increased both the microporosity and mesoporosity the micropore volume and microporous surface area are 0.68 cm g and 1750 m g respectively, and the mesopore volume and mesoporous surface area are 2.04 cm g" ... 

Changes in pore parameters of the carbon aerogels with the heat treatment at high temperatures are listed in Table 13 [153]. With increasing heat treatment temperature, both total surface area and volume decrease, which is mainly due to the decrease in micropores. As a consequence, heat treatment of the carbon aerogel above 2273 K was found to give a carbon containing only mesopores. 

Mesopores of carbon aerogels have possibility for reaction field and thereby carbon aerogels can be applied as template for production of new materials. Thus, carbon aerogels were used as a template for the preparation of highly crystalline zeolites (ZSM-5 and Y) with uniform mesoporous channels [162,163]. 

Other carbon sources have also been used as matrices such as multiwall carbon nanotubes (MWNTs), which led to narrower mesopore distributions than with carbon black pearls. In addition, the use of carbon fibers also allowed cylindrical mesopores to be obtained with low tortuosity. Carbon aerogel monoliths, obtained from resorcinol-formaldehyde gels after drying with COj under supercritical conditions and pyrolysis under nitrogen atmosphere at 1323 K, have also been used as templates for the generation of mesoporosity in zeolites [158]. This method presents the added advantage that the mesoporous zeolite can be synthesized as a monolith. 

Mesoporous Carbons by Sol-Gel Process Aerogels, Xerogels and Cryogels... 

Perez-CabaUero F, Peikolainen A.-L, Uibu M, Kuusik R, Volobujeva O, Koel M (2008) Preparation of carbon aerogels from 5-methylresorcinol-formaldehyde gels. Micropor Mesopor Mat 108 230-236. 

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