Carbon pore texture

These macropores are not effective for adsorption of various molecules, but their presence before activation is preferable for creating micropores in the walls. The pore texture of most activated carbons is illustrated in Figure 2.17b, where macropores (>50nm width) and mesopores (2-50nm... 

In order to control the pore texture in carbon materials, blending of two kinds of carbon precursors, the one giving a relatively high carbonization yield and the other having a very low yield, was proposed and called polymer blend method [112], This idea gave certain success to prepare macroporous carbons from poly(urethane-imide) films prepared by blending poly(amide acid) and phenol-terminated polyurethane prepolymers [113]. By coupling this polymer blend method with... 

The fluorinated carbon-coated AAO film has an interesting adsorption characteristic that has not been reported so far. Figure 3.12 shows N2 adsorption/desorption isotherms at -196°C for the pristine carbon-coated AAO film and the films fluorinated at different temperatures [119]. The isotherm of the pristine film is characterized by the presence of a sharp rise and a hysteresis in a high relative pressure range. Such a steep increase can be ascribed to the capillary condensation of N2 gas into the nanochannels of the AAO films, that is, the inner space of the nanotubes embedded in the AAO films. The amount of N2 adsorbed by the condensation into the fluorinated channels is lower than that of the pristine one. Moreover, the amount drastically decreases with an increase in the severity of fluorination. Since TEM observation revealed that the inner structure of the fluorinated CNTs was not different from that of the pristine nanotubes, the reason why the N2 isotherm was so changed as in Figure 3.12 cannot be attributed to the alteration of the pore texture upon the... 

Thus, regarding the effect of thermal treatment of hydrolytic lignin on the size of lignin carbon pores, the optimum regimes of thermal treatment produce sorbents with a pore size of 0.7-1.0 run. A pilot batch of such tnicroporous carbon was produced, its texture... 

Type of carbon Origin Surface area (m /g) Pore texture... 

Moreno-CastiUa, C., Carrasco-Marin, F., Utrera-Hidalgo, E., and Rivera-UtiiUa, J. (1993). Activated carbons as adsorbents of sulfur dioxide in flowing air. Effect of their pore texture and surface basicity. Langmuir, 9, 1378—83. 

The variety of mechanisms that may be involved in the sorption process of metal ions onto activated carbon induces a great number of factors that control the adsorption the surface oxygen complex content, the pH of point of zero charge, the pore texture of carbon, the solution pH and its ionic strength, the adsorption temperature, the nature of the metal ion given by its speciation diagram, its solubility, and its size in adsorption conditions. The influence of these various conditions is detailed in Section 24.2.1.4. 

Among the characteristics of the adsorbent are its pore texture, surface chemistry, and mineral matter content. The characteristics of the adsorptive are its molecular size, solubility, polarity, pIC, (for electrolytes), and nature of the substituents if it is aromatic. Finally, the solution chemistry factors are the pH and the ionic strength [5]. I shall focus in this section only on the role of the characteristics of the adsorbent, especially its carbon surface chemistry, on the adsorption processes, because although its importance has long been recognized [6, 7], the exact nature of this importance has often been controversial and misunderstood [1]. 

The recent work by Li and coworkers [18] provides a good illustration of the importance of the surface chemistry and pore texture of carbon materials on nonelectrolyte adsorption. They studied the adsorption of trichloroethene (TCE) and methyl ieri-butyl ether (MTBE) on different commercial activated carbons and activated carbon fibers with different porosity and surface chemistry. TCE is a relatively hydrophobic planar molecule. MTBE is tetrahedron-like and relatively hydrophilic. The results of the adsorption from aqueous solutions on the more hydrophobic carbons showed that TCE adsorption was controlled by a pore volume ranging from 0.7 to 1 nm width, as shown in Fig. 25.2. MTBE was primarily adsorbed in pores with widths between 0.8 and 1.1 nm. These micropore ranges were between 1.3 and 1.8 times the kinetic diameter of the adsorptives. 

All these results show the importance of the carbon surface chemistry and pore texture on the adsorption of nonelectrolytic organic solutes. Thus, for hydrophobic carbons, which generally have a low content of surface oxygen complexes, the adsorption of organic molecules is by dispersion and hydrophobic interactions, and the pores involved in the adsorption depend on the molecular size of the adsorptive. Conversely, when the adsorbent s content of surface oxygen complexes increases or its hydrophobicity decreases, there is a preferential adsorption of water on these complexes, which reduces the adsorption capacity of the adsorbent. 

Other important organic electrolytes are the dye molecules. The adsorption of dyes is of interest largely because they are pollutants frequently found in textile wastewaters and because some of them were proposed as molecular probes to characterize the pore texture of carbon adsorbents. However, this last apphcation should be viewed with caution [1] because dye adsorption is profoundly affected by the carbon surface chemistry and solution pH. Thus, Graham [40] found a good linear relationship between a decreased uptake of the anionic metanil yellow and an increased carbon surface acidity. This author concluded that acidic groups on the carbon surface tend to reduce the capacity for anionic adsorbates in general. The adsorption of dyes was subsequendy investigated by other authors [1]. For instance, Nandi and Walker [41] studied the adsorption of acid and basic dyes on different carbon materials and found that the area covered by a dye molecule depended on the nature of the solid surface. 

In many liquid-phase applications, the bacterial colonization of activated carbons can occur quite readily [67]. This colonization [68] is considered to result from (i) the adsorptive properties of carbon, which produce an increase in the concentration of nutrients and oxygen as well as the removal of disinfectant compounds (ii) the pore texture of the carbon particles, which provides the bacteria with a protective environment (iii) the presence of a large variety of functional groups on the carbon surface, which enhances the adhesion of microorganisms and (iv) the nature of the mineral matter content of the carbon, which can favor bacteria adhesion. In general, bacteria attached to carbon particles are very resistant to disinfectants. 

The series of 10 chapters that constitute Part 3 of the book deals mainly with the use of adsorption as a means of characterizing carbons. Thus, the first three chapters in this section complement each other in the use of gas-solid or liquid-solid adsorption to characterize the porous texture and/or the surface chemistry of carbons. Porous texture characterization based on gas adsorption is addressed in Chapter 11 in a very comprehensive manner and includes a description of a number of classical and advanced tools (e.g., density functional theory and Monte Carlo simulations) for the characterization of porosity in carbons. Chapter 12 illustrates the use of adsorption at the liquid-solid interface as a means to characterize both pore texture and surface chemistry. The authon propose these methods (calorimetry, adsorption from solution) to characterize carbons for use in such processes as liquid purification or liquid-solid heterogeneous catalysis, for example. Next, the surface chemical characterization of carbons is comprehensively treated in Chapter 13, which discusses topics such as hydrophilicity and functional groups in carbon as well as the amphoteric characteristics and electrokinetic phenomena on carbon surfaces. 

Carbon materials issued from evaporative drying and pyrolysis of resorcinol-formaldehyde gels were used as metal catalyst supports. These carbons, whose pore texture and composition are controllable, lead to high metal dispersion. Moreover, pore texture control is a great advantage diffusional limitations can be lowered, and even eliminated, by choosing an appropriate pore texture range. 

The composition and pore texture are two key parameters to the carbon performances in the above-cited applications [1]. However, since the origin of the materials used for the preparation of activated charcoals varies constantly, it is difficult to keep these essential parameters unchanged. Indeed, the carbon final texture and composition strongly depend on the raw material chosen. In... 

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.

These results show how it is possible to optimise catalyst selectivity and activity by tuning the textural parameters of the support in a realistic way. Diffusional limitations can be completely avoided by choosing an appropriate pore size range, which is made possible by the pore texture flexibility of carbon supports issued from evaporative drying and pyrolysis of resorcinol-formaldehyde aqueous gels. 

The effect of carbon aerogel pore texture and method of preparation of the supported Pt catalysts on their activity has recently been reported [85]. Two different Pt precursors were used H2[PtCl6] and [Pt(NH3)4](OH)2. For a given Pt precursor, the pore texture of carbon aerogels used had no influence in Pt surface area and ORR activity. The best ORR specific activity was obtained with catalysts prepared with H2[PtCl6]. By contrast, the activity of catalysts prepared from [Pt(NH3)4](OH)2 was low, despite the fact that the Pt dispersion value reached was the highest. These authors [85] indicate that this is probably due to the particle size effect on ORR activity, with a smaller Pt particle size showing a lower activity. 

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]. 

In fuel cell applications, however, the presence of small micropores can reduce the accessibility of the liquid electrolyte to the metal particles placed within them. This may be avoided by developing carbon gels with the appropriate mesoporous network. Hence, the pore texture of carbon gels can be adapted to the reaction under question, and this is possible because of the pore texture flexibility of carbon gels, which can be tailored by controlling all the steps in carbon gel synthesis. This is an advantage over activated carbons, which are generally microporous solids with low meso- and macroporosity, which induces diffusional limitations and diminishes catalyst performance. 

Fievet, R, M. Mullet, and J. Ragetti, Impedance measurements for determination of pore texture of a carbon membrane. Journal of Membrane Science, 1998. 149 pp. 143-150... 

In the field of carbon xerogels, it is also important to remark that, despite commonly accepted ideas, evaporative drying does not always completely destroy the pore texture of phenolic gels. Indeed, by choosing appropriate values of synthesis variables, in particular pH values, it is possible to maintain a large porosity in the dried xerogels. Pore volumes up to 2 cmVg and pore size distributions from around 1 nm up to 200 nm have been observed [50, 66]. 

Yoshizawa N, Hatori H, Soneda Y, Hanzawa Y, Kaneko K, Dresselhaus M S (2003) Structure and electrochemical properties of carbon aerogels polymerized in the presence of Cu. J Non-Cryst Solids 330 99-105 Maldonado-Hodar F J, Moreno-Castilla C, Perez-Cadenas A F (2004) Surface morphology, metal dispersion, and pore texture of transition metal-doped monolithic carbon aerogels and steam-activated derivatives. 

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