Carbon micropore distributions

Dubinin adsorption models have been used to calculate carbon micropore distributions from experimental isotherm measurements of a number of adsorbates, including nitrogen [120-122], carbon dioxide [122,123], methane [123], and several other organic molecules [124]. It is has generally been... 

For most catalysts, mesopores are dominant, whereas for materials derived from zeolites or active carbons, micropores are the most important. Determination of the pore size distribution is indispensable in catalysis research. 

Stoeckli HF. On the description of micropore distributions by various mathematical-models. Carbon, 1989 27(6) 962-964. 

The micropore distribution is performed mostly on the carbon filament surface. Nowadays a program was undertaken to examine the parameters of an active carbon fiber to optimize both the mass uptake of ammonia, methane and hydrogen and the carbon density. 

The pore volumes and the micropore distribution parameters of the carbon samples were analyzed in an automatic volumetric sorption analyzer (model ASAP 2000, Micromeritics Instrument Co. Norcross, GA, USA) using N2 and CO2 adsorption at 77 and 273 K, respectively. Five different experiments were carried out to determine the experimental error with sample CA-3 finding a value around 3%. 

The most recent developments involve similar approaches but now based on density functional theory using statistical thermodynamics [34]. Although the majority of results obtained are related to slit-shaped carbon adsorbents [34,35] the approach holds great potential for rapid and widely applicable assessment of the micropore distribution. 

The ideal carbon fitr pure physisorption can be derived directly fiom the Dubitiin-Radushkevich equation. The two carbon-related parameters are the structural constant B and the micropore volume Wo. Usually these values are obtained from N2-isotherms at 77K. A lower value of B will yield a higher value of We, i.e. a hi er adsorption capacity. In the original DR-equation B is related to the width of the Gaussian micropore distribution but it is also well known that B [K j is directly related to the adsorption energy E [kJ.moT ] by Eq.4, the latter being in turn related to the mean micropore half-width L [nm] by Eq.5 [37]. 

FIGURE 2.31 Micropore distribution of carbon AU-11 obtained from best fit of the adsorption data using Equation 2.128 and Equation 2.132. (Adapted from Dubinin, M.M., Effremov, S.N., Kataeva, L.I., and Ustinov, E.A., Izv. Akad. Nauk SSSR, Sen Khim., 255, 1, 1985). 

FIGURE 4.16 Comparison of micropore distribution of carbon CEP-59 calculated using Dubinin theory of volume filling of micropores and the experimental data obtained fiom molecular sieve experiments in the region of micropore width less than 0.75 nm. (After Stoeckli, F.H. and Kraehenbeuhl, F., Carbon, 19, 353, 1982. With permission.)... 

Carbonization seems to be an effective method to adjust the pore size of PAF-1 to increase the gas selectivity. PAF-1-450 (PAF-1 carbonized at 450 °C), with a narrow micropore distribution of 0.8 nm, shows obvious increased CO2 sorption. Besides, on the basis of single component isotherm data, the dual-site Langmuir-Freundlich adsorption model-based lAST prediction indicates that the CO2/N2 adsorption selectivity may be as high as 209 at a 15 85 CO2 N2 ratio. Also, the CO2/CH4 adsorption selectivity should be in the range of 7.8-9.8 at a 15 85 C02 CH4 ratio at 0adsorption selectivity could be about 392 at 273 K and 1 bar for the 20 80 CO2 H2 mixture (Figure 10.2). ... 

New possibilities for developing better sorbents for methane storage are still available. For example, no reports have appeared on activation of carbon fibers by using molten KOH, which conld yield the optimnm micropore distribution and volume. Single-wall carbon nanotnbes with the right sizes, particularly bundled in the aligned forms, conld resnlt in the ideal sorbent for methane storage. 

Swiatkowski, A, Trznadel, B.J., and Zietek, S., Description of active carbon micropore size distribution based on the Horvath-Kawazoe equation adapted to benzene adsorption isotherms, Adsorpt. Sci. Technol., 14(1), 59-68(1996). 

Kraehenbuehl F, Stoeckli HE, Addoun A, Ehrburger P, Donnet JB. The use of immersion calorimetry in the determination of micropore distribution of carbons in the course of activation. Carbon 1986 24(4) 483 88. 

This review coneluded by stating that mieroporous activated carbons and CMS, can be characterized in terms of their micropore distributions and the degree of hydrophilicity of their surfaces. The value of this work is the comparison made available between data of traditional adsorption methods and calorimetric methods. Examples of this are given in Table 5.6 and Figure 5.47. 

Fig. XVII-31. (a) Nitrogen adsorption isotherms expressed as /-plots for various samples of a-FeOOH dispersed on carbon fibers, (h) Micropore size distributions as obtained by the MP method. [Reprinted with permission from K. Kaneko, Langmuir, 3, 357 (1987) (Ref. 231.) Copyright 1987, American Chemical Society.]...

A major difficulty in testing the validity of predictions from the DR equation is that independent estimates of the relevant parameters—the total micropore volume and the pore size distribution—are so often lacking. However, Marsh and Rand compared the extrapolated value for from DR plots of CO2 on a series of activated carbons, with the micropore volume estimated by the pre-adsorption of nonane. They found that except in one case, the value from the DR plot was below, often much below, the nonane figure (Table 4.9). 

For a second active carbon, AG, the DR plot was convex to the logio(p7p) This carbon was believed from X-ray results to have a wider distribution of pores. It was found that the isotherms of both benzene and cyclohexane could be interpreted by postulating that the micropore system consisted of two sub-systems each with its own Wq and and with m = 2 ... 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

Fig. 2. Pore size distribution of typical samples of activated carbon (small pore gas carbon and large pore decolorizing carbon) and carbon molecular sieve (CMS). A / Arrepresents the increment of specific micropore volume for an increment of pore radius.

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

The stmcture of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aHphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the stmcture even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore stmcture. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and AppHed Chemistry (lUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

Activated carbons for use in Hquid-phase appHcations differ from gas-phase carbons primarily in pore size distribution. Liquid-phase carbons have significantly more pore volume in the macropore range, which permits Hquids to diffuse more rapidly into the mesopores and micropores (69). The larger pores also promote greater adsorption of large molecules, either impurities or products, in many Hquid-phase appHcations. Specific-grade choice is based on the isotherm (70,71) and, in some cases, bench or pilot scale evaluations of candidate carbons. 

Effectiveness of selective adsorption of phenanthrene in Triton X-100 solution depends on surface area, pore size distribution, and surface chemical properties of adsorbents. Since the micellar structure is not rigid, the monomer enters the pores and is adsorbed on the internal surfaces. The size of a monomer of Triton X-100 (27 A) is larger than phenanthrene (11.8 A) [4]. Therefore, only phenanthrene enters micropores with width between 11.8 A and 27 A. Table 1 shows that the area only for phenanthrene adsorption is the highest for 20 40 mesh. From XPS results, the carbon content on the surfaces was increased with decreasing particle size. Thus, 20 40 mesh activated carbon is more beneficial for selective adsorption of phenanthrene compared to Triton X-100. 

Fig. 3.23 shows pore volume distributions of some commercially important porous materials. Note that zeolites and activated carbon consist predominantly of micropores, whereas alumina and silica have pores mainly in the me.sopore range. Zeolites and active carbons have a sharp peak in pore size distribution, but in the case of the activated carbon also larger pores are present. The wide-pore silica is prepared specially to facilitate internal mass-transfer. 

The absorption property exhibited by active carbon certainly depends on the large specific surface area of the material, though an interpretation that it is based solely on this is incomplete. This is borne out by the fact that equal amounts of two activated carbon specimens, prepared from different raw materials or by different processes and having the same total surface area, may behave differently with regard to adsorption. Such differences can be partly explained in terms of the respective surface properties of the carbon samples and partly in terms of their relative pore structure and pore distribution. Every activated carbon particle is associated with at least two types of pores of distinctly different sizes. They are the macropores and the micropores. The macropores completely permeate each particle and act as wide pathways for the diffusion of material in and out of carbon, but they contribute very little to the total surface area. The micropores are more important since they... 

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