Microporous carbons nitrogen adsorption isotherms

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.]...

The support and the catalysts were characterised by means of nitrogen adsorption, XPS, TPD and SEM. The nitrogen adsorption isotherms were determined at 77 K in a Coulter Omnisorp 1000 CX equipment, and were analysed by the BET equation (SBet), and by the t-plot for mesopore surface area (Smeso) and micropore and mesopore volume (Vmicr0, Vmeso), using the standard isotherm for carbon materials. The catalyst samples were previously outgassed at 120 °C. 

Porous carbons were prepared using MCM-48 and SBA-15 mesoporous templates and various carbon precursors (see Chapter 3 for preparation description). Figure 8.11 displays the nitrogen adsorption isotherms at 77 K of SBA-15 and of the corresponding templated carbon obtained by carbonization of sucrose in the template. Both isotherms show a bimodal porosity in the templated carbon, mesopores are generated by the removal of the silica walls, and micropores are present in... 

As explained in Chapter 7, since the multilayer isotherm path is rather insensitive to differences in surface chemistry, for routine mesopore analysis it is possible to make use of a universal form of nitrogen isotherm. However, most activated carbons are highly microporous and the determination of the micropore size distribution remains a more difficult problem. Indeed, as discussed in Chapter 8, even the assessment of the total micropore volume presents conceptual difficulties. We should therefore regard the measurement of a nitrogen adsorption isotherm as only the first stage in the characterization of a microporous carbon. 

Figure 1 shows CO2 adsorption isotherms and Table 1 contains the micropore volume calculated from both nitrogen and carbon dioxide adsorption isotherms. 

The nitrogen adsorption isotherms obtained with both series of activated carbons were of type I, corresponding to microporous materials. The results of the textural characterization are shown in tables 1 and 2. The last column in both tables was calculated from the CO2 adsorption data, the remaining parameters being determined from the nitrogen adsorption data... 

As shown in Fig. 3, nitrogen adsorption isotherms of CMK-1 feature well-pronounced capillary condensation steps similar to those of ordered mesoporous silicas and indicative of high degree of mesopore size uniformity. The isotherms reveal that the CMK-1 carbon has high nitrogen BET specific surface area (1500-1800 m g ), and large total pore volume (0.9-1.2 cm g ) [14]. The adsorption capacity is comparable or larger than that of MCM-48 template. The pore-size analysis (calibrated BJH analysis) shows that typical CMK-1 has uniform mesopores about 3 nm in size, which is accompanied by a certain amount of micropores when sucrose is used as the carbon source. 

Figure 18.4 Low-pressure nitrogen adsorption isotherms of various nonmicroporous carbon blacks (CB) with different graphitic order and of microporous CB. (Adsorption data taken from Refs [[39], [40], and [50]] for the graphitized, the thermal CB, and the furnace, respectively.)...

Lately, potassium hydroxide (KOH) activation has been used to produce activated carbons from cheap and available natural precursors, such as coals and pitch-derived carbonaceous materials. This method efficiently develops the micropores, and aUows to get various pore size distributions depending on the kind of precursor and activation conditions [82—85]. The nitrogen adsorption isotherms (Fig. 23.11) at 77 K of activated carbons obtained from coal (C), coal semicoke (CS), pitch semicoke (PS), pitch mesophase (PM), and a commercial activated carbon (AC), using KOH and carbon in the 4 1 ratio, reveal that a... 

The nitrogen adsorption isotherm recorded on ZSM-5 zeolite prepared without carbon displays almost rectangular shape being typical for purely microporous materials (Fig. 3). The isotherms on all samples prepared using carbon template (Fig. 3) posses both micro- and mesoporous features. The almost vertical rise of the adsorbed amount of nitrogen up to ptp = 0.05 corresponds to the volume filling of micropores. The isotherms exhibit hysteresis loops at plpo > 0.4, which are indicative for the capillary condensation in mesopores. The distribution of these... 

Steam activation is the most widely used method for producing activated carbons in the world. In terms of adsorption capacity, it easily reaches specific surface areas of 1000 m g" at 50% of activation conversion degree (regardless of carbonization yield), when an acceptable raw material with an initial ash content helow 10% is used. Basically, steam activated carbons are microporous materials, with a mlcropore size that increases with the activation degree, but with no mesopore development. As an example. Fig. 11 shows the nitrogen adsorption isotherms of activated carbon produced by the steam activation of an anthracite [33]. Each adsorption isotherm corresponds to an activated carbon with activation degrees of 20, 35, 50, 70 and 80% bum-off (samples AC-720, AC-735, AC-750, AC-770 and AC-780 respectively), the sample AC-700 being the non-activated char. These isotherms not only provide specific numerical parameters correlated with adsorption capacity, but also provide certain qualitative infonnation derived from their shape. 

The Os-plot method provictei an effective and simple way for evaluation of the micropore volume the total surface area S, and the mesopore surfece aura of nanoporous materials. For the purpose of illustraticm. Fig. 8 presents the Os-plot for the nitrogen adsorption isotherms on seladed active (srbcsis at 77 K. The values of Si and S,m evaluated from nitrogen adsorption data for the WV-A900, BAX 1500 and NP-5 active carbons are summarized in Table 4. 

Fig. 12. The HK micropore volume distributions fiar the WV-A900, BAX 1500 and NP5 active carbons obtained from nitrogen adsorption isotherms...

The weak point of computational methods is still that they assume well-defined shape pores, rigid, and not interconnected, and thereof are inaccurate for real microporous carbons. In addition, all correlations presented in literature assume that the porosity of electrodes is identical to that of the carbon powder and do not take into account the additives, that is, binder and carbon black. As shown in Figure 8.6 presenting nitrogen adsorption isotherms recorded at 77 K (Figure 8.6a) and pore size distribution (PSD) (Figure 8.6b) of an industrial AC powder (NORIT,... 

The nitrogen adsorption/desorption isotherms allow the specific surface area, pore size distribution as well as the micro/meso ratio to be estimated. The total surface area is quite similar for the investigated samples and it ranges from 329 to 403 m /g being the most developed for the Nt+3M+F composite as shown in Fig. 9.6. The nitrogen adsorption isotherms showed that the carbon materials are typically mesoporous (apart from the material M+F, that is, without nanotubes), and the amount of micropores is very moderate. The micropore volume values for all the samples are comparable var3dng from 0.152 to 0.174 cm /g. The porosity characteristics of all the composites are illustrated in Table 9.1. 

The Nitrogen adsorption isotherms at 77 K on carbon membranes are shown in (Fig. 4). An incremental trend in total pore volume, Vp, with the activation time is observed. Activation time IS minutes brin about changes in both the micropore and the mesopore volume (0.02 instead of 0.01 cm g ). On the contrary, activation time, betwerai 60 to 90 miiiot l ds to a notable gain in the micropore volume only (rise fiom 0.200 to 0.2SS and finally to 0.3SS cm g ). At this point, it must be noted that no maaopores are observed during die activation process for all the samples. 

The pore size distributions of several types of porous solid have been determined from their nitrogen adsorption isotherms using a new analysis method (ref. 1). In contrast to earlier thermodynamic methods (which break down at small pore sizes), the new approach is based on a molecular model of nitrogen adsorption in a pore. The development of this technique means that, for the first time, the distribution of pore sizes may be calculated over both the mesopore and micropore size ranges using a single analysis method. The use of the new method is illustrated with a series of results obtained from carbon, silica and alumina samples. Its predictions are compared with those obtained using established analysis methods. 

Fig. 3. Nitrogen adsorption isotherms at very low relative pressures for microporous carbons 1, Carbosieve S 2, rayon char JF516I 3, woven Kevlar char.

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

Recently, Lee and Pyun have focused on the characterization of pore fractality of the microporous carbon powder specimens by using nitrogen gas adsorption method based upon the D-A adsorption theory in consideration of PSD with pore fractality. Figure 5 envisages the nitrogen gas adsorption isotherm obtained from the as-reactivated carbon powder specimen prepared by reactivation of the commercially as-activated carbon powder at 1000 °C in an atmosphere of C02/C0 gas mixture for 2 h. The solid... 

The recent contribution by Kaneko et al. (1995) has revealed that it is possible to produce highly hydrophobic fluorinated microporous carbon fibres. Two fluorinated carbons were reported to have apparent surface areas of 420 and 340 m2g and micropore volumes of, respectively, 0.19 and 0.14cm3g 1. These materials gave Type I nitrogen and methanol isotherms, but the adsorption of water vapour was too small to measure at pjp° < 0.8 and the uptake was very low even at p/p° 1. 

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