N2 adsorption data

This difference between H2O and N2 adsorption data has been attributed to either the accessibility of water to... 

Pore size obtained by Horvath-Kawazoe analysis of N2 adsorption data. 

Porosity parameters of the samples estimated from N2 adsorption data... 

Porosity and pore size distributions, both before and after surface modification, are analyzed from N2 adsorption data. Both capacity as well as adsorption free energies are obtained from metal ion adsorption isotherms from aqueous solution as described in the experimental section. 

We thank Dr. Steve Augustine for collection of the N2 adsorption data. JPB and VMG thank NATO for funding of the Advanced Research Workshop where this work was presented. This work was supported in part by a grant from the Petroleum Research Fund administered by the American Chemical Society. 

A t-plot 5 of the N2 adsorption data, shown Figure 4, exhibited three distinct regions. In the first region, the data were fitted to a straight line passing through the origin. The slope of this line yielded an equivalent... 

If we draw the characteristic curve for Nj at 77 K and CO2 at 273 K (at subatmospheric and high pressures), both adsorptives fit the same curve for both materials for (A/p) values lower than 150 KJ/mol, clearly indicating that both adsorptives follow the same adsorption mechanism. If the N2 adsorption data obtained at low relative pressures (10 to 10 ) are plotted it is clear that the resulting characteristic curve falls below the one corresponding to CO2. This observed diffusional problems of N2 molecules to enter a part of the porosity of MCM-41 at 77 K must be due to the presence of narrow micropores (<0.7 nm) which are accessed by CO2 at 273 K (0.14 cm /g micropore volume from the DR equation). Considering that working with N2 at very low relative pressures needs the use of relatively expensive equipment and extreme conditions, and that N2 at 77 K presents diffusional problems, it is clear that CO2 works better for characterizing micropores. 

Figure 4. Particle diameter d determined by TEM ( ) and N2 adsorption data ( ) and aggregate diameter d determined by TEM (A) and N2 adsorption data ( ) versus k. Xerogels are represented by full symbols and aerogels by open symbols...

The CO2 and N2 adsorption data, commented above, have been used to calculate the micropore volume of the materials. Table 3 contains the micropore volumes obtained for the series of zeolites A and B. 

Figure 17.4 Characteristic curves for an activated carbon fiber (ACF) that includes the N2 adsorption data at 77 K (relative pressure from 10 to 1) ( ) and the CO2 adsorption data at 273 K at subatmospheric pressures ( ).

Figure 4. Plot of the N2 adsorption data for a TSLS complex according to the Langmuir equation.

Other researchers [68, 69] have reached similar conclusions. In particular, studies on freeze-dried organic gels led to A 2.55 when the FHH isotherm equation was applied to N2 adsorption data and A > 2.6 when SAXS was used [68]. For three shale samples, Ma et al. [69] found that the A values obtained from N2 adsorption data were significantly lower than those obtained from SANS experiments. The authors [69] suggested that the discrepancies were due to the different properties of the dense liquid phase in the small and large pores and to the different volumes that were occupied by the adsorbed film with respect to that probed by SANS. In conclusion, while SANS (and SAXS) sees the total porosity of a system (including inaccessible pores), adsorbate molecules can only penetrate pores that are both larger than their molecular diameter and accessible. 

Figure 6.2 (a) Values of surface fractal dimensions D, obtained from N2 adsorption data from the authors (diamonds), from [30] (circles), and from [95] (stars), (b) Average adsorption energies of water vapor (data from [96]) on various monoionic forms of montmorillonite as a function of the cation charge Z. Reproduced by permission of the Polish Academy of Sciences. 

Figure 6.4 Correlations between the fractional clay content and the surface fractal dimension A obtained from H2O and N2 adsorption data of several alluvial soils [109]. Reproduced by permission of Elsevier.

Though the selection of an adsorption pressure is most often dictated by economics and cycle conditions, an optimum or near optimum pressure should be selected if at all possible. Based on preliminary calculations, the existence of a pressure at which adsorptive capacity might be maximum was first suggested by Ziegler [8] and at least partially verified in the N2 adsorption data reported by Johnson [1]. 

According to these results, and many others published in the literature [20,23,35,38-44,49-97], to increase the adsorption capacity of an AC high hydroxide/carbon ratios need to be used. However, in addition to the increase in surface area and micropore volume, it is also important to analyze the effect on the MPSD. Eigure 1.11 presents the MPSD calculated by applying the Dubinin-Stoeckli (DS) equation [10,11] to the N2 adsorption data. The higher the KOH/anthracite ratio, the wider the pore size distribution and the higher the mean pore size. These MPSD curves agree with what can be deduced from the difference in the micropore volumes calculated from N2 and CO2 adsorption data. 

FIGURE 1.11 MPSD calculated by applying the Dubinin-Stoeckli (DS) equation to the N2 adsorption data for samples prepared with different KOH/anthracite ratios (redrawn from Lozano-Castello, D., Cazorla-Amoros, D., Linares-Solano, A., and Quinn, D.F. Carbon 40(7) 989-1002, 2002. With permission). 

From their N2 adsorption data, Jaroniec et al. (1991) determined the pore sizes of PAN-based and cellulose-based ACFs, and found that 85% of the pore volumes were composed of uniform miCTopoies with a diameter 10 A. In fact, the pore sizes of ACF in all reported Uteratuie are 10 A. 

Calculation of the micropore volume from the CO2 adsorption data can be done using the commonly applied Dubinin-Radushkevich (DR) equation, while the /-method is used to derive this information from the N2 adsorption data. Expression of the micropore eontribution should preferably be done as a micropore volume. The reason for this is because the micropore surface area has no physical meaning if micropore filling is taking plaee and the cross sectional area of the adsorbed molecule caimot unambiguously be assigned. 

Fig. 2. PSD obtained from N2 adsorption data applying DFT model and t-plot with different refoence materials.

In this work the data measured in the low relative pressures range (p/po< 0.003) were used to fit the DR equation to the CO2 experimental data of adsorption. The micropore volumes obtained are compiled in Table 1. When the micropore volumes obtained by the DR fitting of the CO2 adsorption isotherms, over the aforementioned range of relative pressures, were compared with the narrow microporosity from the N2 adsorption data, a better agreement was obtained than in the case of the application of the DA equation to the CO2 adsorption data (see Fig. 3). 

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