Isotherms of nitrogen adsorption

Some information on adsorption potential of adsorbents with bonded fullerene molecules it is possible to obtain from the nitrogen adsorption isotherms. On Fig. 5 the isotherms of nitrogen adsorption on both adsorbents are presented. These isotherms coincide because of the C constant of BET equation practically the same (C constants of BET equation are 64 and 67 respectively). These values are much smaller than C on graphitized carbon black (C is 1000) and a little smaller than on fullerene crystals (C is 90). So the adsorption potential of such adsorbent is not very high. 

Figure 5. The isotherms of nitrogen adsorption at 77 K on aminosilica gel (filled dots) and on Silasorb Amin (open dots) with bonded fullerene Ceo-...

Fig. 9. Isotherms of nitrogen adsorption at 77 K in different plots for three ACFs. Courtesy of Prof. K. Kaneko of Chiba Univ.

Figure 14. Isotherm of nitrogen adsorption and desorption (left side) and pore-size distribution functions of sample A5 (right side).

FIGURE 33.3 Isotherms of nitrogen adsorption(-F)-desorption( ) for sample 8 (a) and sample 9 (b). 

FIGURE 38.2 (a) Isotherms of nitrogen adsorption-desorption (77.4 K) on fumed silicas of the first series and (b) the as plots for adsorption normalized by dividing by the BET monolayer capacity a. ... 

Until recently, catalyst coking has been generally considered as a harmful side process that causes catalyst deactivation and, in some cases, catalyst destruction [1, 2J. However, catalyst coking (carbonization) is reported in many studies as intentionally performed in order to obtain new systems with useful properties. Thus, alumina was covered by a carbon layer to prepare more inert carriers for desulfurization catalysts [3]. R. Leboda et (see, e.g.,[4]) showed a good performance of sibca with partially carbonized surface as chromatographical adsorbent. Carrott and Sing [5] used carbonized silica to obtain standard isotherms of nitrogen adsorption. 

Figures 1 and 2 illustrate two key signatures of wholly microporous silicon the absence of any mesopores (>2 nm width) by high-resolution transmission electron microscopy (see handbook chapter Microscopy of Porous Silicon ) and the absence of any hysteresis in a type I isotherm of nitrogen adsorption and desorption in a material of very high surface area (see handbook chapter Gas Adsorption Analysis of Porous Silicon ).

No serious attempts have been made to interpret adsorption data correctly. Low values of surface areas of this series of porous carbons comes from the inappropriate use of the isotherm of nitrogen adsorption at 77 K. Incidentally, all isotherms should be made available in all publications. The low values are associated with slow-activated diffusion effects in microporosities of dimensions <0.5 nm. High values of surface area ( 1200m g ) result from multilayer adsorption in the wider of the micropores and in porosities of indefinite shape, approximating to cylinders, cones, spheres, etc. Hence, it seems unreasonable to relate such adsorption processes to monolayer adsorption on surfaces of graphitic microcrystallites. 

Fig. 2.29 Comparison of nitrogen adsorption at 78 K on a carbon black (Sterling FT) before and after graphitization (a) The amount adsorbed on the ungraphitized sample plotted against the amount x, adsorbed on the graphitized sample, at the same pressure, b) The corresponding isotherms O, adsorption, , desorption on the ungraphitized sample (4 runs) A. adsorption A desorption, on the graphitized sample (4 runs).

Fig. 3.3 Adsorption isotherm of nitrogen at 77 K on halloysite. Open circles, adsorption solid circles, desorption. ...

Fig. 3.4 Compaction of alumina powder. Isotherms of nitrogen at 77 K, on (A) the uncompacted powder, and (B) on the powder compacted at a pressure of 1480 GN (96 ton in" ). Open symbols, adsorption solid symbols, desorption.

Fig. 3.20 Pore size distributions (calculated by the Roberts method) for silica powder compacted at (A) Ibtonin" (B) 64tonin (C) 130 ton in". The distributions in (a) were calculated from the desorption brunch of the isotherms of nitrogen, and in (h) from the adsorption branch.

Fig. 3.22 Adsorption isotherms of nitrogen at 77 K on silica powder and its compacts. (A) uncompressed (B) 10 ton in (C) 40 ton in" (D) 50 ton in (E) 100 ton in . Open symbols represent adsorption, solid symbols desorption. (Courtesy Ramsay.)...

Fig. 4.13 The pre-adsorption method (a) adsorption isotherms of nitrogen at 77 K on a sample of Mogul I carbon black charged with different amounts x of pre-adsorbed nonane. Values ofx (mg g (A) 63 (B)48 (C) 29 (D) 16 (E) 0. (See Table 4.5.) (Some points at low pressures omitted for the sake of clarity.)...

Fig. 4J2 Adsorption isotherms of carbon tetrachloride (at 298 K) on ammonium phosphotungstate compact, (1) before, (2) after preadsorption of n-nonane. (3) is the isotherm of nitrogen, after preadsorption, for reference. Open symbols, adsorption solid symbols,...

It is, however, possible to use a Type 111 isotherm of an adsorptive G, say, on a solid S for the evaluation of the specific surface of S, provided a standard sample of the solid is available to enable one to construct a standard a,-curve of G on S. The area of the standard sample must be known, usually from the nitrogen isotherm. 

Fig. 5.14 Adsorption isotherms of water on carbon in (a) to f) with corresponding isotherms of nitrogen in (a), (c) and (J), and of benzene in (f>). (a) Charcoal (b) active carbon AY8 (c) charcoal A (J) charcoal (e) a coal tar pitch kilned at 1200°C (/) a charcoal (S600H). (Redrawn from the diagrams in the original papers.)...

Surface areas are deterrnined routinely and exactiy from measurements of the amount of physically adsorbed, physisorbed, nitrogen. Physical adsorption is a process akin to condensation the adsorbed molecules interact weakly with the surface and multilayers form. The standard interpretation of nitrogen adsorption data is based on the BET model (45), which accounts for multilayer adsorption. From a measured adsorption isotherm and the known area of an adsorbed N2 molecule, taken to be 0.162 nm, the surface area of the soHd is calculated (see Adsorption). 

Surface areas were determined from the adsorption isotherms of nitrogen at 77 K, using a Micromeritics ASAP 200 instrument. Powder X-ray diffraction patterns were obtained with a CGR theta 60 instrument using CuKa monochromated radiation. Reducibility and the amount of Cu species were determined by temperature programmed reduction (TPR) with H2 (H2/Ar 3/97, vol/vol). The experimental set up has been described previously [6]. 

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. 

Characterization. The high resolution TEM images were obtained on a JEOL 2010 electron microscope with an acceleration voltage of 200 kV. Measurement of nitrogen adsorption-desorption isotherms was performed on a Micromeritics ASAP... 

However, there is another consideration which has worried this author as early as 1937 when he and his coworkers measured the adsorption isotherm of nitrogen on nickel and iron at liquid oxygen temperature and found that the isotherm was essentially flat between 10-4 and 10 l mm. Hg pressure. It was about this time that the Emmet-Brunauer method was first made public, and off hand the two results appeared to be irreconcilable. This uncertainty was aggravated by the fact that the nitrogen adsorption isotherm as published by the author and his Coworkers could at that time not be extended to pressures higher than... 

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