Isotherms of nitrogen and argon

Fig. 23. Adsorption isotherms of nitrogen and argon on reduced polycrystalline copper. (After Rhodin, J. Am. Chem. Soc. 72, 4343 (1950).]...

Figure 2. Adsorption isotherms of nitrogen and argon on iron...

Although commercially important, the precipitated silicas have received much less attention in the scientific literature than either the Aerosils or silica gels. In certain respects they are similar to pyrogenic silicas indeed, at one time they were treated as alternative non-porous silicas. Thus, the reversible Type II isotherms of nitrogen and argon obtained by Basset et al. (1968) were assumed to represent uncomplicated monolayer-multilayer adsorption. More recent work (Carrott and Sing, 1984) has shown that the Type II character is here the result of adsorption both on the external surface and within some micropores. 

Essential progress has been made recently in the area of molecular level modeling of capillary condensation. The methods of grand canonical Monte Carlo (GCMC) simulations [4], molecular dynamics (MD) [5], and density functional theory (DFT) [6] are capable of generating hysteresis loops for sorption of simple fluids in model pores. In our previous publications (see [7] and references therein), we have shown that the non-local density functional theory (NLDFT) with properly chosen parameters of fluid-fluid and fluid-solid intermolecular interactions quantitatively predicts desorption branches of hysteretic isotherms of nitrogen and argon on reference MCM-41 samples with pore channels narrower than 5 nm. 

The gas adsorption measurements by Zettlemoyer and co-workers (13, 14) appeared to indicate that some precipitated silicas (e.g., HiSil 233 from Pittsburgh Plate Glass Company) behaved as nonporous adsorbents. Thus, reversible Type II isotherms of nitrogen and argon were obtained by Bassett et al. (14), who concluded that unrestricted monolayer-multilayer adsorption had occurred. More recent work (15) showed that this interpretation is probably an oversimplification of the physisorption process. 

Isotherms of nitrogen and argon were measured at 77-87 K at two distinct ranges ... 

It offers a simple but effective means of testing for the identity in shape of the isotherms of any suitable adsorptive on a given set of samples of the same substance. In view of the widespread use of nitrogen and argon in surface area and porosity studies, data for the construction of the standard a5-curves for these adsorbates on hydroxylated silica are given in tables 2.3 and 2.4. 

The following discussion of some typical data for the adsorption of nitrogen and argon will throw light on these questions and also illustrate die value of using the as-method of isotherm analysis. 

Figure 12.11. Nitrogen isotherm and differential enthalpy of adsorption of nitrogen and argon on AlP04-5 crystals (MOller e al 1989).

Equation 5 is analogous to Eq. 2 discussed above. As illustrated in Figure 4b, the mesopore size distributions determined from nitrogen and argon adsorption data were essentially the same, when calculations were carried out for adsorption branches of isotherms using the corresponding KJS-calibrated relations (Eqs. 2 and 5) [18]. 

On an empirical basis, this Anderson-Brunauer equation can be applied to some isotherms (e.g. nitrogen and argon at 77 K on various non-porous oxides) over a much wider range of pjp° than the original BET equation. 

Pore filling model comparisons have also been reported for other porous solids. The inside pore diameters of MCM-41 type adsorbents have been calculated to a high degree of consistency from nitrogen and argon porosimetry (Fig. 4) by using a DFT model of gas adsorption in cylindrical oxide pores to interpret the experimental isotherms... 

Textural characterization. The nitrogen and argon isotherms were obtained at liquid nitrogen and liquid argon temperature by using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples ( 0.2 g) were outgassed for 16 h at 350 °C under vacuum. 

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