Reference adsorbent

MCM-41 samples usually exhibit certain external surface area, so the amount adsorbed on the external surface also needs to be accounted for. This amount can be described as the amount adsorbed on the macroporous reference adsorbent, vref(p/po), multiplied by the MCM-41 external surface area, Sex, divided by the specific surface area, Srer, of the reference adsorbent used to evaluate Sex. Therefore, the model nitrogen adsorption isotherm for the MCM-41 sample with the capillary condensation pressure of pc/po, primary mesopore adsorption capacity of vp>max and external surface area of Sex is described by the following equation ... 

The equilibrium thermodynamic functions describing the retention process are essentially related to the net retention volume. For example, the intermolecular adsorption of Gibbs free energy, —AGa, for one mole of solute vapor from a reference gaseous state with the partial pressure, Po, to a reference adsorbed state with the equilibrium spread pressure (or two-dimensional pressure), no, is given as [115]... 

Eref (J mol ) is the characteristic energy of the reference adsorbate It is related to the average pore diameter H by equation 10 ... 

The adsorption isotherms have been treated using the first method. Using equations 1, 2 and 3 to calculate V and Srcf (the reference adsorbate is N2) and equations 4, 5, 6 and the R-K-S eos for the calculation of Va, f and f, we obtained the reference characteristic curve. It is presented in figure 1. 

Figure 2 Nitrogen as-plots on samples of carbon cloth. Ungraphitized Vulcan carbon black used as the non-porous reference adsorbent.

Carbon blacks with specific surface areas of up to 100 m /g can be regarded as essentially nonporous [15] since they give reversible Type II isotherms in the lUPAC classification [10]. Early physisorption measurements on carbon blacks [1] were designed to test the validity of surfiice areas determined the BET method [6]. Carbon blacks were considered [5] to be especially suitable for this purpose because the discrete nature of their spheroidal particles allowed electron microscopy to be used for the evaluation of the particle size distribution. Certain well-characterized carbon blacks are still extremely useful as reference adsorbents [11, 16]. 

Here, denotes the maximum amount adsorbed in the micropores, and a° denotes the monolayer capacity of the mesopore surface. For the standard adsorption isotherm measured on a nonporous reference adsorbent, we have [73]... 

Here, 6s(p/p is the relative surface coverage for a nonporous reference adsorbent, a,(p/pc) is the amount adsorbed on the surface of this refference adsorbent at relative pressure p/po, cfs denotes the monolayer capacity evaluated from the standard adsorption isotherm, and a,(0.4) denotes the amount adsorbed on the surface of the reference solid at relative pressure p/pg = 0.4. 

The amount % adsorbed per unit surface area was evaluated from the adsorption isotherm measured on a reference adsorbent, whereas the specific surface area S of mesopores was estimated from the adsorption isotherm [114]. 

A reference non-porous material with similar surface characteristics is chosen to obtain the information on the statistical film thickness as a function of the reduced pressure. The t values for different reference adsorbents as a function of the reduced pressure are available in literature, normally in the table form or a best fit equation. 

It is seen in Eq. (29) that the DR equation requires two types of parameters (1) properties of the adsorbent-micropore volume Vo and characteristic energy Eq-, (2) properties of the adsorbate — saturation pressure Pg, affinity coefficient P, and liquid molar volume. Principally, the first group of parameters are temperature-independent and can be obtained from the adsorption of a reference adsorbate on the same adsorbent, while the second group of parameters are temperature-dependent but can be simply estimated from the thermodynamic properties of each species. Therefore, a limited set of data is required to describe binary adsorption over a wide range of pressure and temperature. Thus the advantages of using the DR equation deserve to be exploited. 

The detailed examination of the MCM-41 porous structure was based on analysis of adsorption branch of isotherms using the method of comparison plots [11]. In this method the amount adsorbed on the solid under investigation (a) is plotted against that adsorbed on a reference adsorbent (a O at the same equilibrium pressure. In this study the adsorption in the pores of MCM-41 was compared with that on the open flat surface of the reference solid material. It was prepared by the thermal destruction of corresponding MCM-41 samples at 1000°C for 2 hours, / s these reference adsorbents had different surface area the adsorption on them was... 

The system of retention indices is also used for characterization of adsorbent selectivity (see Table 3-4). For example. Table 3-4 [28] provides values for McReynolds constants [29] at 200 °C using Chromasorb 106 as the reference adsorbent. The data in parentheses were obtained after aging the columns for 22 days. Polarity of polymer adsorbent changes greatly as follows from the data of Table 3-4. 

To extract information about microporosity of a solid from the characteristic energy E, Dubinin (ref. 1) proposed the use of a reference adsorbate (benzene). His experimental studies (ref. 1) showed that for different activated carbons the ratio of the characteristic energy E of a given adsorbate to the characteristic energy of the reference adsorbate is nearly constant, l.e., B = E/E , where fl is the so-called affinity (similarity) coefficient. Also, Dubinin (ref. 8) proposed the following empirical... 

Dubinin et al. evaluated the amount surface area from the benzene isotherm measured for the nonporous reference adsorbent /7/, we, however estimated the specific surface area Sme of the mesopores from the adsorption isotherm studied /8/, In calculations of the mesopore size distribution and the specific surface area Sme it has been assumed that the parallel - sided slits are rigid and the size distribution does not extend continuously from the mesopore into both the macropore and micropore range. We have used the desorption branch of the hysteresis loop of the isotherm for the computation. The procedure of B.F. Roberts /9/ has been applied. In this computation, which is a rigorous application of the concept of simultaneous capillary condensation and multilayer adsorption, the adsorbed volume is first expressed as a function of pore size then it is converted to pore volume. A standard t - curve /lO/, which represents the benzene adsorption onto nonporous carbon blacks, has been used for correction for multilayer thickness. 

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