Nitrogen adsorption cross-sectional area

In either of the preceding cases, very high or very low C values, any attempt to calculate the effective adsorbate cross-sectional areas from the bulk liquid properties will be subject to considerable error. Nitrogen, as an adsorbate, exhibits the unusual property that on almost all surfaces its C value is sufficiently small to prevent localized adsorption and yet adequately large to prevent the adsorbed layer from behaving as a two-dimensional gas. 

An early normalizing procedure, proposed by Kiselev (1957) to compare adsorption isotherms of hydrocarbons, water vapour, etc. on a series of different adsorbents, was simply to plot the surface excess concentration F (=n/A), obtained from a knowledge of the BET-nitrogen surface area, A (BET), versus p/p°. It is also possible to plot, instead of f, the reduced adsorption , n/nm, which still relies on the BET method to determine the monolayer capacity nm but does not require knowledge of the molecular cross-sectional area a. 

Nitrogen adsorption/condensation measurements were performed using an Autosorb-1 analyzer to calculate sample surface area and pore size distribution. BET analysis at 77 K was applied for extracting the monolayer capacity from the adsorption isotherm and a N molecular cross-sectional area of 0.162 nm2 was used to relate tne monolayer capacity to surface area. PSD s were calculated from the desorption branches of the isotherms using a modified form of the BJH method [18]. Mercury intrusion measurements were performed using an Autoscan-33 continuous scanning mercury porosimeter (12-33000 psia) and a contact angle of 140°. 

The measurements of external and internal specific surface area have already been discussed in Chapter 1, Section 1.1.3. The principles and the isotherm equation of the BET method to measure external specific surface area, including macro- and mesopores, have been presented in Chapter 1, Section 1.3.4.1.5. The external specific surface area is usually determined by nitrogen gas adsorption at the temperature of liquid nitrogen. Both static (one-point) and dynamic (five-point) methods are applied. The calculations are made by Equation 1.75 (Chapter 1), using one or five different pressure values. The external specific surface area is calculated from the maximum number of surface sites, that is, monolayer and the cross-sectional area of nitrogen molecules. 

Adsorption/desorption isotherms of nitrogen at 77 K were measured with an automated apparatus ASAP 2010 (Micromeritics, USA). The specific surface areas, Sbet, were calculated from the linear form of the BET equation, taking the cross-sectional area of the nitrogen molecule to be 16.210 m. Pore size distributions were calculated in the standard maimer by using BJH method [6]. The total pore volumes, Vp, for the samples under study were determined from a single point adsorption at a relative pressure of 0.98 by converting the value of the adsorbed gas to the volume of the liquid adsorbate. 

X-ray powder diffraction (XRD) patterns were taken on a Spectrolab CPS Series 3000 120 diffractometer, using Ni filtered Cu Ka radiation. The nitrogen adsorption isotherms were determined at 77 K by means of a Micromeritics Gemini 2370 surface area analyser. Surface areas were derived from the BET equation in the relative pressure range 0.05-0.25, assuming a cross-sectional area of 0.162 nm" for the nitrogen molecule [ 18]. 

Specific surface area and pore volume distribution were measured by nitrogen adsorption in an Accusorb 2100E Micromeritics adsorption analyzer. The data were interpreted using the BET equation, assuming a cross-sectional area of 16.2 for Nj. 

It has to be stressed that the monolayer surface phase capacity is assumed to be constant over the whole bulk concentration region, i.e., n = const., for x (0,1). Under this assumption we can assess the specific surface areas of the solid adsorbents if the cross - sectional areas of adsorbed molecules are known. However, the following question arises here what molar areas to assign to the different kinds of molecules This problem is similar in the case of gas - solid adsorption and it may be sufficient to refer to the compilation by McLellan and Harnsberger [13]. It has been found that cross - sectional molar areas calculated by means of the molar volumes of the pure components are mostly in agreement with nitrogen surface area values [14]. 

Physisorption measurements were performed with a Micromeritics ASAP 2000 instrument. The BET surfoce areas were determined by nitrogen adsorption at 77 K assuming a cross-sectional area of 0.162 nm for the nitrogen molecule. Before the adsorption measurements the samples were outgassed in vacuum at 423 K for 18 h. 

Some criticism can be made of the assumptions of the B.E.T. adsorption model. If the second and other layers are assumed to be in the liquid state, how can localized adsorption take place on these layers Also, the assumption that the stacks of molecules do not interact energetically seems to be unrealistic. In spite of these theoretical weaknesses, the B.E.T. adsorption expression is very useful for qualitative application to type II and III isotherms, the B.E.T equation is very widely used in the estimation of specific surface areas of solids. The surface area of the adsorbent is estimated from the value of Vm. The most commonly used adsorbate in this method for area determination is nitrogen at 77 K. The knee in the type II isotherm is assumed to correspond to the completion of a monolayer. In the most strict sense, the cross-sectional area of an adsorption site, rather than that of the adsorbate molecule, ought to be used, but the former is an unknown quantity however, this fact does not prevent the B.E.T. expression from being useful for the evaluation of surface areas of adsorbents. 

Brinker and Scherer (8) pointed out that the area of a surface is defined largely by the method of surface area measurement. Many of the measurements of surface areas in work reported before the 1980s were based on the method of determining monolayer capacity of an adsorbent molecule of known cross-sectional area. In the Brunauer-Emmett-Teller (BET) method (45) the apparent surface area is determined from nitrogen adsorption. However, because the nitrogen molecule surface area is 16.2 A2, this definition of the surface excludes microporosity that is accessible, for example, to water molecules. 

Gas adsorption is the preferred method of surface-area determination. An isotherm is generated of the amount of gas adsorbed against gas pressure, and the amount of gas required to form a monolayer is determined. The surface area can tTien be calculated using the cross-sectional area of the gas molecule. Outgassing of the powder before analysis should be conducted very carefully to ensure reproducibility. Commonly, nitrogen at liquid nitrogen vapor pressure is used but, for low surface-area powders, the adsorbed amounts of krypton or xenon are more accurately found. Many theories of gas adsorption have been advanced, but measurements are usually interpreted by using the BET theory [Brunauer, Emmett, and Teller, J. Am. Chem. Soc., 60,309 (1938)]. 

A key material property of powders and fibers in particular is the specific surface area. The extent of adsorption from the vapor and liquid states on a solid surface is determined in part by the specific surface area of the solid. Typically, to determine the specific surface area, a gas adsorption isotherm is measured for example, the adsorption of nitrogen is measured on the substrate of interest at 77 K, the boiling point of nitrogen. The experimental isotherm is then analyzed by the BET (Brunauer, Emmett and Teller) model [42,43] to determine the monolayer capacity of the substrate. The specific monolayer capacity multiplied by the cross-sectional area of the adsorbed gas molecule gives the specific surface area. Amorphous silica gel may have a specific area of 200-300 mVg while carbon fibers may have a value of around 0.1 mVg. 

Total smface areas were measured by nitrogen adsorption at -196 C, using an automated instrument (Omnisorp lOOCX, Coulter Electronics Limited). The cross sectional area of the nitrogen molecule was assumed to be 16.2 x 10 m. Pore type and volume data were also obtained by this method, using t-plot analysis. Metal areas were measured by selective chemisorption of hydrogen at 30 °C in the same instrument. Copper surface areas were measured in a flow system by nitrous oxide chemisorption at 60 C. 

The surface area of the samples was measured by nitrogen adsorption at 77K in a Hicromeritics Digisorb 2500 apparatus, using 0.162 nm as cross-sectional area of the adsorbed nitrogen molecule. 

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