Adsorption of nitrogen on silica

Data for the adsorption of nitrogen on silica-alumina tabulated. V is in cc/g at STP. Find the specific surface by problem P6.01.02. 

The Isosteric Heat of Adsorption. The value of the heat of adsorption is abnormally high in micropores. Sing and Ramakrisbna (173) found that by careful choice of adsorbates and application of the method of analysis, it is possible to distinguish between capillary adsorption and adsorption on high-energy sites. They showed that from p/po 0.01 to 0.2 the isosteric heat of adsorption of nitrogen on silica free of mesopores remains essentially constant at 2.0 kcal mole. On mesoporous gel it drops from 2.3 to 2.0 and on microporous gel from 2.7 to 2.0. The isosteric heat q t is calculated from the adsorption isotherm by the Clausius-Clapeyron equation. 

Figure 28.12 Adsorption of nitrogen on silica as a function of nitrogen pressure (relative to the nitrogen saturation pressure po) at 77 K follows the BET model. Source PC Heimenz, Principles of Colloid and Surface Chemistry, M Dekker, New York (1 977). Data are from DH Everett, CD Parfitt, KSW Sing and R Wilson, J AppI Chem Blotechnol 24, 1 99-21 7 (1 974). 

Calculate the rotational contribution to the entropy of adsorption of ammonia on silica at -30°C, assuming (n) that the adsorbed ammonia retains one degree of rotational freedom and (b) that it retains none. In case (n) assume that the nitrogen is bonded to the surface. 

Figure 5.13 Isotherm and BET plot for the multilayer adsorption of nitrogen on a non-porous sample of silica gel at 77 K...

Fig. 12.3. The adsorption of nitrogen on nonporous samples of silica and alumina, the surface areas ranging from 2.6-11.5 m2 g 1 for silica and from 58-153 m2 g I for alumina. n/nm is plotted against p/p°. n is the number of moles adsorbed per gram of adsorbent and nm is the monolayer capacity, expressed in the same units. Line A = silica + = alumina. Line B the BET isotherm...

Type III is relatively rare—a recent example of nitrogen on ice and seems to be characterised by a heat of adsorption equal to or less than heat of liquifaction of the adsorbate. The example of Type II isotherms are furnished by adsorption of nitrogen on an iron or a platinum catalyst at -195 C and those Type III by adsorption of bromine on silica or alumina gas at 80 C. 

The isotherms and corresponding as-plots in Figures 10.8 and 10.9 are for the adsorption of nitrogen on representative mesoporous and microporous silica gels (Bhambhani et al., 1972). The derived values of the specific surface area are given in Table 10.8. The values of the BET nitrogen area, a(BET), in Table 10.8 are based on the usual assumption that the adsorbed molecules were close-packed in the completed monolayer (i.e. a(N2) 0.162 nm2). The corresponding values of a(S, N2) were calculated from the initial slope of the as-plots by die relation... 

Low-temperature nitrogen adsorption is normally used for the determination of surface area and pore size distribution of porous materials. However, specific field-gradient quadrupole interactions play a significant role in the adsorption of nitrogen on hydroxylated silicas or other polar surfaces (7). Accordingly, some authors (14> 27) have proposed that a nonpolar adsorptive such as argon should be used instead of nitrogen for the determination of surface area. 

Figure 2.1 An apparatus for measuring gas adsorption and some typicai resuits for the multilayer adsorption of nitrogen on a non-porous sample of silica gel at 77 K. (Adapted from Shaw [16]. Copyright (1966), reproduced with permission of Butterworths.)...

To compare different solids, it is convenient to use a term the reduced pressure, which is defined as v, = v/vo.< where Vo.4 is the volume of gas adsorbed at P/Po == 0.4. Carruthers et al. (20) compared the adsorption of nitrogen on four types of silica powders plotting v/v versus v, (where v is the monolayer value from the BET equation) and found direct proportionality. 

Calorimetric studies of microporosity were made by Cancela et al., who measured the heat liberated during the adsorption of benzene on silica. They verified that the energy of adsorption is highest in micropores and measured the area still open to adsorption of nitrogen at various stages of adsorption of benzene (174). 

Figure 7.2 (a) Comparison of two adsorptions that of nitrogen on silica is a physical BET adsorption at 77 K (curve 1), while oxygen s adsorption on charcoal at 150 K (curve 2) is a chemical Langmuir adsorption. Notice that the y-axis is the pressure divided by the saturation pressure, (b) Adsorption isotherm for nitrogen on silica gel in the linearized form of the BET equation, Equation 7.2. From Shaw (1992)... 

Fig. 3.16 Adsorption-desorption isotherm of nitrogen on silica gel empty circles, adsorption black circles, desoiption.

FIGURE 11.24 Experimental measurements of the adsorption of nitrogen on soUd silica gel at 77 K, the NBP of nitrogen [25]. The curve is a simple smooth interpolation. 

Fig. 2.8 The adsorption of nitrogen at 77 K on nonporous samples of silica and alumina, ranging from 2-6 to 11-5 m g for silica and from 58 to 153m g for alumina. n/n is plotted against pjp°. (A) O, silica , alumina. (B) BET isotherm (Equation 2.12) with c values of 100-2(X).

Standard data for the adsorption of nitrogen at 77 K on nonporous hydroxylated silica ... 

Fig. 230 Adsorption of nitrogen at 77 K on a silica powder a) adsorption isotherms b) /-plot. Broken line, uncompacted powder continuous line, power compacted at 2-00 x 10 N m (130 ton in ). (—>—) adsorption (—<-) desorption. / is the ratio of the amount adsorbed on the powder to the amount adsorbed on the compact at the same relative...

Fig. 3.26 Comparison plots for compacts of silica and magnesia. In each case the adsorption of nitrogen at 78 K on the compact is plotted against that on the uncompacted powder, (a) and (b), comparison plot and adsorption isotherm for silica powder compacted at 130 ton in (c) and (d), comparison plot and adsorption isotherm for precipitated magnesia compacted at 10 ton in. Note that the upward sweep of the comparison plot commences at a relative pressure below the inception of the loop.

Data for the adsorption of nitrogen at its atmospheric boiling point on 1.09 g silica gel are correlated by the BET equation as... 

Type II isotherms (e.g. nitrogen on silica gel at 77 K) are frequently encountered, and represent multilayer physical adsorption on non-porous solids. They are often referred to as sigmoid isotherms. For such solids, point B represents the formation of an adsorbed monolayer. Physical adsorption on microporous solids can also result in type II isotherms. In this case, point B represents both monolayer adsorption on the surface as a whole and condensation in the fine pores. The remainder of the curve represents multilayer adsorption as for non-porous solids. 

The first systematic investigations of the adsorption of gases on dehydroxylated silicas were made by Kiselev and his co-workers (Kiselev, 1957,1958). In a study of the adsorption of argon and nitrogen, Aristov and Kiselev (1965) found that, in contrast to nitrogen, the reduced argon isotherm did not appear to depend on the degree of surface hydroxylation. 

The following equilibrium data have been obtained for the adsorption of nitrogen dioxide, N02> on silica gel at 25°C and 1 atm ... 

Adsorption of oxygen, argon, and nitrogen on silica gel was measured and found to be very small compared to the adsorption of ozone. 

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