Nitrogen, adsorption

Many types of vacuum adsorption apparatus have been developed and no doubt every laboratory where serious adsorption measurements are made has equipment with certain unique features. The number of variations are limited only by the need and ingenuity of the users. However, all vacuum adsorption systems have certain essential features, including a vacuum pump, two gas supplies, a sample container, a calibrated volume, manometer and a coolant. 

The sample cell is not completely filled by the adsorbent it contains a void volume which is the volume not occupied by adsorbent, up to stopcock 4. If the calibrated volumes and manifold are filled with nitrogen at pressure Pj and stopcock 4 is opened the pressure will fall because of expansion and adsorption of nitrogen in the evacuated cell. After equilibrium is attained and the mercury in the manometer is returned to the fiducial mark the new pressure P is noted. The number of moles adsorbed is given by 

With pressure in atmospheres and all volumes in cubic centimeters, equation (14.3) can be rewritten in terms of the volume adsorbed at standard temperature (273.2 K) and pressure (1.0 atm) 

Using Vq as the sum of the calibration volumes V, V, and the manifold volume and reducing all gas volumes to standard conditions yields the following, 

For subsequent data points stopcock 4 is closed and additional nitrogen is admitted into the calibrated volumes and manifold. When stopcock 4 is opened the pressure will fall from P2 to P 2 and the volume adsorbed under standard conditions is given by 

Here P is the equilibrium pressure during the adsorption measurements, V the adsorbed volume of N2, the monolayer volume and is a constant. 

Following mono-layer uptake, further increase in pressure results in multi-layer adsorption of N2. For this part of the isotherm, condensation-evaporation equilibrium is assumed to take place, instead of adsorption-desorption equilibrium for each individual layer other than the first layer. This dynamic equiUbria for the first and higher layers and some simplifying assumptions form the basis for the B ET treatment of the multi-layer adsorption isotherm. A lengthy derivation leads to the BET relation between adsorbed volume of N2 and relative pressure. Here relative pressure is defined as the ratio of the equilibrium pressure to the 

A linearized form of the BET plot, typically in P/P° range of 0.05 to 0.25, can then be used to calculate V , hence the BET surface area from the slope and intercept  

As progressively higher pressures are used during N2 adsorption, capillary condensation will occur in pores that are increasingly larger. The Kelvin equation illustrates that the equilibrium vapor pressure is lowered over a concave meniscus of liquid, which is why N2 is able to condense in catalyst pores at pressures lower than the saturahon pressure  

Spectroscopic eUipsometry and XRD measurements require large areas of continuous films. In order to directly measure the properties ofpSi particles, it is possible to exploit the principle of gas adsorption [28], with the most commonly employed 

The super-linear region of the isotherm at higher values of relative pressure is used to determine the pore size distribution. As the super-Unear behavior is due to the early onset of condensation in the pores, a mathematical relationship was developed by Barrett, Joyner and Helena (BJH) which equated the change in desorbed volume from one measurement point to the next to a release of condensate from pores of a particular radius, plus the thinning of the adsorbed layer in pores which were already empty [37]. The pore volume is then calculated according to the following equation [37]  

Nitrogen sorptiometry, also referred to as BET method (named after their inventors Brunauer [202], Brunauer and Emmet [203], and Teller and coworkers [204]), is an approach for the determination of the specific surface area of a (porous) support material based on the multilayer adsorption of nitrogen at the temperature of liquid nitrogen (77 K) according to following procedure  

On dry gels, standard characterization techniques for porous media are used, several of which have been described in Volume 2 of this series helium pycnometry for pore volume determination (Section 6.3.1.2) as well as nitrogen adsorption at 77 K for surface area (Section 6.3.2.2, BET method), for microporosity (Section 6.3.3.2, Dubinin-Radushkevich method), for pore size distribution (Section 6.3.3.3, BJFl method), and for total pore volume (Section 6.3.3.4). When characterizing gels by nitrogen adsorption, other methods are also used for data interpretation, for example, the t-plot method for microporosity (Lippens and de Boer, 1965) and the Dollimore-Heal method (Dollimore and Heal, 1964) or Broekhoff-de Boer theory for mesoporosity (Lecloux, 1981). 

Besides contraction of the sample, insufficient equilibration times may lead to wrong interpretation of nitrogen adsorption data (Reichenauer and Scherer, 2001a) because of incomplete filling of pores, the total pore volume is underestimated. In Fig. 5.8, two identically prepared aerogel samples have been characterized with different total duration of the cycle to prove sufficient equilibration. 

The effect of alkali additives on N2 chemisorption has important implications for ammonia synthesis on iron, where alkali promoters (in the form of K or K20) are used in order to increase the activity of the iron catalyst. 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm 1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

The adsorption in the a-N2 state proceeds mainly via previous adsorption in the y-N2 state. Direct adsorption in the a-N2 state corresponds to a rather low sticking coefficient ( 10 3) but is the only adsorption route at higher 

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Fig. XVII-5. Schematic detector response in a determination of nitrogen adsorption and desorption. A flow of He and N2 is passed through the sample until the detector reading is constant the sample is then cooled in a liquid nitrogen bath. For desorption, the bath is removed. (From Ref. 28. Reprinted with permission from John Wiley Sons, copyright 1995.)...

Fig. XVII-27. Nitrogen adsorption at 77 K for a series of M41S materials. Average pore diameters squares, 25 A triangles, 40 A circles, 45 A. Adsorption solid symbols desorption open symbols. The isotherms are normalized to the volume adsorbed at Pj = 0.9. (From Ref. 187. Reprinted with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

Fig. XVII-31. (a) Nitrogen adsorption isotherms expressed as /-plots for various samples of a-FeOOH dispersed on carbon fibers, (h) Micropore size distributions as obtained by the MP method. [Reprinted with permission from K. Kaneko, Langmuir, 3, 357 (1987) (Ref. 231.) Copyright 1987, American Chemical Society.]...

The nitrogen adsorption isotherm is determined for a finely divided, nonporous solid. It is found that at = 0.5, P/P is 0.05 at 77 K, gnd P/F is 0.2 at 90 K. Calculate the isosteric heat of adsorption, and AS and AC for adsorption at 77 K. Write the statement of the process to which your calculated quantities correspond. Explain whether the state of the adsorbed N2 appears to be more nearly gaslike or liquidlike. The normal boiling point of N2 is 77 K, and its heat of vaporization is 1.35 kcal/mol. 

The numerical values of and a, for a particular sample, which will depend on the kind of linear dimension chosen, cannot be calculated a priori except in the very simplest of cases. In practice one nearly always has to be satisfied with an approximate estimate of their values. For this purpose X is best taken as the mean projected diameter d, i.e. the diameter of a circle having the same area as the projected image of the particle, when viewed in a direction normal to the plane of greatest stability is determined microscopically, and it includes no contributions from the thickness of the particle, i.e. from the dimension normal to the plane of greatest stability. For perfect cubes and spheres, the value of the ratio x,/a ( = K, say) is of course equal to 6. For sand. Fair and Hatch found, with rounded particles 6T, with worn particles 6-4, and with sharp particles 7-7. For crushed quartz, Cartwright reports values of K ranging from 14 to 18, but since the specific surface was determined by nitrogen adsorption (p. 61) some internal surface was probably included. f... 

Some typical examples of BET plots are given in Figs 2.5-2.7. Those in Fig. 2.5 for nitrogen adsorption at 90K on various catalysts, taken from the... 

Specific surface area of carbon blacks before and after graphitization, determined by electron microscopy (A,) end by nitrogen adsorption (A ]t... 

Comparison of specific surface of anatase and zinc oxide determined by electron microscopy A ) and by nitrogen adsorption A )... 

Comparison of particle diameter of colloidal silica by electron microscopy (cf,). by nitrogen adsorption (d ) and by light scattering (d,)... 

Examination of these and other results indicates that the value of a for a given adsorptive which needs to be used in order to arrive at a value of specific surface consistent with that from nitrogen adsorption, varies according to the nature of the adsorbent. The existence of these variations shows that the conventional picture, in which the value of a corresponds to a monolayer which is completely filled with adsorbate molecules in a liquidlike packing, is over-simplified. Two factors can upset the simple picture (a) there may be a tendency for adsorbed molecules to become localized on lattice sites, or on more active parts of the solid surface and (b) the process... 

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).

Comparison of surface areas determined by mercury porosimetry and by nitrogen adsorption ... 

Values of specific surface of alumina gels determined by nitrogen adsorption and by mercury porosimetry ... 

When the values of the BET monolayer capacity calculated from Type III isotherms are compared with independent estimates (e.g. from nitrogen adsorption) considerable discrepancies are frequently found. A number of typical examples are collected in Table 5.1. Comparison of the value of the monolayer capacity predicted by the BET equation with the corresponding value determined independently (columns (iv) and (v)) show that occasionally, as in line 6, the two agree reasonably well, but that in the majority... 

Several properties of the filler are important to the compounder (279). Properties that are frequentiy reported by fumed sihca manufacturers include the acidity of the filler, nitrogen adsorption, oil absorption, and particle size distribution (280,281). The adsorption techniques provide a measure of the surface area of the filler, whereas oil absorption is an indication of the stmcture of the filler (282). Measurement of the sdanol concentration is critical, and some techniques that are commonly used in the industry to estimate this parameter are the methyl red absorption and methanol wettabihty (273,274,277) tests. Other techniques include various spectroscopies, such as diffuse reflectance infrared spectroscopy (drift), inverse gas chromatography (igc), photoacoustic ir, nmr, Raman, and surface forces apparatus (277,283—290). 

Pore Volume by Nitrogen Adsorption or Mercury Penetration... 

Liquid-phase adsorption methods are widely used for quaUty control and specification purposes. The adsorption of iodine from potassium iodide solution is the standard ASTM method D1510-83 (2). The surface area is expressed as the iodine number whose units are milligrams of iodine adsorbed per gram of carbon. It is quite fortuitous that the values of iodine numbers turn out to be about the same as the values for surface areas in square meters per gram by nitrogen adsorption for nonporous carbon blacks. 

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). 

Parkyns and Quinn [20] showed a linear relationship between methane uptake at 25 C, 3.4 MPa and the Dubinin-Radushkievich micropore volume from 77 K nitrogen adsorption for porous carbons,... 

Figure 1.5 shows the cumulative pore volume curve for 5-/rm monosized porous PS-DVB particles with 50, 60, and 70% porosity. The curves were drawn by overlapping the measurements from nitrogen adsorption-desorption and mercury intrusion. A scanning electron micrograph of 5-/rm monosized particles with 50% porosity is shown in Fig. 1.6 (87). 

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