BET adsorption

What information can be obtained from a BET adsorption isotherm ... 

Conventional bulk measurements of adsorption are performed by determining the amount of gas adsorbed at equilibrium as a function of pressure, at a constant temperature [23-25], These bulk adsorption isotherms are commonly analyzed using a kinetic theory for multilayer adsorption developed in 1938 by Brunauer, Emmett and Teller (the BET Theory) [23]. BET adsorption isotherms are a common material science technique for surface area analysis of porous solids, and also permit calculation of adsorption energy and fractional surface coverage. While more advanced analysis methods, such as Density Functional Theory, have been developed in recent years, BET remains a mainstay of material science, and is the recommended method for the experimental measurement of pore surface area. This is largely due to the clear physical meaning of its principal assumptions, and its ability to handle the primary effects of adsorbate-adsorbate and adsorbate-substrate interactions. 

Specific surface areas are then obtained by dividing by the weight of catalyst employed in the experiments in question. It should be pointed out, however, that it is the BET adsorption isotherm that is the basis for conventional determinations of catalyst surface areas. (See Section 6.2.2.)... 

The surface area increase during refining is also easily demonstrated experimentally. The BET adsorption isotherm, for example, shows that there is an approximately 250% increase in specific... 

Brunauer-Emmett-Teller (BET) adsorption describes multi-layer Langmuir adsorption. Multi-layer adsorption occurs in physical or van der Waals bonding of gases or vapors to solid phases. The BET model, originally used to describe this adsorption, has been applied to the description of adsorption from solid solutions. The adsorption of molecules to the surface of particles forms a new surface layer to which additional molecules can adsorb. If it is assumed that the energy of adsorption on all successive layers is equal, the BET adsorption model [36] is expressed as Eq. (6) ... 

Figure 4.42 Calculated mesopore size distribution for the steamed Y-zeolite based on the BET adsorption. The ammonium exchanged Y-zeolite has no mesopores.

As the effects of an annealing step on the properties of cellulose acetate membranes, the increase of crystallinity by means of the X-ray diffraction method (12, 13) and the changes of pore sizes by the BET adsorption method l5) have been reported. 

Fig. 5 (a) shows the nitrogen adsorption isotherms of aluminum hydroxy pillared clays after heat-treatment at 300-500°C. These are of the typical Langmuir type isotherm for microporous crystals. Fig, 5 (b) shows the water adsorption isotherms on the same Al-hydroxy pillared clays [27]. Unlike the water adsorption isotherms for hydrophilic zeolites, such as zeolites X and A, apparently these isotherms cannot be explained by Langmuir nor BET adsorption equations the water adsorption in the early stages is greatly suppressed, and shows hydrophobicity. Water adsorption isotherms for several microporous crystals [20] are compared with that of the alumina pillared clay in Fig. 6. Zeolites NaX and 4A have very steep Langmuir type adsorption isotherms, while new microporous crystals such as silicalite and AlPO -S having no cations in the...

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

Until now, we have focused our attention on those adsorption isotherms that show a saturation limit, an effect usually associated with monolayer coverage. We have seen two ways of arriving at equations that describe such adsorption from the two-dimensional equation of state via the Gibbs equation or from the partition function via statistical thermodynamics. Before we turn our attention to multilayer adsorption, we introduce a third method for the derivation of isotherms, a kinetic approach, since this is the approach adopted in the derivation of the multilayer, BET adsorption isotherm discussed in Section 9.5. We introduce this approach using the Langmuir isotherm as this would be useful in appreciating the common features of (and the differences between) the Langmuir and BET isotherms. 

Physical characterization of the CBI ceramic aggregate was accomplished at the Civil Engineering Department of Stanford University. Samples were analyzed for specific surface area and pore volume, by BET adsorption, and microscopic morphology, by SEM imaging. These analyses were performed on fired samples from the matrix of additive-sludge mixtures described in the previous paragraph. 

Equation 11.60 is the final expression for the BET adsorption isotherm commonly seen in the literature. It gives the total amount of gas phase A that can be absorbed onto a certain surface area of solid material. 

It is easy to see that the BET adsorption isotherm has the correct limits at very high [A] and when multilayer adsorption is negligible. First, consider the case where the pressure of A approaches the value for saturated vapor pressure of A in equilibrium with the liquid. Let the corresponding concentration be designated [A]sa/. The vapor/liquid equilibrium process is written... 

Now consider the form of the BET adsorption isotherm written in Eq. 11.59. If multilayer adsorption were not possible, then Km would be zero. The adsorbed site fraction from Eq. 11.59 becomes... 

George R. Hill In the low temperature physical solution process the surface area would probably be that determined by BET adsorption measurements. In the high temperature process, apparently the coal structure is opened up, and the surface would be the total surface of all the molecular units. This occurs, as the dissolution proceeds, by a combination of chemical bond breaking and solvent action with hydrogen transfer to the free radicals produced. 

The Mg-Al-C03-LDH used as adsorbent and sorbent was prepared with an Mg Al ratio of 2 1 by the coprecipitation method at variable pH [6], The material obtained was characterised by powder X-ray diffraction (PXRD, using a Siemens D-5005 X-ray diffractometer), and elemental and thermal analyses. The material showed the characteristic lamellar structure with a basal spacing of 7.6 A, specific surface area of 87.1 m2 g 1, determined by the N2-BET adsorption isotherm, and an approximate minimum molecular formula [Mg, MAt, (oh) m ](CO, ) 5 2.3 i(h2o) The size distribution and the average size of the LDH particles were determined by light scattering, using a Zetasizer 4 from Malvern. 

Specific surface ranges from 1 to 1000 square meters/gram. It is most often measured by adsorption of nitrogen at Its atmospheric saturation pressure (-195.8 C), with analysis of the data by the BET adsorption equation (problem P6.01.02). Pore diameters of common catalysts range from 10 to 200 Angstroms (10-8 cm) problem P6.01.01 discusses such data. Porosity of a bed of... 

The Brunauer-Emmett-Teller (or BET) adsorption isotherm applies only to the physisorption of vapours but it is important to heterogeneous catalysis because of its use for the determination of the surface areas of solids. The isotherm is given by the following equation,... 

Type E is common for the adsorption of gases. Usually the first concave part is attributed to the adsorption of a monolayer. For higher pressures more layers adsorb on top of the first one. Eventually, if the pressure reaches the saturation vapor pressure, condensation leads to macroscopically thick layers. It can be described by the BET adsorption isotherm equation Eq. (9.37) (see below). 

In Langmuir model, the maximal adsorption is that of a monolayer. Langmuir adsorption isotherms all saturate at high vapor pressures. This is unrealistic for many cases. In order to consider the adsorption of multilayers, Brunauer, Emmett, and Teller extended the Langmuir theory and derived the so-called BET adsorption isotherm [378], The basic idea in the BET theory was to assume a Langmuir adsorption for each of the layers (Fig. 9.8). 

Since the BET adsorption isotherm is so widely used, we describe a simple derivation [1], It is convenient to define two parameters, a and / , according to... 

Figure 9.9 Left BET adsorption isotherms plotted as total number of moles adsorbed, n, divided by the number of moles in a complete monolayer, ri7non, versus the partial pressure, P, divided by the equilibrium vapor pressure, Po. Isotherms were calculated for different values of the parameter C. Right Adsorption isotherms of water on a sample of alumina (Baikowski CR 1) and silica (Aerosil 200) at 20°C (P0 = 2.7 kPa, redrawn from Ref. [379]). The BET curves were plotted using Eq. (9.37) with C = 28 (alumina) and C = 11 (silica). To convert from n/nmo to thickness, the factors 0.194 nm and 0.104 nm were used, which correspond to n-mon = 6.5 and 3.6 water molecules per nm2, respectively.

Usually specific surface areas are determined from adsorption experiments. To illustrate this let us assume that adsorption of a specific sample is adequately described by the Langmuir Eq. (9.22). From fitting experimental results we obtain Tmon in units of mol/g. Then we assume a reasonable value for the cross-section area of a gas molecule a a, and obtain the specific surface from J2 = rmon ANA- In most practical applications the BET adsorption isotherm is used instead of the Langmuir Eq. (9.22) because it fits better. From a fit with the BET isotherm we get Tmon or nmon. Some cross-sectional areas for suitable gases in A2 are N2 16.2 02 14.1 Ar 13.8 n-C4Hi0 18.1. 

An example of the adsorption to one such material is shown in Fig. 9.16. The siliceous material, called MCM-41, contains cylindrical pores [397], With increasing pressure first a layer is adsorbed to the surface. Up until a pressure of P/Po 0.45 is reached, this could be described by a BET adsorption isotherm equation. Then capillary condensation sets in. At a pressure of P/Po 0.75, all pores are filled. This leads to a very much reduced accessible surface and practically to saturation. When reducing the pressure the pores remain filled until the pressure is reduced to P/Pq rs 0.6. The hysteresis between adsorption and desorption is obvious. At P/Po 0.45 all pores are empty and are only coated with roughly a monolayer. Adsorption and desorption isotherms are indistinguishable again below P/Po 0.45. 

Kern and Findenegg measured the adsorption of n-docosane (C22H46) in heptane solution to graphite [403], They used a porous graphite with a specific surface area of 68 m2g 1 as determined from BET adsorption isotherms with N2. Tmax, which is assumed to correspond to monolayer coverage, is found to be 88.9 /.xmol/g. Can you conclude something about the structure of the adsorbed molecules What is the area occupied by one molecule compared to its size ... 

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