BET adsorption data

BET Adsorption Data. A wealth of information about the size and shape of pores may be obtained from adsorption isotherms where the mols of nitrogen adsorbed on the membrane are measured as a function of pressure. However, the use of this techniques is not widespread due to the tedious regimen required in gas adsorption measurements. Further, the hysteresis effects make conclusions about pore-structure ambiguous. 

Each of the treated samples shows increases in nitrogen uptake as compared to the untreated sample, indicating an increase in both surface area and pore volume. The BET surface area, pore size distribution and pore volume were calculated using N2 BET adsorption data, rather than desorption data, in order to discount hysteresis and tensile strength effects (26). These data are reported in Table 2. 

The BET equation filled an annoying gap in the interpretation of adsorption isotherms, and at the time of its appearance in 1938 it was also hailed as a general method for obtaining surface areas from adsorption data. The equation can be put in the form... 

When plotted according to the linear form of the BET equation, data for the adsorption of N2 on Graphon at 77 K give an intercept of 0.004 and a slope of 1.7 (both in cubic centimeters STP per gram). Calculate E assuming a molecular area of 16 for N2. Calculate also the heat of adsorption for the first layer (the heat of condensation of N2 is 1.3 kcal/mol). Would your answer for Vm be much different if the intercept were taken to be zero (and the slope the same) Comment briefly on the practical significance of your conclusion. 

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

In writing the present book our aim has been to give a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The major part of the book is devoted to the Brunauer-Emmett-Teller (BET) method for the determination of specific surface, and the use of the Kelvin equation for the calculation of pore size distribution but due attention has also been given to other well known methods for the estimation of surface area from adsorption measurements, viz. those based on adsorption from solution, on heat of immersion, on chemisorption, and on the application of the Gibbs adsorption equation to gaseous adsorption. 

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). 

Most surface area measurements are based on the interpretation of the low temperature equilibrium adsorption of nitrogen or of krypton on the solid using the BET theory [33,269,276—278]. There is an extensive literature devoted to area determinations from gas adsorption data. Estimates of surfaces may also be obtained from electron micrographs, X-ray diffraction line broadening [279] and changes in the catalytic activity of the solid phase [ 280]. 

Figure 1 shows a sharp decrease of low-pressure hysteresis loop when introducing copper in S-l, pointing to the formation of (CuO)n nanoclusters into the S-l intracrystalline channels and supermicropores. The adsorption data analysis (see Table 1) shows a decrease of both the total (BET) surface area and micropore volume of the CuS-1 sample with respect to the S-l matrix. 

Physical adsorption of nitrogen was carried out on an ASAP 2400 Micromeritics apparatus. Before measurements, samples were evacuated overnight at 350 °C at vacuum of 2 Pa. For all samples the same adsorption data table was used. Collected adsorption data were treated by BET-isotherm in the range 0.05 < P/micropore volume and mesopore + external surface, t-plot method, with master isotherm of nonporous alumina (Harkins-Jura) was used, t-plot was linearized in the range of 0.35 < t < 0.6 nm. 

In general, the BET equation fits adsorption data quite well over the relative pressure range 0.05-0.35, but it predicts considerably more adsorption at higher relative pressures than is experimentally observed. This is consistent with an assumption built into the BET derivation that an infinite number of layers are adsorbed at a relative pressure of unity. Application of the BET equation to nonpolar gas adsorption results is carried out quite frequently to obtain estimates of the specific surface area of solid samples. By assuming a cross-sectional area for the adsorbate molecule, one can use Wm to calculate specific surface area by the following relationship ... 

A number of models have been developed for the analysis of the adsorption data, including the most common Langmuir [49] and BET (Brunauer, Emmet, and Teller) [50] equations, and others such as t-plot [51], H-K (Horvath-Kawazoe) [52], and BJH (Barrett, Joyner, and Halenda) [53] methods. The BET model is often the method of choice, and is usually used for the measurement of total surface areas. In contrast, t-plots and the BJH method are best employed to calculate total micropore and mesopore volume, respectively [46], A combination of isothermal adsorption measurements can provide a fairly complete picture of the pore size distribution in sohd catalysts. Mary surface area analyzers and software based on this methodology are commercially available nowadays. 

Prior to nitrogen adsorption experiment to determine surface properties, ACC sample was degassed at 130°C under vacuum (up to 10 torr) for 12 h. The adsorption data were obtained at the Central Laboratory of Middle East Technical University (METU) with a Quantachrome Autosoib-l-C/MS apparatus over a relative pressure ranging from 10" to 1. The BET specific surface area, total pore volume, micropore volume, mesopore volume, and pore size distribution, PSD, of ACC were yielded by using the software of the apparatus. 

When measured adsorption data are plotted against the concentration value of the adsorbate at equilibrium, the resulting graph is called an adsorption isotherm. The mathematical description of isotherms invariably involves adsorption models described by Langmuir, Freundlich, or Brauner, Emmet and Teller (known as the BET-model). Discussion of these models is given in Part 111, as conditions relevant to chemical-subsurface interactions are examined. 

The BET equation has been derived for multilayer adsorption data. 

The N2 adsorption-desorption isotherms for MSU-Ge-2 show a type-lV adsorption branch associated with a well-defined capillary condensation step at P/Po 0.13, characteristic of uniform mesopores (Fig. 4). The adsorption data indicate a very high Brunauer-Emmett-Teller (BET) surface area of 363 m /g and a pore volume of 0.23 cm /g. Given that the Ge mesostructure is much heavier than the corresponding silica, this surface area is actually equivalent to silica with a surface area of 1,316 m /g. 

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). 

Basically there are three criteria against which the success of the BET theory may be evaluated its ability to fit adsorption data, correct prediction of the temperature dependence of adsorption, and correct evaluation of specific area. We discuss these three issues in this section. 

With monolayer adsorption, we saw how the saturation limit could be related to the specific surface area of the adsorbent. The BET equation permits us to extract from multilayer adsorption data (by means of Equation (77)) the volume of adsorbed gas that would saturate the surface if the adsorption were limited to a monolayer. Therefore Vm may be interpreted in the same manner that the limiting value of the ordinate is handled in the case of monolayer adsorption. Since it is traditional to express both V and Vm in cubic centimeters at STP per gram, we write (see Equation (7.72))... 

Data for calcined samples dioo - XRD (100) interplanar spacing, Sbet - BET specific surface area, V, - total pore volume, Vp - primary mesopore volume, Sex - external surface area, wkjs - primary mesopore diameter. Data for uncalcined samples mreS due - mass percent of residue at 1263 K, mSdir - mass decrease in the temperature range of the surfactant decomposition and desorption of the decomposition products (between about 373 and 623 K). Notes a - no peak on XRD spectrum, d,0o cannot be evaluated, b - no linear region on the Os-plot, which would be suitable for the Vp and Sex evaluation. XRD and adsorption data (except for those for HR-A2 sample) taken from Refs. 24 and 26. Thermogravimetric data for DS-AD taken from Ref. 19. 

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... 

In order to apply real adsorption data to the BET isotherm equation, it is customary to use Equation 6.28a in the linear form... 

Nitrogen adsorption isotherms for the OMMs studied were recorded at 77K using a Micromeritics ASAP 2010 adsorption analyzer. All samples prior to adsorption analysis were degassed at 120°C for 2h under vacuum. The BET specific surface area was calculated from the adsorption data in the range of the relative pressure from 0.04 to 0.2 according to the BET method.46 The total pore volume was estimated from the amount adsorbed taken at the relative pressure about 0.99.47 The pore width was estimated at the pore size distribution maximum obtained by the KJS method.48... 

---