BET measurements

Fig. 3.3.3 (a) Hahn echo ]H intensity during heating cycle of cyclohexane filling the pores of catalyst pellets, (b) Pore size distribution obtained from (a) in comparison with BET measurements. 

The measured NMR signal amplitude is directly proportional to the mass of adsorbate present, and the NMR signal versus pressure (measured at a fixed temperature) is then equivalent to the adsorption isotherm (mass of adsorbate versus pressure) [24-25]. As in conventional BET measurements, this assumes that the proportion of fluid in the adsorbed phase is significantly higher than the gaseous phase. It is therefore possible to correlate each relaxation time measurement with the calculated number of molecular layers of adsorbate, N (where N = 1 is monolayer coverage), also known as fractional surface coverage. 

The specific surface area is usually determined by the BET technique discussed in Section 6.2.2. For the most reliable BET measurements the adsorbate gas molecules should be small, approximately spherical, inert (to avoid chemisorption), and easy to handle at the temperature in question. For economy, nitrogen is the most common choice with measurements usually made at 77 °K, the normal boiling point of liquid nitrogen. Krypton is another material that is frequently employed. 

The way in which a material adsorbs a gas is referred to as an adsorption isotherm. All adsorption isotherms can be described by five representative curves, given in Fig. 1. The isotherm shapes reflect specific conditions for adsorption, such as pore size and heats of adsorption [6]. The most common type of isotherm and the most useful for BET measurements is the Type II isotherm. The inflection point of this isotherm usually indicates monolayer coverage of the adsorbate [9]. 

For multipoint BET measurements, a plot of P/V(P0-P) versus PIPq is obtained using various concentrations of adsorbate and measuring the adsorbed volume. Since the BET equation is in the form of a straight line ... 

The theory for single-point BET measurements is similar however, an assumption is made that the intercept of the PIV(P0-P) versus PIPQ plot equals zero. This assumption is not always valid and can result in surface area values that differ from multipoint values. When the assumption is valid, the single-point method is simpler and faster, and it results in accurate surface area values. 

Substituting the value of Vm into Eq. (8), the total surface area for a single-point BET measurement can be calculated from... 

The specific surface for the single-pont BET measurement is then calculated from Eq. (9). 

Figure 5.5 shows the variation of the pore size distribution as a function of cycles of surface-modification-based N2 adsorption isotherms. The pore size decreases with the modification cycle number. The reduction of the mesopore size for each cycle should be about twice the single-layer thickness. Accordingly, the effective singlelayer thickness is about 6 to 7 A based on the above BET measurements. This value is close to those estimated from the frequency changes of a quartz crystal balance for ultrathin fihns prepared by the surface sol-gel process on 2-D substrates." " ... 

Finally, the other factor that could contribute to the improved performance of the nanotubule electrode is its higher surface area. This higher surface area would make the true current density at the nanotubular material lower than at the control material. In order to access the contribution of surface area, Brunauer-Emmett-Teller (BET) measurements were made on both the nanotubular and control electrodes [124], The specific surface areas were found to be 40 m g (nanotubule electrode) and 13 m g (control electrode). This factor of 3 increase in surface area cannot account for the factor of 12 improvement in capacity observed at the highest current... 

The small, spherical particles of ferrihydrite often pack together to form aggregates >0.1 pm across. The aggregated structure and interparticle porosity create difficulties in measurement of surface area because the internal area is not fully accessible to all measurement techniques. It is rare for more than two methods of area measurement to have been applied to the same sample. Pyman and Posner (1978) obtained an area of 250 m g" using both N2 and water BET measurements. With the EG ME method, however, the same sample had an area of 600 m g". These... 

Clausen, L. I. Fabricius (2000) BET measurements Outgassing of minerals. J. Colloid Interface Sci. 227 7-15... 

The polymerization time as a polymerization parameter for adjustment of the porous properties of thermally initiated copolymers has recently been characterized [111]. A polymerization mixture comprising methylstyrene and l,2-bis(p-vinylbenzyl)ethane as monomers was subjected to thermally initiated copolymerization for different times (0.75, 1.0, 1.5, 2, 6, 12, and 24h) at 65°C. The mixtures were polymerized in silanized 200pm I.D. capillary columns as well as in glass vials for ISEC and MIP/BET measurements, respectively. 

BET measurements (Table 1.2) prove the increase in mesopores, as decreasing the total polymerization time from 24 h to 45 min causes to raise by a factor of 3, resulting in 5p 80mVg, which is comparable to silica particles with a mean pore diameter of 300 A [112,113]. 

Influence of the Polymerization Time on the Porous Properties of Monolithic MS/BVPE Networks, Considering Capillary Columns (80x0.2 mm I.D.) for ISEC and Glass Vial Bulk Polymers for MIP and BET Measurements... 

Kiselev, using the above equation by graphical integration of the isotherm between the limits of saturation and hysteresis loop closure, was able to calculate surface areas for wide-pore samples in good agreement with BET measured areas. For micropores, the absence of hysteresis at the low-pressure end of the isotherm indicates that only adsorption and not condensation occurs, thereby rendering Kiselev s method inapplicable. 

In the example shown the t values were not calculated from equation (8.36) but were taken from a t versus P/Pq plot prepared by deBoer on a sample with a similar BET C value. By choosing t values from a material with a similar but not identical C value the surface area nevertheless agreed to within 1.4% of the BET measured area. 

Both deBoer s t-method and Brunauer s MP method are based on the assumption that the BET measured surface area is valid for micropores. Shields and Lowell, using this same assumption, have proposed a method for the determination of the micropore surface area using mercury porosimetric data. The surface area of micropores is determined as the difference between the BET surface area and that obtained from mercury porosimetry (see Section 11.5). Since mercury porosimetry is capable of measuring pore sizes only as small as approximately 18 A radius, this technique affords a means of calculating the surface area of all... 

Polyacrylic acid Hybrid organic/inorganic network polymers were formed via the reaction of polyacrylic acid with tin(IV). titanium(IV), and silicon (IV) alkoxides and subsequent hydrolysis to form mesoporous materials. Treatment by nitric acid removed the polyacrylate template and produced microporous inorganic hydrous metal oxides Surface areas characterized by BET measurements 130... 

A similar relationship can be observed with promoted M0S2. Each family of catalysts has its own linear correlation, which cannot be compared to each other directly because of the corrosivity problem. More recently, low-temperature oxygen chemisorption has been claimed to be more reliable, but it also lacks a well-determined stoichiometry (52). Oxygen chemisorption has also been applied to tungsten and rhenium sulfides, as well as promoted molybdenum and tungsten sulfides. In the isotropic class, it has been applied only to ruthenium sulfide, in which case it gives approximately the same result as a BET measurement due to the isotropic nature of this sulfide (41). 

The hydrothermal stability of samples A and B was studied by treating the samples in boiling water for l and 2 days and the treated samples were again characterized by XRD and BET measurements. Figure l displays the XRD patterns of samples A and B treated in boiling water at different time. After treatment in boiling water for 2 days, the intensity of X-ray diffraction peaks for samples A and B decreases proportionately with treatment time. It can be clearly observed that the drop in the intensity of [ 100] diffraction peak for sample B is more drastic than sample A. After only l day in boiling... 

Grain Type Median Grain Size (pm) Specific Surface Area(m2g- ) BET-Measured Surface Area / Geometric Predicted Surface Area... 

---