Testing methods specific surface area

In Chapters 4 and 6, several methods of characterization of solids that are normally used for catalyst testing were described. In particular, the parameters which characterize the surface morphology of a porous catalyst are the same that characterize a porous adsorbent, that is, the specific surface area, S [m2/g], the micropore volume, W1 [cm3/g], the sum of the micropore andmesopore volumes, that is, the pore volume, W [cm3/g], and the pore size distribution (PSD), AVp/ADp (see Chapter 6). 

Owing to the last problem the contact angle method has mainly been applied to low-energy surfaces like polymers 1112,122— 126J or silicas grafted with alkyl chains [ 127, 28. Some results are summarized in Table 9. In addition, it is often used to determine the surface tension of materials with a high specific surface area like powders or porous substances 1129-133). In that case the rise of the test liquids in a capillary filled with the substance is measured. 

The X-ray diffraction (XRD) patterns were collected on a RIGAKU diffractometer, operated at 30kV and 20 mA and using CuK radiation (A,=0.15405nm). The specific surface areas of the catalysts were tested on a home-made analyzer, following the BET method from air isotherms determined at liquid nitrogen temperature. The structure properties of the catalysts were characterized by IR spectroscopy using a PE-783 IR spectrometer. The amount of samples were mixed with KBr in form of disks, and the spectra were taken at room temperature. 

For the adsorption tests, a sample of silica (Merk) with a specific surface area of 388 m g measured by the BET method was used. The solid specimens used to measure the contact angles were microscope glass slides and pieces of quartz polished using a rotating plate covered with a polishing cloth impregnated with 10 pm diamond polishing particles. 

Specific surface areas and pore size distributions of mesoporous materials are best probed by nitrogen/argon adsorption and capillary condensation which will be outlined in detail below. It should be emphasized that the concept of specific surface area is not applicable when the size of the sorbed molecules approaches the diameter of the pore. Thus, for microporous substances values for specific surface areas have no physical meaning, but are rather characteristic of the volume of gas adsorbed. Nevertheless, these values are frequently used as practical numbers to compare the quality and porosity of microporous materials. The average pore size of microporous materials has to be probed by size exclusion measurements. For this purpose the uptake of a series of sorbates with increasing minimal kinetic diameter on a solid are explored. The drop in the adsorbed amount with increasing size of the sorbate defines the minimum pore diameter of the tested solid. The method will be described in detail below. 

Carbon blacks with specific surface areas of up to 100 m /g can be regarded as essentially nonporous [15] since they give reversible Type II isotherms in the lUPAC classification [10]. Early physisorption measurements on carbon blacks [1] were designed to test the validity of surfiice areas determined the BET method [6]. Carbon blacks were considered [5] to be especially suitable for this purpose because the discrete nature of their spheroidal particles allowed electron microscopy to be used for the evaluation of the particle size distribution. Certain well-characterized carbon blacks are still extremely useful as reference adsorbents [11, 16]. 

The specific surface areas of the samples studied were determined by the BET method (Table l). Activity tests were carried out 111 a plug-flow reactor at 1 aim. A catalyst sample, 400 mg, was placed in a perforated glass bed of ca. 0.35 cm3 having a depth of 0.2 cm. A mixture of two-thirds He and one-third N20 was passed through the bed at a rate of 180 5cm3 min-1 and a contact time of 0.12s. 

Excipients could usefully be classified or tested according to their properties at three levels, viz. molecular, particulate, and bulk properties. Those are tested for by the manufacturer of a dosage form. It is not clear which of those properties should be covered by the official compendia. Testing of functionality, i.e., at particulate or bulk level, does not seem to be possible yet. Typical tests are bulk density, specific surface area, flowability, and particle size distribution. However, the standardization of methodology in compendia, without specification limits, will probably be of help for both vendor and buyer. Therefore, functionality related tests are now being proposed in the pharmacopoeias. As excipients are getting more complex, their analytical characterization is very important. Interesting opportunities lie ahead, particularly with macromolecular separation, MALDI-TOF-MS, and spectrometric methods such as NIR. 

Important methods for the determination of the specific surface area and of the pore size distribution are based on the measurement of the gas adsorption isotherm [1,2]. The gas adsorption method and the evaluation according to Brunauer, Emmett and Teller using the two-parameter BET equation has been standardized in several countries for a number of years and an ISO standard just appeared. To establish the pore size distribution the method of Barrett, Joyner and Halenda (BJH) is generally accepted. Other methods for this purpose make use of the flow resistance of air through the compressed sample. The Blaine test and other flow tests used to characterize building materials are standardized world-wide. 

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