Specific surface area physical methods

The specific surface area of a solid is one of the first things that must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Such a determination can be made through adsorption studies themselves, and this aspect is taken up in the next chapter there are a number of other methods, however, that are summarized in the following material. Space does not permit a full discussion, and, in particular, the methods that really amount to a particle or pore size determination, such as optical and electron microscopy, x-ray or neutron diffraction, and permeability studies are largely omitted. 

The external surface area of the filler can be estimated from a psd by summing the area of all of the equivalent spheres. This method does not take into account the morphology of the surface. It usually yields low results which provide Htde information on the actual area of the filler that induences physical and chemical processes in compounded systems. In practice, surface area is usually determined (5) from the measured quantity of nitrogen gas that adsorbs in a monolayer at the particle surface according to the BET theory. From this monolayer capacity value the specific surface area can be determined (6), which is an area per unit mass, usually expressed in m /g. 

Physical properties of calcined catalysts were investigated by N2 adsorption at 77 K with an AUTOSORB-l-C analyzer (Quantachrome Instruments). Before the measurements, the samples were degassed at 523 K for 5 h. Specific surface areas (,S BEX) of the samples were calculated by multiplot BET method. Total pore volume (Vtot) was calculated by the Barrett-Joyner-Halenda (BJH) method from the desorption isotherm. The average pore diameter (Dave) was then calculated by assuming cylindrical pore structure. Nonlocal density functional theory (NL-DFT) analysis was also carried out to evaluate the distribution of micro- and mesopores. 

Although there are several methods for analysis of nitrogen physisorption data, the most commonly used is BET surface area. Because for microporous materials the boundary conditions for multilayer adsorption are not fulfilled, the calculated BET surface area has no physical meaning. Such data should be considered proportional to the total micropore volume rather than the specific surface area. The Tplot method can be used to calculate the micropore volume and the mesopore... 

The overall yield is essentially 100 by any of the preparation methods, but the physical characteristics of the ion exchangers are dependent on preparation conditions. For example, sodium titanate prepared by Eqs. la and lb with hydrolysis in one liter of water per mole of Ti(OC3H7)4 has a bulk density of 0.U5 g/cm3 and a specific surface area of lO-UO m /g. The same material prepared by Eqs. la and lb and hydrolyzed in a solution of 100 ml of water in 1000 ml of acetone for each mole of Ti(OC2H7)4 has a bulk density of 0.35 g/cm3 and a specific surface area of 200-UOO m /g. In all cases, the materials consist of agglomerates of 50-100 A particles with the degree of aggregation of the particles determining both the bulk density and surface area. 

Hurd, D. C. and Theyer, F. Changes in the physical and chemical properties of biogenic silica from the central equatorial Pacific-- . Solubility, specific surface area, and solution rate constants of acid-cleaned samples, 211 230, in COLbb, Jr., T. R. P., editor) "Analytical Methods in Oceanography," Adv. Chem. Ser. 147, 1975. 

Surfaces of disperse materials are less well defined than that of mercury. Problems are Incurred with heterogeneity, non-zero solubility, non-inertness on titration, the presence of impurities remaining after the preparation, etc. Some uncertainty as to the specific surface area Ag also presents itself even if a reproducible value is determined when using an established method such as BET (Nj) gas adsorption some reservation remains about its physical meaning do Ions adsorbing from solution "see" the same surface as Nj molecules adsorbing from the gas phase at subzero temperatures on evacuated samples Because of... 

The specific surface area of contact for mass transfer in a gas-liquid dispersion (or in any type of gas-liquid reactor) is defined as the interfacial area of all the bubbles or drops (or phase elements such as films or rivulets) within a volume element divided by the volume of the element. It is necessary to distinguish between the overall specific contact area S for the whole reactor with volume Vr and the local specific contact area 51 for a small volume element AVi- In practice AVi is directly determined by physical methods. The main difficulty in determining overall specific area from local specific areas is that Si varies strongly with the location of AVi in the reactor—a consequence of variations in local gas holdup and in the local Sauter mean diameter [Eq. (64)]. So there is a need for a direct determination of overall interfacial area, over the entire reactor, which is possible with use of the chemical technique. 

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. 

Dithiocarbamate fungicides containing cbfferent metal ions such as Zn, Mn " " and Fe were resolved using a C q column that performed better than the conventional RP-C18 sorbent [123]. However, the method did not allow the speciation of dithiocarbamates that contain no metal. Finally, C q—fullerene was reported to be an effective sorbent material for preconcentrating mercury compounds, and to be preferable to RP-C18 on account of its large specific surface area and volume, which endow it with an increased physical sorption capacity [124]. Also recendy, Pereira et al. [125] developed a new alternative for Cd and Pb determinations at low concentrations, using the preconcentration of a Cjio/CyQ mixture coupled to thermospray flame furnace atomic absorption spectrometry. 

Physical adsorption of N2 gas on surfaces at 77 Kelvin is used as a standard method of measuring surface areas the data for N2 adsorption in the partial pressure range of P/Po = 0.05 to 0.3 are fitted to equation 10.26. The value of v is then calculated, and the specific surface area of the adsorbent is determined from equation 10.24. It has been estimated that d, the adsorption site area, is 16.2 for N2. 

The simplest method of estimating mineral reactive surface area is to equate it to the physical surface area, usually reported in m /gm of substrate. Specific surface area, Ai/W (m /cm of substrate), will be used here for directly comparing surface areas of minerals of differenf ensities. Geochemical models commonly define the physical surface area in terms of solution mass (m ), as in Equation 1, rather than solid phase volume (V ). The two ratios are related by the expression... 

In this work, nanocomposite supports formed by nanometric domains of alumina dispersed on a-Al203 beads were synthesized by a modified incipient wetness impregnation method in order to improve specific surface area and surface reactivily of a-Al203 large porosity precursor. The obtained composites were characterized by conventional physical methods like N2 adsorption-desorption, mercury porosimetry, TEM and SEM, in order to describe the evolution of the composite textural properties with the impregnated phase morphology. 

The chemical and physical methods of activation allow to produce carbon materials with a highly developed specific surface area. Therefore, activated carbons are extensively used as electrode materials for supercapacitors [11,25,30-37]. Considering Eq. (1) showing that capacitance depends on the surface area of the electrodes, it should be expected that higher the specific surface area of an activated carbon, higher the capacitance. However, practical... 

The average particle-size distributions for a number of fireclays are shown in Table 14. Particles larger than 2(jl equivalent spherical radius are determined by a method based on settling under gravity, but for smaller particles a centrifugal method has to be employed. It is unfortunate that more information on the distribution below 2(jLis not available, since it is this fraction that contributes most to the specific surface area and related properties. Nevertheless, the data available does correlate to some extent with other physical properties. 

Physical properties, notably the specific surface areas, have been proposed by some authors as a measure for the activity of catalysts. This correlation is successful only when applied to catalysts which resemble one another in their composition and in their method of preparation. That surface area cannot be considered to be of exclusive importance to catalytic activity is demonstrated by the rather extreme examples given in Table VII. On the other hand, the fact that the capacity for quinoline chemisorption is quantitatively related to the activity of cracking catalysts is shown by Fig. 8 obtained with catalysts of various compositions, methods of preparation, and activities. The amount of quinoline chemisorbed thus measures a general property of this entire class of catalysts which is fundamentally related to their ability to act as catalysts. 

PHYSICAL METHODS. The principal physical methods for measuring specific surface areas of soil clays are electron microscopy and X-ray diffraction. In both techniques, the objective is to determine the shapes and dimensions of representative soil particles and then to calculate the... 

Although the positive adsorption methods offer the advantage of convenient determination in heterogeneous samples, they suffer from the uncertainty involved in the calibration of the packing area (which usually must be done by comparison with results from a physical method using reference clay materials) and from the fact that the monolayer parameter is model-dependent through Eqs. 1.6 and 1.7. It must also be remembered that the specific surface area determined by positive adsorption is ultimately a function of the reaction between surface functional groups and some probe molecule. If the experimental conditions of the reaction are close to those under which the surface behavior of a sample is of interest, then this estimate of specific surface area has surface chemical relevance. 

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