Pore surface area measurement

The single-point BET surface area measurement was used to check for open pores. The results for some soft and hard carbon samples heated at 700°C and 1000°C are presented in Table 2 for comparison. The hard carbon samples studied here have about ten times more open porosity than the soft carbons. 

The principle underlying surface area measurements is simple physisorb an inert gas such as argon or nitrogen and determine how many molecules are needed to form a complete monolayer. As, for example, the N2 molecule occupies 0.162 nm at 77 K, the total surface area follows directly. Although this sounds straightforward, in practice molecules may adsorb beyond the monolayer to form multilayers. In addition, the molecules may condense in small pores. In fact, the narrower the pores, the easier N2 will condense in them. This phenomenon of capillary pore condensation, as described by the Kelvin equation, can be used to determine the types of pores and their size distribution inside a system. But first we need to know more about adsorption isotherms of physisorbed species. Thus, we will derive the isotherm of Brunauer Emmett and Teller, usually called BET isotherm. 

It has been found that a plot of pg / Vg [1-pgl vs pg is hnear for the pressure range of 0.05 to 0.4, with aslope of (C - 1) / (Vmono C ) and intercept of 1/ (Vmono C ). Let us now do a simple calculation using BEH data obtained. Suppose we have a 20 gm. sample having a density of 2.0. We measure the surface area as 6 m. From the area of a sphere. A = r d2, and the volume of a sphere, V = 4/37tD3., we find the total volume of n spheres to be 10 cc, i.e.- n 4/3 r D = 10. The surface area of n 7rD2 spheres is 6 m3. The total number of spheres present, n, is the same in both formulas. Therefore, by substitution, we find D= 10 p. If we obtain a particle diameter by some other method and find that it is mueh smaller than that of the BET method, we infer that the peatieles are porous. We thus speak of the porosity and need to correct for the pore surface area if we are to meike a reasonable estimate of the true diameter by the BET method. 

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. 

Illustration 6.2 indicates how void volume and surface area measurements can be combined in order to evaluate the parameters involved in the simplest model of catalyst pore structure. 

Methods for synthesizing highly porous microspheres were investigated, and surface area measurements were used to confirm the porous nature of the samples [19]. A high surface area was measured and was compared with the calculated surface area value. The measured value was 35 times that of a nonporous particle, indicating the extensive porosity of the spheres. The surface area was also used to explain the drug release mechanisms in the pores of these systems. 

Most research on the structure of skeletal catalysts has focused on nickel and involved methods such as x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), electron diffraction, Auger spectroscopy, and x-ray photoelectron spectroscopy (XPS), in addition to pore size and surface area measurements. Direct imaging of skeletal catalyst structures was not possible for a long while, and so was inferred from indirect methods such as carbon replicas of surfaces [54], The problem is that the materials are often pyrophoric and require storage under water. On drying, they oxidize rapidly and can generate sufficient heat to cause ignition. 

Nienow (1983a) observed a delay in the start of particle growth when binder was added to a bed of porous particles and stable fluidization under conditions which produced quenching with non-porous particles. Nitrogen adsorption measurements showed that the pore surface area of alumina decreased as spraying proceeded, indicafing that an effective reduction in pore volume was taking place. 

The parameters D and Dk > whether for macro (denoted by subscript m) or for micro (denoted by subscript ju) regions, are normal bulk and Knudsen diffusion coefficients, respectively, and can be estimated from kinetic theory, provided the mean radii of the diffusion channels are known. Mean radii, of course, are obtainable from pore volume and surface area measurements, as pointed out in Sect. 3.1. For a bidisperse system, two peaks (corresponding to macro and micro) would be expected in a differential pore size distribution curve and this therefore provides the necessary information. Macro and micro voidages can also be determined experimentally. 

The most definitive surface area measurements are probably those made by nitrogen adsorption using the BET theory. Neither the Brunauer, Emmett and Teller (BET) theory nor equation (11.5), used to calculate surface area from mercury intrusion data makes any assumptions regarding pore shape for surface area determinations. When these two methods are compared there is often surprisingly good agreement. When... 

The mean primary particle sizes of pigment blacks he in the range 10-100 nm specific surface areas are between 20 and 1000 m2/g. The specific surface area, determined by N2 adsorption and evaluation by the BET method [4.29], is often cited as a measure of the fineness of a black. Blacks with specific surface areas >150 m2/g are generally porous. The BET total specific surface area is larger than the geometric surface area measured in the electron microscope, the difference being due to the pore area resp. the pore volume. 

The concept of the surface diameter may be mostly used in the field of adsorption and reaction engineering, where the equivalent surface exposure area is important. The determination of the surface area depends on the method of measurements for example, permeametry can give a much lower area than does gas adsorption. The latter often includes the contribution of pore surface area, which is accessible to the gas molecules. The determination of particle surface area by gas adsorption is given in 1.2.2.4. The fundamentals of gas adsorption are further covered in 1.4.1. 

The surface area measurements were performed by adsorption, using nitrogen as the adsorbate. The samples were previously degassed to below 50 mmHg at room temperature and the analyses were performed at 77 K, using liquid nitrogen. The equilibrium interval was 5 s. The surface area was calculated using the Brunauer, Emmett, and Teller (B.E.T.) method. Pore volume and area distributions based on BJH calculation (8) were evaluated by the B.E.T. apparatus software (NOVA 1200-Quantachrome). 

Tables I and II list major typical physical and adsorptive properties of the powdered active carbon. Effective surface area, measured by the BET method using a Digisorb 2500, is consistently in the range of 3000 to 3400 m /gm. This exceeds the theoretical area of about 2600 m /gm as calculated by the area of one gram of a graphitic plane because of multilayer adsorption and pore filling in a highly microporous structure.

The specific surface area calculated here only involves the walls of the pores of the bed and excludes the pores within the particles. Therefore, the surface area measured in this method can be much smaller than the total surface area measured by gas adsorption methods [24],... 

The measurement of pore size distributions is well established. However, the use of BET surface area measurements for zeolitic materials has been called into question due to potential multiple adsorption and nonconformity of monolayer adsorption implicite in the BET theory. The type of gas used, the method of data analysis, and even the use of the term surface area for a zeolitic material has been seriously questioned lately. On the other hand, most commercial manufacturers supply a surface area determined often by a three point or even a one point procedure that some researchers feel tells something about the material. 

In the pore model developed by Bhatia and Perlmutter, the rate of the gasification reaction per unit pore surface area is characterised by the reaction rate constant, K,. As the original work addresses structurally based effects only, Kj may well be assumed constant throughout the gasification stage and, under kinetic control, the char reactivity is then a direct measure of the available surface area. To allow the description of additional (i.e., non-porous) phenomena, we follow a semi-empirical approach in which we assume that Kj can vary with time, the cause of which can either be structural or catalytic in nature. Accordingly, we define Ks(t) = KsoucnirtCt) Strictly... 

Coronene adsorbs on catalyst sites present on both the alumina support and on the active NiMo sulfide phase 3). It has been found that adsorption decreases with coke content to a very Jow value at high coke levels (4), Therefore, it appears that coronene adsorption on the coke is nil. On this basis, it is assiimmed that the loss in adsorption with increasing coke is proportional to the loss in pore surface area due to coverage by coke. The results of coronene adsorption measurements on the VGO-coked catalysts show an initial drop for the 2% C sample, but little change thereafter for higher coked catalysts (Fig. lA) This implies that the coke Is concentrated near the mouth of the pores, On the other hand, catalyst dlffusivity measurements show a continual and sig-... 

Five Ni-Mo/y-Al203 catalysts with wide variations in pore size distribution were used in the present study. They were eill in the form of extrudates. The catalysts were characterized according to standard procedures. A mercury porosimeter (Quantachrome - Model-Autoscan 60) was used to determine pore size distribution. A Quantasorb adsorption unit was used for BET surface area measurements. The chemical composition and physical dimensions of various catalysts used in the present study were not appreciably different. 

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