Measurement of surface area

Section 2 of this chapter describes the characterization of carbonaceous materials by powder X-ray diffraction, small-angle-X-ray scattering (SAXS), measurements of surface area, and by the carbon-hydrogen-nitrogen (CHN) test, a chemical analysis of composition. In this section, we also describe the electrochemical methods used to study carbonaceous materials. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The characterization of evaporated alloy films can be carried out at widely different levels of sophistication. At the very least, it is necessary to determine the bulk composition, probably after the film has been used for an adsorption or catalytic experiment. Then various techniques can be applied, e.g., X-ray diffraction, electron diffraction, and electron microscopy, to investigate the homogeneity or morphology of the film. The measurement of surface area by chemisorption presents special problems compared with the pure metals. Finally, there is the question of the surface composition (as distinct from the bulk or overall composition), and a brief account is given of techniques such as Auger electron spectroscopy which might be applied to alloy films. 

The simplest measure of surface area is that obtained by the so-called point B method. Many isotherms of Type II or IV show a straight section at intermediate relative pressures. The more pronounced the section, the more complete is the adsorbed monolayer before multilayer adsorption begins. As may be seen from Figure 17.9, the lower limit of the straight section is the point B which was identified by Emmett 24 and Yang 3 as corresponding to a complete monolayer. This interpretation is accepted as a convenient empiricism. 

Having obtained a measure of surface area, a mean pore size may be calculated by simplifying the pore system into np cylindrical pores per unit mass of adsorbent, of mean length Lp and mean pore radius rp. 

Experimental measurements of surface area are usually carried out by BET gas absorption techniques (see Delany et al., 1986, for an appropriate treatment). 

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... 

The elemental composition of CuCr204 Cu 27.44%, Cr 44.92%, O 27.64%. The catalyst is analysed by measurement of surface area and pore volume also by differential thermal analysis, thermogravimetric analysis and x-ray studies. 

The success of kinetic theories directed toward the measurements of surface areas depends upon their ability to predict the number of adsorbate molecules required to exactly cover the solid with a single molecular layer. Equally important is the cross-sectional area of each molecule or the effective area covered by each adsorbed molecule on the surface. The surface area then, is the product of the number of molecules in a completed monolayer and the effective cross-sectional area of an adsorbate molecule. The number of molecules required for the completion of a monolayer will be considered in this chapter and the adsorbate cross-sectional area will be discussed in Chapter 6. 

The experimental results obtained from the measurement of surface area remaining after each lathe cut can be plotted as cumulative surface area against radius. If iS/ is the surface area per cm. of radial distance at radius r, the cumulative surface area is given by... 

Measurement of Surface Area. The Teachability determined by these methods is usually reported as g/cm day. The total surface area of particulate material can be assessed 1) by assuming a particle shape e.g.spherical) and estimating the number of particles, or 2) by measurements using the Brunauer-Emmett-Teller (BET) nitrogen adsorption technique ( ). Unfortunately, the BET method measures the area of surfaces to which nitrogen has access this is not necessarily the same as the area to which a solution has access. Access by solutions requires much larger pore areas. 

FIGURE 3 Measurement of surface area by the BET gas adsorption method. (A) Typical adsorption isotherm with a relatively flat curve in the region of monolayer adsorption. (B) Plot of the linear form of the BET equation between p/p0 = 0.05 and 0.3 used to calculate the monolayer coverage Vm. 

TABLE 13.2 Advantages and Disadvantages of IGC for the Measurement of Surface Area... 

SYSTEMS FOR THE AUTOMATIC MEASUREMENT OF SURFACE AREA AND POROSITY BY THE VOLUMETRIC METHOD... 

Measurement of surface-area by the adsorption method is accomplished by the van der Waals. adsorption isotherm of a suitable gas (usually nitrogen because of its chemical inactivity) at a temperature... 

The measurement of surface areas of soils and soil colloids by the use of low temperature van der Waals adsorption isotherms. Soil Science, 45 57-65. 

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

E. Carroll made the BET measurements of surface area and R. E. Johnson and G. Hughes the electrokinetic measurements. Oral R. Van Buskirk directed most of the spray drying experiments. H. J. McQueston, W. B. Hambleton, Jr., R. E. Polini, J. Watson, P. W. Johnston, Jr., and D. Nickerson assisted in the experimental work. 

H. F. Leach As mentioned briefly in the paper, the catalysts were examined by x-ray diffraction, and for all the samples used there was no apparent loss of crystallinity relative to the parent NaX. The catalysts were also characterized by measurement of surface area the value for NaX was 870 m gram" and for all the other zeolites lay within the range 840-900 m gram" We considered that these results indicated that very little decomposition of the zeolite to amorphous silica or silica-alumina occurred. 

Different physical properties and molecular models have been used to define the molecular surface the most common are reported below together with the descriptors proposed as measures of surface areas and molecular volume (- volume descriptors). Molecular surface area and volume are parameters of molecules that are very important in understanding their structure and chemical behaviour such as their ability to bind ligands and other molecules. An analysis of molecular surface shape is also an important tool in QSAR and - drug design-, in particular, both - molecular shape analysis and - Mezey 3D shape analysis were developed to search for similarities among molecules, based on their molecular shape. 

When closely packed particles are heated to temperatures around the Tammann temperature [33], which is approximately 0.5r,n (where is the melting temperature), the particles join at the points of contact (sinter) and the intervening pores formed gradually disappear with time. In addition to macroscopic measurements of surface area and pore volume, the progress of sintering can be measured [34] from (i) the size of the "neck" that forms between particles (Figure... 

BET) technique. The measurement of surface area is much more consistent than that of... 

Figure 23 is a schematic representation of the ranges of application of different analyzers of dispersion characteristics. In the coarse range, screening methods dominate. Most alternatives exist in the range between 1 and lOOfim. Below 1 fim only electron optical procedures and centrifugal sedimentation are still applicable. Methods for the measurement of surface area are also mainly available in the range between 1 and 100 im. 

The third step, the measurement of changes in catalytic activity, is usually simple and conventional, except that particular features of the irradiation technique may suggest or exclude certain reactions. Here, as elsewhere in catalysis, activation energies are likely to be better indicators of important changes in the catalyst than gross rates, and supporting measurements of surface area and adsorption are frequently... 

Theoretically, for a given chemical nature, the immersion energy of a nonporous solid should be proportional to the surface area and the corresponding coefficient should be available from a reference solid of known surface area. Nevertheless, the detailed surface composition and structure of solids with similar bulk composition and even crystaUinity can be very different because of their chemical, mechanical or thermal history. Therefore, it would be unwise to use the relative measurement of surface area when both the surface and the immersion liquid are polar. Conversely, nonpolar hquids can be used for such a determination because the corresponding immersion energies are not sensitive to minute variations of the surface chemistry. This point will be addressed again later on in the case of microporous samples. 

The static volumetric method is very precise, and is considered a very accurate technique to evaluate the surface area and the pore size in the region of micro- and mesopores. However, it is not advisable whenever a fast measurement of surface area is required, because this method involves a long analysis time to produce highly accurate and reliable results. 

Titanium sulfate supported on zirconia catalysts were prepared by drying of powdered Zr(OH)4 with titanium sulfate aqueous solution followed by calcining in air at high temperature. The characterization of prepared catalysts was performed using Fourier transform infrared (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and by the measurement of surface area. The addition of titanium sulfate to zirconia shifted the phase transition of ZrOa from amorphous to tetragonal to higher temperature because of the interaction between titanium sulfate and zirconia. The catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. 

Measurement of surface area involves the principles of physical adsorp-tion which differ from those of chemical adsorption, as shown in Table... 

Catalyst characterization tests include measurement of surface areas, chemisorption, pore-size distributions, crystal structure as determined by X-ray crystallography, reaction mechanisms as revealed by kinetics, and isotopic tracers and diagnostic catalytic reactions to test functional capabilities. These have been interpreted in terms of variation of catalyst preparation-structure-performance relationships. 

O. 8 and 1.0) were prepared by Co-precipitation method. Characterisation was done by X-ray phase analysis and DRS studies and by measurements of surface area and electrical conductivity. The amount of metallic copper and monovalent copper were estimated by reversible adsorption of CO respectively. Hydrogenation of nitrobenzene to aniline was carried out at 250°C in a fixed bed flow type reactor. Comparison of hydrogenation activity with CO adsorption data show that monovalent copper is more active than metallic copper for the hydrogenation of nitrobenzene. 

Brinker and Scherer (8) pointed out that the area of a surface is defined largely by the method of surface area measurement. Many of the measurements of surface areas in work reported before the 1980s were based on the method of determining monolayer capacity of an adsorbent molecule of known cross-sectional area. In the Brunauer-Emmett-Teller (BET) method (45) the apparent surface area is determined from nitrogen adsorption. However, because the nitrogen molecule surface area is 16.2 A2, this definition of the surface excludes microporosity that is accessible, for example, to water molecules. 

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