Surface area, measurement

Most surface area measurements are based on the interpretation of the low temperature equilibrium adsorption of nitrogen or of krypton on the solid using the BET theory [33,269,276—278]. There is an extensive literature devoted to area determinations from gas adsorption data. Estimates of surfaces may also be obtained from electron micrographs, X-ray diffraction line broadening [279] and changes in the catalytic activity of the solid phase [ 280]. 

The surface area of the product is also dependent upon the atmosphere prevailing during reaction, particularly the availability of water during dehydration processes [281—283] which permits or which facilitates recrystallization. Decomposition of low surface area compounds can provide a route for the preparation of solids of high surface area and high catalytic activity [284,285]. 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area 1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify- 

An impressive property of colloids, including layer silicate minerals, is their large area of reactive surface. Various physical and chemical properties, including water retention and cation exchange capacity, are highly correlated with the surface area of soils. Several techniques estimate the amounts of reactive surface area of soils and are briefly described below. 

Colloid chemists commonly measure surface area by the adsorption of N2 gas. The adsorption is conducted in vacuum and at temperatures near the boiling point of liquid nitrogen (—196° C). The approach is based on the Brunauer-Emmett-Teller (BET) adsorption equation, and has been adapted to a commercially available instrument. Unfortunately, the technique does not give reliable values for expansible soil colloids such as vermiculite or montmorillonite. Nonpolar N2 molecules penetrate little of the interlayer regions between adjacent mineral platelets of expansible layer silicates where 80 to 90% of the total surface area is located. Several workers have used a similar approach with polar H2O vapor and have reported complete saturation of both internal (interlayer) and external surfaces. The approach, however, has not been popular as an experimental technique. 

Soil chemists more commonly measure the retention of polar liquids such as ethylene glycol or glycerol by soils. The basic procedure involves applying excess and then removing all but a monolayer from the mineral surfaces. The excess is removed under vacuum in the presence of a desiccant, to eliminate competition with H2O for retention sites. Some workers advocate a glycol-CaCU mixture to maintain a relatively constant vapor pressure of glycol in the evacuated system, and hence to provide a more reproducible endpoint. 

Surface areas have also been measured by anion repulsion or by adsorption of certain organic solutes from aqueous solution. A particularly promising solute is cetyl pyridinium bromide, which orients differently on external and internal (interlayer) surfaces and can thus aid in distinguishing between the two types of surface. 

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Figure XVI-1 and the related discussion first appeared in 1960 [1], and since then a very useful mathematical approach to irregular surfaces has been applied to the matter of surface area measurement. Figure XVI-1 suggests that a coastline might appear similar under successive magnifications, and one now proceeds to assume that this similarity is exact. The result, as discussed in Section VII-4C and illustrated in Fig. VII-6, is a self-similar line, or in the present case, a self-similar surface. Equation VII-21 now applies and may be written in the form...

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

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. 

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

The Heterogeneity of Catalyst Surfaces for Chemisorption Hugh S. Taylor Alkylation of Isoparaffins V. N. Ipatieff and Louis Schmerling Surface Area Measurements. A New Tool for Studying Contact Catalysts P. H. Emmett... 

Specific Surface Area Measurements (M2/gm) Microscope Micromerograph Sorptometer Scd(1)... 

Surface Area Measurements. For a discussion of procs used see above under Fisher Sub-Siever and Nitrogen Adsorption. For AP, the Fisher Sub-Siever is most suitable for samples having surface areas from 0.05 to 0.46sqm/g, and the Nitrogen Adsorption method for finer mat, up to 30000 sqcm/cc (Ref 49)... 

Isothermal a—time curves for the decomposition of U02(CH3C02)2 in air (513—573 K) [1018] showed two approximately linear regions, 0.0 < a < 0.2 and 0.2 < a < 0.9, for which the values of E were 107 and 165 kJ mole-1, respectively. In nitrogen, the earlier portion of the curve was not linear and E = 151 kJ mole-1 for the later interval. The zero-order kinetic behaviour was explained by growth of nuclei in thin, plate-like crystals, which were shown by microscopic and surface area measurements to fragment when a > 0.85. The proposed initial step in the decomposition was fission of bonds between the U02+ and the (OCO CH3) species [1018]. 

Various in situ and ex situ methods have been used to determine the real surface area of solid electrodes. Each method10,15 32 67,73 74 218 is applicable to a limited number of electrochemical systems so that a universal method of surface area measurement is not available at present. On the other hand, a number of methods used in electrochemistry are not well founded from a physical point of view, and some of them are definitely questionable. In situ and ex situ methods used in electrochemistry have been recently reviewed by Trasatti and Petrii.73 A number of methods are listed in Table 3. 

In principle any standard catalytic metal surface area measuring technique, such as H2 or CO chemisorption can be used to measure the metal/gas interface area Aq or Nq. This is because solid electrolytes such as YSZ chemisorb practically no H2 or CO at any temperature. 

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. 

Two approaches were taken to provide Insight to the uniformity of these samples 1) perform extensive visual Inspection of the materials to ensure that no large palladium crystallites were present, which was the case for both samples, and 2) perform surface area measurements using hydrogen TPD to establish the relationship between the observed data by STEM and the estimated surface area from theoretical considerations. 

X-ray diffraction and surface area measurements suggest that these W-atom surface densities correspond to saturation coverages, which markedly inhibit zirconia sintering and tetragonal to monoclinic transformations at high temperatures. Zr02 surface areas after 1073 K calcination are 4 m g" and increase to an asymptotic value of 51 m g for W surface densities above 5-6 W-atoms nm (Figure 4). Similarly,... 

In order to investigate the relationship between the surface area of skeletal copper and activity, the same sample of catalyst was tested in four successive runs. Rate constants was compared with that of another sample prepared in the same way but pretreated in 6.2 M NaOH at 473 K before use. Figure 4 shows that the first order rate constants, calculated so as to take into account the mass of catalyst relative to the volume of solution, decreased in the first three cycles but then stabilised. The surface areas, measured on small samples taken after reaction, mirrored this pattern. The rate constant, and the surface area, for the pretreated catalyst was similar to those obtained in cycles 3 and 4. It is apparent that activity and surface area are closely related for the unpromoted skeletal copper catalyst and that the pretreatment in NaOH at 473 K is approximately equivalent to three repeated reactions in terms of stabilising activity and surface area. 

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. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

In this way the number of -OH groups per gram of support that are available for reaction with the transition metal alkyl, can be measured. If this information is combined with surface-area measurements, the number of... 

A B.E.T. surface area measurement(37) was carried out on tfie activated Ni powder showing it to have a specific surface area of 32.7 m /g. Thus it is clear that the highly reactive metals have very high surface areas which, when initially prepared, are probably relatively free of oxide coatings. 

Figure 8 shows the spectra of CO adsorbed on our samples they were taken at a CO pressure of about 1 torr. To facilitate comparison, the extinction per square meter of nickel surface, as calculated from the spectra and the analytical data of the samples, has been plotted versus the wave-number. It should be borne in mind that in the calculation of the extinction values, use has been made of the experimentally determined nickel surface areas. Hence, all inaccuracies in the surface area measurements will be reflected in the values of extinction per square meter (E/m2). 

The observed ratio aaiioy/aNi agreed well with the Ni content predicted.) As discussed later, it was believed that hydrogen chemisorption was proportional to the surface nickel concentration (see Section IV). It is clear, however, that chemisorption as a method of surface area measurement must be used with discretion in the case of alloy films. 

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