Active surface area measurement

ECSAcoi Electrochemically active surface area measured by CO2 detection... 

Surface Area. Overall catalyst surface area can be determined by the BET method mentioned eadier, but mote specific techniques are requited to determine a catalyst s active surface area. X-ray diffraction techniques can give data from which the average particle si2e and hence the active surface area may be calculated. Or, it may be necessary to find an appropriate gas or Hquid that will adsorb only on the active surface and to measure the extent of adsorption under controUed conditions. In some cases, it maybe possible to measure the products of reaction between a reactive adsorbent and the active site. Radioactively tagged materials are frequentiy usehil in this appHcation. Once a correlation has been estabHshed between either total or active surface area and catalyst performance (particulady activity), it may be possible to use the less costiy method for quaHty assurance purposes. 

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]. 

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. 

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. 

Alternatively, it may be possible to demonstrate for the pure metals that the catalytic activity is independent of film weight in a certain weight range. For example, rates of ethylene oxidation were constant over pure palladium films, deposited and annealed at 400°C and weighing between 4 and 40 mg (73). Then, if electron micrographs show that the crystallite size is relatively independent of composition, a satisfactory comparison of catalytic activity can be made at the various alloy compositions. Finally, surface area measurements are less urgently needed when activity varies by orders of magnitude, or where the main interest lies outside the determination of absolute reaction rates. 

Comparative methods may be effectively used for measurements of partial surface areas, Ac, of components in porous composites, for example for active surface area in supported catalysts. The traditional methods of Ac measurements are based on chemisorption of H2, 02, CO, NOr. and some other gases that chemisorb on an active component, and have negligible adsorption on a support [5,54], The calculation of Ac is fulfilled by an equation similar to Equation 9.18 assuming some values of w and atomic stoichiometry of chemisorption [54]. But, unfortunately chemisorption is extremely sensitive to insignificant variations of chemical composition and structure of surface, which alters the results of the measurements. 

Surface area measurements of natiual and thermal activated clay minerals were made and the results are given Table 20.2. 

By active surface area we meant the kinetically active part of the total surface area. According to Helgeson et al. (1984), this area is restricted to etch pits. Alternative estimates of surface areas may be obtained from measurements of specific surface area s whenever solid particles have a narrow size range. The specific surface area for spherical particles is given by... 

While and 17 can be rationalized on the basis of kinetic parameters, LHE depends on the active surface area of the semiconductor and the cross section for light absorption of the molecular sensitizer. In practice, the IPCE measurements are performed with monochromatic light, and 1 ( ) values are calculated according to Equation 17.9. 

Zeolite samples that were exhaustively treated were gray in color (materials A). Portions of each treated catalyst (0.3 gram) were calcined at 500°C in oxygen for 6 hours, giving a white product (material B). X-ray analysis showed no deterioration in crystal structure on treatment with TMS and subsequent calcination. Surface area measurements using N2 adsorption at — 196°C and the Point B method are also given in Table I. The value for HY zeolite, activated as described, prior to TMS treatment was 840 m2/gram. 

The best approach is normally an in situ determination based on voltammetry or charging curves, usually within the hydrogen adsorption region [96]. It is of course necessary to know the actual value of 0H for absolute determinations, but the method is practicable on a relative basis. The method becomes absolute only in a few cases, in particular for Pt electrodes [97] for which the catalytic activity per metal atom, which is the parameter really needed to evaluate electrocatalytic effects, can be calculated [98]. Sometimes, results are reported relative to the surface area measured on the basis of the limiting current for a redox reaction [99], but what is obtained is only the macroscopic surface in which asperities of a height higher than the diffusion layer thickness can only be accounted for. 

The latter two factors in this equation depend on kinetic factors, while LHE(A) depends on the nature of the light-absorbing sensitizer and the active (surface) area of the nanocrystalline surface. Measurements are normally carried out by using monochromatic light and the IPCE values are then calculated using the following equation ... 

Indeed, lattice parameters of both the copper and the zinc oxide were found to depend on the catalyst composition. The lattice extension of copper was attributed to alpha brass formation upon partial reduction of zine oxide, and an attempt was made to correlate the lattice constant of copper with the decomposition rate of methanol to methyl formate. Furthermore, the decomposition rate of methanol to carbon monoxide was found to correlate with the changes of lattice constant of zinc oxide. Although such correlations did not establish the cause of the promotion in the absence of surface-area measurements and of correlations of specific activities, the changes of lattice parameters determined by Frolich et al. are real and indicate for the first time that the interaction of catalyst components can result in observable changes of bulk properties of the individual phases. Frolich et al. did not offer an interpretation of the observed changes in lattice parameters of zinc oxide. Yet these changes accompany the formation of an active catalyst, and much of this review will be devoted to the origin, physicochemical nature, and catalytic activity of the active phase in the zinc oxide-copper catalysts. 

The early conflicting reports on the activity of pure copper metal could not be reconciled without the simultaneous or concurrent measurements of activity, surface area, and surface composition. Moreover, it became evident that it is important to use unsupported copper as the reference material to avoid support-metal interactions that may influence the catalytic properties of the latter. 

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.

---