Selwood susceptibility

Magnetic susceptibility of paramagnetic particles is used to determine the concentration of ion-radicals but yields no structural information. The method often demands solid samples of ion-radical salts. Many ion-radical salts are unstable in the solid state, and this requirement turns out to be a decisive limit. Fortunately, there are special ways to determine magnetic susceptibility of paramagnetic particles in solutions (Selwood 1958). However, instruments for such measurements are rarely used in chemical laboratories. Besides, special devices should be used to conduct investigations at different temperatures. 

From the changes in magnetic susceptibility of nickel—silica catalysts during ethylene adsorption at room temperature, Selwood [55] has concluded that ethylene exists both as an associatively and a dissociatively adsorbed species. On increasing the temperature, the dissociative adsorption becomes more important. Thus at 100° C, the susceptibility changes are consistent with the formation of six bonds to the surface for each adsorbed ethylene molecule, suggesting the following process... 

Table 3.8 Atom and group increments to the diamagnetic susceptibility, -x in 10"6 cm3 mol"1 (Selwood 1956)...

In all of this work there was little suggestion that the surface states of the palladium might behave differently from bulk states. Selwood (17) indicated that, from some sorption-magnetic susceptibility data for hydrogen sorbed on palladium which was finely dispersed on alumina gel, the ultimate sorption capacity was approximately at the ratio 2H/Pd. Trzebiatowsky and coworkers (25) deposited palladium on alumina gel in amounts ranging from 0.46 to 9.1% of gel weight. They found the palladium to be present in a normal crystal lattice structure, but its susceptibility was less than for the bulk metal. This suggested to the present authors that the first layer of palladium atoms laid down on the alumina gel underwent an interaction with the alumina, which has some of the properties of a semiconductor. Such behavior was definitely shown in this laboratory (22) in the studies on the sorption of NO by alumina gel. Much of this... 

Susceptibilities were determined by the Gouy method, extrapolated to infinite H (though the effect was small), and corrected for core diamagnetism with Selwood s values. Measurements on Lal2.42 between 1.2 and 80 °K. were made by the mutual inductance method. 

Here, m is the mass of the sample, x is the mass susceptibility, H the field strength and dH/dS the field gradient. Selwood et measured the force by the... 

In summary, the oxygen chemisorption studies, described above, are consistent with the oxidation studies of Weller and Voltz (25) and the magnetic susceptibility measurements of Eisohens and Selwood (5) since they demonstrate that, in a chromia-alumina catalyst, the chromia tends to form clumps or crystallites on the alumina surface. The extent of chromia area is a function of the preparational method, as well as of the composition of the catalyst, and thus its measurement by oxygen chemisorption can be a useful guide in catalyst preparation. 

Historically, the first major advance in our understanding of the physical-chemical structure of a chromia-alumina catalyst resulted from a series of magnetic susceptibility studies carried out by Selwood and Eischens (5). They prepared catalysts by both coprecipitation and impregnation techniques, and measured the magnetic susceptibility as a function of catalyst composition. From the results they were able to draw important conclusions concerning the valence state of the chromium, and the manner in which this chromium was combined with the diamagnetic alumina. The same technique has since been applied by other workers with considerable success. 

It would be expected that chromia-alumina catalysts prepared by coprecipitation techniques should differ from impregnated catalysts, and this difference has been demonstrated by Eischens and Selwood (5) who measured the magnetic susceptibility of a chromia-alumina catalyst (35 wt % Cr) prepared by coprecipitation with ammonium hydroxide from a solution of aluminum nitrate and chromium nitrate. The susceptibility of the reduced catalyst indicated a much greater dispersion of the chromium than was characteristic of the impregnated catalysts. This was attributed to the presence of a three-dimensional dispersion of the chromium in the coprecipitated catalysts, as compared to a two-dimensional dispersion in the case of the impregnated catalysts. 

Below about 10 per cent copper the susceptibilities of copper in the several samples accurately follow the Curie-Weiss law, from which it is (Selwood and Dallas, 24.)... 

Any type of open, porous structure, allowing intimate contact between the solid and aqueous phase, is likely to yield a hydrous phase. In the interaction between water and aluminum, for instance, transmission electron microscopy studies indicate that the initial product consists of very fine fibrils, ca. 3.5 nm thick similarly, Selwood has postulated a threadlike or strand structure to account for the unusually high degree of dispersion evident from magnetic susceptibility studies of various hydrous metal oxide systems. A further obvious possibility here is that open, three-dimensional, cage-type structures (as found, for instance, in zeolites) may be involved. 

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