The Pressed-Salt Method for Obtaining Spectra of Adsorbed Molecules

The Pressed-Salt Method for Obtaining Spectra of Adsorbed Molecules 

The pressed-salt method has attained wide application in studies of the infrared spectra of solids. In this method the solid sample is mixed with a powdered halide salt such as KI or KBr and the mixture is pressed into a disk at high pressures 53-55). This method reduces scattering because solid-gas interfaces are replaced by solid-salt interfaces. When this method is used,-the particle size of the solid is not of critical importance and most ordinary silica or alumina catalysts can be used without the necessity of any particle-size separation. Although it is simple experimentally, the pressed-salt method will probably never attain a major importance in catalytic work, because once the sample is embedded in the salt, it cannot be subjected to further treatment. 

A recent paper describes the use of self-supporting disks of alumina as samples for the infrared study of adsorbed water 56). These self-supporting disks were prepared in the same type of die as is used for the pressed-salt method. The self-supporting disks will undoubtedly prove to be far superior to pressed-salt disks because the former are pervious to gas and the complications caused by the presence of salt are eliminated. Satisfactory self-supporting disks were made by pressing 0.2 gm. of alumina in a 1-in. diameter die at a pressure of 30-40 tons/sq. in. No preliminary particle-size separation is necessary, and the alumina may be impregnated with metal salts either before or after pressing in the die. 

The study of flotation by the pressed-salt method was illustrated by the spectrum of oleic acid adsorbed on CaF2. This spectrum shows a band at 

which was attributed to oleic acid chemisorbed as an oleate and associated with surface Ca atoms. The bands due to the C=C double bond were not observed in the spectrum of chemisorbed oleic acid. The failure to observe bands due to the C=C double bond was attributed to lateral polymerization of the chemisorbed species. It appears more reasonable to assume that these bands were not observed, because the extinction coefficient of the C=C band is small compared with that of the carboxylate ion. 

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VI. The Pressed-Salt Method for Obtaining Spectra of Adsorbed Molecules.. 45... 

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