Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

218 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSby Peri and HannanI6'. These samples have the advantages of greater ease ofmanipulation and smaller scattering losses than powder and can be readily impregnatedwith metals. However, this type of preparation is not yet fully reproducible.Commercial spectrophotometers are suitable for the measurement of adsorptionbecause it is the sample rather than the instrument which limits the resolution.Special types of sample cell are required and it is desirable that the cell is attachedto a vacuum line and a gas handling system to effectively manipulate the gaseousambient. Eischens and Pliskin16* used a Pyrex cell with calcium fluoride windowsat both ends for measurements in the infrared region. A calcium fluoride plateheld the powder sample and coula be heated by a tungsten heating element woundon a quartz tube. This type of cell has two disadvantages. It must be mountedvertically to prevent loss of powder, which requires modification in the normalspectrometer arrangement, and it cannot be baked out at high temperatures becauseof the glyptal resin used to seal the calcium fluoride windows to the Pyrexbody. For compressed discs and massive plates, the cell need not be verticallymounted. Cells which allow the sample to be outgassed in one section and thenmoved to the measuring section containing the salt windows have been des~ribed'~~-''~.A design used by Peri and Hannan16' is illustrated in Fig. 22.The infrared cell is constructed of Vycor and quartz. Calcium fluoride windowsare sealed to thin silver flanges and to the Vycor cell with silver chloride. Theaerogel plate can be reproducibly moved between the furnace section and theinfrared beam by an enclosed magnet. In this way, spectra of the gas phase alonecan be obtained. Disadvantages are the use of wax to seal the ground joint andInfrared beamYITo vacuum Nlchrqme saAple holdersy tem winding,sn(quqrtz)I n!~y:~hk~Quartz-enclosed magnet18 in. ._______IFig. 22. Infrared cell for the study of solids under vacuum or in the presence of gaseous ambients.From ref. 169.