Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLIPGAS INTERFACE 197I , , -, IN52 IM IMFig. 9. Desorption spectrometer using 10 kc bridge. R,, R, and R, are part of standard impedancebridge. C, = 30 pf, C, = 0-100 ppf. From ref. 88.Apparatus suitable for the study of evaporated metal films under ultra highvacuum conditions has been described by Roberts" and by Hayward et al.".In addition to an ion gauge for pressure measurement, both authors employ amass spectrometer to characterise the species present in the gas phase. This isparticularly important when studying hydrocarbonsg1 which may decompose inthe system. However, there are also problems associated with the measurement ofhydrogen and oxygen. The standard tungsten filament found in an ion gauge reactswith both gases. Hydrogen is dissociated into atoms at the emission temperature,and this must be avoided by the use of a coated filament. Lanthanum boride coatedon tantalum, tungsten or rhenium is suitable for this purpose and gives a satisfactoryelectron emission at a temperature of 1300" C93-95. Oxygen reacts withthe carbon present in a tungsten filament to form CO and C0227*90. The carboncan be depleted by operation of the filament for many hours in a low partialpressureof oxygen"i"'. An alternative is the use of an iridium filament.2.1.4 Measurement of adsorbed weightMeasurement of weight changes is a very direct method of obtaining adsorptionkinetics, and is also used to follow the course of other heterogeneous reactions(see Section 3). It is particularly suitable for adsorption measurements with smallsurface areas at relatively high pressures, where the volumetric method is insensitive.Several comprehensive reviews are availableg6 which discuss the theory,References pp. 270-278-'*