Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLIDGAS INTERFACE 217tion curve, or the maximum intensity, both of which are a functionof the concentrationof the adsorbing species. This is usually expressed by Beer's law,log(l,/l) = CELwhere I,, Z are the intensities of the incident and transmitted light respectively,C is the concentration in mole. 1-', E is the molar extinction coefficient and L isthe path length. Log Z,/Z is called the optical density or adsorbance.In general, chemisorption will produce new spectral bands which are notcharacteristic of the adsorbate or the adsorbent. However, absence of such bandscannot be taken as evidence of an absence of chemisorption. A difficulty presentin any attempt to make kinetic measurements is that extinction coefficients areoften significantly altered as a result of adsorption. These changes, which cannotas yet be interpreted theoretically, make it difficult to correlate the observedabsorbance with the coverage of adsorbed molecules. The change in extinctioncoefficient is dependent on both the adsorbate and the adsorbent. For example, anincrease of E was observed with increasing coverage for ethylene adsorbed oncopper oxide, whereas the reverse occurred with nickel oxide' 63.Since chemisorption results only in the formation of a monolayer, the radiationmust traverse many layers for the adsorption bands to be detectable. This may beachieved in three ways (a) by transmission through a highly dispersed medium,(b) by multiple reflections, or (c) by a series of total internal reflections. Applicationsof the latter two methods to the study of adsorption has been limited up to thepresent time'64* lS5. The Grst method requires the preparation of the adsorbatein the form of a powder or a porous glass. In order to reduce light scattering to aminimum, the maximum size of the particles must be smaller than the wavelengthbeing used. In the infrared region, this implies particles smaller than one micronfor non-absorbing species. For species which absorb in the infrared, such as metals,even smaller particles are required. On moving towards the ultraviolet, the scatteringlosses increase rapidly, thus making it even more important to obtain verysmall particles.Eischens and Pliskin160 prepared samples by spreading a slurry of the powderon a calcium fluoride disc and evaporating the volatile liquid. Fahrenfort andHazet?roek'66 supported the powder on a 200 mesh screen, claiming that sampletreatment was possible over a wider temperature range, with no wavelengthlimitations due to strong absorption by the catalyst. Some powders can be pressedinto wafers and used dire~tly'~'. Metals are generally supported on silica, aluminaor titania. Cabosil (SOz) and Alon C (y-Al,O,) with particle sizes less than 200 Aare suitable. The metal is deposited by impregnation with a metal salt solution,drying and reduction in hydrogen. It is preferable that the size of the final metalparticle be less than 100 A. Transparent massive samples of silica gel and silicaaluminahave been prepared by Leftin and and transparent y-aluminaReferences pp. 270-278