Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 189vacuum to atmospheric in less than 0.05 seconds with an error of f 1 torr. Theresponse characteristics of the complete system allow the adsorption cell to attain95 % of its final pressure in 0.3 seconds.Table 3 gives results which have been obtained with evaporated metal films, indicatingthe type of adsorption found for various gas-metal combinations. Chemisorptionon oxides also shows fast and slow effects. The situation is more complicatedthan for metals because both the metal ions and the oxide ions are potentialchemisorption sites. Usually, oxides are prepared as powders and the conceptof a clean surface is necessarily different from that for metals. It is difficult to obtaina stoichiometric oxide surface and diffusion effects are often important. The dataof chemisorption on oxides is given in Table 4.The kinetics of adsorption can be categorised as follows:(1) Non-activated adsorption. This is characterised by (a) zero or negative temperaturecoefficient of velocity, (b) initial rate independent of coverage, and (c)rate proportional to pressure.(2) Activated adsorption without a precursor. This is characterised by (a) exponentialincrease in rate with increasing temperature, (b) continuous fall in ratewith increasing coverage, and (c) rate directly proportional to pressure.(3) Activated adsorption with a precursor. This is characterised by (a) exponentialincrease in rate with increasing temperature, (b) continuous fall in rate withincreasing coverage, and (c) no simple dependence of rate on pressure.Although part of the data for activated adsorption, particularly on metal surfaces,can be ascribed to contamination or diffusion effects, there are some experimentalresults which cannot be explained in this way. The theoretical reasons for thistype of behaviour have been discussed by Bond3 and by Hayward and Trapnell'.In general, such chemisorptions obey the Elovich' equationdN _- ae-bNdtwhere N is the amount adsorbed and a and b are constants. This equation can bederived for a uniform and a'non uniform surface on the basis of a variation ofactivation energy with amount adsorbed.(b) Ultra high vacuum techniqueAs noted in Section 2.1.2, measurement of the kinetics of chemisorption on cleanmetal surfaces generally requires ultra high vacuum techniques, in order to accomplishthe experiment in a reasonable period of time. The variant of the classicaladsorption method known as the flash-filament technique has been developed byseveral groups of workersE4-'' and recently summarised by Ehrlich".The kinetic data obtained from the flash filament technique depend on threeparameters, the absolute rate of adsorption (AdN/dt) occurring on a sample ofReferences pp. 270-278