Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

194 EXPERIMENT A L MET H o D s F o R HE T E R o GE NE o u s RE A c T I o N sIf desorption is complete before gas can leave the system, (i.e. Q, = 0), the gasdensity rises to a peak (N2-Nt) giving the amount of gas adsorbed directly.Such rapid heating is not essential for determining the amount adsorbed. Whenthe original steady state is restored N, = No, Qi = Q, and the shaded areas inFig. 5 must be equal. The identity of these two quantities serves as a convenientcheck on experiment.The ideal systems described are difficult to achieve in practice. In a real closedsystem adsorption occurs on the walls and gas is pumped by the pressure gauge.The walls may degass as the pressure falls, presenting a difficult problem. Thereforethe equation for a closed system is really represented by the equation for a flowsystem, where FA and S, now denote gas flow or pumping of any type other thanthat due to adsorption on the filament. Closed systems are also limited becausefor a given initial pressure, only a small range of surface concentrations can bemeasured. Fig. 6 shows a typical apparatus for adsorption measurements. Theflow into the adsorption cell is controlled by valve V, and the pumping rate can bealtered by adjusting the magnetically operated valve E. A much higher pressureis maintained in the gas reservoir B so that the admittance rate does not fallduring the experiment. In order to avoid pressure gradients, the gauge and filamentmust be brought close together, the limit being set by the need to shield thefilament from heat generated by the gauge and from stray electrons.The type of apparatus described can also be used for making desorption studies.Because the heating of the sample, to effect desorption, changes the temperatureof the flament, processes which occur at different temperatures can be recognised.6 1E111 IFig. 6. Flow system for adsorption studies, with independent control of entrance and exit speed.F, adsorption sample; I, ionisation gauge. Flow out of gas reservoir (B) and into cell controlledby valve VA; gas flow out of cell regulated by adjustable exit port E. From ref. 88.