Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

196 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONSadsorbed gas. First order desorption is independent of initial concentration andsecond order desorption gives a characteristic s curve. If the pumping rate cannot beneglected by comparison with the desorption rate, the analysis becomes more complicatedand is best followed using a heating curve such that 1/T remains linearwith time. Ehrlich8* gives an analysis of the kinetics under these conditions andillustrates the technique by reference to the desorption of nitrogen from tungsten.A suitable apparatus for flash desorption studies is illustrated in Fig. 8. TheBayard-Alpert gauge is coupled with a high speed ion current detector to providean instantaneous record of gas density. The filament and gauge need to be mountedin separate containers to minimise electrical interference, but connected by widebore tubing to prevent pressure gradients. Control and measurement of temperatureis of prime importance. This can be accomplished by means of a simple bridgecircuit8’ or preferably by means of the ‘desorption spectrometeryg0 shovn in Fig. 9.The heating and measuring functions of the filament are separate, direct current isused for heating, but resistance is measured by a 10 KC impedance bridge.Flashing of the filament results in an unbalance signal proportional to the resistance.This is amplified, detected, filtered and displayed on one axis of an oscilloscope.In converting filament resistance to sample temperature, it should be rememberedthat the temperature is not uniform over the whole sample. Correctionsfor end losses become smaller as the rate of filament heating is increased and thefilament diameter decreased.Fig. 8. Flash desorption cell and ultra high vacuum system. C, desorption cell with sample F,mounted on glass covered tungsten leads L, and gauge G. Volume of cell is defined by groundglass port D and E. A and H, ion gauges for leak checking and pressure monitoring; B, Granville-Phillips valve; I, liquid nitrogen trap; S, selective getter bulb. From ref. 88.