Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

200 EXPERIMENTAL METHODS FOR HETEROGENEOUS REACTIONS2.1.5 Measurement of work function2"l6The electron work function of a solid, er$, is defined as the work necessaryto remove an electron from the highest occupied level inside the solid to a pointin uacuo just outside the surface. (Further details of the theory of the work functioncan be obtained by reference to works on solid state physics"'~"*.) Whenadsorption takes place, dipoles are created, altering the potential barrier which theelectron must penetrate to escape the surface. 4 is increased if the dipole producedon adsorption is directed away from the surface (electronegative layer), ordecreased if the dipole is directed towards the surface (electropositive layer). Byconsidering the adsorbed layer as a parallel plate condenser, it can be shownthat the potential drop (AV) across the layer is given byAV = 4nN,8pwhere N, = number of surface sites per cm2, 8 is the fraction of surface coveredand p is the dipole moment of the adsorbed molecule. eAV = eA+, the change inenergy required to remove an electron from the solid after forming the adsorbedlayer. ThereforeThe kinetics of adsorption can be studied from the rate of change of + providedp is constant. However, this is not necessarily the case except over a limited range.It must aIso be remembered that each crystal plane wilI have a different workfunction in the clean state. If more than one crystal plane is exposed, an averagevalue will be obtained. Added to this is the fact that many gases adsorb differentlyon different crystal planes which makes the interpretation of results rather complicated.Methods of work function measurement are of two types, electron emissionmethods and condenser methods. In the former method heating, irradiation withlight of a suitable wavelength or application of sufficiently strong electrical fieldsis used to cause electrons to tunnel through the surface potential barrier. The lattermethod consists of measuring the contact potential difference between the surfaceunder study and a reference electrode.(a) Thermionic emissionFor thermionic emission from a uniform surface, the saturation current densityj extrapolated to zero applied field is given byj = ~ ( 1 - t ) exp(-e4/kT) ~ ~