AC ELECTRICAL CHARACTERIZATION OF HETEROGENEOUS CATALYSTS

At operating temperatures, heterogeneous catalysts behave like lossy dielectrics. The technique of impedance spectroscopy has been used in conjunction with other AC methods to differentiate between different loss processes. At low frequencies and high temperatures, thermally activated hopping of relatively free charges is dominant, whilst at higher frequencies and lower temperatures relaxation losses from dipoles at the surface and in the bulk of the material prevail. The principal use of impedance spectroscopy is in the determination of geometry-independent parameters. These help to elucidate the roles of the insulating and conducting components in a catalyst and define the frequency and temperature ranges over which various processes predominate. In general, the dielectric-frequency relationships are in agreement with the models for the "universal" dielectric behaviour of solids. The electrical parameters are highly sensitive to inherent changes in catalysts and hence these AC methods offer potential in-situ techniques for studying factors such as chemisorption, deactivation, activity, selectivity, phase changes, and solid-state reactions. © 1993 Academic Press, Inc.