The Compensation Effect in Heterogeneous Catalysis

This chapter discusses the compensation effect (C.E.) in heterogeneous catalysis. C.E. indicates that an increase in the enthalpy of activation frequently has not the expected result of a considerable decrease in the rate constant, because there occurs a simultaneous increase in the entropy of activation or of the frequency factor, which compensates partly or entirely for the change in the exponent. It is assumed that the activated complex has the same composition for both the homogeneous and the heterogeneous reaction. Reactions involving the tunneling of electrons among ions of different valence states may serve as an example for the possibility that a tunnel effect may account for the occurrence of C.E. In many cases, a C.E. is indicated on comparison of the rates of the same reaction at the same type of catalyst after the latter has undergone special pretreatments or has been changed in its composition. Moreover, an apparent C.E. can result from errors in the experimental data used for an Arrhenium plot.