Compensation effects and compensation defects in kinetic and mechanistic interpretations of heterogeneous chemical reactions

Abstract

AbstractA case is presented for kinetic compensation effects (KCEs) reported for sets of heterogeneous rate processes to be classified. Because many of these KCEs do not conform to the ideal of isokinetic behavior, a quantitative method for describing the degree of compensation (κ) is introduced. In the case of solid‐promoted catalytic reactions, it is suggested that (unlike simple homogeneous reactions) the effective concentrations and dispositions of surface intermediates can vary with temperature. Thus, the rate constants vary differently with temperature for the individual reactions of a set and, thereby, influence apparent magnitudes of the measured Arrhenius parameters. As a consequence, compensation is observed for sets of chemically related reactions occurring within a common temperature interval. KCEs are thus linked to mechanistic information and, depending on the magnitude of κ, for identifying shared features of catalytic rate controls. In this respect, the classification of KCEs is useful and can be based on similarities or differences between the reactants, catalysts, or experimental conditions in a given set of measurements. These ideas are demonstrated for selected sets of catalytic reactions. KCEs reported from a range of thermal decompositions of solids are also discussed. It is argued that, for mathematical as well as term definition reasons, compensation in this type of heterogeneous process is often artifactual and, therefore, cannot provide information of mechanistic value. Overall, it is concluded that a more thorough interpretation of compensation trends should lead to greater understanding of heterogeneous processes, particularly at the mechanistic level. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 464–473, 2006