Apparent Activation Energy in Electrochemical Multistep Reactions: A Description via Sensitivities and Degrees of Rate Control

Abstract

Activation energy is a well-known empirical parameter in chemical kinetics that characterizes the dependence of the chemical rate coefficients on the temperature and provides information to compare the intrinsic activity of the catalysts. However, the determination and interpretation of the apparent activation energy in multistep reactions is not an easy task. For this purpose, concepts based on the definition of degree of rate control are convenient, which comprise mathematical approaches for analyzing reaction mechanisms and chemical kinetics. Although these concepts have been used in catalysis, they have not yet been quantitatively applied in electrocatalytic systems, whose ability to control the potential across the solid/liquid interface is the main difference with heterogeneous catalysis, and the electrical current is commonly used as a measure of the reaction rate. Herein, we use the definitions of sensitivities and kinetic degree of rate control to address some of the drawbacks that frequently arise with interpreting apparent activation energy as a measure of intrinsic electrocatalytic activity of an electrode. For this, a Langmuir–Hinshelwood-like electrokinetic model is used for making numerical experiments and verifying the proposed ideas. The results show that, to improve the catalytic activity of an electrode material, one must act upon the reaction steps with the highest normalized electrochemical sensitivities. On the other hand, experiments at different applied voltages showed that if the electroactive surface poisoning processes take place, changes in Eapp should be carefully used to compare the catalytic activity of the electrodes considering the effects by surface coverages. Finally, the importance of making measurements at steady-state to avoid difficulties in the calculations of apparent activation energy is also discussed.

Topics & Concepts