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

preprint
submitted on 25.05.2020, 23:03 and posted on 27.05.2020, 09:40 by Alfredo Calderón-Cárdenas, Enrique A. Paredes-Salazar, Hamilton Varela

Activation energy is a well-known empirical parameter in chemical kinetics that characterises 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, the concept of degree of rate control is convenient, which comprises a mathematical approach for analyzing reaction mechanisms and chemical kinetics. Although this concept has been used in catalysis, it has not yet been applied in electrocatalytic systems, whose ability to control the potential across the solid/liquid interface is the main difference with heterogenous catalysis, and the electrical current is commonly used as a measure of the reaction rate. Herein we use the definition of ‘degree of rate control for elementary step’ to address some of the drawbacks that frequently arise with interpreting apparent activation energy as a measure of intrinsic electrocatalytic activity of electrode. For this, an electrokinetic model Langmuir-Hinshelwood-like is used for making numerical experiments and verifying the proposed ideas. The results show that to improve the catalytic activity of an electrode material, it must act upon the reaction steps with the highest normalised absolute values of degree of rate control. On the other hand, experiments at different applied voltages showed that if the electroactive surface poisoning process take place, changes in 𝐸𝑎𝑝𝑝 can not be used to compare the catalytic activity of the electrodes. Finally, the importance of making measurements at steady-state to avoid large errors in the calculations of apparent activation energy is also discussed.

Funding

São Paulo Research Foundation (FAPESP) #2013/16930-7

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) #306060/2017-5

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) - Finance Code 001

History

Email Address of Submitting Author

hamiltonvarela@usp.br

Institution

Universidade de São Paulo

Country

Brazil

ORCID For Submitting Author

0000-0002-6237-6068

Declaration of Conflict of Interest

no conflict of interest

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