The Role of Self-Organized Reaction Rates in the Micro-Kinetic Description of Electrocatalytic Reactions
Authors: Calderón Cárdenas Alfredo , Paredes Salazar Enrique Adalberto , Varela Hamilton 
Published: July 2021

Kinetic modeling of electro-catalytic reactions based on micro-kinetic approaches allows at obtaining information on reaction mechanisms and determine how some parameters, such as rate coefficients, are affected by the conditions under which the catalyst operates.1 Unlike a kinetic model, which only considers the overall reaction rate, micro-kinetic modeling provides access to fundamental information of elementary reaction steps. In this sense, we present a micro-kinetic model linked to a proposed reaction mechanism for the formic acid electro-oxidation reaction (FAEOR) on platinum. The model was tested by numerical simulations under voltammetric and oscillatory regimes.2

We formulated the micro-kinetic model using the following workflow stages: i) Gathering information from literature, including spectroscopy and computational chemistry studies; ii) Mechanism proposal and model description that consists of a set of ordinary differential equations involving charge and mass balances on the electrocatalytic surface; iii) Numerical resolution of the model using a set of test parameters; iv) Comparison with different sets of experiments including cyclic voltammetry and potential oscillations under the galvanostatic regime. The initial electrokinetic parameters, associated with the rate coefficients, were adjusted and re-evaluated by an iterative procedure to improve the description of the experimental observations.

The type of electrochemical experiments performed in the micro-kinetic model is fundamental to the success of validation. Under oscillatory conditions, galvanostatic experiments are an excellent complement to other electrodynamic techniques because the rate of several processes that are happening in the electrode-solution interface can be associated with oscillatory features such as frequency, amplitude, and oscillation format.3 Thus, the self-organized mode offers a sensitive tool to evaluate the proposed reaction mechanism. In this work, we studied the oscillatory characteristics in the electrochemical response of the FAEOR to determine the rate coefficients as a function of the electrode potential. The consistency of the numerical solution of our model (Figure 1), namely frequency and amplitude of the potential oscillations, with the experimental response makes our proposal a plausible possibility for the FAEOR, providing evidence to the clarification of this controversial process through a micro-kinetic approach.

References

(1) Campbell, C. T. Micro- and Macro-Kinetics: Their Relationship in Heterogeneous Catalysis. Top. Catal.
1994, 1 (3–4), 353–366. https://doi.org/10.1007/BF01492288.

(2) Calderón-Cárdenas, A.; Hartl, F. W.; Gallas, J. A. C.; Varela, H. Modeling the Triple-Path Electro-Oxidation of Formic Acid on Platinum: Cyclic Voltammetry and Oscillations. Catal. Today
2021, 359, 90–98. https://doi.org/10.1016/j.cattod.2019.04.054.

(3) Machado, E. G.; Varela, H. Kinetic Instabilities in Electrocatalysis. Encyclopedia of Interfacial Chemistry; Elsevier, 2018; pp 701–718. https://doi.org/10.1016/B978-0-12-409547-2.13369-4.

Figure 1