A model for the electrochemical reduction of 2-ethylpicolinate under galvanostatic control

Abstract

A model is presented for the variation in reactant concentrations during the electrochemical reduction of 2-ethylpicolinate on a lead cathod in sulphuric acid solutions under galvanostatic conditions. The electrolyses were performed in a laboratory filter-press reactor. Successive or parallel electrochemical reactions coupled with chemical reactions are taken into account, according to a reaction scheme in agreement with the experimental results. Analytical expressions are used to describe the progress of the various reactions, taking into account both chemical and electrochemical kinetics and transfer properties. All reactions are assumed to be first or pseudo-first order. The variations in charge-transfer rate constants are considered as functions of reactant conversion. The effects of acidity, current efficiency, initial concentration of 2-ethylpicolinate and temperature are presented. The model aims at estimating the yield of electrolysis products under the experimental conditions necessary for obtaining 2-hydroxymethylpyridine in optimum quantities.Symbols C X concentration of species X in the cathodic compartment (mol m )3 ) C 0 X initial concentration of species X in the cathodic compartment (mol m )3 ) C XS concentration of species X at the surface of electrode (mol m )3 ) c width of cathodic compartment (direction perpendicular to flow) (31Â 10 )3 m)electrode potential/ECS (V), at I=0 for the electrochemical reaction i F=96487Faraday's constant (C mol )1 ) h thickness of the cathodic compartment (direction perpendicular to flow) (3Â 10 )3 m) [H 2 SO 4 ] sulphuric acid concentration (mol dm )3 in the formulas) I electrolysis current (A) j electrolysis current density (A m )2 ) j i current density of the electrochemical reaction i (A m )2 ) k dX mass-transfer coefficient of species X (m s )1 ) k fi charge-transfer constant for the electrochemical reaction i (m s )1 ) k fi0 charge transfer constant without overvoltage for the electrochemical reaction i (m s )1 ) k i rate constant for the chemical reaction i k¢ i apparent rate constant for the chemical reaction i M X molar mass of species X (g mol )1 ) n number of electrons exchanged p parameter for the expression of R f Q charge at time t per mol of reactant (F mol )1 ) Q V flow rate (12.8Â 10 )6 m 3 s )1 ) r i rate of the chemical reaction i R=8.314 ideal gas constant (J K )1 mol )1 ) XmReynolds number R f faradaic yield R X chemical yield in species X s slope