Validation of Mixed Potential Theory Using Formic Acid and Ferric Ion As a Redox Couple
Authors: Chauhan Nilam L. , Sarkar Arindam , Juvekar Vinay A , Mahajani Sanjay M. , Suresh A.K. 
Published: February 2020

The present work explores the possibility of validating mixed potential theory using the redox reaction between formic acid with ferric sulphate (HCOOH + 2Fe3+ ⇋ CO2 + 2H+ + 2Fe2+) on platinum catalyst. It is hypothesized that the catalytic oxidation of formic acid by ferric sulphate can be viewed as two distinct electrochemical reactions proceeding simultaneously on the electrode, one being the electrochemical oxidation of formic acid and the other, electrochemical reduction of ferric ions. Moreover, for electroneutrality, the rate of oxidation of formic acid must match exactly with that of the reduction of ferric sulphate. The manifestation of this phenomenon is the unique electrode potential generally referred as the mixed potential. The value of the mixed potential can be determined easily by measuring the open circuit potential (OCP) of the electrode which is immersed in a solution containing both formic acid and ferric sulphate. Further, in the present work an attempt has been made to investigate the correlation between catalytic chemical reaction and the individual electrochemical halves in terms of conversion of ferric ions.

The catalytic oxidation of formic acid by ferric sulphate was studied at rotating disk electrode on which platinum black catalyst was drop casted. The reaction was studied through open circuit potential (OCP) measurement (where no external electromotive force is applied) at different rotational rates of the electrode. The number of moles of ferric ions was determined spectroscopically over a period of five hours. It was observed that OCP increased with time due to decrease in the concentration of ferric ions. These experiments have been designated as chemical experiments. Simultaneously, the two electrochemical halves of the above redox reaction were studied independently at rotating disk electrode. Formic acid oxidation studies were performed through chronoamperometry and that of ferric ion reduction was studied via cyclic voltammetry technique. These sets of experiment are refereed as electrochemical experiments. From the electrochemical data obtained for reduction of ferric ions, key kinetic and transport parameter were determined and these were used further to model the current-voltage behavior for reduction of ferric ions to ferrous ions over time, assuming a first order reaction. Using this model in conjunction with OCP time data it was possible to predict the number of moles of ferric ion consumed as function of time during the OCP experiment. These predictions were compared with the number of moles of ferric ions actually consumed during OCP experiment. These comparisons were made at different rotation rates of the electrode and very good agreement between the measured and predicted values was observed at all rates of rotation. This validates the hypothesis that the kinetics of a catalytic chemical reaction can be obtained from the individual current potential relationship for the two electrochemical reactions.

The present work suggests that catalytic chemical reactions follow an electron transfer mechanism. This approach would enable prediction of chemical kinetics for primarily catalytic reactions through few electrochemical parameters. Further, it opens up possibility of estimation of key electrochemical parameters for several catalytic systems and then the kinetics can be predicted by selecting the required parameters for corresponding oxidation as well as reduction reaction.

Figure 1