Electrochemical study of carboxylic acids with Nb-supported boron doped diamond anode. Part 2: Electrochemical oxidation associated to DFT calculations

Abstract

This work investigates electro-oxidation (EO) of carboxylic acids (acetic acid (AA), oxalic acid (OA) and formic acid (FA)) in aqueous solution by using Nb-supported boron-doped diamond (BDD) electrode. Voltammetric studies conducted with Nb/BDD anode show that AA oxidation takes place at a potential very close to that of the discharge of water, while OA and FA are oxidized before the discharge of water. Galvanostatic electrolyzes of aqueous solutions containing AA, OA, FA in HClO4 were conducted to determine the elimination of carboxylic acid concentration, mineralization degree and oxidation kinetics by measuring total organic carbon (TOC) and liquid chromatography (HPLC). Results showed that OA and FA were quasi-completely mineralized; while lower concentrations of intermediates were detected during AA oxidation. The association of electrochemical measurements with computational simulations led us to understand the EO pathway of AA, OA and FA on BDD surface. Theoretical calculations demonstrated that, HOMO energies are related to the difficulty to oxidize (losing an electron) a specific compound. Analyzing by computational calculations the EO reactions attained, it was possible verify that AA and OA are oxidized by a direct electron transfer with the anode surface coupled with OH radicals, while FA is most probably oxidized via OH radicals generated by water discharge. In the case of AA, oxidation pathway possibly involves the production of intermediates that avoid a complete elimination of this compound.