Estudo experimental e teórico das interações de espécies químicas na superfície do eletrodo de diamante dopado com boro para promover a formação de espécies fortemente oxidantes

Abstract

Diamond films receive special attention in electrochemistry due to their peculiar properties (low capacitive current, wide potential window in aqueous medium, rapid electron transfer kinetics in multiple redox systems, weak molecular adsorption (low passivation) and corrosion resistance) , especially when the boron doped diamond surfaces were employed. Recent studies have demonstrated that the oxidation mechanisms of organic compounds as well as the formation of strong oxidant species are strongly influenced by the characteristics of the conducting layer and the substrate properties of diamond anodes. The main objective of this project is to study by experimental procedures and theoretical calculations the interactions of different species on the surface of the BDD electrode. The calculations were performed at theoretical levels MP2 (with the base set of aug-cc-pVTZ (ATZ)) and DFT (with the functional ones: BHandHLYP, PBE1PBE and X3LYP) implemented in the Gaussian package 09. The solvent was described by two implicit models: PCM and SMD. The reaction of hydrogen atom release of the oxalic acid molecule by the hydroxyl radical, the reaction sequence reaches the relatively high energy barrier of about 0.55 eV. The overall degradation reaction of oxalic acid is exothermic about 7 eV, which should make this a very efficient path. The theoretical results are basically applicable to any inert electrode in which hydroxyl radical formation and oxalic acid degradation proceed through the outer sphere mechanism without adsorption of intermediates. The concentration of sulfate in solution is an important parameter during the degradation of organic compounds. Solutions in low concentration allowed the conductivity necessary for the passage of the electric current applied to promote oxidation of oxalic acid. However, the organic compound molecules were degraded directly on the surface of the diamond electrode by electron transfer. In addition, the production of hydroxyl radicals was not sufficient to promote a mediated electrochemical degradation. These behaviors were evidenced by the polarization curves and the monitoring of the formation of the RNO-●OH adduct when the hydroxyl radicals were produced in the solution. On the other hand, when the sulfate concentration was significantly high in the solution (1 × 10 -2 M), the degradation of the oxalic acid occurs by oxidation mediated by the radical SO4•-. The theoretical calculations were obtained in a solvent, predominantly of water molecules, with sufficient conductivity to allow the development of electrochemical reactions on the surface.