Understanding the electrochemical oxidation of dyes on platinum and boron–doped diamond electrode surfaces: experimental and computational study

Abstract

Anodic oxidation (AO) approach proceeds via direct and indirect electrochemical pathways and their subsequent reactions. The interest to elucidate the mechanisms for removing dyes from water contributes to the understanding of more complex reactions involving organic pollutants towards anode surfaces. The present study was motivated by the reports that promote the use of AO for removing different organic compounds but no considerations about the influence of different functional groups in their structure have been discussed. Therefore, we have evaluated the influence of different functional groups in the dye structure (Reactive Orange 16, Reactive Violet 4, Reactive Red 228, and Reactive Black 5) by potentiodynamic measurements and by computational analyzes using density functional theory (DFT). The computational studies have allowed to carry out morphological studies on the frontier orbitals where the electrons are more energetic and then, the electron-transfer to electrode surface is achieved, which was associated to the electrochemical measurements (current-potential profiles). Also, the theoretical studies were used to understand the bulk electrolysis, in terms of mineralization. The results clearly demonstrate that organic molecules can be degraded in different way and level due to the oxidants electrochemically generated as well as the interaction of dyes with anode surface by adsorbed/non-adsorbed intermediates. Conversely, the decolorization mechanisms, which are related to the fragmentation of chromophore group, are associated to the direct AO approach, favoring different order of elimination, as already reported in our previous work. The results were discussed in light of the existing literature.

Photovoltaic Electrochemically Driven Degradation of Calcon Dye with Simultaneous Green Hydrogen Production

“…Meanwhile, no cathodic peaks were registered at the high overpotential region in the latter approach, which corresponds to the indirect oxidation mechanism. This electric-diamond surface feature is interesting because it confirms that the EO of organics does not only depend on the general rule for active and non-active anode classification [ 26 , 43 , 46 ]. Instead, the EO also depends on the chemical structure of the pollutants, the properties of diamond film, and the electrolyte and its concentration, as well as the potential or current in which the EO is promoted.…”

“…The four observed anodic peaks were located at +0.63 V (E pa 1), +1.00 V (E pa 2), +1.54 V (E pa 3) and +1.85 V (E pa 4), while only two cathodic peaks were detected at +0.26 V (E pc 1) and +0.81 V (E pc 2). The CV profile clearly evidenced the versatile behavior of the BDD electrode due to the effective promotion of direct and indirect oxidation routes of the azo dye at its surface [ 43 ]. Anodic peaks (E pa 1 = +0.63 V and E pa 2 = +1.00 V) at the lower overpotential region (<1.6 V) are associated with direct electron-transfer on the diamond surface, while the current-voltammetric signals (E pa 3 = +1.54 V and E pa 4 = +1.85 V) at the high overpotential region (>1.62 V) should be related to the indirect oxidation approach via the participation of free heterogeneous • OH (which are electrogenerated via water discharge (Equation (2)) in the Nernst layer (commonly named “reaction cage” [ 44 , 45 ]).…”

“…Firstly, a direct electron transfer occurs at lower potentials, triggering a reversible reaction that converts Calcon into two possible by-products, which is then reduced to release the Calcon again. This mechanism favors a partial adsorption stage due to the exitance of an interaction on the BDD surface [ 43 , 46 ]. The chemical structure of the target organic compound, the electrical density of the functional groups, and the surface characteristics of the diamond electrode may all play a role in this interaction, as already shown by other authors [ 43 , 52 ].…”

“…This mechanism favors a partial adsorption stage due to the exitance of an interaction on the BDD surface [ 43 , 46 ]. The chemical structure of the target organic compound, the electrical density of the functional groups, and the surface characteristics of the diamond electrode may all play a role in this interaction, as already shown by other authors [ 43 , 52 ]. A facilitated oxidation process is preferred at higher positive potentials where the • OH radicals can be electrogenerated [ 26 ].…”

Recent developments in hydrogels containing copper and palladium for the catalytic reduction/degradation of organic pollutants

“…9–11 Scheme 1 provides the different approaches for the elimination of pollutants including absorption, chemical reactions, adsorption, photocatalysis, and filtration. 12–23 Some of these methods are simple, inexpensive, with low energy consumption and high yield, and do not produce toxic products or require complex instruments; however, eco-friendly approaches are preferable. 24 Chemical degradation is one of the most extensively utilized strategies for removing pollution from the environment, including ozone/UV, irradiation/H 2 O 2 oxidation, photocatalytic degradation, supercritical water oxidation, and electrochemical methods.…”