Cathodic hydrogen production by simultaneous oxidation of methyl red and 2,4-dichlorophenoxyacetate in aqueous...

In this work, results concerning hydrogen gas production during the oxidation of methyl red (MR) and sodium 2,4-dichlorophenoxyacetate (2,4-DNa), is presented, emphasizing not only the amount of hydrogen gas that was produced but also the kinetic and efficiency parameters involved in this process. For this purpose, a two-compartment electrochemical cell was used with a Nafion® membrane as separator in order to collect H₂ without other chemical species (only with traces of water vapor). Under these experimental conditions, it was possible to guarantee the purity of the H₂ collected. The electrochemical oxidation of MR and 2,4-DNa solutions was carried out by applying 30 mA cm^-2 at 298 K, using different non-active anodes (Si/BDD, Pb/PbO₂, or Sb-doped SnO₂) and different cathodes (Pt mesh, 316-type stainless-steel, or Pt-10%Rh) in order to investigate the effect of the electrocatalytic materials and experimental conditions. Thus, the H₂ produced was measured as a function of the electrolysis time and compared with the values estimated by Faraday's law. The results showed that the hydrogen production rate r(H₂) is independent of the nature of the anodic material, although an important effect on the oxygen production was observed on the BDD anode by using sulfuric acid as supporting electrolyte. The effect was discussed through the formation of sulphate-oxidizing species (SO₄ ^-˙ and S₂O₈ ^2-) which interfere in the oxygen production step on BDD anodes. The use of different cathodes showed small changes in the hydrogen production rate r(H₂), which were basically associated with the differences in hydrogen adsorption energy prior to its evolution. The results were discussed in light of the existing literature.