Photoelectrocatalytic degradation of diazo dyes on nanostructured WO3 electrodes

Photoelectrocatalytic degradation of Remazol Black B (RBB) dye in aqueous solutions was investigated. The degradation was carried out on a highly efficient n-WO3 photoelectrode, which was synthesized by electrochemical processing. Effects of various synthesis and post-treatment conditions were investigated using electrochemical techniques and AFM imaging. Photocurrent measurements have shown that Butler–Gartner equation is fulfilled for n-WO3 films, which allowed us to determine the flat-band potential (Efb = 0.35 V versus Ag/AgCl, for pH = 0). The WO3 film electrode was found to be a better photelectrocatalyst than TiO2 with similar AFM surface roughness. Kinetic measurements of photoelectrocatalytic RBB degradation have shown that a generalized Langmuir–Hinshelwood model applies to the system, leading to the overall rate constant k′ = 1.1 × 10−9 mol cm−2 s−1, with RBB adsorption equilibrium constant K = 3.35 × 104 mol−1 L and Gibbs free energy of adsorption ΔG° = −6.16 kcal/mol (−25.8 kJ/mol). In mechanistic considerations of possible lines of oxidative attack on RBB, we looked at electrolyte composition and properties of electronic structure of RBB, by utilizing quantum mechanical calculations of molecular orbitals. The ab initio SCF Hartree–Fock calculations of molecular orbitals were carried out to evaluate most vulnerable areas of RBB molecule for electrophilic and nucleophilic attacks, as well as for a direct charge transfer oxidation process. Several electronic properties were mapped onto the total electron density surface and analyzed to reveal initial reaction pathways for oxidative degradation of RBB.