Solar photoelectrocatalytic degradation of Acid Orange 7 azo dye using a highly stable TiO₂ photoanode synthesized by atmospheric plasma spray

A TiO₂ coating composed of 29% rutile, 9% anatase and 62% of Ti₇O₁₃ on stainless steel support has been prepared by atmospheric plasma spray technology. This novel photoanode was coupled to an air-diffusion cathode that generates H₂O₂ in a photoelectrochemical cell submitted to direct sunlight irradiation to degrade 100mL of Acid Orange 7 (AO7) azo dye solutions in 0.05M Na₂SO₄ by solar photoelectrocatalysis (SPEC). The photoanode presented excellent mechanical properties as well as large stability and long durability up to 2.0mAcm^-2. The decolorization process in SPEC was synergistic of the individual processes in solar photocatalysis and anodic oxidation with generated H₂O₂ under comparable conditions owing to the larger production of OH from the higher amounts of photogenerated holes that can be separated of photogenerated electrons. The best operating variables for SPEC were 15mgL^-1 AO7, pH 7.0 and anodic current density (j_anod) of 1.0mAcm^-2. Under these conditions, the azo dye disappeared in 100min and the solution was totally decolorized in 120min, but only 40% mineralization was attained after 240min of electrolysis. The AO7 decay followed a pseudo-first-order reaction as found by reversed-phase HPLC and it was accelerated with increasing j_anod due to the higher amounts of OH generated from the production of more photogenerated holes and the enhancement of anodic oxidation of water. Ion-exclusion HPLC allowed the identification of generated carboxylic acids like phthalic, tartaric, succinic, acetic and oxamic, which were the main components of long-time electrolysis. Their persistence explains the low mineralization of AO7 in SPEC. The initial N of the azo dye was mineralized as NH₄⁺ ion, in larger proportion than NO₃^- ion, although it was mainly loss as volatile species.