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 100 mL of Acid Orange 7 (AO7) azo dye solutions in 0.05 M Na2SO4 by solar photoelectrocatalysis (SPEC). The photoanode presented excellent mechanical properties as well as large stability and long durability up to 2.0 mA cm(-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)-O-center dot from the higher amounts of photogenerated holes that can be separated of photogenerated electrons. The best operating variables for SPEC were 15 mg L-1 AO7, pH 7.0 and anodic current density (j(anod)) of 1.0 mA cm(-2). Under these conditions, the azo dye disappeared in wanocl, -100min and the solution was totally decolorized in 120 min, but only 40% mineralization was attained after 240 min 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)-O-center dot generated from the production of more photogenerated holes and the enhancement of anodic oxidation of water. Ion-exclusion I-IPLC 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. (C) 2012 Elsevier B.V. All rights reserved.