Self-doped TiO2 nanotube arrays for electrochemical mineralization of phenols

Self-doped TiO2 nanotube arrays (DNTA) were prepared for the electrooxidation of resistant organics. The anatase TiO2 NTAs had an improved carrier density and conductivity from Ti3+ doping, and the oxygen-evolution potential remained at a high value of 2.48 V versus the standard hydrogen electrode, and thus, achieved a highly enhanced removal efficiency of phenol. The second anodization could stabilize Ti3+ and improve the performance by removing surface TiO2 particles. Improper preparation parameters (i.e., a short anodization time, a high calcination temperature and cathodization current density) harmed the electrooxidation activity. Although boron-doped diamond (BDD) anodes performed best in removing phenol, DNTA exhibited a higher mineralization of phenol than Pt/Ti and BDD at 120 min because intermediates were oxidized once they are produced with DNTA. Mechanism investigations using reagents such as tert-butanol, oxalic acid, terephthalic acid, and coumarin showed that the DNTA mineralization resulted mainly from surface-bound (OH)-O-center dot, and the DNTA produced more than twice the amount of center dot OH compared with BDD. The free (OH)-O-center dot on the BDD electrode was more conducive to initial substrate oxidation, whereas the adsorbed (OH)-O-center dot on the DNTA electrode mineralized the organics in situ. The preferential removal of p-substituted phenols on DNTA was attributed mainly to their electromigration and the aromatic intermediates that are hydrophobic were beneficial to mineralization. (C) 2019 Published by Elsevier Ltd.