Self-doped TiO2 nanotube arrays for electrochemical mineralization of phenols.

Self-doped TiO 2 nanotube arrays (DNTA) were prepared for the electrooxidation of resistant organics. The anatase TiO 2 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 TiO 2 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, and the DNTA produced more than twice the amount of OH compared with BDD. The free OH on the BDD electrode was more conducive to initial substrate oxidation, whereas the adsorbed OH 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. Image 1 • The DNTA showed excellent oxidation efficiency by dopping appropriate Ti3+. • DNTA exhibited lower removal but higher mineralization efficiency for phenol than BDD. • Surface-bound.•OH was a dominant mechanism for the mineralization. • The electrooxidation efficiencies of different p-substituted phenols were investigated. [ABSTRACT FROM AUTHOR]