Electrocatalytic reduction of nitrobenzene using TiO2 nanotube electrodes with different morphologies: Kinetics, mechanism, and degradation pathways

Here we report on one of the first studies utilizing self-doped TiO2 nanotube arrays (NTAs) for electrocatalytic reduction of water pollutants with nitrobenzene (NB) as the model compound. In particular, we focused on the effects of morphological and crystallographic characteristics on the electrochemical reactivity of TiO2 NTAs towards the target pollutant. The TiO2 NTAs with different morphologies and exposed facets were synthesized by tuning a suite of anodization parameters and applying a post-anodization treatment. A series of electrochemical reduction tests using TiO2 NTAs as the cathode were carried out to investigate the effects of nanotube morphologies and operating conditions. Results indicate that the {0 0 1}-exposed facet, longer nanotube length, and larger nanotube diameter are the structural and morphological features that can lead to enhanced performance of TiO2 NTA electrodes. With the applied cathode potential of -1.20 V/SCE and an initial concentration of 300 mg/L,> 95% NB can be efficiently degraded at the energy consumption of 2.07 kWh/m(3) (E-E/O), 7.67 kWh/kg (NB), and 30 kWh/kg (C), all of which are much lower than those of electrochemical oxidation processes. The good stability of the TiO2 NTA cathode was also demonstrated in 10 cyclic runs. Furthermore, CV measurements and scavenger tests were employed to study the reaction mechanism. It was found that both the direct reduction and surface adsorbed H-* contributed to NB degradation. The NB transformation pathway and degradation kinetics of NB and the intermediate products were also investigated. Results obtained from this study suggest that the self-doped TiO2 NTA is a promising cost-effective electrode material for electrocatalytic reduction of water pollutants with high efficiency and stability.