Electrochemical oxidation of lignin at electrochemically reduced TiO2 nanotubes

Abstract

For this study, the electrochemical oxidation of lignin was investigated at the surfaces of electrochemically (EC) reduced TiO2 nanotube arrays. The effects of nanotube lengths on the lignin oxidation were explored by growing the nanotubes with different lengths through anodization. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and electron paramagnetic resonance (EPR) techniques were employed to characterize the fabricated TiO2 nanotubes. The electrochemical behaviors of the nanotubes before and after the EC treatment were assessed by various electrochemical methods, including cyclic voltammetry (CV), chronopotentiometry (CP), and impedance spectroscopy. The TiO2 nanotubes were treated by applying a cathodic current (5 mA cm−2) for 10 min, which significantly increased its electrocatalytic activity. It was also determined that the nanotube length was linearly increased with the increase of the anodization time, and that the longer the nanotubes and the larger the double layer capacitance. The length of the nanotubes had a significant effect on the level of lignin oxidation, and an optimal TiO2 nanotube length that enabled the most efficient oxidation of lignin was determined. The efficiency of the TiO2 electrodes towards the oxidation of lignin was also compared to a Pt electrode. The TiO2 nanotubes that were grown for 16 h by anodization with ~13.5 μm length exhibited the lowest impedance and the highest lignin oxidation efficacy. The total organic carbon (TOC) of the lignin solution under different oxidation times was also measured to further evaluate the efficiency of the electrochemical degradation of lignin, and 70% TOC removal was achieved in 3 h. The TiO2 electrodes were shown to outperform the Pt electrode in all the lignin oxidation studies. Moreover, the activation energy required for the electrochemical oxidation of lignin was investigated by performing the oxidation under various temperatures and found to be 21.0 kJ mol−1. The EC reduced TiO2 nanotube electrode showed high activity and stability, promising for environmental applications.