Structured Titanium Oxynitride (TiO N ) Nanotube Arrays for a Continuous Electrocatalytic Phenol-Degradation Process: Synthesis, Characterization, Mechanisms and the Chemical Reaction Micro-Kinetics

Abstract In this study a novel titanium oxynitride electrocatalyst was synthesized and its electrocatalytic activity for the degradation of phenol was evaluated. A highly conductive and efficient Ti–O–N electrocatalyst was prepared in a three-step synthesis. A titanium coil was anodized to grow TiO2 nanotubes, which were then annealed in air to convert the amorphous structure to anatase and afterwards annealed in ammonia to obtain the final Ti–O–N catalyst. This was characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). To evaluate its degradation capabilities, the electrocatalytic oxidation of the phenol was performed inside a coil-type electrocatalytic microreactor. Phenol conversions of up to 95% of the initial 0.4 mmol L–1 of phenol were achieved at the studied applied electric potentials (1–16 V), NaCl concentrations (0.51–5.12 g L–1) and flow rates(20–500 μL min–1). The mechanism of electrocatalytic oxidation was proposed in a three-dimensional reactor model that accurately describes the electrocatalytic degradation of phenol at the Ti–O–N anode in the presence of NaCl in the phenol solution. It was shown that both OH*– and OCl*–mediated reaction mechanisms contribute to the phenol’s degradation, while at high NaCl concentrations (5 g L–1) the latter is dominant. In addition, the optimal reactor design is determined by studying the mass-transfer limitation with the model.