Photoelectrocatalytic Oxidation of Urea on a Photoanode Made of Titanium Dioxide Nanotubes

Nanotube photoanodes made of titanium dioxide (TNTs/Ti) are fabricated by anodic treatment of a titanium foil at a voltage of 60 V in an electrolyte based on ethylene glycol according to a two-step scheme that includes intermediate removal of an amorphous coating and subsequent calcination at 450°C. TNTs in the form of anatase have a length of 20–22 μm, an averaged diameter of 90–100 nm, and a wall thickness of 20 nm. The activity of the obtained photoanode in a reaction of photoelectrocatalytic oxidation of urea in aqueous 0.1 M KOH and 0.9% NaCl solutions is studied. A scheme of parallel oxidation of urea with hypochlorite formed during photoelectrooxidation of chloride ions is proposed. It is found that the overvoltage of the reaction of photoelectrooxidation of urea on TNTs/Ti in neutral chloride and alkaline solutions is 0.6 V lower in comparison with that upon electrochemical oxidation on a platinum electrode. It is shown that a TNTs/Ti photoanode provides efficient photoelectrooxidation of urea, including from solutions with low concentrations of the latter, which is an advantage in comparison with electrooxidation on a Pt electrode.