Mechanism and kinetics of phenol photocatalytic, electrocatalytic and photoelectrocatalytic degradation in a TiO2-nanotube fixed-bed microreactor

Phenol photocatalysis, electrocatalysis and photoelectrocatalysis were performed under different conditions (UV-light intensity, applied electrical potential, and flow rate) inside an in-house-developed, coil-type, photoelectrocatalytic microreactor. The main part of the microreactor is a photocatalytically active unit that was placed into a channel in a UV-transparent housing. The degradation reactions took place on the surface of a photoanode coil that is made up of TiO2 nanotubes. The mechanisms of photocatalysis and photoelectrocatalysis were proposed and validated with a mathematical model, which described the governing processes occurring during the microreactor’s operation. In the case of the photocatalysis there was not enough oxygen for complete phenol degradation, whereas phenol was successfully mineralized with the help of an applied electrical potential. The applied electrical potential successfully prevented electron–hole recombination, which allowed for unhindered hydroxyl radical formation and a high phenol degradation rate. The highest initial phenol concentration used, approximately 45.7 mg L−1, was completely mineralized at an applied potential of 16 V, a UV-light intensity of approximately 2.8 mW cm−2, and a flow rate of 50 μL min−1. Finally, an energy-efficiency study was performed to identify the optimal photoelectrocatalytic microreactor’s operating conditions for the phenol degradation.