Differentiating the reaction mechanism of three-dimensionally electrocatalytic system packed with different particle electrodes: Electro-oxidation versus electro-fenton

Recently, there are still some controversial mechanisms of the 3D electrocatalytic oxidation system, which would probably confound its industrial application. From the conventional viewpoint, the Ti 4 O 7 material may be the desired particle electrodes in the 3D system since its high oxygen evolution potential favors the production of • OH via H 2 O splitting reaction at the anode side of Ti 4 O 7 particle electrodes. In fact, the incorporation of Ti 4 O 7 particles showed phenol degradation of 88% and COD removal of 51% within 120 min, under the optimum conditions at energy consumption of 0.668 kWh g −1 COD, the performance of which was much lower than those in many previous literatures. In contrast, the prepared carbon black-polytetrafluoroethylene composite (CB-PTFE) particles with abundant oxygen-containing functional groups could yield considerable amounts of H 2 O 2 (200 mg L −1 ) in the 3D reactor and achieved a complete degradation of phenol and COD removal of 80% in the presence of Fe 2+ , accompanying a low energy consumption of only 0.080 kWh g −1 COD. It was estimated that only 20% of Ti 4 O 7 particles near the anode attained the potential over 2.73 V/SCE at 30 mA cm −2 based on the potential test and simulation, responsible for the low yield of • OH via the H 2 O splitting on Ti 4 O 7 (1.74 × 10 −14  M), and the main role of Ti 4 O 7 particle electrodes in phenol degradation was through direct oxidation. For the CB-PTFE-based 3D system, current density of 10 mA cm −2 was sufficient for all the CB-PTFE particles to attain cathodic potential of −0.67 V/SCE, conducive to the high yield of H 2 O 2 and • OH (9.11 × 10 −14  M) in the presence of Fe 2+ , and the • OH-mediated indirect oxidation was mainly responsible for the phenol degradation. Generally, this study can provide a deep insight into the 3D electrocatalytic oxidation technology and help to develop the high-efficiency and cost-efficient 3D technologies for industrial application.