Coeffect of Co-Mn in bimetallic oxide catalyst for ultrafast electrochemical reduction of nitrate: Insight into ion interaction

Co3O4 has been considered as promising catalysts for electroreduction of nitrate (ERN), which is an emerging tactic to close the nitrogen cycle on earth. However, the nitrate reduction efficiency of Co3O4 is less than satisfactory, which could be enhanced by assistance of Mn element. Herein, we synthesized Co3O4, Mn2O3 and Mn1.5Co1.5O4 nanoparticles for electrodes via a simple sol–gel method using metal nitrates. Moreover, a special electrode 3-Mn/Co was fabricated by physical mixed Co3O4 and Mn2O3. Mn1.5Co1.5O4 and 3-Mn/Co electrodes presented 78.6 % and 64.09 % nitrate removal efficiency after 3 h, 115.46 % and 75.68 % higher than that of Co3O4, proving the existence of Co-Mn coeffect. The optimum Mn1.5Co1.5O4 endures operating parameters experiments of applied potential, level of NO3– concentration, pH value and Cl- concentration and maintains solid stability through ten batches of reaction. Eventually, the ion valence changes of electrodes before and after electrocatalysis verifies that both Co and Mn function in the reaction. The ERN performance variation between electrodes, doped and physically mixed with Mn, and Co3O4 further elucidates that Mn3+ takes major effect in ERN process, while Mn4+ would blocks the electrons transferred to NO3–. In general, a novel process of Co3+/Mn3+- Co2+/Mn4+ reaction cycle in Co and Mn bimetallic and monometallic oxides for ERN performance is proposed as a clue for electrode fabrication.