Efficiently electrochemical removal of nitrite contamination with stable RuO2-TiO2/Ti electrodes

- RuO2 serves as the electrocatalytic active component for RuO2-TiO2/Ti electrode.
- The electrode exchanging strategy greatly improves the stability of electrode.
- Epitaxial spread of RuO2 on TiO2 stabilizes the RuO2 in the electrocatalytic uses.
- Oxidation and reduction of NO2− give NO3− (main product) and NH4+, respectively.

RuO2-TiO2/Ti electrode was prepared for the electrocatalytic removal of nitrite in this work. The influences of calcination temperatures on crystal phase and morphology of RuO2-TiO2 composite were explored by XRD and TEM. The formation of RuO2 epitaxial layers on the surface of TiO2 by calcination at 400 °C reduces the impedance (EIS test) and improves the electrocatalytic activity for RuO2-TiO2. The CV test shows that the electrochemically surface active sites increase along with the increase of RuO2 content from 0 to 2.0 wt%. The nitrite removal rate of 2.0 wt% RuO2-TiO2/Ti electrode is found ca. 6.7 and 2.5 times faster than those of 0.02 wt% and 0.1 wt% RuO2-TiO2/Ti electrodes, respectively. Oxidation of the active sites on the RuO2 results in an obvious activity decrease for RuO2/Ti and RuO2-TiO2/Ti electrodes in just 3 reaction cycles (30 min/cycle). By exchanging the anode and cathode after every cycle, the catalytic activity of corresponding 2.0 wt% RuO2-TiO2/Ti electrode remains almost unchanged after 50 cycles. Although the electrocatalytic service life of RuO2/Ti electrode is also greatly improved, its electrocatalytic activity decreases much after 50 cycles. The relatively longer service life of 2.0 wt% RuO2-TiO2/Ti electrode should own to the intensified interaction between the epitaxially spread RuO2 and TiO2, which stabilizes the chemical states of RuO2. The main product for the electrocatalytic removal of nitrite is nitrate by indirect oxidation, while a small amount of ammonium is also produced at the cathode. The as-produced ammonium will be oxidized into nitrogen molecule, which is released from the aqueous solution afterwards and contributes the decrease of the total content of N in the solution.

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