Ni(II)/Ni(III) redox couple endows Ni foam-supported Ni2P with excellent capability for direct ammonia oxidation

Abstract

Converting ammonia in wastewater into harmless nitrogen is a green strategy and electrochemical advanced oxidation processes (EAOP) based on electron transfer are the important means to realize this strategy. As a typical EAOP, ammonia oxidation catalyzed by high-valence transition metal anodes is one of the most effective and greenest conversion measures. Hence, in this study we constructed an electrocatalytic ammonia oxidation system using a nickel phosphide anode (Ni2P/NF). When the initial concentration of ammonia was 1000 mg l−1, and the current was 10 mA, the Faraday efficiency of Ni2P/NF in ammonia oxidation catalysis reached 52.8%. In addition, the Ni2P/NF anode could stabilize the electrolysis of ammonia for up to 24 h. When the voltage was higher than 1.44 V vs. RHE, two peaks appeared at 479 cm−1 and 558 cm−1 in the in situ Raman spectrum and the corresponding current on the CV curve increased rapidly, which revealed that Ni oxyhydroxides formed on the reconstructed surface of Ni2P/NF were the real active sites for catalyzing the ammonia decomposition. The generated intermediates nitrate and nitrite were detected based on the in situ FTIR and spectrophotometric analysis. According to the experimental findings, we proposed a possible pathway for ammonia removal based on the participation of the Ni(II)/Ni(III) redox couple. This study enriched the in-depth understanding of ammonia oxidation and provided a very promising way to treat ammonia containing wastewater.