Identification of the role of Cu site in Ni-Cu hydroxide for robust and high selective electrochemical ammonia oxidation to nitrite

Abstract

Ammonia oxidation was essential in nitrogen recycle, and electrochemical oxidation with artificial, none-noble, robust, high selective ammonia oxidation catalyst was quite attractive while bimetallic Ni-Cu hydroxide, NixCu1-x hydroxide, is known to be a quite reactive ammonia oxidation catalyst by previous work. Here we identify the role of Cu site in Ni-Cu hydroxide/oxyhydroxide for its electrochemical oxidation of ammonia. CV and Tafel results revealed that the ammonia oxidation was catalyzed by NiOOH at high onset potential (1.52 V vs. RHE), which was high activity, while the doping of Cu site lowered the onset potential to 1.40 V and achieved a lower Tafel slope. EIS results revealed a large resistance on Cu(OH)2 while NiOOH was proved much more conductive for electron transfer. The in-situ FTIR further revealed that the ammonia oxidation on Cu(OH)2 follows -N intermediate oxidation mechanism with a detection of the nitrite formation on Cu(OH)2. This route was totally different from that on NiOOH which follows -N2Hy intermediate dehydrogenation mechanism. The performance on Ni0.8Cu0.2 hydroxide/oxyhydroxide was proved with confidence robust and high selective during long-time electrolysis experiments. 98% of ammonia was electrochemical oxidized into nitrite on Ni0.8Cu0.2 oxyhydroxide at 1.53 V vs. RHE, indicating a very similar route with Cu(OH)2 but even much more robust comparing with that on Cu(OH)2 or NiOOH. The results also suggested that Cu sites were responsible for high selectivity towards nitrite via -N intermediate while Ni sites charged for catalyzing and played a role of electron transfer tunnel on bimetallic Ni-Cu oxyhydroxide during electrochemical ammonia oxidation.