Nitrogen reduction reaction (NRR) has become a popular research topic over the past a few years, since it allows the direct conversion of renewable electric energy into chemical energy stored in ammonia, thus streamlining the procedures and enhancing the process efficiency with zero CO₂ emission. However, the achievements in this area are still far from meeting the requirements for its commercial application, majorly owing to the serious competing of the hydrogen evolution reaction (HER). Furthermore, the mechanistic study is significantly limited for this reason, even though understanding the reaction mechanism on the electrocatalysts’ surfaces is vital for the rational design of more advanced electrocatalysts.

In this thesis, the nitrogen reduction mechanism on noble metals su...[ Read more ]

Nitrogen reduction reaction (NRR) has become a popular research topic over the past a few years, since it allows the direct conversion of renewable electric energy into chemical energy stored in ammonia, thus streamlining the procedures and enhancing the process efficiency with zero CO₂ emission. However, the achievements in this area are still far from meeting the requirements for its commercial application, majorly owing to the serious competing of the hydrogen evolution reaction (HER). Furthermore, the mechanistic study is significantly limited for this reason, even though understanding the reaction mechanism on the electrocatalysts’ surfaces is vital for the rational design of more advanced electrocatalysts.

In this thesis, the nitrogen reduction mechanism on noble metals surfaces (Au, Pt, Ru, Rh) is studied for the first time by the in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and differential electrochemical mass spectrometry (DEMS). On an Au surface, the N-N stretching band at 1109 cm^-1 is detected during the nitrogen reduction reaction in an alkaline electrolyte, indicating that the N₂H_y (3≤y≤4) is a reaction intermediate on Au surfaces and that the nitrogen reduction reaction on Au surface follows an alternative pathway. While on Ru and Rh surfaces, the N=N stretching bands at ~1940 and ~2020 cm^-1 are detected in acidic and alkaline electrolytes, respectively. N₂H_x (0≤x≤2) is a reaction intermediate on Ru and Rh surfaces, which could be oxidized at potential below 0 V. Additionally, the differential electrochemical mass spectrometry signal of N₂H₂ fragment (m/e=29) further confirmed the formation of N₂H₂, and also indicates that the N₂H₂ could be desorbed from the Rh surface and decomposed into nitrogen and ammonia in the electrolyte. A general NRR reaction pathway on metallic surfaces is generated based on the spectroscopic studies. Besides, a series of chromium oxynitrides are synthesized as the electrocatalysts for nitrogen reduction reaction, which were evaluated in a home-made proton exchange membrane electrolyzer (PEMEL). High ammonia formation rate of 8.9×10^-11 mol s^-1 cm^-2 and 15.56 μg h^-1 mg^-1_cat is achieved, demonstrating that metal nitride-based materials could be promising electrocatalysts toward nitrogen reduction reaction, and that doping oxygen into the metal nitrates could facilitate the reactivity of the nitrogen atom on the metal nitrides surfaces.