THESIS
2020
xvii, 138 pages : illustrations ; 30 cm
Abstract
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 CO
2 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...[
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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 CO
2 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 N
2H
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. N
2H
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 N
2H
2 fragment (m/e=29) further
confirmed the formation of N
2H
2, and also indicates that the N
2H
2 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
-1cat 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.
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