Novel nanomaterials for efficient photocatalytic ammonia synthesis
Ammonia is a key industrial raw material for fertilizers, chemicals and energy. The annual artificial ammonia synthesis via the Haber-Bosch process causes about 2% of global energy consumption and can lead to 1.6% CO₂ emission. Therefore, it is urgent to develop low-cost and environmentally friendly approaches for artificial ammonia synthesis under ambient conditions. In this thesis, we summarized the current research status. Besides, a new concept of “artificial nitrogen cycle process based on photochemical and electrochemical reactions” was proposed. Through nanostructure control, metal modification, small π-conjugated molecule modification, plasma modification, several kinds of novel nanomaterials were developed and achieved highly efficient artificial ammonia synthesis under ambient conditions. A facile approach was used to prepare defective g-C₃N₄ nanorods with a narrower bandgap and a sub-gap, which can significantly enhance the light utilization ratio. More importantly, the defects of g-C₃N₄ nanorods can also enhance light absorption and boost the cleavage of N₂ molecules, which is the rate-determining step of nitrogen fixation. Compared with bulk g-C₃N₄, the photocatalytic N₂ reduction rate of defective g-C₃N₄ nanorods as the catalysts was increased by 3.66 times. We also report a novel bismuth bromide oxide (BiOBr)-Tetracyanoquinodimethane (TCNQ) photocatalyst prepared via a facile self-assembly method. Due to the well-match band structure of TCNQ and BiOBr, the separation and transfer of photogenerated electron-hole pairs were significantly boosted. The highest ammonia yield of the optimized sample reached 2.617 mg/h/gcat, which was 5.6-fold as that of pristine BiOBr and higher than the reported BiOBr-based photocatalysts. Nitrate is a crucial environmental pollutant and its risk on the ecosystem keeps increasing. In this thesis, we reported a green and facile synthesis of novel metallic ruthenium particle modified graphitic carbon nitride photocatalysts. Compare with bulk graphitic carbon nitride, the optimal sample had 2.93-fold photocatalytic nitrate reduction to ammonia activity. We also report a facile synthesis of carbon/bismuth/bismuth oxide photocatalyst via a one-pot hydrothermal reaction without using reducing reagent. Compared with bismuth oxide (α-Bi₂O₃), the photocatalytic ammonia yield of the optimum sample increased 3.65 times. In addition, the ammonia selectivity increased from 65.21% to 95.00%. The highly enhanced photocatalytic performance was attributed to the surface plasmon resonance of metallic bismuth. Meanwhile, the formation of carbon enables to boost the transfer of electrons significantly. The results and research findings of these works will contribute to the green artificial ammonia synthesis under ambient conditions.
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