Nonmetallic Modification Of G-C₃N₄ For Efficient Nitrogen Fixation As In-situ Foliar-applied Ammonium Fertilizer

Nitrogen element in human nucleic acids and proteins has been derived from the organic nitrogen of plants,and it could further track back to the synthetic ammonia in high temperature and pressure.Therefore,with the aggravation of energy shortage and environmental pollution,the photocatalytic technology has been a research hotspot in application of ammonia generation with the use of solar energy in mild conditions.In this study,considering the combination of the photocatalytic technology and foliar fertilization,it was proposed for the first time the idea of in-situ photocatalytic ammonia production in sunlight as“foliar nitrogen fertilizer”.The in-situ foliar ammonia supply system made the full use of low-concentration ammonia and solar energy.It would develop the actual application of photocatalysis and establish a foundation for the further research of the in-situ foliar fertilization technology with the advantages of rapid absorption,strong pertinence and high utilization efficiency.According to these,metal-free catalyst graphitic carbon nitride(g-C₃N₄),a stable,nontoxic,harmless and environmental friendly solar photocatalyst,has been selected as a part of the novel“foliar fertilization”system.The present research article endeavors to increase the specific surface area and reduce the recombination rate to enhance the ammonia generation of g-C₃N₄.Further,the physicochemical properties of prepared catalysts have been systematically explored based on the various material characterization techniques as well as density functional theory(DFT)calculation methods.The g-C₃N₄ catalysts with the large specific surface area were prepared via a facile two-step thermal treatment in air without templates and co-catalysts.Larger specific surface area leads to more active sites for nitrogen adsorption and reduction,which enhanced the photocatalytic activity a lot.The optimal g-C₃N₄ catalyst with secondary calcining of 120 min showed an 18-fold larger specific surface area and 4.2 times enhanced ammonium evolution rate compared with bulk g-C₃N₄.Besides,the DFT results showed that the nitrogen molecule is more likely to horizontally adsorb above the two-coordinated nitrogen atom site on the surface of g-C₃N₄ with an adsorption energy of-72.59 k J·mol^-1.Oxygen-doped g-C₃N₄ catalysts were synthesized with microwave hydrothermal method to reduce the recombination rate of electrons and holes.With the treatment of hydrogen peroxide solution(10%)in preparation,the efficiency of ammonia production of catalyst OCN-10 was the highest(3.98 mmol·g^-1·h^-1),which was 2.46 times of the non-doped catalyst.The results of characterizations and DFT calculation showed that part of the nitrogen atoms were displaced by oxygen atoms in g-C₃N₄ structure and the energy gap of catalyst decreased,hence the absorption and utilization ability of visible light were enhanced.In addition,a proper amount of oxygen doping could be served as the“electron aggregation center”to inhibit recombination of photo-generated carriers.Moreover,when nitrogen molecules adsorbed above electron-rich oxygen atoms,electrons could more easily migrate from oxygen atoms to N₂,which facilitated their activation and reduced the energy required for reduction reaction.