The Preparation And Performance Study Of Modified G-C₃N₄ Catalyst For Photocatalytic Nitrogen Fixation

Ammonia is an essential component of carbon-based organisms for the construction of amino acids,genetic material（DNA,RNA）and other biomolecules.Ammonia has also received great attention as a potential fuel in the energy sector due to its high hydrogen content（17.6wt%）.As the main raw material for ammonia production,although occupy about 78%of the atmosphere in volume,nitrogen,in molecular form,is unable be used directly because of the extremely high dissociation energy of its nonpolar N≡N covalent triple bond,Need to complete the conversion of molecular nitrogen to fixed nitrogen in order to be further utilized.The efficient conversion of nitrogen molecules to fixed nitrogen is a key step in completing the global nitrogen cycle.At present,mainstream industrial nitrogen fixation mainly through the Haber-Bosch process,the energy consumption for this process accounts for 1.4%of global energy supply and produces 2.3 tons of greenhouse gases.In contrast,natural organisms can catalytically reduce nitrogen to ammonia under mild conditions via nitrogenase in vivo,while the Fe-protein can provide electrons that convert nitrogen to ammonia,the lower activation barrier of the biological N₂ fixation process inspires us that it is highly feasible to achieve nitrogen fixation under mild conditions,if the electrons and nitrogen adsorption sites required for reduction are simultaneously provided.Herein,we demonstrate an effective approach to enhancing the activity of g-C₃N₄materials for nitrogen fixation under visible light by means of oxygen-containing groups doping and carbon dots decoration of the carbon nitride catalyst.For instance,after modified the g-C₃N₄ nanosheets prepared by conventional procedures（g-CN-NS）,we observe a significant increase in N₂fixation efficiency.The XPS and FT-IR spectra results indicate the existence of C=O,C-OH and C-O-C functional groups in the novel defect-rich and ultrathin O doped graphitic carbon nitrogen nanosheets（O-CN-NS）.From the experimental results,nitrogen fixation activity is positively correlated with the oxygen content,In order to clarify which of the three oxygen-containing groups plays a major role in the reaction,carboxyl group（Adipic acid）and hydroxyl group（1.6-Hexanediol）are introduced respectively under the same amount of carbon-doping,it indicate that the carboxyl oxygen play a more indispensable role in the reaction.Furthermore,EPR results showed that with the introduction of oxygen-containing groups,a large number of nitrogen defects appeared on the surface of carbon nitride,while hydroxyl and carboxyl groups tended to produce nitrogen vacancies at different positions of the catalyst.The explanation for this phenomenon can be summarized as two points,on one hand,negatively charged carboxyl groups and positively charged terminal amino groups can be rearranged by electrostatic interaction during ball milling,and then shape the laminated structure according to the principle of electrostatic adsorption,on the other hand,carboxy oxygen and hydroxyl oxygen will form the hydrogen bonds（NH··O）with terminal amino groups in melamine,the hydrogen bond will be destroyed during high temperature polymerization of g-C₃N₄,And promote the formation of nitrogen vacancy.Then,carbon sources with different carbon chain lengths were introduced into carbon nitride to prepare carbon nitride catalysts with different hydrophobicity,and the performance of different hydrophobicity catalysts in nitrogen fixation was compared,which proved that the hydrophobicity of the material is positively correlated with the performance.By designing the interface contact experiment,we proved that the essence of the nitrogen fixation process is a three-phase interface reaction based on water,gas phase nitrogen and solid catalyst.The carbonation-treated g-C₃N₄ is highly hydrophobic,thus providing abundant three-phase contact points for the nitrogen fixation reaction.The effective adsorption of gaseous nitrogen is a crucial step.At the same time,an oxygen-containing group is introduced in the carbonization process,in which the C=O-O group acts as a hydrogen pump in the reaction,and a large amount of hydrogen protons are provided by hydration,thereby reducing the potential energy required for the reaction.Finally,iron doping experiments were performed on the original citric acid modified samples,and the photocatalytic nitrogen-immobilized catalyst containing iron oxide carbon nitride heterojunction was successfully prepared by hydrothermal method and pyrolysis method.Tested,Fe as the active center of biological nitrogenase,effectively promotes the nitrogen reduction reaction.