| Nitrogen fixation reaction is the second most important reaction in nature after photosynthesis,which ensures the orderly transformation of nitrogen from free state to absorbable state.At present,Haber nitrogen fixation method is the main nitrogen fixation method,but it brings huge energy consumption and massive carbon emission due to its harsh reaction conditions.In contrast,photocatalytic technology can use solar energy to achieve green and low-energy nitrogen fixation under mild reaction conditions.Graphite carbon nitride(g-C3N4)is a nonmetal photocatalyst,which is widely used in the field of catalytic nitrogen fixation due to its stable physicochemical properties,low cost and easy availability.However,the defects of conventional g-C3N4,such as the photogenic holes and photoelectron complex,the poor absorption capacity of light wave with wavelength greater than 470nm and small specific surface area,affect the nitrogen fixation efficiency of g-C3N4.To improve the nitrogen fixation performance of g-C3N4,this paper proposes a variety of modification means of g-C3N4 in view of the above defects,and determines the specific modification mechanism by means of characterization method.Meanwhile,the photocatalytic nitrogen fixation experiment is designed to prove the effectiveness and rationality of the modification means,as well as the preliminary discussion on the nitrogen fixation mechanism.The specific research contents are as follows:(1)Original g-C3N4 was prepared by thermal polymerization with melamine as the precursor,and g-C3N4 nanometer sheets with rich carbon vacancies were prepared by secondary roasting under ammonia gas(NH3),and g-C3N4 before and after modification was characterized by XRD,BET,XPS,pL and uv-vis.Compared with the original g-C3N4 on the surface,it was about 6 times higher(-33.86 m2·g-1),and the abundant carbon vacancy as the active center effectively inhibited photogenic holes and photogenic electron migration,and increased the adsorption capacity of the catalyst on N2.In the experiment of photocatalyst activity,it was found that the type of trapping agent had a significant effect on the nitrogen fixation efficiency,among which ethylene glycol had the best trapping effect on the holes,and the optimal trapping effect was the ethylene glycol with 0.1%volume concentration.Under the simulated sunlight,the nitrogen fixation performance of g-C3N4 modified by NH3 was 5.78 times higher than that of the unmodified g-C3N4.(2)Bulk g-C3N4 was treated with the strong oxidization and corrosivity of ozone(O3),and a semiconductor catalyst with narrower bandgap width and better light absorption capacity was obtained.Through characterization,carbon vacancy was found in ozone-treated materials.The catalytic experiment showed that g-C3N4 had the best nitrogen fixation efficiency after 2 hours of ozone treatment.It is about 2.7 times more efficient than the original g-C3N4.In addition,after four cycle experiments,the modified g-C3N4 still maintains high efficiency and stable nitrogen fixation efficiency,indicating that it has a good application prospect.(3)With three different precursors,dicyandiamide,thiourea and melamine,three different photocatalysts were synthesized by using the strong oxidization of sodium persulfate and the blowing action of the airflow generated by decomposition.Through characterization,it was found that sodium persulfate not only affected the crystal structure and chemical composition of g-C3N4,but also affected the specific surface area and pore diameter distribution.The three precursors produced nitrogen vacancies to different degrees after the action of sodium persulfate,among which the precursors were g-C3N4 of melamine.Under simulated sunlight irradiation,the nitrogen fixation rates of g-C3N4 modified with dicyandiamide,melamine and urea as precursors were 4.76,7.63 and 2.71 times that of the unmodified g-C3N4,respectively.In addition,it was found that the concentration of hydrogen ions in the reaction solution was also an important factor affecting the reaction rate.In a certain range,the lower the pH value was,the higher nitrogen fixation efficiency was,and the mechanism of photocatalytic nitrogen fixation was preliminarily discussed.Figure[50]table[12]reference[140]... |
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