| Ammonia is an essential chemical raw material for industrial and agricultural production.Nitrogen fixation is the main way to obtain ammonia.However,the nitrogen fixation methods that exist in nature cannot meet the production needs,and artificial methods are needed for nitrogen fixation to produce ammonia.At present,Haber-Bosch method is the main artificial nitrogen fixation technology used in industry.The process consumes a lot of energy and produces a large amount of greenhouse gases,which cannot meet the new requirements of energy saving,green and environmental protection in modern industry.Therefore,it is of great scientific significance and application value to find a new method of artificial nitrogen fixation.The photocatalytic nitrogen fixation technology,which utilizes renewable solar energy and the by-product is clean and pollution-free,is considered as a new energy-saving and environmentally friendly artificial technology.Graphitic carbon nitride(g-C3N4)has a wide range of raw materials,low price,and simple synthesis process.It is widely recognized as a metal-free semiconductor catalyst that is expected to achieve large-scale production.In this thesis,g-C3N4 as the matrix,a series of related catalysts were synthesized and their photocatalytic nitrogen fixation capacity was studied.The main research and conclusions are as follows:(1)Taking advantage of the characteristic that dicyandiamide would decompose when heated in an aqueous solution above 80℃,the temperature was set at a critical point,and the solid obtained by evaporation was calcined to obtain g-C3N4 containing a small amount of cyano groups(-C≡N),which was introduced into the g-C3N4 molecular chains to form structural defects.Compared with the intrinsic g-C3N4(bulk-GCN),the g-C3N4 prepared by this method showed larger specific surface area,redshifted light absorption edge,and higher separation efficiency of photogenerated electrons and holes.The photocatalytic nitrogen fixation rate of the catalysts in ultrapure water could reach 1.54 mg·L-1·gcat-1·h-1 at room temperature,which was about 5.7 times that of bulk-GCN.(2)The ruthenium(Ru)single atoms supported g-C3N4 composite catalyst was successfully synthesized by solid phase in-situ reduction.The results showed that Ru was dispersed on g-C3N4 as single atoms,and there were structural defects caused by the introduction of-C=N in the molecular structure of g-C3N4.With the loading of Ru single atoms,the light absorption range of g-C3N4 was widened and the absorption intensity was enhanced,which made the composite catalyst g-C3N4 supported by Ru single atoms had better photocatalytic activity.When the loading of Ru was 0.05 mmol and the calcination temperature was 550℃,the prepared Ru single atoms supported g-C3N4 had the best photocatalytic activity.Its photocatalytic nitrogen fixation efficiency in ultrapure water was reached to 2.14 mg·L-1·gcat-1·h-1,which was 2.7 times that of g-C3N4 catalysts with no Ru atoms loading.Meanwhile,Ru atoms loaded g-C3N4 also showed good stability in nitrogen photofixation.In addition,the photocatalytic nitrogen fixation mechanisms of the catalysts were investigated by PL,EIS and other characterization methods.(3)Using HBO3 as B source,B doped multi-defect g-C3N4 catalyst was synthesized by one-step in situ method.A variety of characterization results showed that B was successfully doped into g-C3N4.With the increase of B,the content of-C≡N in the catalysts increased,and nitrogen vacancies were detected in the catalyst(B-CN-0.72)with 0.72 g of HBO3 added.Impurity B,cyano groups and nitrogen vacancies all acted as electron acceptors and could be used as active sites for nitrogen adsorption and activation.The photoabsorption edges of the catalysts prepared by this method were redshifted.The photogenerated carrier separation efficiency was higher and photogenerated electrons transfer rate was faster.B-doped multi-defect g-C3N4 catalyst had good photocatalytic activity.The photocatalytic nitrogen fixation rate of B-CN-0.72 in ultrapure water without hole scavengers was highest,reaching 39.28 mg·L-1·gcat-1·h-1,which was 25.5 times that of pure g-C3N4.Besides,B-CN-0.72 also had good photocatalytic nitrogen fixation stability.(4)On the basis of the successful preparation of B-doped multi-defect g-C3N4(B-CN)catalyst,further modification of the catalyst by CeO2 was studied.CeO2 precursor was added in the preparation process of B-CN,and the composite(CeO2/B-CN)catalysts were successfully prepared after calcination.Compared with B-CN,CeO2/B-CN had higher light absorption intensity in the region of ultraviolet(UV),more effective separation efficiency of photogenerated carriers and smaller resistance of charge transfer,which were favorable for the photocatalytic nitrogen fixation.The highest nitrogen photofixation rate was 45.40 mg·L-1·gcat-1·h-1,which was 1.2 times that of pure B-CN. |
PDF Full Text Request