Photocatalyst Synthesis and Study of Catalytic Mechanism on the Base of the Conversion of Small Molecules

Author:Sun Xiang

Supervisor:wang wen zhong


Degree Year:2019





With the development of modernization of our country,the energy shortage and environmental pollution have became increasing concerns in recent years.As one of the most green and environmental friendly way to harness solar energy,photocatalysis has drown a lot of attention from researchers.Besides,the advantages of this technology are mild reaction conditions and readily available raw materials.Nowadays,there is already a huge quantum of researches shown that the semiconductor photocatalysis has potential in pollutant removement,energy conversion,organic synthesis as well as other aspects.The results of research show that the photo-generated electrons and holes can reduce N2 and oxidize the organic molecular to generate ammonia and high value-added product,respectively.On the one hand,ammonia is currently synthesized from N2 and H2via Haber-Bosch process,which requires the forcing reaction pressure(1535 MPa)and temperature(350550oC).The massive world energy supply consumption(12%)and excessive CO2 emission derived from fossil is undesirable towards the upcoming renewable energy era.As an alternative strategy,the photocatalytic conversion of N2 from air to NH3at mild condition(25oC,1 atm)is of great importance.On the other hand,the selective conversion of benzyl alcohol is an integral part of organic synthesis field,which plays the important role in foodstuff,medicine,species,dyes and other chemical products.Developing of the efficient photocatalyst is still of great importance.So,it is vital to design and develope an efficient and novel photocatalyst which could produce the oxidative radicals for benzyl alcohol selective conversion.In the aspect of N2 and O2 photo-reduction,based on the basic steps including N2and O2 activation,electron and proton transfer,and product desorption,the photocatalyst of LaFeO3,POM and g-C3N4 were selected to improve the efficiency of N2 photofixation.The N2 photofixation efficiency was improved by so called“Pull and Push”mechanism,immobilization of catalyst as well as surface modification of polyoxometalates.In the aspect of benzyl alcohol conversion,skeletal C in K+doped g-C3N4 is enriched by more delocalized electrons,presenting nucleophilicity and prompting the C-C coupling reactions between benzaldehyde.The main contents of this paper include:(1)Our work provides an alternative N2 activation strategy that weaken the triple N≡N bonds under light irradiation by taking“pull and push”strategy of nitrogenase to the photocatalysis fields.The N2 photofixation performance(250μmol g-1 h-1)was achieved over phosphate modified(P1-LFO)as the positive results of the photocatalyst designation.The excellent N2 photofixation performance is contributed to the synergistic effect of hydrogen bonding from phosphate and transition metals of LaFeO3,which enhances N2 adsorption and activation.Phosphate modification might serve as a universal strategy for the designation of the photocatalysts with sustainable N2conversion performance.(2)The“PVDF bridge”was conctructed in a SW12/V-C3N4 nanocomposite by wet-chemical process.By introducing the“PVDF bridge’’on the interface as an electron transport chain,the charge spatial separation was obviously enhanced.It was attributed to the fact that PVDF bridge,which can transfer photogenerated electrons from V-C3N4to SiW12,thus reduces the recombination rates of photogenerated carries.Furthermore,bridged SiW12 increases the rate of H2O2 reduction reaction due to that these“stored”electron and protons in SiW12,which could be synergistically coupled to the actived N2absorbed on V-C3N4.(3)The K+doped graphic carbon nitride was prepared by KOH molten method.K+intercalation not only facilitated the light harvesting as well as the transport of charge carriers,but also induced surface deprotonation of g-C3N4 and thus remarkable nucleophilicity for prompting the C-C coupling reactions.This work sheds light on the design of earth-abundant inorganic photocatalysts for C-C coupling reactions such as the green synthesis of benzoin under ambient conditions.