| Nowadays,ammonia has become an indispensable chemical product in the modern society.Lighting and rhizobia in nature fix nitrogen into ammonia,while artificial nitrogen fixation follows Haber-Bosch process.Nitrogen fertilizers produced from ammonia is feeding half of the global population.However,Haber-Bosch process requires high temperature and high pressure,which causes excess energy consumption and carbon emissions.In order to reduce environmental problems caused by artificial nitrogen fixation,researchers have explored the photocatalytic strategy driven by solar,that fixes nitrogen and water into ammonia at normal temperature and pressure.However,the conversion efficiency of light energy to chemical energy,visible light utilization efficiency,stability,and raw material cost of photocatalysts still hinder the practical application of photocatalytic nitrogen fixation strategies.Therefore,it is of great significance to explore more practical nitrogen-fixing photocatalytic materials.This investigation discovered that Ce-UiO-66 was activated under the irradiation of ultraviolet light,that is,the benzene-C bonds on Ce-UiO-66 particles breakage and form nitrogen fixation active sites.The phenomenon of ultraviolet-activation was proved by structural characterizations and theoretical calculation.Although Ce-UiO-66 catalyst exhibits excellent photocatalytic performance due to ultraviolet-activation,uncontrolled ultravioletactivation also spawns collapse of Ce-UiO-66-based catalysts.Therefore,it is necessary to consider not only improving the light-chemical energy conversion efficiency,but also controlling ultraviolet-activation in the modification strategies of Ce-UiO-66-based catalysts.Meanwhile,in order to follow the application requirements of nitrogen fixation,the utilization efficiency of visible light in solar should be discussed in the designing of Ce-UiO-66-based catalysts.The raw material cost of Ce-UiO-66-based catalysts is also important in the investigation.1.Ce-UiO-66-based catalyst was modified by the strategy of graphene-embedding(GSCeact).As conductor,graphene controls the transfer of photogenerated electrons,and the composite form of embedding increase the contact specific area between graphene and CeUiO-66 particles.Therefore,compared with pure Ce-UiO-66 catalyst(CeMOFact),the process of activation on GSCeact has been effectively controlled.GSCeact maintained 91.83%of nitrogen fixation efficiency after 7 × 24 h of photocatalytic reaction,while the nitrogen fixation efficiency of CeMOFact decreased to 0%.Moreover,graphene-embedding also significantly reduces the recombination of electrons and holes in Ce-UiO-66-based catalyst.Thus,GSCeact showed remarkable apparent quantum efficiency of 9.25%under the irradiation of 365 nm light,while the apparent quantum efficiency of CeMOFact was 6.75%.GSCeact was utilized to cultivate rice seedlings as solar ammonia fertilizer and achieved the similar effect as standard ammonia fertilizer.2.Ce-UiO-66-based catalyst was modified by the strategy of coating black TiOx particles(B-TiOx-Ceact).Compared with anatase TiO2 particles,black TiOx particles absorb visible light and excite photogenerated electrons.Ac cording to an S-type heterojunction is formed between UV-activated Ce-UiO-66-layer and black TiOx particles,the electrons are transferred to the active sites on Ce-UiO-66-layer.Meanwhile,the stable electron transfers control the activation process of Ce-UiO-66-based catalyst.Formic acid-pretreated black TiOx particles also promote nitrogen fixation reaction by releasing carbon center free radicals.Therefore,BTiOx-Ceact showed excellent visible light utilization efficiency and exhibited apparent quantum efficiency of 5.32%under the irradiation of 455 nm,which exceeded 1.96%of CeMOFact and 3.45%of GSCeact.3.Ce-UiO-66-based catalyst was modified by the strategy of compositing vermiculite(VMCeact),which is a low-cost natural mineral.The lamellar structure of vermiculite protects the Ce-UiO-66 particles on its surface.Thus,this strategy improved the stability of Ce-UiO66-based catalyst by reducing the mechanical damage in the photocatalytic process.Meanwhile,iron oxide in vermiculite absorbs visible light and excites photogenerated electrons.The heterojunction between iron oxide and Ce-UiO-66 transfers electrons from iron oxide to active sites on Ce-UiO-66 particles.Therefore,VMCeact exhibited apparent quantum efficiency of 4.49%under the irradiation of 455 nm.4.Ce-UiO-66-based catalyst was modified by the strategy of mixing organic ligands BDC and BTC(Ce-BDCBTCact).Ultraviolet-activated Ce-BDCBTCact forms columnar nanoparticles with the exposing of oxygen vacancies on cerium metal clusters,and the surface is covered with active sites of benzene-C bonds breakage.Ce-BDCBTCact with two kinds of actives sites showed extremely low recombination rate of photogenerated electrons and holes.Therefore,Ce-BDCBTCact exhibited high apparent quantum efficiency of 8.68%under the irradiation of 365 nm light.5.Ce-UiO-66-based catalyst was modified by the strategy of constructing Ce-UiO-66 film on CeO2 layer(CeO2-MOFact-ITO).After ultraviolet activation,Ce-UiO-66 is covered on the surface of CeO2 layer in the form of thin film,which abundant nitrogen fixation active sites are generated on the film.The morphology of film avoided the mechanical damage of CeUiO-77-based catalyst in the process of photocatalytic nitrogen fixation,while CeO2 layer controlled the activation of Ce-UiO-66 by controlling the transfer of photogenerated electrons.Therefore,CeO2-MOFact-ITO maintained 99.99%of nitrogen fixation efficiency after 7 × 24 h of photocatalytic reaction.The five modification strategies of graphene-embedding,coating black TiOx particles,compositing vermiculite,mixing organic ligands and constructing Ce-UiO-66 film on CeO2 layer have successfully obtained advantages for Ce-UiO-66-based catalysts in one or more application requirements,including light-chemical energy conversion efficiency,visible light utilization efficiency,stability and raw material cost.Moreover,Ce-UiO-66-based catalysts performed well in the crop cultivation experiments,which shows the broad prospects of CeUiO-66-based catalysts as solar nitrogen fertilizer. |
