Disordered Mesoporous Materials Synthesized By Nano-confined Gas-releasing Reaction And Their Applications In Heterogeneous Catalysis

With the increasing energy crisis and environmental pollution,mesoporous materials play an important role in solving these serious problems.Mesoporous materials can not only improve the efficiency of fossil energy,but also show excellent performance in the conversion of new energy.Therefore,the development of mesoporous material is particularly critical.At present,the synthesis of mesoporous materials is relatively mature,but there are still some problems to be solved.For example,in the synthesis of mesoporous materials using hard-template method,there is a certain obstacle in the diffusion of precursors in the ordered pores,which results in the need of pretreatment to the templates or some special conditions in synthesis.And the yield of product is ususlly low.In addition,in some catalytic reactions the ordered pores of catalyst also limite the contact of reactants and active centers,resulting in lower catalytic activity.Regardless of the preparation of mesoporous catalysts or catalytic process,how to improve the diffusion of substances in mesopores is the key to the development of mesoporous catalysts.Based on the above problems,this thesis will offer some attempts in following four parts:1.Synthesis of mesoporous g-C₃N₄ nanorods and their catalytic performance in photocatalysis.In the past years,much attentions are focused on the mesoporous g-C₃N₄,because it can utilize visible light and has higher photocatalytic activity than bulk g-C₃N₄.In this work,g-C₃N₄ nanorods with disordered mesoporous structures are prepared through a nano-confined thermal condensation of cyanamide inside the SiO₂nanotubes and relevant experiments and characterizations are performed to vertify the formation mechanism of porous structures in nano-confined reaction.In this method,cyanamide loaded in mesoporous SiO₂ nanotubes is condensed to form g-C₃N₄ at high temperature and a large amount of NH₃ is produced.Due to the confinement of SiO₂shell,NH₃ can not release quickly.A large amount of tiny bubble are produced among g-C₃N₄ and thereby the mesoporous structures of g-C₃N₄ are formed.Thanks to the large cavity of nanotubes,much cyanamide can be loaded in it and the yield of g-C₃N₄is high.The disordered mesopores can improve the light absorption and charge separation efficiency of g-C₃N₄ and the sharp edges in it facilitate electrons and holes to escape from the surface and participate in the redox reactions.Therefore,g-C₃N₄nanorods show high photocatalytic activity in water splitting and phtodegradation of RhB,and the apparent quantum yield at the wavelength of 420 nm reach 5.43%.2.Synthesis of mesoporous carbon nanorods and their catalytic performance in hydrogenation of phenol as supports of Pd catalyst.Mesoporous carbon nanorods with disordered mesoporous structures are prepared using nano-confined reactions.In the synthetic process,furfuryl alcohol loaded in SiO₂ nanotubes is polymerized under the catalysis of oxalic acid to produce polyfurfuryl alcohol,and then polyfurfuryl alcohol is carbonized to generate carbon at high temperature with the releasing of water vapor.Due to the confinement of SiO₂ shell,water vapor can not release quickly.A large amount of tiny bubble are produced among carbon and thereby the mesoporous structures of carbon are formed.This process fully conforms to the mechanism of nano-confined reaction.The polymerization process of furfuryl alcohol proves that the porous structure of target product can be obtained only when the gas releasing and the product generation are synchronized in the nano-confined reaction.Using this method,mesoporous N-doped carbon nanorods with different N contents are prepared through adding different amounts of guanidine hydrochloride in furfuryl alcohol solution.Pd catalysts supported on N-doped mesoporous carbon were used for the hydrogenation of phenol.As the nitrogen content increases,the size of Pd particles gradually decreases and the catalytic activity gradually increases.This is mainly due to the size control of Pd particles by N atoms and the change of surface electron density of Pd nanoparticles.3.Synthesis of Ni catalysts supported on the mesoporous SiO₂ nanotubes and their catalytic performance in dry reforming of CH₄.Ni catalysts with ultra-small Ni particles?2 nm?and porous structure are prepared through the nano-cpnfined decomposition and high temperature reduction of Ni?NO₃?₂ in SiO₂ nanotubes.The decomposition of Ni?NO₃?₂ in SiO₂ nanotubes conforms to the nano-confined reaction.With less Ni?NO₃?₂ dosage,highly dispersed NiO particles are produced instead of NiO porous structures.After the reduction,highly dispersed Ni catalyst is formed.With the decrease of the porosity and more disordered of the SiO₂ structure,the Ni particles are more dispersed and the particles are smaller,indicating that the disordered mesoporous structure of SiO₂ is more favorable to the loading of the Ni catalysts.The N atoms in the residual PEI in SiO₂ have strong coordination with Ni precursor,which effectively increases the dispersion of the Ni catalysts and enhances the interaction between Ni particles and SiO₂ support.The catalysts show high catalytic activity and strong resistance to carbon deposition in dry reforming of methane.The conversion of CH₄and CO₂ reached 85.9%and 89.7%at high space velocity and no significant carbon deposition is observed after 100 hours.The high catalytic activity and resistance to carbon deposition can be attributed to highly dispersed Ni particles and permeable porous structures of catalysts.4.Catalytic performance of Ni/CeO₂-SiO₂ catalysts in dry reforming of methane.Porous CeO₂-SiO₂ nanotubes are prepared through the decomposition reaction of Ce?NO₃?₃ in SiO₂ nanotubes.And highly dispersed Ni/CeO₂-SiO₂ catalyst are finally obtained through the nano-confined decomposition of Ni?NO₃?₂ using CeO₂-SiO₂nanotubes as supports.When the content of CeO₂ is 42 wt%,Ni/CeO₂-SiO₂ catalyst shows highest catalytic activity and best stability.The conversion of CH₄ and CO₂ reach59.5%and 70%,and remain unchanged after 6-hour reaction at the mass-space velocity of 550 Lg^-_cat.1h^-1.The increase of catalytic stability is attributed to the strong interaction between Ni and CeO₂ to stabilize the highly dispersed Ni particles.