Preparation And Catalytic Mechanism Study Of High-Efficiency Bifunctional Catalyst For Liquid Organic Hydrogen Carriers Hydrogenation/Dehydrogenation

In the context of carbon neutrality,energy transition is urgent in order to cope with the growing problem of environmental pollution and energy crisis.Hydrogen energy has the advantages of high combustion calorific value,wide source and renewable,which makes it an ideal green energy medium.However,the problem of safe and efficient storage and transportation of hydrogen energy has limited the large-scale development of the"hydrogen economy".The new organic liquid hydrogen storage technology is based on unsaturated compounds and enables safe and efficient storage and release of hydrogen through reversible catalytic hydrogenation and dehydrogenation reactions.Organic liquid hydrogen storage materials have similar physical properties to petroleum and can be used to store and transport hydrogen at ambient temperature and pressure using existing fossil energy infrastructure.However,the material suffers from both a high thermodynamic temperature for dehydrogenation and a slow kinetic rate of dehydrogenation.Therefore,the development of efficient catalysts is a key aspect of the commercialization of organic liquid hydrogen storage technology.The specific surface area of the catalyst carrier,the nature of the acid site,and the content of the catalyst active ingredient and the size of the metal particles all affect the metal dispersion,the metal-carrier interaction,and the adsorption capacity of the catalyst to the reactants,thus significantly affecting the catalyst activity.In this thesis,different carriers ofγ-Al₂O₃,SiO₂-C_COOH and NC-SiO₂ were selected and loaded with Ru and Pd active components to prepare catalysts with bifunctional of hydrogenation-dehydrogenation.The main study contents and results are as follows:（1）The Ru_xPd_y/Al₂O₃-H series catalysts were prepared by usingγ-Al₂O₃ as the carrier combined with double reduction method.The effects of secondary reduction and metal loading on the structure of Ru Pd/Al₂O₃ catalysts were investigated,as well as the effect of catalyst application on catalytic NPCZ/12H-NPCZ with dual catalytic activity of hydrogenation/dehydrogenation.The results show that the Ru_2.5Pd_2.5/Al₂O₃-H catalyst prepared by the dual reduction method has more strong acid sites and active sites（Ru⁰,Pd⁰）,and the stronger metal-carrier interaction promotes the movement of Al in the carrier to the surface and the formation of cavities,which facilitates the diffusion of reactants to the active center for the catalytic reaction.Ru_2.5Pd_2.5/Al₂O₃-H catalyzed NPCZ hydrogenation for 40 min to achieve 5.30 wt%hydrogen storage and catalyzed 12H-NPCZ dehydrogenation for 180 min to achieve 5.31 wt%hydrogen release,and no activity decay after the hydrogenation-dehydrogenation cycle.The apparent activation energies of all levels of the 12H-NPCZ dehydrogenation catalytic reaction at 160-190°C were 55.09 k J/mol,116.73 k J/mol and 133.87 k J/mol,respectively.Combined with DFT calculations,it was found that 8H-NPCZ and 4H-NPCZ molecules formed stable adsorption structures on the surface of Ru₃Pd₃/γ-Al₂O₃（100）,and the reaction energies at all levels of conversion of 12H-NPCZ to NPCZ were 1.16 e V,1.42 e V and 1.63 e V,respectively.Therefore,the bimetallic Ru_2.5Pd_2.5/Al₂O₃-H catalyst has excellent bifunctional activity for hydrogenation-dehydrogenation.（2）The SiO₂-C_COOH composite carrier was prepared by mechanical ball milling method,combined with impregnation method to prepare the Ru Pd/SiO₂-C_COOH catalyst.The effects of ball milling speed,ball milling time,and doping amount of carboxylated carbon nanotubes on the catalyst structure were investigated,as well as the effects of catalyst application for catalytic NPCZ/12H-NPCZ double catalytic activity.The results show that the SiO₂-C_COOH composite carrier has uniform acidic site distribution due to the uniform dispersion of the carbon microcrystals formed by ball milling in the SiO₂pore structure,while the interaction of the-COO^-group with Ruⁿ⁺and Pd²⁺can effectively anchor the metal and improve the metal dispersion,showing good catalytic hydrogenation-dehydrogenation activity.Among them,the metal dispersion in Ru Pd/SiC-10 catalyst is as high as 21.23%and the metal particle size is only 5.25 nm,which can achieve 5.26 wt%hydrogen storage by catalyzing the NPCZ hydrogenation reaction for40 min and 5.31 wt%hydrogen release by catalyzing the 12H-NPCZ dehydrogenation reaction for 120 min.The apparent activation energies of the various levels of the catalytic12H-NPCZ dehydrogenation reaction at 150-180°C were 60.21 k J/mol,77.48 k J/mol,and 137.84 k J/mol,respectively.Under high temperature dehydrogenation conditions,the catalytic activity of Ru Pd/SiC-10 catalyst decays due to the decomposition of carboxyl groups and the shedding of metal particles from the catalyst surface.（3）NC-SiO₂ carrier with irregular"pomegranate-shaped"hollow spherical particles was prepared by the"one-pot method",combined with impregnation method to prepare the Ru Pd/NC-SiO₂ catalyst.The effects of annealing temperature,carbon content and nitrogen content on the catalyst structure and the catalytic application on the hydrogenation/dehydrogenation dual catalytic activity of catalytic NPCZ/12H-NPCZ were investigated.The results show that the Ru Pd/NC-SiO₂ catalyst possesses a strong acidic site of Bronsted,and the strong interaction between the nitrogen atom and the metal has an anchoring effect on the Ru and Pd metals,which improves the dispersion of the metals and has good catalytic activity.Among them,the metal dispersion in Ru Pd/N₂CSicatalyst is as high as 47.34%and the metal particle size is only 2.34 nm,which can achieve 5.43 wt%hydrogen storage by catalyzing the NPCZ hydrogenation reaction for10 min and 5.42 wt%hydrogen release by catalyzing the 12H-NPCZ dehydrogenation reaction for 120 min.The apparent activation energies of the various levels of the catalytic12H-NPCZ dehydrogenation reaction at 150-180°C were 49.63 k J/mol,137.08 k J/mol,and 173.83 k J/mol,respectively.Catalyst cycling tests revealed that the strong interaction of nitrogen atoms with metals contributed to the excellent hydrogenation-dehydrogenation cycling stability of Ru Pd/NC-SiO₂catalyst.DFT calculations showed that 12H-NPCZ,8H-NPCZ,4H-NPCZ,and NPCZ molecules formed stable adsorption structures on the Ru₁Pd₂/CN surface and the reaction energy of 12-NPCZ to 8H-NPCZ conversion was 0.48 e V,which was favorable for the catalytic dehydrogenation reaction to occur.