Preparation,Characterization And Catalytic Performance Of Palladium Anchored To Several Kinds Of Fe₃O₄@SiO₂-based Coreshell Composites

As a green and renewable new energy source,hydrogen has attracted much attention and has a very bright future for replacing the traditional energy.Among various hydrogen storage materials,ammonia borane?AB?attracts much attention because of its unique advantages including high hydrogen storage density?19.6 wt%?.However,AB is relatively stable at room temperature,and its hydrolysis occurs in the presence of suitable catalysts.Therefore,an efficient and stable catalyst plays an important role in the hydrolytic dehydrogenation of AB.Notably,the magnetic materials,which possess the advantages of high activity,magnetic recyclability and easy reuse,have been widely used in the fields of organic synthesis,catalytic chemistry and pharmaceutical synthesis.Among them,the magnetic core-shell material with Fe₃O₄ as the core and SiO₂ as the shell layer becomes a class of excellent support due to its high stability and modifiability.Compared with other precious metals,Pd is more suitable for the hydrolysis of AB because of its low cost and satisfactory catalytic performance.In this thesis,a series of magnetic supports were prepared by compositing Fe₃O₄@SiO₂with several carbon and silica-based porous materials.Then,the magnetic palladium nanocatalysts supported on the as-prepared composites were prepared by an in-situ reduction strategy.Their catalytic performance for the hydrolysis of AB were investigated.In addition,the corresponding dehydrogenation reaction kinetics were also studied.The main conclusions are as follows:1.Fe₃O₄ magnetic particles were prepared by a solvothermal method using Fe Cl₃·6H₂O as Fe source.These particles were used to synthesize Fe₃O₄@SiO₂ with a core-shell structure by a sol-gel method using TEOS as Si source.Then,using Fe₃O₄@SiO₂ as support,Pd/Fe₃O₄@SiO₂magnetic catalyst was prepared by the in-situ reduction of Pd Cl₂ with Na BH₄.XRD,TEM and other characterization results indicated that SiO₂ as a shell was successfully coated on Fe₃O₄ spherical core,and the Pd nanoparticles?NPs?were successfully anchored to Fe₃O₄@SiO₂.Kinetic studies showed that the reaction orders in terms of the catalyst concentration and AB concentration were 1.06 and 0.18,respectively.The apparent activation energy was41.6 k J mol^-1.In addition,the catalytic activity of Pd/Fe₃O₄@SiO₂ had not obvious decrease even after 5 runs,implying good recyclability.2.Using glucose as the carbon source,Fe₃O₄@SiO₂-C was successfully synthesized with a hydrothermal method,and further used as support to anchor Pd NPs.The characteristic results showed that the Pd NPs were uniformly dispersed on the composite support.Catalyzed by the as-prepared Pd/Fe₃O₄@SiO₂-C catalyst,the hydrolysis of AB was 1.05 order in catalyst concentration and 0.08 order in AB concentration,and the apparent activation energy was 18.4 k J mol^-1.The Pd/Fe₃O₄@SiO₂-C catalyst had high stability and good magnetism,and it still maintained high catalytic activity after 5 runs.3.Using glucose as the carbon source and zinc chloride as the activator,a series of glucose-derived porous carbon?PC?materials were obtained through an impregnation-calcination process.PC was combined with Fe₃O₄@SiO₂ core-shell material to obtained Fe₃O₄@SiO₂-PC composite supports.Then,Pd particles were successfully anchored to the Fe₃O₄@SiO₂-PC composites with the in-situ reduction of Pd Cl₂.Characterization analysis showed that the particle sizes of Pd NPs were in the range of 2.3-4.3 nm in the Pd/Fe₃O₄@SiO₂-PC catalyst,which possessed a hierarchical porous structure with high specific surface area(626.0 m² g^-1)and large pore volume(0.61 cm³ g^-1).The optimal mass ratio of glucose to Zn Cl₂was 1:8based on the catalytic activity in the hydrolysis of AB.Catalyzed by the optimal catalyst,the apparent activation energy was 28.4 k J mol^-1,and the TOF was 46.2 concentration and AB concentration was 1.10 and 0.04 orders,respectively.At the same time,the Pd/Fe₃O₄@SiO₂-PC catalyst was still highly active even after 10 runs,revealing excellent reusability.4.Mesoporous molecular sieve MCM-41 was synthesized by using cetyltrimethylammonium bromide?CTAB?as template and TEOS as Si source.Then,a series of Fe₃O₄@SiO₂-MCM-41 composites were prepared by adjusting the relative amounts of Fe₃O₄@SiO₂ to MCM-41.The corresponding Pd/Fe₃O₄@SiO₂-MCM-41magnetic catalysts were obtained by anchoring Pd NPs to the magnetic composites by the in-situ reduction of Pd Cl₂.Characterization results showed that the Pd particles were uniformly dispersed on the composite supports.When the mass ratio of Fe₃O₄@SiO₂ to MCM-41 was 1:1,the corresponding catalyst exhibited the highest activity.Catalyzed by the optimal Pd/Fe₃O₄@SiO₂-MCM-41 for AB hydrolysis,the reaction order for the catalyst concentration was 0.84,and for the substrate concentration was 0.The apparent activation energy was 30.2 k J mol^-1.The catalyst still maintained 67%of its initial activity in the fourth run.5.Using melamine as raw material and ammonium chloride as activator,porous g-C₃N₄ was synthesized with a calcination procedure,and used to prepare a series of Fe₃O₄@SiO₂-g-C₃N₄composites.Then,the Pd NPs were anchored to the composite supports by the in-situ reduction of Pd Cl₂ with Na BH₄.The characterization results showed that the size of the Pd NPs in Pd/Fe₃O₄@SiO₂-g-C₃N₄ were in the range of5.4-9.8 nm,and the catalyst had a specific surface area of 47.5 m²g^-1 and a total pore volume of 0.13 cm³g^-1.The catalyst possessed the highest catalytic activity when the mass ratio of Fe₃O₄@SiO₂ to g-C₃N₄ was 1:1.The catalytic hydrolysis of AB was0.93 order in terms of the catalyst concentration and 0.22 order with AB concentration over the optimal catalyst.The corresponding TOF value was 33.7 runs,the catalyst still maintained a high catalytic activity,indicating that the catalyst had high recyclability.6.Compared with Pd/Fe₃O₄@SiO₂,the Pd NPs supported on the Fe₃O₄@SiO₂composites modified by glucose-based carbon displayed higher catalytic activity for AB hydrolysis.In particular,the activity was greatly improved when the Fe₃O₄@SiO₂-PC composite is used as support.7.Although the preparation processes for Pd NPs supported on Fe₃O₄@SiO₂-based composited with PC,MCM-41 and g-C₃N₄ were similar,these catalysts showed different catalytic activities.Among them,the Pd/Fe₃O₄@SiO₂-PC catalyst demonstrated the best catalytic performance,and the corresponding TOF was as high as 46.2 mol_H2· mol_Pd^-1· min^-1.