Preparation Of Porous Materials Supported Noble Metal Composite Nanoparticle Catalysts For Catalytic Hydrogen Generation

The production of hydrogen by decomposing chemical hydrides such as formic acid?FA?and ammonia borane?AB?is one of the effective ways to solve the energy problem.A variety of supported noble metal?Pd,Au,Pt etc.?catalysts exhibit excellent properties in the chemical hydride decomposition to produce hydrogen.Considering the scarcity and high price of noble metal,many efforts have been devoted to improving the utilization efficiency and specific activity of noble metal species.Reducing the size of noble metal particles?forming nanoparticles or even subnanoparticles?can often significantly improve the activity of the catalysts.However,small nanoparticles are prone to agglomeration or leaching during the catalytic reaction,which leads to the decrease of catalytic performance.To solve the above problems,recent research focuses on how to design and prepare supported noble metal catalysts with better performance and higher stability by selecting more suitable supports,preparation methods and adding additives.In this thesis,several kinds of supported binary metal composite nanoparticles?PdAu?PtPd?PdCo etc.?catalysts with excellent catalytic properties for hydrogen production were prepared by using amines modified mesoporous Zr SBA-15 and polyacrylonitrile beads as supports.The composition,structure,distribution state and chemical environment of the noble metal composite nanoparticles were studied by characterization means of XRD,TEM and XPS etc.The effects of the component synergistic effect of binary metals and the interfacial interaction between metal nanoparticles and supports on the activity and stability of the catalysts were discussed.The nature of the active centers and the catalytic mechanism of the catalysts were clarified.The detailed contents and conclusions of this thesis are as follows:1.A series of amines modified ZrSBA-15 supported PdAu composite nanopartices?NPs?catalysts were prepared by NaBH₄ coreduction method,and their catalytic properties were studied for the hydrogen generation from formic acid in aqueous solution without any additives.The catalytic activities of the supported PdAu catalysts vary with the change of amines and Pd/Au ratios.The optimized catalyst Pd₆₀Au₄₀/ZrSBA-15-AP?AP,?3-Aminopropyl?trimethoxysilane?achieves a very high efficiency under ambient conditions with a initial TOF?turnover frequency?value of 1185 h^-1,which is superior to other types of catalysts,and it can even work well at a temperature near the freezing point of the solution.During the preparation process of the catalysts,the amino groups distributed on the surface of mesoporous supports could coordinate with the cations of Au and Pd to produce metal complexes with different structure stabilities and redox properties,and finally result in the formation of Pd Au NPs with different particle sizes and dispersion states.During the term of catalytic reaction,the organic amines play the role of a proton trap in the activation of formic acid molecules,which is beneficial to breaking the O-H bonds in formic acid molecules.The co-existence of small Pd Au alloy NPs and the suitable amino groups within the single solid support could provide highly efficient bifunctional sites for synergistically activating formic acid molecules to release hydrogen.2.Using AP-modified ZrSBA-15 as support,a series of supported Pt Pd composite nanoparticle catalysts with different Pt/Pd ratios were prepared by NaBH₄ coreduction method.Their catalytic properties were investigated through the dehydrogenation of ammonia borane and the cascade hydrogenation reaction of p-nitrophenol.Introducing amino groups on the surface of ZrSBA-15 is of benefit to achieving high dispersion of Pt Pd NPs,and can effectively prevent the overgrowth of Pt Pd NPs during the preparation and catalytic reaction processes.The resulting hybrid catalysts exhibit very high catalytic activity and stability for H₂ generation from aqueous ammonia borane,and for the reduction of p-nitrophenol using ammonia borane as hydrogen source.Among them,Pt₈₅Pd₁₅/Zr SBA-15-AP shows the highest activity for ammonia borane dehydrogenation under ambient condition,while Pt₅₀Pd₅₀/ZrSBA-15-AP exhibits the highest efficiency for the reduction of p-nitrophenol with ammonia borane.These results suggest that highly efficient interface active sites could be constructed by adjusting the composition of bimetallic Pt Pd NPs and the chemical surrounding of mesoporous ZrSBA-15,which demonstrate effective synergistic catalysis for ammonia borane dehydrogenation and p-nitrophenol reduction.3.Millimeter size polyacrylonitrile?PAN?beads obtained by ball-dropping method were used as supports for preparing several kinds of supported Pd-based composite nanoparticle catalysts through an impregnation-NaBH₄ reduction process.All the PAN beads supported Pd nanoparticles catalysts exhibit high catalytic activity for the additive-free formic acid dehydrogenation and the cascade reduction of organic dyes?methylene blue and rhodamine B?at ambient condition.Doping suitable content of Co or Fe species could further enhance the catalytic activity of the PAN supported Pd-based catalysts.The abundant-CN groups existed on the surface of PAN beads are beneficial to dispersing and immobilizing Pd NPs,and may also act as proton scavengers to provide a suitable basic environment for activating formic acid molecular.During the catalytic reaction process,the intermediate species produced by in-situ activation of formic acid molecules on the surface of the catalyst can act as hydrogen sources in the direct reduction system of dye molecules,thus efficiently realizing the cascade reactions of dehydrogenantion of formic acid and hydrogenation of dye molecules under very mild conditions?ambient temperature and atmospheric pressure?.