Designing The Interface Structure Of Metal-based Catalysts To Regulate The Dehydrogenation/hydrogenation Activity

In heterogeneous metal catalytic system,usually metal is the main catalytic active centers,and the strong interaction between metal and support(SIMS)throughout the contact interface is one of the important strategies used to control the electron density on the surface of metal,and to strengthen the catalytic performance toward the absorption,activation and transformation of substrate moledules.To be specific,the interfacial interaction could be tuned from two aspects such as metals(e.g.the structure,size and crystal phases of metal nanoparticles)and support materials(e.g.doping,crystallinity,pore structure,band structure and etc)to control the electron density of metal sites.Compared with several other support materials like metal oxides,activated carbon,non-metal oxides,metal organic framework and so on,nitrogen-doped carbon(NC)materials are considered as a kind of ideal catalyst supports with the advantage of adjustable electronic structure,specific pore structure,corrosion resistance,low cost and easy chemical modification.So,the key research issues of this thesis are focused on the design of metal/NC interface to promote the catalytic performance of metal centers toward the specific reactions by enriching or lowering the electron density of metal centers via the electron transfer throughout the contacting interface.Under the guidance of the above discussed ideas,the research fields of this thesis will focus on a series of scientific problems encountered in the projuect of"CO₂ cycle to achieve the controlled release and chemical storage of H₂",that is,the mechanism of dehydrogenation/hydrogenation and to activation of small molecules over metal centers.Specifically,we will tune the electron-donating or-withdrawing capacity of the suppot by adjusting the band structure,the coordination type and concentration of doped heteroatoms in the carbon framework as well as structure optimization of metal phase.So,a series of Mott-Schottky type metal/semiconductor catalytic systems have been developed,and successfully applied in several typical and important dehydrogenation/hydrogenation reactions like dehydrogenation of formic acid and hydrogenating CO₂.By studying the“structure-activity relationship”,the regulation mechanism of metal centers'electronic structure and catalytic performance and further the reaction mechanism at the metal/semiconductor rectifying interface were studied systematically,so we believe this thesis could advance the design and development of highly efficient,stable and low-cost heterogeneous catalysts in the field of energy and environment.(1)Electron-rich Pd nanoparticles facilitating the dehydrogenation of formic acid A“sol-gel”method was employed to synthesize the semiconductor carbon nitride at the temperature as low as 300°C,thus a larger temperature control window(300 to500°C)has been achieved.And a series of carbon nitride materials with different band structures were developed by changing the thermal polymerization temperature.Furthermore,we compounded carbon nitride with metal Pd to construct the rectifying contact interface between Pd nanoparticles and C₃N₄ support.Most importantly,the catalytic dehydrogenation activity of Pd nanoparticles could be greatly enhanced as more electrons were transferred to the surface of Pd nanoparticles due to interface effect.(2)Controlling the electron density on single Fe atoms to adjust selective dehydrogenation of primary aminesWe have adopted a“bottom-up”method to prepare the heteroatom B doped singleFe nanomaterials from Fe salt,glucose,boric acid and dicyandiamide.It was found that the state of B bonding to single Fe atoms or not had a significant effect on the electronic structure of Fe centers.To be specific,only the proper electron-deficient state of Fe centers could strengthen the catalytic dehydrogenation performance to achieve the highly selective synthesis of nitrile.We believe this work could offer a new idea and strategy to develop carbonaceous single atom catalysts.(3)High-efficiently activating and hydrogenating CO2 over electron-rich Mo CnanoparticlesWe have designed a“PMo₁₂-melamine”supramolecular self-assembly method tosynthesize the nanocomposite ultrafine Mo C nanoparticles supported on the nitrogen doped carbon materials.And the schottky barrier at the interface of Mo C and carbon support was formed in-situ.The content of nitrogen doped in the carbon support was optimized to induce more electron to the surface of Mo C nanoparticles to activate CO₂and further to hydrogenate into formic acid high-efficiently.(4)Two-dimonsional Mo2C nanocatalysts facilitating the activation andcarbonylation of CO2Based on the former research work about“CO₂ activation”,we further extended the application of CO₂ chemical transformation.A“partially oxidize-etch”method was developed to optimize the internal structure of metal phase to obtain the two-dimonsional Mo₂C nanocatalysts.The specific structure could make more catalytic active sites exposed and accelerate the reactant molecules transfer theoretically.To our surprise,the two-dimonsional Mo₂C nanocatalysts displayed high activity and selectivity in the model reaction of the carbonylation of o?phenylenediamines with CO₂to 2?benzimidazolones.