Controllable Construction Of Two-dimensional Mesoporous Transition Metal Compound Nanocatalysts And Their Applications In Energy Catalysis

Nanocatalysts with high activity and stability are greatly concerned subjects for development in fields of catalytic sciences and technologies.Non-noble metal-based materials have attracted a wide range of interest as alternative catalysts owing to their similar electronic structure to that of noble metals.Despite of much efforts focussing on these types of catalysts,the improvement of their activity,electrical conductivity,and chemical stability is still a challenge to successfully achieve sufficient catalytic efficiency for real applications.Two-dimensional（2D）layered nanomaterials have opened great opportunities to build a new class of materials with many superior properties in material science and nanotechnology owing to their special properties of large specific surface area,more exposed active sites as well as sufficient contact areas between reactants and catalysts.Therefore,studies on synthesis of transition metals（TMs）based nanocatalysts with suitable 2D configurations have been largely boosted for the development of nanocatalysts.In this paper,we constructed a series of TMs-based nanocatalysts with 2D mesoporous structures,regulated compositions,variable morphologies and conformable architectures.The performance of these nanocatalysts for catalyzing the hydrogen evolution reaction,the oxygen reduction reaction,and the ammonia decomposition for hydrogen production were systematically studied.Moreover,we also in-depth explored the correlation among their composition,structure,and catalytic performance.The study in this Dessertation may provide a theoretical guidance and a technical support for design of novel highly efficient TMs-based catalysts.In the first Chapter,the research progress of transition-metal compounds based nanocatalysts was extensively reviewed and the strategies to improve their catalytic performance and their potential applications in various fields were summaried.Based on the research progress,the research motivation and focus of this Dessertation were proposed.In the second Chapter,we developed a universal strategy to synthesize a highly active and durable precious-metal-free mesoporous Mo₂C/graphene（m-Mo₂C/G）electrocatalyst with a two-dimensional layered structural feature using glucose as a carbon source and in situ assembled mesoporous KIT-6/G as a template.The obtained m-Mo₂C/G electrocatalyst possesses unique well-organized mesoporous structure of ultrasmall Mo₂C nanocrystals duplicated from a mesoporous KIT-6 template hybridizing on the surface of graphene sheets during the synthetic process.The unique mesoporous structural feature of Mo₂C and the compact integration of Mo₂C nanocrystals with graphene sheets may result in much faster charge transfer rate and larger electroactive area,accounting for the promotion of its catalytic performance for the HER.The highly active and durable m-Mo₂C/G electrocatalysts may be regarded as a promising HER catalyst in practical application.In the third Chapter,we constructed 0D/2D heterojunctions of ternary uniform（Mo₂C）_x-（WC）_1-x-QDs decorated on nitrogen-doped graphene（NG）via introducing WC QDs into the above mentioned m-Mo₂C/G electrocatalysts for the HER.In comparison with our previously reported m-Mo₂C/G electrocatalysts,the as-prepared（Mo₂C）_x-（WC）_1-x-QDs/NG electrocatalysts possess similar 2D layered structural features with abundant mesopores,large surface area,and ultrasmall nanocrystals dispersed,whereas their performance in the HER is remarkably improved.The enhanced HER activity may be ascribed to the redistribution of valence electrons and an increase in conductivity on the introduction of highly conductive tungsten carbide.In addition,the abundant N dopants from the precursors could not only lead to a downshift in the valence bands of active carbon atoms in graphene with the accompanying formation of structural defects but also function as an electron acceptor to assist the C atoms adjacent to molybdenum–tungsten carbide,which is immensely conducive to enhancements in activity in the HER.In the forth Chapter,we fabricated a series of 2D layered mesoporous transition metal nitrogen-carbide/nitrogen-doped graphene（meso-M-N-C/N-G,M=Fe,Co,and Ni）electrocatalysts using 4,4-bipyridine as nitrogen and carbon sources and mesoporous KIT-6/N-G as a template,which was generated by in-situ formation of KIT-6 on graphene nanosheets.The catalytic activities of electrocatalysts for oxygen reduction reaction（ORR）were systematically investigated and the mechanism of different active site for catalyzing ORR was elucidated.The meso-Fe-N-C/N-G electrocatalyst showed super electrocatalytic performance for ORR.Excitingly,its catalytic activity and durability were superior to those of Pt/C,making it a good candidate as an ORR electrocatalyst in fuel cells.In the fifth Chapter,we synthesized two-phase molybdenum nitride（η-MoN andγ-Mo₂N）nanocrystals,which were embedded in 2D mesoporous silica/graphene hybrid nanosheets via filling ammonium molybdate into SBA-15/G hybrids followed by the nitridation treatment in NH₃ atmosphere.The unique 2D structure can offer sufficient contact areas between reactants and electrocatalysts as well as shortened diffusion distance of substances.It was concluded that the function of rGO as electron donor to molybdenum nitride induced a downward shift of Mo 3d states and,therefore,weakened the Mo–N bond.Consequently,the 2D MoN/SBA-15/rGO and Mo₂N/SBA-15/rGO exhibited exceptionally high catalytic activity and good stability among Mo-based catalysts reported to date for NH₃ decomposition.Density functional theory（DFT）calculations revealed the complete reaction pathway of NH₃decomposition onη-MoN（100）andγ-Mo₂N（100）surfaces and the reaction occurred on Mo₂N was more favorable than MoN due to the lower energy barrier.In the sixth Chapter,we developed a facile approach to fabricate 2D ultrathin Co–Fe spinel oxides nanosheets encapsulated with mesoporous silica shells(Co_xFe_3-xO₄@mSiO₂)as active and stable catalysts for NH₃ decomposition using FeCo-LDHs@SiO₂/CTAB hybrids as precursors.The unique 2D structure of Co_xFe_3-xO₄@mSiO₂ nanosheets offered high specific surface area and intimate contact with NH₃ when a dimensional confinement was imposed.The tunable stoichiometry of Co_xFe_3-xO₄ nanosheets regulated the electron structure and optimized the nitrogen desorption ability to further enhance the catalytic activity and durability.The structural feature of encapsulation of mSiO₂ shells not only effectively prevented the coalescence of FeCo-LDHs nanosheets during the calcination process to facilitate the generation of ultrathin Co-Fe spinel oxides nanosheets with abundant active sites,but also availably protected Co_xFe_3-xO₄ nanosheets from detachment and agglomeration during the NH₃ decomposition to further enhance their catalytic durability.Furthermore,confining Co_xFe_3-xO₄ nanosheets in mesoporous SiO₂ shells may induce to generate defects and dislocations,thus providing more active sites to subsequently enhance the NH₃ decomposition performance.