Synthesis And Application Of Chainmail Catalyst

Catalysis is one of the core fields of chemistry and plays an indispensable role in promoting the development of national economy and society.Developing catalysts with high activity and good stability is eternally the major pursuit.In recent years,"nanocatalysis"has become a hot topic in the fields of materials and catalysis.However,the survival environment in the practical application is harsh for nanocatalyst.For example,the liquid medium is strongly acidic or alkaline with high overpotential in electrocatalysis,high temperature is indispensable in many heterogeneous catalytic reactions,or toxic molecules exist in the reaction environment,which will lead to the deactivation of nanocatalysts due to the leaching or agglomeration of nanoparticles(NPs),and excessive oxidation or strong adsorption of intermediates on acltive sites.Two-dimensional materials(mainly graphene)encapsulated metal catalysts have the unique"chainmail structure" and exhibit excellent activity and stability in the reaction.Graphene coating prevents the direct contact between metal particles and reactant molecules or medium,thus protecting the metal particles from agglomeration,leaching,and strong adsorption of poisonous molecules in harsh reaction conditions.At the same time,the different work function between metal and graphene enables the electron of metal to penetrate through the graphene layer,and increase the density of states near Fermi level of graphene,which can enhance the adsorption of reactant molecules and promote the catalytic reaction on the external surface of graphene shell.In consideration of the above advantages,we believe that it is of great significance to explore the application of"chainmail catalyst"in heterogeneous catalysis.The main purpose of this thesis is to develop the preparation strategies of chainmail catalyst and explore the structure-performance correlation of these catalysts in heterogeneous catalysis.First,the thesis showed that the Pt-based nanocatalyst with chainmail structure(Pt@C-MWCNTs)was prepared via ethylene glycol reduction and thermal treatment,and Pt@C-MWCNTs exhibited higher activity and stability than bare Pt catalyst(Pt/MWCNTs)in the selective oxidation of ethanol and glycerol under harsh conditions(60-150 ℃,0.5-2.0 MPa 02,pH<1).The carbon film encapsulating outside of Pt NPs would suppress the over-oxidation of Pt NPs when exposed to O2,while 02-TPD analysis indicated that the inert PtO2 species formed on the surface of bare Pt NPs.Raman analysis of the acetic acid-adsorbed Pt catalysts implied that carbon film could separate the organic carboxylic acid(product of alcohols oxidation)from Pt NPs,thus inhibiting the strong coordination adsorption of acetic acid on bare Pt NPs and promoting the alcohol oxidation process.Moreover,carbon film could also retard the sintering and leaching of Pt NPs.In order to simplify the preparation process and explore the activity of chainmail catalysts in different reactions,the third chapter of this thesis reported a N-doped nanotubes-graphene encapsulated Ni NPs hybrid(Ni@N-CNTs-GS)which was synthesized via simple pyrolysis technology using cheap agar,melamine and Ni(en)3(NO3)2 as raw materials.Those newly formed CNTs were rooted in and anchored by the graphene sheets,and the carbon film-encapsulated Ni NPs were dispersed evenly both in the chambers of CNTs and on the surface of the graphene sheets.It was found that Ni@N-CNTs-GS was very active and stable for the selective hydrogenation of nitroarenes to anilines via a direct route.The fourth and fifth chapter of this thesis reported the synthesis of N-doped graphene encapsulated Ni NPs catalysts(Ni@NC)and N-doped carbon film encapsulated Co NPs catalysts(Co@NC)from M2(bcd)2(ted)MOFs,and explored their catalytic performance in the hydrogenation of nitrobenzene and glycidol,respectively.Detailed characterization results indicated that the thickness of graphene shell,the size of Ni NPs,and the valence state of Ni depended strongly on pyrolysis temperature.The chainmail structure of Co-based catalysts could directly affect the adsorption form of H2 molecules:on the surface of bare Co NPs(Co/AC),H2 was dissociated into H atoms,which was very active and led to the over-hydrogenation of glycidol to n-propanol;while the carbon film in Co@NC catalyst could inhibit the dissociation of H2,and the molecular H2 adsorbed on carbon film promoted the selective hydrogenation of glycidol to 1,3-propanediol.In addition,the composition of chainmail(graphene shell)could also influence the performance of Co-based catalyst:in the nitrogen-rich Co@NG-ZIF catalyst,the large number of N atoms in graphene shell stimulated the dissociation of H2,leading to the over-hydrogenation of glycidol to n-propanol.