ZIF-67-Derived Cobalt-Based Nanocomposites:Synthesis And Their Catalytic Applications

Metal-organic frameworks（MOFs）have attracted considerable attention due to their composition and structural diversity,as well as their widespread application in catalysis,drug delivery and gas separation.The composition diversity,deriving from the metal nodes and organic ligands,allows MOFs to be used as a flexible precursor for the synthesis of metal-based nanocomposites.In addition,their morphological and size diversity enable them to be a useful class of template for the synthesis of polymers and metal-organic complex.In this thesis,ZIF-67 was used for the purpose for synthesis of different types of Co-based nanocomposites.The catalytic performance of the as-prepared Co-based materials was examined for energy conversion applications,including chemical production of H2,oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）.This thesis can be devided into following three sections:1.SiO2-encompassed Co@N-doped porous carbon assemblies as recyclable catalysts for efficient hydrolysis of ammonia borane.Considering that it is difficult to control the size and distribution of metal particles after calcining MOF materials by using traditional synthesis methods.In this section,a new type of recyclable catalyst is obtained by calcination of ZIF-67@SiO2 nanotubes at high temperatures under an N2 atmosphere.It is found that the surface layer of SiO2 in the precursor nanotubes play an vital role in decreasing the size of Co nanoparticles by supplying an additional surface for their dispersion in the growth.In addition,the SiO2 layer can give rise to a highly ordered arrangement of Co@N-doped porous carbon within the catalysts.This kind of structure is likely to enable the ease of mass transfer of ammonia borane within the catalysts.A set of remarkable catalytic benefits are found in the optimized catalysts obtained via calcination at 800°C,including a high hydrogen generation rate of 8.4 mol min^-11 mol^-1_（Co）,a relatively low activation energy of 36.1 kJ mol^-1,and a remarkable reusability（at least 10times）.2.MoO2@Co3（PO4）2/N-C core-shell composite as a high-performance ORR catalyst.Direct calcination of MOFs leads to formation of limited catalytically active species in the nanocomposites.In this section,MoO3@ZIF-67@Co-phytic acid（PA）heterostructures are reported to be used as a class of precursor materials for the preparation of multifunctional composites.MoO3 nanorods were first used as support for the growth of ZIF-67,which were further used as a self-sacrifice template for the preparation of MoO3@ZIF-67@Co-PA under mild conditions.Calcination of MoO3@ZIF-67@Co-PA under an atmosphere of H2/Ar（95 v%）at high temperatures for 2 h leads to the formation of MoO2@Co3（PO4）2/N-C.MoO3 nanorods are reduced to MoO2 and the supported ZIF-67@Co-PA is transformed to ordered Co3（PO4）2/N-C hollow structure after the high-temperature treatment.The as-prepared MoO2@Co3（PO4）2/N-C composites are found to be a high-performance ORR catalyst.The experimental results show that the half-wave potential is located at 0.846 V,which is comparable with the Pt/C.The excellent ORR performance is a result of a combination of the enhanced conductivity by MoO2 and abundance of catalytic sites of Co3（PO4）2 in the composites.As an added benefit,the stability and methanol endurance of the as-prepared composite catalysts outperform the state-of-the-art ORR catalyst of Pt/C.3.Hollow Co-PA polyhedral frameworks for enhanced OER performance.In this part,a facile method for the synthesis of hollow Co-PA polyhedral frameworks is developed by direct addition of an aqueous solution of PA into a methanol dispersion of ZIF-67.In this method,ZIF-67 was not only used as a self-sacrificing structural template but also explored as a Co source.PA was used for etching ZIF-67 to form Co-PA cross-linked complexes due to the strong affinity of Co²⁺and PA.The as-synthesized Co-PA hollow nanomaterials exhibit electrocatalytic activity towards OER,with small overpotential of 351 mV and Tafel slope of 39 mV/dec.It is important to note that the OER performance can be further improved by addition of Fe³⁺into the Co-PA cross-linked complexes through cation exchange,as supported by decreases in both overpotential to 306 mV and Tafel slope to 36 mV/dec.This work is potentially to be use for the preparation of other transitional metal-PA complexes for energy conversion applications as a versatile catalyst.