Preparation And Catalytic Properties Of Mo/W Based Catalysts Using POM@ZIFs As Precursors

China is rich of molybdenum?Mo?and tungsten?W?reserves,especially in Henan Province.However,the development and utilization of molybdenum and tungsten resources is still in the primary stage of minerals rough processing,and the industry of molybdenum and tungsten urgently need industrial upgrading.In fact,due to the unique electronic structures,high stability and redox chemistry,Mo/W based metal carbides and oxides are widely used in various catalytic reactions and are excellent catalysts for oxidation and reduction reactions.In order to take full advantage of China's Mo/W mineral resources and improve its industrial value-added,we have designed and synthesized a series of efficient catalysts using Mo/W based materials,which can be applied to the olefin epoxidation and electrocatalytic methanol oxidation and electrocatalytic hydrogen evolution reaction.Epoxidation of olefins not only has wide applications in pharmaceuticals,epoxy resins,plasticsand other high value-added chemical products,but also has important value in the environmentally friendly chemical process.Due to the sustainability,low cost and clean pollution-free,design and synthesis high performance epoxidation catalysts with H₂O₂ as oxidant have become the research priority.It is noticeable,however,the electricity for industrial use is still generally from thermal power generation,which requires a large amount of coal,oil and others fossil energy.With the over-extraction and utilization of fossil energy,its reserves are gradually exhausted,the price fluctuates accordingly.Furthermore,the usage of fossil fuels leads to serious environmental pollution problems.High-efficient and clean renewable energy resources are thus needed to replace the traditional fossil fuels.Fuel cells are the most promising alternative fuel due to the high energy exchange efficiency,cleanness and device simple,including hydrogen fuel cell and direct methanol fuel cell?DMFC?.One of the biggest problems facing hydrogen fuel cells is the supply of hydrogen energy.Electrochemical water splitting has great significance for hydrogen production.As an important anode reaction,electrocatalytic methanol oxidation reaction?MOR?is the key to realize high-performance DMFC.Precious metal Pt is found to be an ideal catalyst for the hydrogen evolution reaction?HER?and MOR,however,its widespread application was hindered because of the high cost and low abundance.Thus,the preparation of highly efficient,low cost and stable non-noble metal catalysts for the MOR and HER have become the research priority as well.In this paper,we choose the polyoxometalates?POMs?and zeolitic imidazolate frameworks ZIF-8as research object.Based on the space-confinement strategy,we successfully obtained the efficient catalysts for olefins epoxidation,methanol oxidation reaction and hydrogen evolution reaction.Therefore,the research content of mainly divided into the following three chapters:1.Firstly,the Keggin-type K₅[?-BW₁₂O₄₀]11.4 H₂O(BW₁₂)was encapsulated inside the cavity ofZIF-8.Based on the space-confinement pyrolysis and post-modification oxidation strategy,we successfully fabricated a H₂O coordinated WOx epoxidation catalyst H₂O-WO_x@C.With the unique H₂O coordinate environments,the catalyst exhibits high activities in olefin epoxidations when using H₂O₂ as oxidant.In epoxidation of cis-cyclooctene tests,the turnover frequency?TOF?of the as-synthesized catalyst reached as high as 949 h^-1,which is a 120-fold increase compared with the commercial WO₃.Spectroscopic evidence and catalytic results strongly indicate that the HOO-WO_x is the most reasonable active intermediate in the as-synthesized catalyst.2.Secondly,the catalyst precursor SiW₉Ni₃@ZIF/C was synthesized by using transition-metal-substituted Keggin-type polyoxotungstate K₁₀[SiW₉O₃₇Ni₃?H₂O?₃]?denoted as SiW₉Ni₃?and ZIF-8.Then SiW9Ni3@ZIF/C precursor was further pyrolyzed,and successfully obtained WC/Ni@C catalyst.In particular,WC/Ni@C showed high catalytic activities in methanol oxidation reaction,the onset potential+0.35V?vs Ag/AgCl?,and when potential reached+0.60V?vs Ag/AgCl?,the current density was highest at310 mA cm^-2.The long-term stability tests show that the WC/Ni@C catalyst can be used for 110 min without obvious deactivations.3.At last,the Keggin-type?n-Bu₄N?₃[PMo₁₂O₄₀](denoted as PMo₁₂)was encapsulated inside thecavity of ZIF-8.High temperature pyrolysis obtained molybdenum carbide nanoparticles catalyst MoC@NC,which showed a good catalytic performance for HER.In 0.5M H₂SO₄,it exhibited a low overpotential of 138mV?vs RHE?at 10 mA cm^-22 and Tafel slope of 68.9 mV dec^-1.The long-term electrolytic tests demonstrate that MoC@NC catalyst has favorable stability.Moreover,the MoC@NC also showed favorable performance towards HER in 1M KOH,with a small overpotential of 136 mV?vs RHE?at 10 mA cm^-2 and a low Tafel slope of 68.7 mV dec^-1.