Two-dimensional MOF Nanosheets And MOF Carbonized Materials Are Prepared And Applied To Electrocatalytic Reduction Of Carbon Dioxide

Fossil fuels release a large amount of carbon dioxide during their combustion.If they are not controlled into the atmosphere,they will lead to a continuous increase in CO2 concentration,causing a series of environmental problems such as acid rain and greenhouse effect.In 2017,global CO2 emissions have reached 41 billion tons.Therefore,it is urgent to research on CO2 pollution control technology for fossil fuel combustion process.Among them,the electrochemical reduction method can not only reduce the CO2 content in the exhaust gas,but also obtain the effective products,such as formic acid,acetic acid,carbon monoxide,etc.,as an important resource.Therefore,this method has important research value and application prospects.In view of the current research status of electrocatalytic reduction of CO2,it is necessary to select a catalyst with simple synthesis,good selectivity and high catalytic efficiency.In this thesis,a two-dimensional metal-organic framework(MOF)nanosheet and MOF carbonized composite electrocatalyst are designed and synthesized with the aim of low-cost,high-performance,and it is systematically investigated to research on catalytic performance in the process of reducing carbon dioxide.The main research contents of the thesis are as follows:1.As a highly active catalyst,single atoms are widely used in electrocatalytic reduction of CO2,but their active atomic content is low.Therefore,we first time have applied MOF Ni(Im)2 with a similar structure to a single atom for electrocatalytic reduction of CO2.MOF nanosheets is synthesized to different thicknesses by ultrasonic stripping.The FE of CO increased from 33.7%of the bulk MOF to 78.8%of the MOF nanosheets,and the TOF is as high as 2521 h-1.Atomic force microscopy(AFM)characterization shows that the thinner the surface layer,the higher the catalytic activity.In addition,the comparative experiments show that the catalytic activity of Ni-N4 structure MOF Ni3(HITP)2 and Zr-TCPP(Ni)is lower than that of Ni(Im)2,it indicated that the type of organic ligand has a significant effect on the activity of the central metal.At the same time,the catalytic activity can also be changed by adjusting the amount of ammonia water to obtain MOF structures of different phase states.Therefore,the catalytic activity of the MOF can be enhanced by selecting the different organic ligands and changing the amount of the auxiliary agent to change the chemical environment of the central metal.2.In order to overcome the shortcomings of poor conductivity,low surface active sites and low catalytic activity of bulk MOF,we use the characteristics of ultra-thin,high conductivity and many active sites of two-dimensional MOF Cu2(CuTCPP)(TCPP=5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin)nanosheets for the first time to applied to electrocatalytic reduction of carbon dioxide(CO2RR).It has high activity and selectivity to the liquid product formate and acetate,and the total FE reaches 73.8%.However,the current continued to increase in the first hour during the reaction,and the total FE is much less than 100%.It is concluded that the two-dimensional MOFs nanosheets have undergone structural change.Therefore,the performance of the catalyst after the structural transformation is tested by pre-electrolysis for 1.5 hours.The total FE of the two liquid products increased to 85.2%,and the current remained substantially constant with the total FE approaching 100%.The TOF of catalytic reduction to formate and acetate can be as high as 2037 h-1 and 148 h-1,respectively.The catalyst produced by this pre-electrolysis has much higher catalytic efficiency than the common Cu2O,CuO,Cu and porphyrin-Cu(II)complexes.In addition,X-ray powder diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and transmission electron microscopy(TEM)confirmed the cathode electrolysis of 2-D MOF nanosheets through Cu(HCOO)2 and Cu(OH)2 intermediate converted to a mixture of CuO,Cu2O and Cu4O3.At the same time,it also verified by fourier transform infrared spectroscopy(FT-IR)and nuclear magnetic resonance(NMR)that the CuTCPP complex still exists on the mixed catalyst after electrolysis.In addition,physical mixing of CuTCPP with CuO also verified that CuTCPP can synergistically increase the activity of the catalyst.The above studies provide a research paradigm for the structural changes in the catalytic reduction of carbon dioxide by MOF.3.In order to further improve the formate selectivity,it is necessary to develop a high-efficiency catalyst,and Sn,as an inexpensive metal,has a high electrocatalytic performance for reducing CO2.We first time have applied Sn-MOF carbonized materials to electrocatalytic reduction of CO2.And a series of Sn/SnO2/C composite catalysts are formed by carbonization at different temperatures under argon atmosphere.The catalyst's FE for the preparation of formate by Sn/SnO2/C-400 reached 87.6%,and the catalyst stability is up to 20 hours.The TOF of the formate-reducing reaction is as high as 2277 h-1.The structure of Sn/SnO2/C composites are investigated by XRD,SEM,XPS,HRTEM,and FT-IR.The Sn(101)crystal plane is the strongest in the catalyst and contains a small amount of SnO2.The catalytic efficiency of the composite catalyst formed after carbonization is much higher than that of Sn,SnO2 and the precursor Sn-MOF.In addition,the effect of structural changes on the catalytic activity of formate are investigated by controlling the carbonization time of Sn/SnO2/C composites.It confirmed that the Sn(101)crystal plane has a certain selectivity for the enhancement of formate.However,when the intensity of Sn(101)crystal plane reaches a certain level,it will reduce the CO2RR reaction and enhance the HER reaction.