Preparation Of MXene-based Composite Photocatalysts And Their Nitrogen-fixing Properties

As a carrier of green energy and chemical raw materials,ammonia plays an important role in industry,agriculture and pharmaceutical fields.At present,industrial nitrogen fixation "Haber-Bosch"occupies an irreplaceable position in synthetic ammonia.The process requires high-purity nitrogen and hydrogen to be carried out on Fe-based catalysts under high temperature and pressure conditions.Thus causing 2%of global energy consumption,and the reformation of nature gas and coal produce 300 million tons of CO2 emissions,annually.Photocatalytic nitrogen fixation provides a promising approach to sustainable artificial nitrogen fixation under mild conditions using sunlight as energy,water as proton source,and nitrogen as nitrogen source.Two-dimensional nanomaterials are endowed with excellent catalytic performance due to their atomic thickness,high specific surface area,unique electronic structure.Two-dimensional transition metal carbides and nitrides(MXene)is a new type of two-dimensional layered nanomaterials,with excellent conductivity and hydrophilicity,showing a broad application prospect in the fields of energy storage and environmental catalysis.In this work,two-dimensional transition metal carbides(Ti-MXene)was used as a precursor to design a nitrogen-fixing photocatalyst from the perspectives of element doping and defect control.It provides a new idea for the design of nitrogen-fixing photocatalyst.The main research contents are as follows:(1)MXene not only has ultra-low work function and negatively charged surface,but also has the characteristics of high conductivity,hydrophilic surface,good stability,excellent electrochemical activity and easy functional modification,making it a potential catalyst and carrier material.The MXene/TiO2/Co composite materials used Ti3C2 as a precursor,TiO2 was grown in-situ on the surface of MXene using the Ti source of MXene,and then the transition metal element Co was loaded by the impregnation method.The results show that the accordion-like MXene and TiO2 grown in situ on its surface form an intimate MXene@TiO2 heterojunction.The structure improves the separation of photogenerated carriers,and the good conductivity of MXene increases the transmission of photogenerated carriers,therefore reducing the recombination of carriers.Co modification adjusts the balance of the chemical adsorption of catalyst between the N2and NH3.Eventually,the efficient photocatalytic nitrogen fixation achieved.The nitrogen fixation performance of MXene/TiO2/Co-0.5%in pure water without hole sacrificial agent under N2 atmosphere is improved to 110.45 ?mol·g-1h-1,which is 5 times,1.4 times and 3.5 times of MXene,MXene/TiO2 and TiO2 nitrogen fixation performance,respectively.(2)Metal organic frameworks(MOFs)are a type of crystalline material with periodic network structure formed by self-assembly process of inorganic metal ions and organic ligands,which can be designed with low density,high specific surface area,structure and function,adjustable hole size and other features,MOFs are expected to become a new generation of photocatalytic materials.However,MOFs have poor conductivity,the faster spraration of photogenerated electrons can be achieved by combining with MXene.In this work,a zirconium-based MOFs(UiO-66)/MXene composite material was prepared by solvothermal method and mixed impregnation.The results showed that after light irradiation,a large number of cluster defects and ligand defects are introduced into the UiO-66 material,which made the catalyst has the excellent ability of nitrogen fixation in the full-light,the nitrogen fixation performance in the visible light has been significantly improved especially,reaching 256?mol·g-1h-1 and 95?mol·g-1h-1,respectively,and the nitrogen fixation yield in the visible band is better than most reported catalysts.The effect of different defect types produced by light on the performance of nitrogen fixation was studied by the post-synthetic ligand exchange process(PSE),which demonstrated that the unsaturated coordination metal catalytic active centers induced by intrinsic defects of metal organic frameworks(MOFs)play an important role.The recombination of MXene increases the effective separation of photo-generated electrons and reduces the recombination probability of electrons and holes and its nitrogen-fixing performance is further improved from 190?mol·g-1h-1 to 210 ?mol·g-1h-1.