Preparation Of Metal-organic Frameworks And Its Electrochemical Catalytic Performance For Nitrogen Fixation

Conversion of naturally abundant nitrogen to ammonia is essential for economic development.Haber-Bosch is traditional industrial nitrogen fixation method that consumes a lot of energy and emits a lot of CO2.Therefore,it is important to develop a new low-energy and environmentally friendly nitrogen fixation method.The electrochemical nitrogen reduction reaction(NRR)can produce NH3 under ambient conditions and is a promising green alternative to Haber-Bosch.However,current catalyst performance is poor,so the nitrogen fixation rate and Faraday efficiency(FE)are both low.Metal-organic frameworks(MOFs)have high porosity,high specific surface area and abundant coordination unsaturated metal sites,which are potential NRR catalysts.Using ferric chloride and 2-aminoterephthalic acid(or terephthalic acid),NH2-MIL-88B-Fe(or MIL-88B-Fe)is synthesized by hydrothermal method.The particle size of the biconical prismatic NH2-MIL-88B-Fe is smaller than that of spindle-shaped MIL-88B-Fe.The maximum NH3 production rate of NH2-MIL-88B-Fe is 12.05 × 10-11 mol s-1 cm 2,which is higher than that of MIL-88B-Fe(3.575 × 10-11 mol s-1 cm-2).The FE of NH2-MIL-88B-Fe is also higher than that of MIL-88B-Fe.Introducing-NH2 to MIL-88B-Fe can increase the coordination unsaturated sites to improve NRR performance.The MOF-74 catalysts with three metal centers of Ni,Co and Zn were synthesized by hydrothermal method using metal nitrate and 2,5-dihydroxyterephthalic acid.The flower-shaped spherical Ni-MOF-74 particles are evenly distributed,and the particle size is smaller than that of hexagonal columnar Co-MOF-74 and massive Zn-MOF-74.Ni-MOF-74 has the best NRR performance,the highest NH3 production rate is 6.68 × 10-11 mol s-1 cm-2,and the FE is up to 23.69%.This is due to Ni-MOF-74 has the smallest particle size,the largest electrochemical specific surface area,and the largest content of metal-oxygen bonds.Four types of Zr-based metal-organic frameworks,i.e.,UiO-66,UiO-66-NH2,UiO-66-NO2 and UiO-66-Br were prepared by hydrothermal method using zirconium chloride and functionalized terephthalic acid.UiO-66 is the regular octahedron.After the introduction of functional groups,the crystal morphology changes.UiO-66-NH2 has the best NRR performance,its maximum NH3 generation rate is 20.7 × 10-11 mol s-1 cm-2,and FE is 85.21%.This is because UiO-66-NH2 is the uniformly distributed small crystals with the most oxygen vacancies and unsaturated sites.