Preparation And Photocatalytic Nitrogen Fixation Performance Study Of Modified MIL-68-In Derivatives

Ammonia is not only an important chemical intermediate in industrial and agriculture,but also a good carrier of renewable energy.The most commonly used artificial method at present,the Haber-Bosch method,consumes a lot of fossil fuels and emits a lot of harmful gases.Therefore,photocatalytic nitrogen fixation with mild reaction conditions and environmental friendliness has attracted wide attention from researchers.In this study,based on the problems of no active site,low stability,blocked electron transmission,low catalytic activity and unclear mechanism in the field of photocatalytic nitrogen fixation,In-based MOFs(MIL-68-In)was selected as the precursor to prepare a series of derivatives.The catalytic activity was improved by introducing active sites,enhancing material stability and electrical conductivity,and the catalytic mechanism was clarified.As well as the formation process of catalyst,it provides a more comprehensive idea and experimental basis for the performance improvement and structural design of nitrogen reduction photocatalyst.Specific research contents are as follows:(1)Preparation of Ag-loaded In2O3 composites by MIL-68-In precursor system and its photocatalytic nitrogen fixation performance.Precursor MIL-68-In was synthesized with In3+ and H2BDC at low cost.The hollow tubular In2O3+5%Ag composite was synthesized by Muffle furnace calcination and light deposition.In2O3+5%Ag has relatively high photocatalytic nitrogen fixing performance of(35.56 μmol g-1 h-1.cat.).Moreover,it has better electron transport rate,strong photoresponse and large electrochemical active area.The results show that the LSPR effect of Ag is beneficial to broaden the range of light absorption and increase the intensity of light absorption.The Ag loading provided the active site for the catalyst and enhanced the adsorption and activation of N2.In addition,when the Ag load is too much,the material agglomeration phenomenon occurs,and the specific surface area of the catalyst becomes smaller,thus affecting the nitrogen fixing performance of the catalyst.(2)Preparation of In2O3@C composite by MIL-68-In precursor system and its photocatalytic nitrogen fixation performanceTwo kinds of catalyst In2O3@C and In2O3 were prepared by using MIL-68-In as precursor,NaOAc was added to adjust the size of the precursor,and argon and air were heat treated respectively.By comparison,it can be seen that rod-like In2O3@C has higher photocatalytic nitrogen fixing performance of(32.54 μmol g-1 h-1.cat.)It is better than In2O3,and has better conductivity,stronger photoresponse and larger electrochemical active area,so it is a stable photocatalyst with higher nitrogen fixing activity.Experimental investigation shows that In2O3@C has a higher specific surface area due to the protection of carbon layer,and In2O3@C contains more oxygen defects.DFT calculation results prove that the existence of oxygen vacancy is conducive to the conductivity of the catalyst,and the active site of nitrogen fixation reaction is In3-around the oxygen vacancy.(3)Preparation of In/In2O3@C composites by MIL-68-In precursor system and its photocatalytic nitrogen fixation performanceIn/In2O3@C composites with different content of indium were prepared by heat treatment of MIL68-In in a tubular furnace with smaller diameter,and the calcination temperature was controlled at the same time.In/In2O3@C-500℃ has the highest photocatalytic nitrogen fixing performance of(51.83μmol g-1 h-1.cat.).Its better photocatalytic nitrogen fixing activity is due to the improvement of light absorption of the catalyst by LSPR effect of In,the improvement of electrical conductivity of the catalyst and nitrogen adsorption by indium and oxygen vacancy.The active site of nitrogen fixation reaction is In3+around the oxygen vacancy,the electron flow is In→ In2O3,and the Speed step is the formation of*NNH.According to the characterization results,the phase transformation process of catalyst calcination was proposed:the precursor began to pyrolyze to form In2O3 at 400℃.At 500-600℃,MOFs decompose completely,metal ion In3+ forms In2O3 with O of the organic ligand,and some of the In2O3 is reduced to In by carbon in the precursor.At 700℃,more and more In2O3 is reduced to In by carbon.At the same time,the particles begin to aggregate,the degree of graphitization of carbon intensifies,and the morphology also shrinks.At 800-900℃,almost all carbon is reduced to In,the phenomenon of particle aggregation and thermal shrinkage becomes more obvious,and the degree of graphitization of carbon is higher.