Synthesis And Properties Of Nanocatalysts For Electrocatalytic Olefin Epoxidation

Epoxide is an important multifunctional intermediate and value-added product in the industry.The emerging electrocatalytic epoxidation is one of the important research directions of electrocatalytic synthesis,which uses water as the oxygen source to epoxidize olefins at a specific potential,and at the same time,produces green hydrogen in high purity at the cathode,which has the advantages of safety,high added value,and controllable process.Aiming at the problems of low activity,low selectivity,and the unclear mechanism of current electrocatalytic olefin epoxidation catalysts,the capacity and structure-activity relationship of electrocatalytic epoxidation of cyclooctene by mono-/bi-metallic compound were systematically explored.A flexible and simple potential control activation method was adopted to improve the Faraday efficiency of the catalysts,which provided a feasible way and a scientific basis for the development of high-performance and low-cost electrocatalysts for olefin epoxidation.The research results obtained in this paper are divided into the following three aspects:（1）A series of transition metal oxides（Fe₂O₃,Fe₃O₄,Co O,Co₃O₄,Ni O,Mn₃O₄）were synthesized,the electrocatalytic potential range was determined by cyclic voltammetry scanning test（CV）,and the epoxidation performance of the catalyst was evaluated at the constant potentials.The comprehensive results show that Fe₂O₃ and Mn₃O₄ have the best electrocatalytic oxidation performance.The highest Faradic efficiency（F.E.）of Fe₂O₃ is 26%at 1.9 V with an epoxidation selectivity of 63%;The F.E.of Mn₃O₄ at 1.9 V and 2.5 V is 22%,and the epoxidation selectivity is 65%and 62%,respectively.According to the adsorption evolution mechanism of oxygen evolution reaction and the metal center valence-changing oxygen transfer mechanism of epoxidation,the demand for adsorption energy of oxygen-containing species in electrocatalytic epoxidation is just the opposite of that in oxygen evolution reaction,that is,metal compounds with poor oxygen evolution reaction such as Mn and Fe often have high epoxidation activity,which is also in line with the activity evaluation results.In this chapter,a monometallic oxide with excellent electrocatalytic epoxidation of cyclooctene was obtained.The evaluation method and optimization appraisal conditions of catalytic performance were determined,which clarified the main body and basic method of the material for the subsequent optimization work.（2）Based on Fe₂O₃ and Mn₃O₄ electrocatalysts obtained by performance screening,bimetallic oxides such as Mn Co₂O₄,Ni Co₂O₄,Ni Fe₂O_4,and Co Fe₂O₄were synthesized to further improve their properties.The series of catalysts were evaluated by the same method:Mn Co₂O₄ has the highest Faraday efficiency of epoxidation at 1.5 V,which is 29%,and better than the highest value of monometallic oxide（F.E.is 26%at 1.9 V for Fe₂O₃）,and the required potential is lower.Except for the poor performance of Ni Co₂O₄,the selectivity of bimetallic oxide at a high potential of 2.5 V is better than that at a low potential,with the F.E.of 20～26%and the selectivity of about 70%.The comparative experiments show that the existence of two kinds of valence-changing metals may widen the potential range of the valence-changing oxygen transfer process in the metal center of the catalyst so that the catalyst can still have a highly selective epoxidation electrocatalytic process at a higher overpotential.The bimetallic oxide catalysts can maintain high selectivity and F.E.in addition to high oxidation current at high potential,which provides a feasible strategy for improving the performance of epoxidation electrocatalysts.（3）A cyclic voltammetry scanning activation pretreatment method based on different electrochemical intervals was adopted.The evaluation results at 1.9V after electrochemical activation show that the F.E.and current of epoxidation of single and bimetallic oxide catalysts are improved to varying degrees after activation.The F.E.of Fe₂O₃ with high initial performance is increased by one time compared with that of the unactivated product,reaching 41%,and the epoxidation selectivity is 75%.The performance of activated Mn₃O₄ is also comparable to that of activated Fe₂O₃.The structural characterization of the activated catalyst revealed that the pretreatment method improved the catalytic efficiency by increasing the number of active intermediates needed for the reaction.