Theoretical Study On Silver Single Atom Catalysts And Nanoparticle Catalysts For Ethylene Epoxidation Reaction

Ethylene epoxidation is an important chemical reaction for the industrial production of ethylene oxide.At present,the α-Al2O3 supported silver-based catalyst is the only catalyst used in the industrial application of ethylene epoxidation reaction,and has excellent catalytic activity.However,there are two key problems with the use of silver-based catalysts:firstly,although the selectivity of the catalyst has been greatly improved compared with that of pure silver catalysts by adding additives and other means,it still does not meet the higher requirements for selectivity in efficient production;secondly,the stability against aggregation of the catalyst particles is poor,and the catalyst is easy to sinter and become inactivate under reaction conditions,resulting in the shortening of its service life.These two problems restrict the efficient progress of catalytic reactions,resulting in an increase in the amount used of precious metal silver and a shortening of the production cycle,which increases the production cost.For industrial catalytic reactions,studying the mechanism and performance of catalytic reactions requires complex experiments with a large number of influencing factors,which is cumbersome and has a long research cycle.The theoretical calculation can easily simulate the reaction process at the catalytic active center under reaction conditions,and give the direction of performance tuning of catalysts,saving a lot of time and resources.Therefore,this dissertation aims at the two key problems in the industrial application of silver-based catalysts for ethylene epoxidation,based on density functional theory calculation,Wulff-Kaichew construction,microkinetic simulation,sintering kinetic simulation and other theoretical calculation methods,and put forward catalyst development and modification schemes from the perspectives of supported silver single atom catalysts and nanoparticle catalysts,respectively,so as to facilitate the experimental realization of the reduction application of precious metal silver.The main conclusions are as follows:1.A silver single atom catalyst(Ag1/α-Al2O3)supported by α-Al2O3 was screened as a potential stable and efficient catalyst for ethylene epoxidation.It has 100%selectivity for ethylene oxide and exhibits higher activity per unit mass of silver than oxygen reconstructed Ag(111)surface.Through microkinetic analysis,it was found that the reaction on Ag1/α-Al2O3 was more inclined to occur through the oxygen association-Eley-Rideal mechanism,which avoided the formation of oxametallacycle(OMC)intermediates,and is the source of the excellent catalytic performance of Ag1/α-Al2O3.Through the analysis of electronic structure,it is found that the unstable oxametallacycle intermediate on Ag1/α-Al2O3,the unequal charge distribution of oxygen and its spin polarization adsorption state are the reasons for the superiority of the oxygen association-Eley-Rideal mechanism.2.The ethylene epoxidation reaction through the Eley-Rideal mechanism on the surfaces of five silver single atom catalysts with supports with different reducibility,Ag1/CeO2(110),Ag1/CeO2(111),Ag1/MgO(100),Ag1/ZnO(100)and Ag1/TbO2(111)were studied,and the support tuning rules of supported silver single atom catalysts were given.The order of activity of the five silver single atom catalysts to catalyze the ethylene epoxidation reaction is as follows:Ag1/TbO2(111)>Ag1/CeO2(110)>Ag1/CeO2(111)>Ag1/ZnO(100)>Ag1/MgO(100),and the order of selectivity for the formation of ethylene oxide is as follows:Ag1/ZnO(100)>Ag1/CeO2(111)>Ag1/MgO(100)>Ag1/TbO2(111)>Ag1/CeO2(110).By analyzing the factors affecting the activity and selectivity,it was found that the oxygen vacancy formation energy of the support can be used as a descriptor of the catalyst activity,and the silver single atom catalyst with a support with lower oxygen vacancy formation energy has higher catalytic activity.The difference between the C-O bond lengths of the two transition states for ethylene oxide and acetaldehyde can be used as a descriptor of the selectivity of the reaction,and the larger the difference in the C-O bond length of the transition states,the higher the selectivity of the reaction to form ethylene oxide.3.A kind of method for simulating the real-time reaction kinetics of supported nanoparticle catalysts have been independently developed,and the accuracy of the developed simulation method is verified by experiments.The sintering behavior of the α-Al2O3(0001)-supported silver nanoparticles simulated by this method is in good agreement with the experimental results.Among them,with the extension of sintering time,the reaction rate of silver particles first decreases sharply and then slowly.By analyzing the changes in the reaction rate contribution of different active centers,the sintering induced inactivation of supported silver nanoparticle catalysts were attributed to the decrease in the number of active sites on the surface of oxygen-reconstructed Ag(111)surface and the decrease in the proportion of sites at the Ag-Al2O3 interface caused by particle migration and coalescence.4.The effects of different tuning factors on the stability and activity of the catalysts were analyzed through the developed method for simulating the real-time reaction kinetics of supported particle catalysts.The conclusions are as follows:1)The silver surface has little effect on the stability against aggregation and activity of the catalyst,and the support surface may have a great influence on the stability against aggregation and activity of the catalyst.The stability against aggregation and activity of α-Al2O3(0001),(1120)and(1102)surfaces-supported silver particles were similar to each other,while theα-Al2O3(1010)surface-supported silver particles have relatively higher stability against aggregation and catalytic activity.2)The addition of additives to the support surface will affect the stability against aggregation and real-time activity of the supported silver particle catalyst.After the addition of Cs,K,Na or Re additives to the α-Al2O3(0001),(1120)and(1102)surfaces,the stability against aggregation of the particles was enhanced and the catalytic activity was increased.The degree of influence of the four additives on the stability against aggregation and activity of silver particles is Re>Na>K>Cs.However,after the addition of the four additives to the α-Al2O3(1010)surface,the stability against aggregation of the supported silver particle catalysts decreased and the catalytic activity of the particles decreased,and the influence of the four additives on the stability against aggregation and activity of silver particles is Re>Na>Cs>K.3)The strain of the support will have an effect on the stability against aggregation and activity of the particles.On the α-Al2O3(0001),(1120),(1102)and(1010)surfaces,the stability against aggregation of the particles is enhanced with the increase of the stretch degree of the support.In terms of activity,the support strain leads to a decrease in the catalytic activity of the supported silver particles on the α-Al2O3(0001)surface;and on the α-Al2O3(1120)and α-Al2O3(1102)surfaces,the strain of the support will lead to the increase of the catalytic activity of the particles;while on the α-Al2O3(1010)surface,the stretching of the support is conducive to the increase of the catalytic activity of the particles,and the compression of the support will lead to the decrease of the catalytic activity of the particles.