First-principle Study On Thepromotion Of Silver Catalyst And Theprocess Of Ethylene Epoxidation

Silver is currently the only efficient catalyst to produce ethylene oxide from ethylene.Improving the performance of silver catalyst,especially its selectivity,can save the costs of ethylene a lot,whieh is of great economic significance.Such purpose can be achieved by adding promoters,among which Cs,Re.and C1 are the most widely used and have the best performace.Adjusting the types and proportions of the promoters can achieve highly selectivity of silver catalysts,but such regulation is often empirical and hard to reach a consensus on the specific modification mechanism of the promoters.These defects can be offset by quantum chemical calculation,which provides a theoretical guidance for catalyst design and performance regulation at molecular level.In this thesis,Quantum-Espresso based on density functional theory was employed to study the oxidation process of ethylene on the promoted Ag(111)surfaces and the(001)surfaces of three transition metal oxides.The optimal adsorption sites of O,Cs,Re and Cl atoms on the Ag(111)surface were determined firstly which are all fce sites.By calculating the adsorption energies of all key species on the Ag(111)surface,it was found that C2H4,EO and AA were all physically adsorbed,while the OMC intermediate was in a chemisorbed state.The adsorption strength of the OMC intermediate followed the order:Cs,Cl-Ag(111)<Cs-Ag(111)<Re,Cl-Ag(111)<Re-Ag(111)<Ag(111),which indicated that the addition of Re or Cs weakened the adsorption of the OMC intermediate,which could be further weakened as C1 was added.The selectivity on Ag(111)surfaces modified by different promoters followed:Ag(111)<Re-Ag(111)<Re,C1-Ag(111)<Cs-Ag(111)<Cs,C1-Ag(111).It can be found that the additon of the promoters improved the selectivity of the Ag(111)surface,while the ?Ea value on the Ag(111)surfaces promoted by Cs-C1 and Re-C1 increased by 0.12eV and 0.04 eV respectively compared to the Ag(111)surfaces promoted by Cs and Re,indicating that the selectivity of the Ag(111)surfaces was further improved as C1 was added,and there is a synergistic effect between C1 and Cs or Re;In addition,the promotion effect of Cs was better than Re.There was a good correlation between the adsorption strength of the OMC intermediate and the selectivity of the silver surface.The adsorption properties of each key specie on the An2O(001),Ag2O(001)and Cu2O(001)surfaces during reaction were investigated,as well as the effect of an oxygen vacancy.It was found that C2H4 adsorption was physically exothermic on each(001)surface while the adsorption of EO,AA and OMC intermediate were all chemically endothermic.On the metal-oxide surfaces containing oxygen vacancies,the adsorption energy of each key species was larger than that in the absence of oxygen vacancies,indicating that the presence of oxygen vacancies enhanced the adsorption strength of those species.