Theoretical Study On Structural Control Of Au/TiO₂ Catalysts And Their Activity For Propene Epoxidation

As the basic organic chemical reagent,propene oxide is widely used in production and daily life.Traditionally,chlorohydrin and organic hydroperoxide methods have been used for the industrial production of PO;however,these processes are complex,expensive,environmentally unfriendly,and generate large amounts of by-products and waste materials.In recent decades,researchers have been looking for green,highly efficient catalysts for the preparation of propene oxide.Moreover,the molecular oxygen is environmentally friendly and has high atomic economy,so using O2 as the oxidant has become the most direct,effective and promising method.However,owing to the diverse reaction systems and complex reaction processes,the reaction mechanism of propene epoxidation is unclear.In recent years,with the development of theoretical calculation methods and computer technology,more and more researchers have turned from the original experimental researches to theoretical simulation,in order to get more micro interpretation and theoretical guidance for exploring the reaction mechanism.In this paper,we systematically studied the interrelationship between structural regulation of supported Au/TiO2 catalysts and the activity of propene epoxidation by density functional theory calculations,and provided a new strategy for the development of high-efficiency and green catalysts experimentally.For propene epoxidation,we first investigated the the effect of metal/support interface and reaction atmospheres on Au7 cluster supported on perfect and defective anatase TiO2(001)surfaces.Then,based on the experimentally controversial reaction mechanism,we calculated the propene epoxidation at different reaction sites(metal/carrier interface,oxide surface)on a series of Au3/TiO2 catalysts.And the effect of different oxygen species on epoxidation of propylene has also been explored.The main research content and conclusions are as followsIn the first chapter,the research background and significance of this dissertation are mainly discussed,including the industrial production methods and corresponding problems of propene oxide,the significance and research status of direct propene epoxidation with O2,and the research progress of supported heterogeneous catalyst surface/interface microstructure regulation and its application in propene epoxidation.At the end of this section,we pointed out the shortcomings of the current researches and briefly summarized the research content of this paper.In the second chapter,the density functional theory(DFT)and generalized gradient approximation(GGA)theory are described in detail.Then,we introduced the VASP calculation software package.In the third chapter,the effect of interface and reaction atmosphere on propene epoxidation on Au7/anatase TiO2-x(001)catalysts is studied using density functional theory calculations.The results indicate that propene epoxidation occurs on topmost Au atoms on perfect Au7/TiO2(001)catalyst.Propene epoxidation with O2 alone has higher barrier and reaction energy,while in the presence of H2,the hydrogenation of O2 to OOH is a feasible pathway for propene epoxidation from both kinetic and thermodynamic viewpoints.On defective Au7/TiO2-x(001)-Vo catalyst,oxygen vacancy regulates geometric/electronic structures of interfacial sites,and propene epoxidation instead occurs at the interface of Au7 and TiO2-x(001)-Vo.Under O2 atmosphere,O atom fills oxygen vacancy and significantly reduces the energy of entire catalytic system;however,this has little effect on kinetics of epoxidation.O2-H2 mixture results in the lowest barrier owing to activation of O-O bond and interfacial synergy.Our results suggest that moderate and rational regulation of oxygen vacancy on oxide surface can provide highly active sites and better interfacial synergy for propene epoxidation,and highlight the essential role of reaction atmosphere,which can be utilized to design high-efficiency heterogeneous catalysts.In the fourth chapter,we studied the propene epoxidation at different reaction sites of Au3 cluster supported on the TiO2(101),(110),and(201)surfaces.Meanwhile,we also explored the effect of various oxygen species that may exist during the experiment(such as OOH,02,O and OH)on the propene epoxidation.Our results showed that the adsorption states of the reactants are different on different catalysts,resulting in diverse reaction sites for propene epoxidation.On Au3/TiO2(101),propene epoxidation occurred only at the interface of Au3/TiO2(101),and OOH and O atom have higher epoxidation activity.On Au3/TiO2(110),it is possible that propene epoxidation takes place at both the interface and the(110)surface.However,propene epoxidation has better reactivity only when O2 is used as an oxidant at the interface.As for Au3/TiO2(201),same as Au3/TiO2(1 10),propene is also epoxidized at the interface and(201)surface.Moreover,the optimal reaction site is the(201)surface and OOH is the most suitable oxidant.This study displayed that the adsorption of reactants depended on the crystal plane effect of the oxide,leading to propene epoxidation occurring at different reaction sites on diverse catalysts.Importantly,propene epoxidation has a tendency to transfer from the interface to the oxide surface as the surface energy is increasing.In addition,the most suitable oxygen species of propene epoxidation are different under different conditions.Our work revealed that the reaction mechanism of propene epoxidation at different sites of supported catalysts and explained the effect of oxygen species,providing a detailed theoretical explanation and guidance for experimental phenomena.In the fifth chapter,we summarized the main conclusions and the innovation of our work,and proposed the future studies.