HREELS Study Of The Growth And Properties Of MnO_x On The Ni(111) Surface And Activation Of Small Molecules

The complexities of heterogeneous catalysts increase the difficulties of studying its structure-activity relationship and reaction mechanism.In the field of surface chemistry,researches on a single crystal surface with unique surface structure and active sites to simulate realistic catalysts can provide more reliable information to understand heterogeneous catalysis at the molecular and atomic level,which has a great significance for the design and development of catalysts.In this thesis,surface structures,surface-adsorbed species,adsorption sites,and interactions between surface oxygen species and CO(or H2)on Ni(111)and the prepared MnO_x/Ni(111)were studied in UHV(ultra-high vacuum)system.High surface sensitive HREELS,LEED and AES were used.The results are as follows:There are two kinds of oxygen species after the oxidation of Ni(111),in which the energy loss peak at 54 meV is ascribed to the surface chemisorbed oxygen species,and the peak at 69 meV to the surface nickel oxide.It was found that the surface chemisorbed oxygen species is less stable when annealing in vacuum or interacting with CO.As the amount of oxygen increases,the surface chemisorbed oxygen species at 54 meV shifts to 58 meV,and it can change to the surface nickel oxide after annealing in vacuum.The surface oxygen species prefers at the tri-hollow site resulting in more a-top adsorbed CO in the co-adsorption of CO and oxygen,and it can react with CO at room temperature.However,the surface nickel oxide is less active,and can be reduced only at a higher temperature and higher partial pressure of CO.The pre-adsorption of CO can suppress the adsorption and oxidation of the Ni(111)surface.It can be gradually removed and replaced by oxygen in the presence of O2.MnO_x/Ni(111)thin films were prepared by post-oxidation at oxygen partial pressures of 1×10-8 Torr and 5×10-8 Torr respectively,at low and high temperatures.High temperature is benefical to the formation of an ordered structure while it favors the investigation of more species of surface manganese oxide at low temperature.Among the energy loss peaks of the prepared MnO films,the loss peak at 47 meV for the MnO_x with a low oxidation state is the most unstable while the loss peak at 64 meV for the Ni-O-Mn structure is the most stable when annealing in vacuum.CO has a stronger ability to reduce MnO_x than H2.The loss peak of the outermost Mn-O-Mn at 75 meV can be converted by annealing and exposing to oxygen,indicating its reversible redox properties.