In Situ Study In Surface And Interface Chemistry Of Two Dimensional Nanostructures On Metal Surfaces

As an ideal model system in surface science studies,two-dimensional(2D)nanostructures grown on metal surfaces have attracted much attention in the fields of surface chemistry and catalysis.However,there is still a lack of in-depth understanding of the key scientific issues such as the dynamic evolution of the model catalyst structure and the mechanisms of the catalytic reactions under near ambient pressure conditions.In this thesis,we focused on the studies of 2D graphene and 2D manganese oxide(MnOx)nanonstructures grown on metal surfaces by surface science characterizations including near ambient pressure-photoemission electron microscopy(NAP-PEEM),X-ray photoelectron spectroscopy(XPS)and scanning tunneling microscopy(STM).Our research topics include the oscillation of CO oxidation reaction in the confined space,the activation of CO at the oxide/metal interface and the depth distribution of oxygen vacancies in the metal oxide.The main results are as follows.(1)I participated in the whole process of building and testing of the first NAP-PEEM system,and combined it with a tunable deep ultraviolet laser to build the DUV-NAP-PEEM system.We observed CO and O2 intercalation process at the interface of graphene/Pt(110)by NAP-PEEM under the near ambient pressure condition.Compared with CO.the intercalation of O2was found to be easier.It was found that the CO oxidation reaction can take place in a 2D nano-confined space between a single layer of graphene and the Pt(110)surface.The chemical oscillation behavior under graphene overlayer was observed for the first time(2)2D MnOx ultra-thin nanostructures with different oxidation states on Au(111)surface were grown on Au(111).Using STM and XPS characterizations it was found that the transformation between MnO/Au(111)and Mn3O4/Au(111)can be achieved by annealing in O2 atmosphere and ultra-high vacuum.The activation of CO at MnOx/Au(111)surface interface was studied by quasi in situ XPS combined with treatment in high pressure cell(HPC).MnOx/Au(111)can activate and dissociate CO to produce deposited carbon at near ambient pressure CO above 473 K.The effect of MnOx coverage on CO activation was investigated.A volcano curve of carbon deposit amount as a function of MnO coverage was clearly presented,indicating that MnO island edges are the active centers.The active sites and the related reaction mechanism of CO activation were studied by density functional theory(DFT)calculation.(3)Mn3O4 thin film was grown on Au(111)surface.The depth distribution of oxygen vacancy(Vo)in CO-reduced Mn3O4 films was investigated by a combination of ion sputtering treatment and variable-angle XPS measurement.It was affirmed that O atom was favorable to locate at the surface sites and thus more Vos would exist in the deep region.XPS and XRD(X-ray diffraction)were used to study the reduced Mn3O4 powder under the same treatment conditions and the bulk phase in deep region was found to be reduced preferentially.Meanwhile,DFT calculation showed that the energy of oxygen filling at Vo(Eof)was more favorable on the surface than that in the bulk,which was in accordance with the experimental results.