In-situ Studies On The Structures And Thermal Stabilities Of Cu-based Model Catalysts By Surface Science Techniques Including Synchrotron Radiation Spectroscopies

In the past decades,traditional oxide-supported Cu based catalyst have received considerable attention due to their unique catalytic properties and have been applied in many important catalytic reactions,such as water gas shift reaction(WGSR),CO oxidation and N2O decomposition.However,due to the complicated surface composition and their internal interactions of the traditional catalysts are limited by the in-situ characterization techniques,the interfacial structures of Cu/oxide,the interactions between Cu and oxide support,and the synergistic effects as well as their determining factors of Cu/oxide catalysts are largely unknown.Inverse strategy is a novel method which has been widely used recent years.This approach is to employ inverted systems in which a material that normally serves as an oxide support is deposited onto the metal single crystal surface as an active material.The inverted model catalysts provides an important way to investigate mechanisms for interfacial reactions.It not only decrease the complexity of the surface,but also emphasize the control of oxide/metal interfacial sites.In order to gain a fundamental understanding of Cu/oxide interfacial properties at the molecular level,we employed model catalyst systems consisting of CuxO/Cu(100),FeO/Cu(100),Cu/CeO1.75(111)to investigate the interactions between Cu and different well-ordered oxide by using synchrotron radiation photoemission spectroscopy(SRPES)together with X-ray photoemission spectroscopy(XPS),low energy electron diffraction(LEED)and scanning tunneling microscopy(STM).Steam reforming of methanol to produce hydrogen is an important way to produce clean energy,which has great significance for sustainable development.In methanol steam reforming reaction,copper based catalysts show high selectivity and activity.But the actual catalytic process of methanol steam reforming is pretty complex.The reaction intermediates is seriously affected by the catalysts surface structure.As one of the most important intermediates,the adsorption structure of formic acid on copper surface and reaction process are still unclear.Therefore,we prepared several Cu(100)surfaces with different structures,and further explained the influence of surface structures on catalytic reactions by using SRPES,infrared reflection-absorption spectroscopy(RAIRS),temperature programmed desorption(TPD)and other techniques.The main achievements in this thesis are summarized as follows:1.Formic acid adsorption and decomposition routes on CuxO/Cu(100)surface with different structures.Several Cu surfaces with different structures were prepared in ultra-high vacuum environment,namely clean Cu(100)surface,Cu(100)-(2(?)×(?))R45°-O surface and Cu(100)-c(2×2)-O surface.The effects of oxygen atoms on the adsorption and decomposition of formic acid were analyzed in detail.Our results show that HCOO-vertically adsorb on Cu(100)surface with a disordered structure,while inclined adsorb on Cu(100)-c(2×2)-O surface with an ordered structure.The Cu(100)-(2(?)×(?))R45°-O surface is inert for formic acid dissociate adsorption.The local c(2×2)-O region not only dramatically increased the adsorption density of HCOO-on the surface,but also changed the decomposition route of HCOO-.On clean Cu(100)surface,formic acid decomposes into CO2 and H2 with the increase of surface temperature.On the Cu(100)-c(2×2)-O surface,proton H which are generated by C-H bond fracture of formic acid further reacts with surface oxygen atoms to generate H2O.In addition,compared with clean Cu(100)surface,more CO2 was produced on Cu(100)-c(2×2)-O surface,and the desorption temperature of CO2 was lower,which indicate that Cu(100)-c(2×2)-O structure promoted the adsorption and decomposition of formic acid on the surface at 300 K,and also indicate that the structure of oxygen atoms on Cu(100)surface played a crucial role in the decomposition process of formate.This work is of great significance for us to further understand the methanol steam reforming reaction and water gas shift reaction.2.FeOx growth on Cu(100)surface and oxygen activity at the interface.The growth and interfacial interaction of FeO on Cu(100)surface were investigated by SRPES,LEED and STM.Firstly,FeOx thin films were prepared on Cu(100)substrate.It was found that FeOx exist in the form of FeO on the surface when the coverage of FeOx is lower than 3 ML,and exist in the form of Fe3O4 on the surface when the coverage of FeOx is higher than 3 ML.The submonolayer FeO forms an orthogonal stripes structure with an orientation of(111)on the Cu(100)surface,in which the width of a single stripe is about 5 nm.Fe3O4 shows a hexagonal island structure.The addition of submonolayer FeO increases the oxygen activity at the FeO/CuxO interface and promotes the structural transition from Cu(100)-(2(?)×(?))R45°-O to Cu(100)-c(2×2)-O at the interface sites during annealing.This work clearly describes the oxygen activity at the FeO/CuxO interface,which will provide theoretical support for the design of more new Cu-based and Fe-based catalysts.3.Electronic structure and interfacial properties of Cu/CeO2-x model catalyst.Well-ordered CeO1.75(111)thin film with thickness of about 2 nm was prepared on clean Cu(111)surface,and then Cu was vapor deposited onto the CeO1.75(111)surface gradually.It was found that Cu particles grew two-dimensional on the CeO1.75(111)surface and showed different charge transfer directions at different adsorption sites.When the Cu coverage is low(<0.12 ML),the Cu nanoparticles prefer to adsorb at the step edge and terrace stoichiometric sites of CeO1.75 thin film.The electron is transferred from Cu to CeO1.75 thin film as well as Cu is oxidized to Cu+.The concentration of Ce3+ increases gradually in this step.When Cu coverage is high(>0.12 ML),Cu gradually adsorbs on the oxygen vacancy site of the CeO1.75 film.At this step,the electron is transferred from CeO1.75 film to Cu,and the concentration of Ce3+gradually decreases and finally becomes constant.This indicates that the oxygen vacancy on the surface of CeO 1.75 has an important effect on the electronic structure of Cu.Compared with CeO2 film,there are more defects on the surface of CeO1.75 film,and these defects provide more nucleation sites for Cu.Therefore,the mean particle size of Cu on the CeO1.75 surface is significantly smaller than that on the CeO2 surface.During annealing,the Cu nanoparticles supported by the CeO1.75 surface undergo sintering with Ostwald ripening process,which means large Cu nanoparticles grow at the expense of smaller Cu particles.In addition,C u shows obvious sintering at the defect site when annealed to 700 K,indicating that the defects on the surface of CeO1.75 are of great significance for regulating the thermal stability and particle size of Cu nanoparticles.