Growth,Structure And Reaction Properties Of Cu And Cu₂O Thin Films On Single Crystal Metal Surface

Cu-based catalysts have been subjected to numerous investigations due to their applications for many reactions,such as CO oxidation,methanol-synthesis,and water-gas-shift reactions.However,there is a severe lack of experimental characterizations of the nature of the active sites and the reaction mechanisms,which hence prohibits the revealing of the structure-property relationships in these systemes.It is known that the atomic structure and the related electronic property of a catalyst can dramatically affect both its activity and selectivity in reactions.While in a real catalytic reaction,the active metals can usually form surface alloy or thin oxide films,depending on the composition and the applied reaction condition.Therefore,it is important to construct model systems with clear surface structures and look into their physicochemical properties directly at atomic scale.The achieved knowledge from these studies would in turn help to uncover the catalytic mechanism and reveal the structure-property relationship of the catalyst surface.In this thesis,we applied mainly low-temperature scanning tunneling microscopy technique?LT-STM?,but also combined with other surface characterization techniques and density functional theory calculation?DFT?,to investigate the surface structures and chemical properties of a few Cu-based bimetallic systems including Au/Cu?111?and Cu/Au?111?thin films,Au-Cu monatomic surface alloys,and Cu/Pt?111?thin films,and also ultrathin oxide film such as Cu₂O/Pt?111?.The main results are summarized in the following:1.We investigated the atomic and electronic structures of both the Au/Cu?111?and Cu/Au?111?thin films with LT-STM and synchrotron radiation photoelectron spectroscopy?SRPES?.Atomic-resolution STM images clearly revealed that the Au films evaporated on Cu?111?at room temperature grow with compressed lattices,which gradually evolve and restore to that of the bulk gold until the fourth layer.Both STM and SRPES evidenced that there are considerable Cu atoms incorporated into each layer of the Au films,whose concentrations decrease stepwise along with the film thickening.As a reversed system,the growth of Cu films on Au?111?starts with agglomerating at the subsurface and adopting a?1×1?lattice within submonolayer coverage.The lattice quickly shrinks to that of bulk Cu?111?from the second layer,yet the electronic property is restored slowly until the third layer.In each Cu film,there were also intermixed Au atoms coming from the substrate,and their concentrations also decrease along with the film thickening.We also compared the CO adsorption on both the Au/Cu?111?and Cu/Au?111?submonolayer films considering their similar compositions at the surface but different substrates.We found that on these films CO adsorption was significantly increased than bare Au?111?but still weaker than the Cu?111?surface.Moreover,the adsorbed CO molecules were apparently connected to the incorporated Cu atoms in the surface layers,yet the enhanced CO bindings were closely related to the electronic properties of the films.2.Based on the results of bimetallic films,we successfully prepared single-atom alloys of Au/Cu?111?and Cu/Au?111?by lowering down the coverage and annealing to high temperature,and performed LT-STM experiments to investigate the atomic structures as well as the local electronic properties.Atomically resolved STM images directly recognize the randomly distributed Au atoms in the Cu?111?surface.Scanning tunneling spectroscopy?STS?measurements reveal that in both surface and subsurface locations the Au atoms display an overall similar electronic structure compared to the Cu?111?substrate,except at-0.5 V where an enhanced surface state were observed.Adsorption experiments show that the presence of Au atoms reduces the binding strength of CO.Correspondingly,for the singly dispersed Cu atoms on Au?111?surface,the adsorption probability of CO is also found significantly weakened compared to Cu?111?surface.No CO adsorption can be observed even at 77 K,possibly due to the influence of Au?111?substrate.3.We have further applied STM and X-ray photoelectron spectroscopy?XPS?to study the growth of Cu film on Pt?111?substrate and investigate the activation and reaction of O₂ over it.High-resolution STM images revealed that at room temperature the submonolayer Cu film grows with exactly the same lattice constant as the Pt?111?surface.Adsorption experiments show that CO preferentially adsorbs on an atop site of Pt?111?substrate while O₂ adsorbs preferentially on the Cu/Pt?111?film.Moreover,O₂ is found to dissociate readily even at 77 K and form?2×2?-O adsorption structure by occupying the fcc hollow sites of the Cu island.Further exposure to O₂ at room temperature leads to the formation of Cu-0 clusters incorporated in the Cu islands.Therefore,co-exposure to CO and O₂ leads to separated adsorption of both molecules on distinct surface regions which then react at elevated temperatures.4.By oxidizing the Cu/Pt?111?film at elevate temperature under O₂ exposure we prepared a highly ordered copper???oxide film with Cu₂O?111?-like honeycomb structure.STM experiments clearly evidenced that the Cu₂O/Pt?111?film is highly sensitive to CO interaction even at a low temperature of?200 K.High resolution STM images,in combination with XPS and temperature-programmed desorption spectroscopy?TDS?measurements demonstrated that CO₂ is released during the reaction and the Cu₂O film is reduced into a Cu/Pt?111?adlayer covered by chemisorbed oxygen.Density functional theory?DFT?calculations and CO exposing experiments revealed that CO interacts weakly with the stoichiometric Cu₂O film.Therefore,the reaction is proposed to proceed via CO diffusing from the Pt?111?surface to the periphery of the Cu₂O islands and reacting with the low-coordinated oxygen ions.Thus,the Cu₂O/Pt system presents a potential bifunctional catalyst wherein the Pt surface activates CO while the Cu₂O film supplies oxygen.