Surface Structures And Interface Properties Of Ceria Supported Zr,Co,Co-Au Bimetallic Model Catalysts

Ceria,as an active catalyst and support for catalytic metals or multicomponent alloys,finds applications with extraordinary performance in many important catalytic processes.These applications,to a large extent,benefit from the peculiar redox properties and oxygen storage capacity of ceria.However,in the real catalytic environment,pure CeO2 nanoparticles undergo seriously sintering at high temperature,which brings the loss of its essential oxygen storage capacity.The insertion of zirconium ions into ceria is found to be an effective approach to enhance the thermal stability and oxygen storage capacity of ceria.Ceria-supported Co,owing to its low price and easy preparation,is considered to be a promising alternative for noble metal catalyst for certain reactions,such as CO oxidation,steam reforming of ethanol(SRE)and water gas shift reaction(WGS).Ceria-supported bimetallic Co-Au system is known to exhibit superior catalytic activity compared to individual Co and Au counterparts due to the synergistic effects between the two metals as well as the interaction between the metal and oxide.Therefore,we present a systematic study on ceria-supported Zr,Co and Co-Au bimetallic model catalysts prepared under ultrahigh-vacuum condition using modern techniques of surface science in this thesis.The studies of the interfacial structures,thermal stability as well as their determing factors,the synergistic effects between the two metals and the interaction between the metal and oxide are of significance to explore the catalytic machnism of ceria-supported metal catalysts,improve the catalysts' efficiency and design new catalysts with high performances.The major research results of this thesis include:1.The interfacial interaction between Zr with CeO2(111)and CeOx(111)(1.5<x<2)thin film surfaces were investigated by STM,XPS and work function measurements.Ceria thin films in stoichiometric(CeO2)and under stoichiometric(CeOx)(1.5<x<2)form,which were about 3 nm thick,were epitaxially grown on a clean Ru(0001)substrate in ultrahigh vacuum,followed by the physical vapor deposition of Zr nanoparticles onto the ceria thin film surfaces.The deposition of Zr on the CeO2(111)surface leads to charge transfer from Zr to ceria,accompanied by partial reduction of Ce from Ce4+ to Ce3+ states and oxidation of metallic Zr to Zr4+.A mixed Zr-Ce-O oxide is formed with Ce3+ and Zr4+ chemical states rather than a mixture of single non-interacting zirconium and cerium oxides.Subsequent deposition of Zr causes the formation of Zr suboxides with consecutive lower chemical states—Zr?+ and metallic Zr0.STM images show that Zr grows two-dimens ionally(2D)on the CeO2(111)thin film due to the strong interaction of Zr with ceria.At room temperature,the deposition of Zr on the CeOx(111)thin film surfaces results in the extensive reduction of the partially reduced ceria surface.However,the interaction of Zr with ceria surfaces are highly determined by the nature of ceria supports.2.The interaction of Co with CeO2(111)and CeOx(111)(1.5<x<2)thin films were investigated in the the temperature range between 300 and 800 K by STM,SRPES and XPS.The interaction of Co with the CeO2(111)surface is found to be strong even at room temperature,leading to oxidation of Co to Co2+ due to electron transfer to the CeO2 substrate and partial formation of Ce3+ at low Co coverages.As the Co coverage increases,both the concentrations of Co2+ and the metallic Co increase,but the latter becomes dominant at higher Co coverages.Upon annealing to higher temperatrues,Co2+ ions diffuse into the CeO2 film,while the small metallic Co islands agglomerating into larger one occurs.Compared to the fully oxidized ceria substrate surface,the interaction between cobalt and the CeO1.82 surface is weaker,and metallic Co is the dominant species on the reduced CeOx(111)(1.5<x<2)thin films.Upon annealing the Co/Ce01.82(111)system,the inward diffusion of Co into the cerium oxide film is more hindered.3.The growth and sintering behavior of pure Au clusters and bimetallic Co-Au on CeO2(111)and CeOx(111)(1.5<x<2)thin films were studied by SRPES,XPS and STM.The deposition of Au on the CeO2(111)surface causes electron transfer from Au to ceria,accompanied by partial reduction of Ce from Ce4+ to Ce3+ states and oxidation of metallic Au to Au?+.While,upon the deposition of Au on the CeO1.82(111)thin film surface,Au?-is formed by electron transfer from Ce3+ ions to Au clusters.Annealing of the Au/CeO2(111)system in UHV leads to significant aggregation of Au small clusters into large particles.Au particles are more thermally stable on the CeO1.82(111)surface than on the CeO2(111)surface due to the stronger bonding of Au atoms on the surface defect sites.The Co-Au bimetallic clusters are grown on CeO2(111)and CeO1.82(111)by sequential deposition of Co,followed by Au.The surface of the bimetallic nanoparticles is Au-rich due to the lower surface energy for Au compared to Co.Our XPS results show that Co-Au bimetallic surfaces exhibit a different sintering behavior on the CeO2 thin film from that on the CeO1.82 thin film.The addition of Co to the Au/CeO2 surface inhibits Au clusters sintering at high temperature in comparison with that of pure Au particles.However,the presence of Au does not prevent Co from diffusing into the substrate.Au particles are less stable on the Co/CeO1.82 layer than on CeO1.82 surface.A possible explanation is that Au is more easily encapsulated in the presence of surface Co or Au is diffusing into the bulk of the clusters upon annealing.