Surface Science Study Of Ceria-Supported Metal Model Catalysts

Ceria can be served as a catalyst or a support for catalytically active metals or alloys in many catalytic reactions owing to its special redox properties and oxygen storage capacity.Ceria-supported Ag-Ni bimetallic system is of particular interest in important catalytic processes due to the low cost and easy preparation,such as the reduction of organic pollutants.Its unusual performance benefits from the ligand and ensemble effects between Ag and Ni as well as the interactions between Ag,Ni and the ceria.It has been well-demonstrated that insertion of samarium or calcium ions into ceria can enhance the thermal stability,catalytic activity and electronic performance of ceria.Compared with Ni/ceria catalysts,Sm,Ca-doped ceria-supported Ni catalysts are observed to have better catalytic performance.In order to design new catalysts with higher performance and improve the ceria-based catalysts' efficiency,we employed model catalysts prepared in ultrahigh vacuum(UHV)to have fundamental-level investigations of the ceria-supported Ag-Ni and Sm,Ca-doped ceria-supported Ni systems by using modern surface science microscopic and spectroscopic techniques.We mainly focus on the morphologies,the interactions between metal and the oxide or two metals,the influence of Sm(Ca)on the catalytic performance of ceria,the thermal stabilities and their determing effects of the above-mentioned catalysts.The major results of this thesis contain:1.The growth and annealing behavior of Ag,Ni,and Ag-Ni bimetallic clusters on oxidized CeO2(111)and reduced CeOi.95(111)were systematically investigated by X-ray photoelectron spectroscopy(XPS),scanning tunneling microscopy(STM)and low energy electron diffraction(LEED).In particular,the effect of Ni on the growth and thermal stability of Ag clusters on CeO2(111)and CeO1.95(111)was examined.The oxidized CeO2(111)films were epitaxially grown on a clean Cu(111)surface in UHV.The reduced CeO1.95(111)was obtained by vacuum annealing of CeO2(111).At 0.05 ML,Ag clusters exhibit three-dimensional growth on CeO2(111)and mainly nucleate at step edges.In contrast,Ni clusters randomly disperse on the ceria surface and prefer a two-dimensional growth at low coverages.The different growth behaviors between Ag and Ni clusters on CeO2 are attributed to the fact that CeO2 interacts stronger with Ni than Ag.Moreover,upon the deposition of Ni on CeO2,part of metallic Ni is oxidized to Ni2+.On the CeO1.95 surface,an increase ratio of metallic Ni to Ni2+ is observed.The Ag-Ni clusters were obtained by deposition of Ag on the Ni pre-deposited ceria surface.Compared with Ag clusters on CeO2,the presence of Ni enables the incoming Ag clusters to exhibit smaller sizes with higher density at room temperature,and moreover,it hinders the sintering of Ag clusters on CeO2.On the CeO1.95 surface,the amount of Ag-Ni clusters with smaller sizes increases and the thermal stability of Ag clusters is significantly enhanced.2.The electronic structure,growth and thermal stability of Sm on CeO2(111)/Cu(111)were investigated by means of XPS,LEED and STM,and the Sm-doped ceria films were prepared.Metallic samarium was deposited on the CeO2(111)surface by physical vapor deposition in UHV at room temperature.Upon the deposition of Sm on CeO2,metallic Sm is oxidized to Sm3+,accompanied by the reduction of Ce4+ to Ce3+.With increasing the Sm coverage,the concentration of Ce3+ increases monotonically.After depositing 6.00 ML of Sm,only Ce3+ is observed within the detection depth of XPS.At room temperature,Sm clusters randomly nucleate on CeO2 and exhibit two-dimensional growth at low coverages.Annealing to higher temperatures leads to the agglomeration of Sm clusters,diffusion of Sm into the CeO2 film and reverse oxygen spillover from CeO2 to the Sm surface.The reduced Sm-doped CeOx was obtained by heating 0.1 ML Sm/CeO2 to 800 K in UHV.The oxidized Sm-doped CeO2 was acquired by exposure the reduced Sm-doped CeOx to oxygen at 600 K.Upon deposition of Ni on the Sm-doped ceria films,it is found that the existing Sm clusters on ceria act as the nucleation centers for part of Ni atoms growth.In addition,the results show that the oxidation of Ni is only related to the reduction degree of ceria,suggesting that the interaction of Ni with ceria is stronger than that with samaria.Upon heating,small Ni clusters aggregate into large ones,while Ni2+ ions diffuse into the subsurface of Sm-doped ceria.3.The interaction between Ca and the CeO2(111)/Cu(111)films was studied by XPS and STM upon the deposition of metallic Ca on CeO2.The results indicate that Ca would react with CeO2,leading to the oxidation of Ca to Ca2+ and partial reduction of CeO2.A mixed Ca-O-Ce oxide layer is formed upon the deposition of low coverages of Ca on CeO2 at room temperature.Heating 1.2 ML Ca/CeO2 to different temperatures results in the formation of Ca-doped CeO2 films with various structures,surface components and oxidation states.Large CaO nanofilms are observed on the CeO2 surface upon annealing 1.2 ML Ca/CeO2 to 600 K,which are higher than 1.0 nm.Further annealing to 800 K,more Ca2+ ions migrate into the CeO2 films and the mixed CaO-CeO2 oxide is formed.Upon CO2 adsorption on CeO2,CeO1.87 and Ca-doped CeO2 surfaces at 180 K,carboxylates and carbonates are formed on ceria(CeO2 and CeO1.87)and Ca-doped CeO2 films,respectively,which are identified by synchrotron radiation photoemission spectroscopy(SRPES)and XPS.The presence of Ca on the ceria surface is observed to be beneficial for the CO2 adsorption.When Ni is evaporated on the Ca pre-covered CeO1.87 surface,part of incoming Ni atoms anchor on the Ca islands,and metallic Ni is the only species.Moreover,introducing Ca into Ni/CeO1.87 system can enhance the thermal stability of CeO1.87-supported Ni clusters.