Studies Of CO/NO Adsorption And Reaction On The Surfaces Of CeO₂ Supported Pd Model Catalysts

Ceria-supported Pd catalysts are widely used in many catalytic reactions such as automotive exhaust emission-control reactions,where ceria-supported Pd Single-Atom-Site catalysts especially show excellent activity in the reactions.However,the nature of active sites,the structure-property relationships as well as reaction mechanisms in these reactions are still inconclusive.The use of surface science techniques combined with ultra-high vacuum in situ sample preparation technology provides an effective way for fundamental understanding of the structure-property relationships and reaction mechanism of metal/oxide catalysts at the atomic-molecular level.To thoroughly understand these key scientific issues,in this dissertation,we adopted a“model catalyst”approach by preparing a series of CeO₂-supported Pd model catalysts,including Pd nanoparticles,Pd O nanoparticles and single-atom Pd catalysts on well-defined CeO₂（111）thin films.The surface morphologies,electronic structures and thermal stabilities of these catalysts were systematically studied by synchrotron radiation photoemission spectroscopy（SRPES）,scanning tunneling microscopy（STM）and infrared reflection absorption spectroscopy（IRAS）.In addition,the adsorption,reaction and desorption of CO,NO and CO+NO on these catalysts were comprehensively studied by SRPES,IRAS and temperature-programmed desorption（TPD）.Through these studies,the fundamental understanding of the structures,adsorption/reaction properties of Pd/CeO₂ catalysts were achieved at the atomic-molecular level,which is of great importance to develop new catalysts with higher activity and better stability.The specific contents are summarized as follows:First,several CeO₂-supported Pd model catalysts were prepared on the surface of CeO₂（111）thin films by molecular beam epitaxy technique in ultra-high vacuum（UHV）system,including Pd nanoparticles with different coverages supported on CeO₂（111）（denoted as Pd NPs/CeO₂（111））,Pd O nanoparticles supported on CeO₂（111）（denoted as Pd O NPs/CeO₂（111））,and single-atom dispersed Pd-CeO₂ nanoparticles supported on CeO₂（111）（denoted as Pd₁-CeO₂ NPs/CeO₂（111））model catalysts.The growth model of Pd on CeO₂（111）,surface morphologies,electronic structures and the thermal-stability of these model catalysts were systematically studied by a series of surface analysis techniques such as SRPES,STM and IRAS.The results indicate that the Pd NPs demonstrate a three-dimensional island growth model on the CeO₂（111）surface and sintering as the temperature increases.Pd O NPs are unstable on the CeO₂（111）surface,and undergo partial reduction as the temperature rises.It is found that the Pd₁-CeO₂NPs/CeO₂（111）surface shows excellent thermal-stability.Second,the adsorption of CO and NO at low temperature（120 K）and their reaction and desorption behaviors on the surfaces of these model catalysts were comprehensively studied by SRPES,IRAS and TPD.The results show that the amount of adsorbed CO increases with Pd loading.While the low-temperature transformation/decomposition of CO mainly occurs at the interfaces of Pd-CeO₂.Pd²⁺is identified as the active site of the low-temperature CO decomposition.Regarding NO adsorption on the surfaces of these model catalysts,N₂O was identified as the primary product at the low-temperature（120K）,with the NO dimer serving as the intermediate in the generation of N₂O.The addition of single atom Pd in CeO₂ NPs promotes the low-temperature conversion of N₂O to N₂.The adsorbed NO on Pd NPs partially dissociates into N and O atoms above 400 K.The atomic N recombine into N₂ and desorb from the surface at higher temperature.Third,the CO+NO reactions on the Pd NPs/CeO₂（111）and Pd₁-CeO₂ NPs/CeO₂（111）model catalysts at ambient temperature（300 K）are explored by SRPES,TPD and IRAS.The results reveal that at room temperature,CO and NO predominantly molecularly adsorbed on the surface of Pd NPs/CeO₂（111）.Upon annealing the surface,both CO and NO dissociation ocuur,resulting in the formation of CO₂,N₂ and N₂O.However,on the surface of Pd₁-CeO₂ NPs/CeO₂（111）,only the production of CO₂ and N₂are observed.Furthermore,CO₃^2-and N₂O₂^2-were identified as intermediates in the generation of CO₂and N₂.These findings serve as direct experimental evidence supporting the reaction mechanism proposed in prior density functional theory（DFT）calculations.