Synthesis,Surface-Interface Oxygen Activation And CO (Preferential) Oxidation Of Cerium-Based Catalysts

Since the 19th century,scientists have made significant progress in the science of catalysis through the development and production of catalysts,the selection and regulation of the catalytic reaction process,and the investigation and understanding of the catalytic mechanisms.Many crucial domains in the development of modern industrial society have been facilitated by catalysis science,such as energy production and utilization,environmental protection,etc.It can be said that the advancements in catalysis science have promoted modern industrial growth and scientific development to a great extent.Therefore,catalysis science has always been a popular area of research in academia and industry.The surface adsorption theory proposed by Langmuir and the active center theory proposed by Taylor provided the basis for developing modern catalysis theory.Based on this theory,the catalysts’surface properties significantly impact the catalytic process.Cerium dioxide（CeO₂）material has unique intrinsic surface properties;it can rapidly generate oxygen vacancies because its oxygen vacancy formation energy on the surface is low.Similarly,the Ce element has stable valence（III and IV）,redox characteristics,and high oxygen mobility.However,this catalyst material has low catalytic activity at low temperatures and has a narrow complete conversion window.The effective reduction of the reaction barrier at low temperature is the key to improving the reaction efficiency,which also leads to the exploration of the low temperature reaction activity of the catalyst in this paper.This problem can be effectively overcome by regulating the CeO₂surface,which can be accomplished in several ways,including the regulation of the exposed lattice plane of CeO₂ nanocrystals,generation of surface defects（such as oxygen vacancies）using synthetic chemicals,generation of more unsaturated sites by increasing the edge steps of CeO₂ crystal using size effects,and the regulation of the interaction between CeO₂ and noble metals by introducing the composite transition metal oxides,which could produce a highly active lattice oxygen interface to facilitate the interaction.From the perspective of industrialization,CeO₂ has abundant reserves and high thermal stability.It is considered as an excellent composite catalyst support and has been industrialized in many fields,including energy production and environmental protection.Given the excellent intrinsic properties of CeO₂ materials,chemical methods such as controlled synthesis,crystal face adjustment,and surface modification can be used to exploit the catalytic performance potential,investigate the catalytic mechanism,and develop excellent CeO₂-based catalytic materials.In this study,we initiated our experimental work by catalyzing the classic template reaction of CO oxidation and the preferential oxidation of CO under hydrogen-rich conditions.The highly active oxygen species at the surface and interface of CeO₂ were effectively produced,which broadened the low-temperature reaction window of CeO₂-based composite catalysts and improved the efficiency and selectivity of the catalyst.Moreover,we systematically investigated the process of CO oxidation on CeO₂-based composite catalysts.First,in order to study the impact of size effect on the surface properties and the catalytic activity of CO oxidation.The chemical characterization and fitting calculation exhibited that the growth mechanism of CeO₂ nanoparticles followed Ostwald ripening.Controlled synthesis of CeO₂ nanoparticles with a particle size of11 nm to 100 nm can be carried out by regulating the reaction temperature,and controlled synthesis of CeO₂ nanoparticles with a particle size of 30 nm to 55 nm can be accomplished by regulating the reaction time.Based on Scherrer formula and Arrhenius formula,the growth kinetic equation of CeO₂ nanoparticles can be obtained as D⁵=16.2⁵+3.6×10²⁰t exp（-344.20/RT）.The CeO₂ with a particle size of 19 nm obtained by the treatment at 700°C showed the best CO oxidation performance after loading 1%Pt.It indicates that the size effect of CeO₂ can significantly affect the content of oxygen species on the surface of CeO₂,which in turn acts on the catalytic process and significantly impacts the catalytic performance.The results of this study may provide new insights into the controlled synthesis of a specific size of CeO₂nanoparticles and the impact of size effect on the surface properties of catalysts.Second,a synthesis method of anchoring CeO₂ quantum dots to Fe₂O₃ nano sheets to construct active interfaces was developed.We successfully prepared CeO₂quantum dots by thermal decomposition,and dispersed them onto Fe₂O₃ nanosheets to construct special interfacial oxygen activation.Under anhydrous and oxygen-free conditions,CeO₂ quantum dots with a size of less than 5 nm were prepared by thermally decomposing ceric ammonium nitrate using oleylamine as a capping agent.Then,the CeO₂ quantum dots were dispersed on Fe₂O₃ nanosheets using a surfactant,and the Fe₂O₃-CeO₂ catalyst support with a particular interface was obtained after rapid annealing.The outcomes revealed that the interaction between CeO₂ quantum dots and the Fe₂O₃ interface resulted in lattice distortion on the surface of Fe₂O₃nanosheets.This interfacial effect is very effective in activating oxygen species.Since the activated oxygen species are directly involved in the catalytic process of CO preferential oxidation,the catalytic activity is observed to be significantly improved,which shows that the catalytic performance still exists at a low temperature（-50°C）,and the complete oxidation of CO can be achieved at room temperature.This research effectively widened the low-temperature window of the catalytic reaction.It exhibited that a high conversion efficiency for the complete conversion from room temperature to zero could be achieved efficiently.Next,the gradient control of the oxygen vacancy concentration on the surface of CeO₂ was carried out to explore the effect on the Pt/Ce interaction and catalytic activity for CO oxidation at low-temperature.Firstly,the CeO₂ nanoplate composed of aggregated nanoparticles was successfully prepared by adding urea as a precipitant to a cerium salt aqueous solution using the hydrothermal method.The modification of CeO₂surface,lattice oxygen activation,and Pt/CeO₂ interaction were simultaneously achieved through gradient adjustment of the hydrogen reduction temperature.When the hydrogen reduction temperature reached 400°C,the degree of lattice oxygen activation,redox capacity,and metal support interaction all reached the most appropriate state and showed excellent catalytic performance.The activated lattice oxygen effectively participates in the CO oxidation reaction,which is critical for the catalyst to achieve complete CO oxidation at a low temperature of 80°C.Finally,in order to further reduce the reaction barrier under low temperature condition and to find out the key factor of low temperature catalytic activity,we prepare a highly active CeO₂ interface by confining Fe O_x in porous CeO₂ nanospheres.Based on the solvothermal synthesis of porous CeO₂ nanospheres with an average diameter of 120 nm,a surface area of 168 m²g^-1,and pore size of 18.7 nm,Pt O_y-Fe O_x/CeO₂-H was prepared by modifying the pores with Fe O_x.The composite catalyst exhibits excellent CO preferential oxidation performance,reaching 100%CO conversion at room temperature with almost no decay during long-term operation,which the catalyst cannot achieve without Fe O_x modification.The beneficial effects of the Pt O_y-Fe O_x/CeO₂-H composite catalyst in the reaction process include sufficient contact at the interface,the stable Fe（II）species,oxygen activation on the surface of the CeO₂ support,the reduction of the hydrogen spillover effect.And the CO was more easily to reacted with activated oxygen and then desorbed from Pt.