Synthesis Of Mesoporous CeO₂ Supported Pd And Their Low-Temperature CO Catalytic Oxidation

It is of great practical and therical value to explore CO catalytic oxidation since it has been widely employed to the fields of environmental protection,energy,chemical engineering,material,military industry and basic research.Single-component and mixed transition metal oxides,transition metal nitride and metal organic frameworks（MOFs）have been employed to catalytic oxidation of CO.As a result,catalysts with exceerllent CO catalytic activity at ambient congdition or ultra-low temperature are generally instable,while a stable one is generally inactive below 100℃.The price of palladium is relatively lower than a majority of platinium group metals（PGM）.In addition,over 100%of CO catalytic conversion could be contained by platinium-based catalysts at below 100℃ or even lower than ambient temperature under moisture condition for a long time.Therefore,palladium-based catalysts have been widely applied to CO catalytic oxidation.To date,a substantial amount of difficulty and focus are concentrating on reducing Pd content and cut down the price of supported Pd catalysts via the improvement of specific area,alteration of pore structure,nano particle structure.Thus,the present work attempted to explore the influence of different precipitant,hydrothermal temperature and surfactants on composition and structure of CeO₂ in the procedure.As-prepared CeO₂ was supported with Pd（1%wt）and employed to CO catalytic oxidation below 100℃.In conclusion,we have accomplished the following works:（1）A series of CeO₂supports have been prepared via a hydrothermal method using oxalic acid,urea,ammonium carbonate and ammonia as precipitant assisted with different surfactants（P123,F127,PVP,CTAB）.As-prepared CeO₂ supports have been impregnated with Pdimplemented to explore the catalytic oxidation of CO.Pd/CeO₂ prepared using ammonia as precipitant assisted with P123via a hydrothermal method treated at 100℃ in 12 h exhibited the best catalytic activity with T₁₀₀ of 50℃.XRD analysis indicates no crystalline phase of Pd species could be found from all catalysts.The crystallization of CeO₂ increased with the increase of hydrothermal temperature,when oxalic acid was employed as precipitant.PVP surfactant assisted synthesis of CeO₂ in the procedure can fcilitate reducing the crystallinity.Nitrogen adsorption desorption tests indicate that all catalysts are mesoporous Pd/CeO₂.Syn－thesis of CeO₂ using urea or ammonium carbonate as precipitant leads to ultra large surface area（96-160 m²/g）of the Pd/CeO₂ catalysts.While using ammonia as precipitant may lead to small surface area（47-87 m²/g）of the Pd/CeO₂ catalysts.It can be inferred from TEM analysis that synthesis of CeO₂ using ammonia as precipitant can facilitate the formation of high-crystallinity CeO₂ nano particles with uniform morphology and particle size distribution（20-30nm）.While synthesis of CeO₂ using urea and ammonium carbonate as precipitant leads to nonuniform distribution of particles（of which sizes above 100 nm）and the uniform,narrow distribution of mesoporous.XPS analysis indicate that urea,ammonium carbonate as precipitant in the synthesis of mesoporous CeO₂ can facilitate the formation of PdO and PdO₂.While synthesis of CeO₂ via ammonia method can facilitate to the formation of PdO.Under the experimental conditions,nano CeO₂ particles supported Pd catalysts synthesized via ammonia as precipitant is in favor of CO catalytic oxidation at ambient temperature.（2）We prebed catalytic oxidation of CO over rare earth oxygen storage material(CeZrO₂ and CeZrAl_0.1O_x)supported Pd catalysts and found that the eddition Zr oxis to CeO₂ can reduce the T₁₀₀ of CO catalytic conversion from 125℃ to 60℃.A further addition of aluminum oxide to CeZrO₂ in a proper extent could reduce the T₁₀₀ of CO catalytic conversion to 55℃.The specific surface area of Pd/CeZrO₂ and Pd/CeZrAl_0.1O_x was significantly higher than that of Pd/CeO₂,Although the surface oxygen abundance of Pd/CeZrAl_0.1O_x was lower than that of Pd/CeO₂ and Pd/CeZrO₂,whileT90 and T₁₀₀ of Pd/CeZrAl_0.1O_x is significantly lower than that of Pd/CeO₂ and Pd/CeZrO₂.In addition,pore size distribution and pore volume of Pd/CeZrAl_0.1O_x is extended.