The Synthesis Of Advanced Ceria Based Material And The Study Of The Palladium-Ceria Based Material Interaction

Due to the promulgation and implementation of"the strictest"National ? motor vehicle pollutant emission standard,the development of vehicle exhaust catalyst is facing unprecedented challenges.Noble metal supported catalyst,as an important representative,needs to have much higher activity and stability to meet the requirements.Therefore,we aim to obtain the ceria-based materials with high performance,and improve the hydrothermal stability of catalysts.The effect of synthesis conditions on the physical and chemical properties of ceria-based materials was investigated.Then,the noble metal of palladium(Pd)was loaded on the supports,and the nature of Pd-support interaction was revealed.It can provide the theoretical bases and thoughts for the reduction and stabilization of noble metals.The effect of supports with different cerium contents on the Pd species was investigated.We found that the Pd-support interaction is reflected as the Pd-O bonding.The higher active oxygen content on the surface can lead to the stronger interaction,and Pd is most likely to exist as Pd²⁺.After thermal aging,the supports are agglomerated and the amount of active surface oxygen decreases,leading to the interaction weakens sharply.Pd species is agglomerated into the metallic palladium or palladium oxide particles.The reaction mechanism of CO oxidation and the active site is the same for different samples.Zirconia-rich ceria-zirconia material(CZ),which shows the best thermal stability in gasoline vehicle aftertreatment,was selected as the subject to investigate the effect of ripening duration on the properties of CZ and the state of Pd supported on it systematically.We found that ripening for a long time could reduce the surface energy of CZ grains,and then reduce the crystalline size and increase the specific surface area.However,it can change the distribution of active oxygen species,and make the oxygen vacancy defects concentrated on the surface.It results that the dynamic oxygen storage&release(DOSC)values of materials and the TWC activity of the corresponding Pd-supported catalysts follow a volcanic-type trend with the aging time prolonging.The sample with 5 hours ripening shows the best performance.In order to improve the stability of the supports further,the pore forming and granulation was added in the synthesis process.The effects of the operations on the properties of CZ materials were invstigated.The results showed that the pore forming and granulation could increase the number of mesoporous over 10 nm without affecting the grain properties.It can improve the surface area and stability of CZ,resulting that it has a high surface area and DOSC value after 1000? aging.It can also reduce the decline of TWC activity of Pd/CZ catalyst after aging.In order to control and stabilize the state of noble metal,cationic adsorption and palladium colloidal loading method were used instead of the impregnation method to load Pd on ceria-based material.A series of Pd-supported catalysts with the same total Pd content but different palladium species concentrations was prepared.The Pd-support interaction was then studied systematically from the view of noble metal.The results showed that the interaction between ionic Pd²⁺and support is stronger,which can improve the activity of oxygen species in support.On the contrary,the interaction between metal Pd⁰ and support is weaker than that of Pd²⁺.Pd⁰ can absorb the reactant of CO and C₃H₆ more easily.The catalyst with an appropriate Pd²⁺/Pd⁰ concentration can present a superior reaction activity and better hydrothermal stability.