Preparation, Characterization And Three-way Catalytic Performance Of The Core-shell Structured Catalysts

As the motor vehicle exhaust emission standard is becoming more and morestrict and precious metal resources increasingly scarce, three way catalysts （TWCs）industry faces the challenge of reduction in precious metals and increases the catalystactivity and thermal stability. In recent years, the development of nanometer catalyticscience and application of nano TWCs brought dawn to meet this challenge. Mazdahad prepared a TWCs catalyst, in which all precious metal particle size was smallerthan5nm, showing good three-way catalytic activity and thermal stability, so thedosage of precious metals could be reduced70-90%. However, nano TWCs fordomestic research has just started. The ultrasonic membrane diffusion method（UAMR） can realized controllable synthesis of nano-noble metal particles, and theprepared RhAu/γ-Al₂O₃-UAMR catalysts possessed much higher catalytic activitywith low dosage of precious metal. However, how to improve the thermal stability ofnano metal catalyst is a problem. This topic is based on the controllable preparation ofmetal nanoparticles through designed experiments to find package type Pd@SiO₂nanoparticles controllable preparation conditions, expounds the influence law of thesynthesis parameters on the dispersion. The Pd@SiO₂nanoparticles were loaded onthree kinds of carriers, gamma Al₂O₃, Ce_0.4Zr_0.6O₂and gamma Al₂O₃（70%）+Ce_0.4Zr_0.6O₂（30%）, respectively. The activities of the prepared catalysts that calcinedat different temperatures were characterized. The results showed that:1. The Pd nanoparticles was prepared by chemical reduction method, the size ofthe particle is4⁶nm. Ethyl orthosilicate （TEOS） as silica source to prepare thePd@SiO₂nanoparticles, and the average thickness of the shell is about12nm, thepackage rate of Pd nanoparticles is100%, and each SiO₂shell package only one Pdnanoparticle.2. The Pd/CZ (Ce_0.4Zr_0.6O₂)-500and Pd@SiO₂/CZ-500catalysts were preparedby immersion method, and the aged samples were represented as Pd/CZ-900and Pd@SiO₂/CZ-900. The three-way catalytic activity and characterization results showthat the chemical state of Pd0, PdOxand PdO were both exist at the catalysts preparedfrom nanoparticles. The three-way catalytic activity of Pd/CZ-900and Pd@SiO₂/CZ-900catalysts were higher than the corresponding fresh samples Pd/CZ-500and Pd@SiO₂/CZ-500, at the same time, the activity of Pd@SiO₂/CZ-900is higher than Pd/CZ-900catalyst, the presence of SiO₂shell increases the thermal stability ofthe catalyst.3. Also, the Pd/A （Al₂O₃）-500and Pd@SiO₂/A-500catalysts were also made, theaged samples represented as Pd/A-900and Pd@SiO₂/A-900. The results showed thatthe Pd@SiO₂/A-500and Pd@SiO₂/A-900catalysts possessed lower catalytic activitythan Pd/A-500and Pd/A-900catalysts, the presence of SiO₂shell failed to increasethe thermal stability.4. At last, the Pd/A+CZ-500and Pd@SiO₂/A+CZ-500catalysts were made, the agedsamples represented as Pd/A+CZ-900and Pd@SiO₂/A+CZ-900. The results showedthat the Pd@SiO₂/A+CZ-500and Pd@SiO₂/A+CZ-900catalysts possessed lowercatalytic activity than Pd/A+CZ-500and Pd/A+CZ-900, the presence of SiO₂shellfailed to increase the thermal stability.