Study On The Preparation And Three-way Catalytic Performances Of Pd-doped Catalysts

The using of three-way catalysts is an effective means for controlling of automotive exhaust emissions. Noble metals of Pt, Rh and Pd are the major active components for three-way catalytic reaction. With the increase of the number of automobiles, the demand for precious metals goes up which led to higher noble metal prices. Furthermore, a better durability is required for three-way catalysts because the mileage specified in the forthcoming state V standard will increase from 100,000 to 160,000 kilometers. Therefore, besides high catalytic activities, how to reduce the dosage of precious metals and how to improve the thermal stability and durability are the faced problems in TWCs industry at present.To deal with the above two problems, Pd doped catalysts with various pore structures were fabricated based on our previous work. The physicochemical properties of obtained catalysts were characterized by SEM, TEM, XRD, H2-TPR, UV-Raman and XPS techniques, etc. At the same time, three-way catalytic performance and thermal stability were investigated. The main research contents and results are as follows:(1) A series of 3DOM Ce0.7-x Zr0.3PdxO2(x=0,0.01,0.02,0.05,0.07,0.1) catalysts were fabricated by colloidal crystal templating method. The 3DOM structures were successfully obtained for all samples. The doping amount of Pd has a threshold. The samples of x≤0.05 were of cubic fluorite structure. Furthermore, either Pd or PdO species were detected. However, small traces of PdO could be detected in the samples with x≥0.07. When the value of x was equal to 0.05, the 3DOM Ce0.65Zr0.3Pd0.05O2 sample showed best low temperature performances among all 3DOM samples. The complete conversion temperatures(T90) of CO, HC and NO on 3DOM Ce0.65Zr0.3Pd0.05O2 were 153, 223 and 216 oC, respectively, which were similar to the corresponding T90 on 3DOM Ce0.6Zr0.3Pd0.1O2.(2) In order to investigate the thermal stability of 3DOM Ce0.65 Zr0.3Pd0.05 O2 Pd-doped catalyst, 3.42 wt%Pd/3DOM Ce0.7Zr0.3O2 Pd-loaded catalyst with the same Pd content as Pd-doped catalyst was prepared by the impregnation method. Three-way catalytic activities and thermal stabilities of the two different types of Pd based catalysts were compared. It is showed that 3DOM structures were kept well even if Pd was introduced into either doped or loaded catalysts. Both fresh and aged samples, Pd-loaded catalysts had better catalytic activities than Pd-doped ones. However, for HC and NO conversion, the ?T50 and ?T90 of Pd-doped catalyst before and after being aged were less than those of Pd-loaded catalyst, which means Pd-doped catalyst has better thermal stability than Pd-loaded catalyst. It is because the linkage of Pd-O-Ce was formed by Pd2+ ions entering into the lattice of CeO2, so that the free migration of Pd species was inhibited.(3) 3DOM-meso、3DOM and nonporous Ce0.65Zr0.3Pd0.05O2 were synthesized by soft and hard templating method, colloidal crystal templating method and sol-gel method, respectively. Then the physicochemical properties, three-way catalytic activity and thermal stabilities were studied. Meanwhile, the effects of pore structure on three-way catalytic activities were investigated. It is showed that 3DOM-meso Ce0.65Zr0.3Pd0.05O2 has better three-way catalytic performances and higher thermal stability for the conversion of HC and NO. All the three pollutants could be completely converted under 220 oC, after being aged, T50 and T90 increased by about 40 oC.(4) To further improve the catalytic activities of Pd-doped catalysts, the ratio of Ce to Zr was adjust, thus a series of Ce0.5-x/2Zr0.5-x/2PdxO2 catalysts with different Pd dosage were synthesized by sol-gel method. Moreover, the optimum doping amount of Pd was probed. It is indicated that the obtained samples are nano particles which sizes are about 10 nm. There is no linear relationship between doping amount and three-way catalytic activities. The three-way catalytic activities of Ce0.495Zr0.495Pd0.01-O2 was similar to those of Ce0.46Zr0.46Pd0.08O2 catalyst. The completely converted temperatures for the conversion of CO, HC and NO on Ce0.495Zr0.495Pd0.01O2 are 156, 223 and 196 oC, respectively. However, the amount of Pd in Ce0.495Zr0.495Pd0.01O2 is only 20% compared to that of Ce0.46Zr0.46Pd0.08O2. The above results can provide data support for the reduction of noble metal dosage.