Fabrication, Characterization And CO Oxidation Catalytic Activity Of Supported Pd Nano-Catalys

Three-way catalyst can convert CO, NOx and hydrocarbons in the automotive exhaust gas to nonhazardous substances simultaneously with high efficiency. However, due to the shortage of noble metal resources and the increasingly strict vehicle emission control regulations. Therefore, the TWCs should preserve lower content of precious metals with higher catalytic activity than the TWCs prepared by traditional method. The traditional method of preparing TWCs should be improved. Hereon, we investigated the synthesis of Pd colloids by UAMR (ultrasonic-assisted membrane reduction , UAMR) method, the preparation of supported Pd nanocatalysts from a typical nano-sized Pd colloid, then characterized, and investigated its catalytic performance in the three way model reactions.Firstly, the orthogonal test method was used to explore the conditions for preparing Pd nanoparticles by the UAMR method, and the obtained Pd nanoparticles were characterized by SEM, TEM and laser particle size analyzer, to obtain the best conditions of the UAMR method to prepare Pd nanoparticles. The results showed that Pd nanoparticles with a nearly spherical shape and narrow particle size distribution can be achieved by UAMR method. The nano-sized and narrowly dispersed Pd nanoparticles could be synthesized under such conditions: the flow rate of the constant flow pump is about 10 mL·min-1, the rotating speed of the peristaltic pump is about 100 r·min-1, the solution concentration is 4×10-4 mol·L-1, the molar ratio of NaBH4/Pd is about 5:1, the molar ratio of PVP/Pd is 20:1, the ultrasonic frequency is 0 Hz and the micropore size of membrane is 40 nm.Secondly, the fabrication of supported Pd catalyst from a typical nano-sized Pd colloid with 1 wt% Pd loading was studied; several carriers such as Al2O3, TiO2, SiO2, CeO2, and Ce0.5Zr0.5O2 were selected. The catalysts were characterized by a combination of N2 adsorption/desorption, X-ray diffraction (XRD), Temperature-Programmed Reduction by H2 (H2-TPR) and Inductively Coupled Plasma-atom Emission Spectrometer (ICP-AES), and the catalytic behavior of the samples in the CO oxidation reaction was investigated. The results showed that the catalyst of Pd/Ce0.5Zr0.5O2 exhibited the lowest metal loading, the best catalytic activity for CO oxidation among the prepared supported Pd catalysts. This method of preparing catalyst provides a chance to reduce the usage of precious metal and to improve the catalytic activity of supported Pd catalysts. Finally, the investigation on several Pd/Al2O3 and Pd/Ce0.5Zr0.5O2 catalysts prepared by different processes was carried out. The effect of the catalysts structure, morphology, active components composition, chemical state (oxidized or reduced), and metal dispersion on the activity of supported catalysts was investigated. The results showed that the catalytic activity was effected by the oxidation state and the surface relative content of the surface active component. The high oxidation state of Pd was conducive to high CO oxidation reaction on the catalyst surface, the Pd species on the surface of the catalyst play an important role in catalytic activity.