| In this thesis,the oxidation of CO and HCs involved in automotive exhaust catalysis was studied over supported ceria-based catalysts.Different types of Pt species(Pt-O-Ce,PtOx,and Pt nanoparticles(Pt NPs))and Pd species(PdxCe1-xO2-δ and PdO)supported on nano-ceria was synthesized and the catalytic mechanism was characterized.The evolution of chemical state of the metal species during CO oxidation reaction and their correlation with CO oxidation performance was investigated.Furthermore,the influence of other components(H2O,C3H6)on the CO oxidation reaction over different types of metal species and the reaction mechanism was also explored.This thesis aims to deepen the understanding of the relationship between composition,structure and catalytic performance of supported ceria-based catalysts.The main findings are as follows:(1)Different types of Pt or Pd species can be obtained on the surface of nano CeO2 by impregnation with different Pt or Pd complex precursors.In acidic aqueous solution,the anionic Pt complexes(PtCl62-,PtCl42-)is mainly electrostatically adsorbed on the protonated CeO2 surface,and highly dispersed Pt-O-Ce species can be generated after drying and calcination.The cationic Pt complex(Pt(NH3)42+)exhibits a weak interaction with the CeO2 surface under neutral pH conditions,and amorphous PtOx species are mainly formed after drying and calcination.Similarly,the strong electrostatic adsorption between anionic PdCl2-and CeO2 at acid solution lead to ionic Ce1-xPdxO2-δ formed after calcination.By adding sodium carbonate precipitant during loading,the Pd species are deposited CeO2 surface by precipitation,and PdO species are mainly obtained after calcination.(2)The oxidized Pt-O-Ce species was only slightly reduced to the ionic Ptn+-O-Ce(n is slightly less than 2)during heating up to 350℃ in 1%CO/1%O2/He flow,and the related catalysts show very poor activity below 200℃.The PtOx species could be reduced to metallic Pt0 after the reaction temperature was raised to 150℃,and the CO oxidation activity also increased rapidly.In contrast,metallic Pt NPs obtained by reduction of the oxidized Pt-O-Ce and PtOx with H2 exhibit good low-temperature activity for CO oxidation.However,the metallic Pt NPs(especially generated by the reduction of Pt-O-Ce species with H2)would be reoxidized into the less active Ptn+-OCe species during the reaction.The characterization results showed that the oxidation of metallic Pt NPs arise from oxygen reverse spillover from the CeO2 support.Compared with the Pt/CeO2 catalysts reduced with H2,the catalysts reduced with of CO or liquid phase reducing agent(NaBH4 and ethylene glycol)leads to a decrease the degree of reduction of the CeO2 support,hence the oxidation of metallic Pt NPs by CeO2 is not favored.In situ FTIR spectroscopy and CO-TPR characterization reveal that the improved CO oxidation performance of metallic Pt NPs is due to the higher reducibility of Pt NPs-ceria interface compared with oxidized Pt-O-Ce and PtOx,so that the CO adsorbed on Pt can reacted with lattice oxygen of CeO2 at low temperature.Similar to the Pt/CeO2 catalysts,the Ce1-xPdxO2-δ species on the Pd/CeO2 catalyst mainly exist as ionic Pd2+during the CO oxidation reaction,while the PdO species can be immediately reduced to metallic Pd0.However,compared to the Pt-O-Ce species,the Ce1-xPdxO2-δ species shows higher low-temperature reducibility,and therefore exhibits better CO oxidation activity.These results indicate that the reducibility of the Pt and Pd species on CeO2 play an important role on the CO oxidation reaction.(3)For the Pt/CeO2 catalysts,the addition of H2O in the CO oxidation reaction can significantly promote the catalytic activity of PtOx and metallic Pt NPs species.In situ FTIR spectroscopy and H218O isotope-labeling experiment show that the reaction between the lattice oxygen of CeO2 support and CO is inhibited in the CO oxidation in the presence of H2O,and the oxygen is activated on the metallic Pt at the Pt-support interface by the proton transfer of H2O molecules.Furthermore,since the CeO2 surface is covered by H2O,the oxidation of metallic Pt(especially generated by the reduction of Pt-O-Ce species with H2)to the less active Pt-O-Ce by CeO2 is greatly suppressed.However,the addition of H2O does not promote the CO oxidation reaction for the PtO-Ce species on CeO2,because ionic Pt is not conducive to the oxygen activation regardless of whether there is water in the reaction.For the CO oxidation on Pd/CeO2 catalysts,the addition of H2O shows a smaller effect on the CO oxidation for ionic Ce1xPdxO2-δ species,while it significantly promotes the CO oxidation reaction for PdO species.This is mainly due to the water-assisted O2 activation process is facilitated on the metallic Pd which is generated by the reduction of PdO during CO oxidation.Benefit from an alternative oxygen activation pathway in presence of water on PdO/CeO2,the coexistence of C3H6 shows less inhibitory effect on the CO oxidation activity.Moreover,for the individual oxidation of C3H6,the PdO species also exhibits higher activity than the Ce1-xPdxO2-δ species since the PdO species is more easily reduced to Pd during the reaction.These results indicate that the highly dispersed metallic Pd on the ceria-based support possess better catalytic oxidation performance than the Ce1-xPdxO2-δ species for the simultaneous oxidation of CO and HCs in presence of H2O. |
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