Preparation And Characterization Of Sintering-resistant Rh-Pr₆O₁₁/SiO₂ And Ni-CeO₂/SiO₂ Catalysts And Their Performance For Methane Partial Oxidation To Syngas

This thesis focus on the studies of preparing highly-dispersed and sintering-resistant supported catalysts using a simple impregnation method.The Pd/Al₂O₃,Rh-LnOn/SiO₂?Ln=Gd,Pr,Eu,Nd,Y?and Ni-CeO₂/SiO₂ catalysts were prepared from metal acetylacetonate.The properties and the interaction between the components of the catalysts were characterized by thermogravimetry?TG?,X-ray powder diffraction?XRD?,raman spectroscopy?IR?,temperature-programmed reduction?H₂-TPR?,Brunauere-Emmette-Teller?BET?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?.The performance of the catalysts for the partial oxidation of methane?POM?to syngas was also investigated.The main results of this thesis were summarized as follows:?1?A series of Pd/Al₂O₃ samples with different Pd loadings were prepared by impregnation method using Pd?acac?₂ as a precursor.The results showed that the Pd?acac?₂/Al₂O₃ samples decomposed at 203? in air.When the Pd loadings increased from 0.25%to 5%,the size of Pd nanoparticles also increased from 4.1 nm to 6.3 nm after the samples calcined at 800?.That's to say,Pd/Al₂O₃-acac samples didn't have high thermal stability compared to Pd/SiO₂-acac samples,but they were better than Pd/Al₂O₃ samples prepared by Pd?NO3?2.?2?The highly-dispersed and sintering-resistant Rh-LnOn/SiO₂ catalysts were prepared using co-impregnation method with Rh?acac?₃and Ln?acac?₃?Ln=Gd,Pr,Eu,Nd,Y?as precursors.After 50 h of POM reaction,the Rh nanoparticle size of the 0.1%Rh/SiO₂-800 catalyst increased from 2.4 nm to 3.5 nm.The sintering-resistant ability and dispersion of Rh species were improved dramatically due to the addition of LnOn.The Rh-LnOn/SiO₂ catalysts also showed excellent performance of POM reaction:after 120 h of POM reaction at 700?,the Rh nanoparticle size of the 0.1%Rh-10%Pr6011/SiO₂-800 catalyst was still smaller than 3 nm.The H₂-TPR results indicated that the reduction peak of Rh2O3 species shifted from 118? to 127? after adding Pr6011 to 1%Rh/SiO₂-800 catalyst which suggested that there was a strong interaction between Rh2O3 and Pr6O111.?3?CeO₂/SiO₂ samples were prepared by impregnation method using different Ce compounds as precursors.10%CeO₂/SiO₂ samples prepared by Ce?acac?₃ and Ce?CH₃COO?₃ possessed high dispersion and favorable redox stability.Meanwhile,the CeO₂ nanoparticle size could maintain at approximately 2.5 nm after calcined at 800? in air for the CeO₂/SiO₂ samples prepared by Ce?acac?₃ when the CeO₂ loading was below 10%.Whereas,CeO₂ prepared by Ce?NO3?3,CeCl₃ and citric acid had a poor dispersion on SiO₂ surface.The CeO₂ nanoparticle size decreased after H₂-TPR characterization and calcined at 600? in air.The cause of CeO₂ nanoparticle size decreased might be the formation of cerium silicates which promoted the CeO₂ nanoparticles redispersed on SiO₂ surface.What's more,the XPS results showed that the percentages of Ce³⁺species of 10%CeO₂/SiO₂-acac samples were much higher than 10%CeO₂/SiO₂-NO3 and 10%CeO₂/SiO₂-Cl samples.That's to say,there were more oxygen vacancies on the surface of 10%CeO₂/SiO₂-acac samples.Ni/SiO₂ catalysts prepared by Ni?acac?₂ exhibited good performance of POM reaction compared with other precursors?Ni?NO3?2,NiCl2,Ni?CH3COO?2 and citric acid?.The intensity of XRD peaks of NiO decreased and the performance of POM reaction improved by adding CeO₂ into Ni/SiO₂ catalysts.The H₂-TPR and XPS results showed that both the reduction peak of NiO and the binding energy of Ni 2p3/2 on 1%Ni-10%CeO₂/SiO₂-acac-800 catalyst were higher than 1%Ni/SiO₂-acac-800 catalyst which indicated that there was a strong interaction between NiO and CeO₂.