Preparation And Characterization Of Sinter-resistant Supported Metal Catalysts From Metal Acetylacetonate And Performance For Partial Oxidation Of Methane To Syngas

Exploring a simple method that can synthesize the highly dispersed and sinter-resistant catalysts still remains a tough challenge.This thesis mainly prepares the Ni/SiO₂,Rh-Ce02/SiO₂ and Rh-Sm2O₃/SiO₂ catalysts using metal acetylacetonate as precursors.The preparation and catalytic properties of catalysts and the interaction between promoters and metals,are characterized in detail by in situ infrared spectroscopy?IR?,thermogravimetric analysis?TG?,N2 physisorption?BET method?,X-ray powder diffraction?XRD?,temperature-programmed reduction?H₂-TPR?,transmission electron microscopy?TEM?and X-ray photoelectron spectroscopy?XPS?.The performance of the catalysts for the partial oxidation of methane?POM?and methane reforming with carbon dioxide to syngas is also investigated.The main results of this thesis are summarized as follows:?1?The results show that the highly dispersed and sinter-resistant Ni/SiO₂ catalysts can be prepared by the conventional impregnation method.Compared with other precursors?Ni?NO₃?2 and NiCI2?.the catalysts prepared by acetylacetonate shows better dispersion.The in situ IR results show that when SiO₂ is impregnated in the Ni?acac?2 sollution,the surface silanol groups of SiO₂ can act as the centers to interact with Ni?acac?2 molecules,thus causing the formation of a hydrogen-bonded Ni?acac?2 layer on the Sio2 surface.TG-DTG results show that monolayer Ni?acac?2 decomposes into highly dispersed NiO when heated to approximately 350 °C in air.This may be responsible for the reason that Ni nanoparticles nearly have the same average particle size of?2.6 nm when the loadings is below 5%or calcination temperature isn't over 800 °C.?2?The sinter-resistant Rh-CeO₂/SiO₂ and Rh-Sm2O₃/SiO₂ catalysts can be synthesized using the conventional impregnation method with Rhodium acetylacetonate?Rh?acac?3?and Cerium acetylacetonate?Ce?acac?3?,Samrium acetylacetonate?Sm?acac?3?as precursors.The characterization results are compared with the catalysts prepared by nitrate.The adding of rare earth oxides?CeO₂ and Sm2O₃?can improve the sinter-resistant ability and dispersion of Rh species,so the Rh-CeO₂/SiO₂ and Rh-Sm2O₃/SiO₂ catalysts have good POM and DRM performance even the loading of Rh decreased to 0.025wt%.No aggregation of Rh species is observed after reaction and the average particle size is about 2.6 nm.The surface silanol groups of SiO₂ acts as the centers to interact with M?acac?3?M=Rh,Ce,Sm?molecules when SiO₂ is impregnated in the M?acac?3 solution,causing the formation of a hydrogen-bonded M?acac?3 layer on the SiO₂ surface.In this experiment,the monolayer coverage of Sm?acac?3 on the SiO₂ surface is equal to a Sm?acac?3 loading?mass fraction?of approximately 31%,which in turn corresponds to a Sm2O₃ loading of approximately 15%.When a Sm?acac?3/SiO₂ sample with Sm?acac?3 loading below 31%is heated in air to approximately 360 °C,the monolayer Sm?acac?3 species decomposes into highly dispersed Sm2O₃ species on the SiO₂ surface,which displays exellent stability against sintering even at high temperature.No obvious aggregation of the Sm2O₃ species is observed even when the sample is heated to 800 °C in air.Whereas the Rh-CeO₂/SiO₂-NO₃ and Rh-Sm2O₃/Si02-NO₃ catalysts appear obvious aggregation problems after reaction,thus the activity of the catalysts decreases apparently.The H₂-TPR and XPS results show that there is strong interaction between Rh and CeO₂ or Sm2O₃,therefore the reduction peak of Rh2O₃ shifts toward high temperature.Moreover,the binding energy of Rh 3d of Rh-CeO₂/SiO₂-acac and Rh-Sm2O₃/SiO₂-acac catalysts is also increased about?1.4 eV and?0,6 eV respectively.The CeO₂/SiO₂-acac samples show excellent dispersion when the loading is below 15%,for there is no obvious diffraction peak of CeO₂ is detected.While,the CeO₂/SiO₂-NO₃ samples appear diffraction peak of CeO₂ when the loading is only 5%.Calibrating the TPR for H₂ consumption we can find that when the reduction temperature is below 900 ??the surface CeO₂ species of CeO₂/SiO₂-acac is almost reduced to Ce2O₃,whereas the bulk CeO₂ species is partly reduced.By comparison,the bulk CeO₂ species of CeO₂/SiO₂-NO₃ samples is more easier to be reduced,which may owing to that bulk CeO₂ species decomposed by Ce?acac?3 is too large to be reduced.