Morphology Control And Modification Of CeO₂ And Low-Concentration Methane Catalytic Properties Over The Pd/CeO₂ Catalysts

In recent years,global warming has aroused people's wide concern,the resulting environmental problems are increasingly serious.Hence,reducing the emission of greenhouse gas is an effective route to solve the problem.Methane is also one of the main greenhouse gases except CO₂,and the warming potential of methane is 21-23 times higher than that of carbon dioxide.Environmental problems caused by methane cannot be ignored.In china,the coal resources are very rich,and a large amount of coal bed methane is released in coal mining?the main component is CH4?,meanwhile,about 70% of methane is emitted through mine ventilation air with extremely low-concentration methane?0.1%-1.0vol.%?,which not only causes the waste of energy resource,more importantly,poses a threat to environment.Therefore,reasonable use of low-concentration methane in mine ventilation air is an effective way to solve this problem.Presently,catalytic oxidation technology is an effective method for the utilization of low-concentration methane.Therefore,the key issue is to prepare catalysts with high active and stability for methane oxidation.Ceria has important application value in the field of catalysis due to its unique redox property and high oxygen storage and release capacity.And the catalytic property strongly depends on the morphologies and structures of ceria materials.Therefore,the work mainly foucses on the synthesis of ceria with various morphologies and structures by hydrothermal method.The catalytic performances of Pd-based catalysts were studied in detail.Furthermore,influences of the ceria morphologies and structures were elaborated and the relationships between carrier structures and catalytic properties were also established.The main research contents and conclusions are as follows:?1?Octahedral CeO₂ possessing a stair-like microstructure comprising self-assembled nano-sized rectangular blocks was synthesized.The samples were analyzed by X-ray diffraction,scanning electron microscopy and nitrogen adsorption/desorption techniques.Catalytic property of the octahedral Ce O₂ supported Pd catalyst was investigated by the catalytic oxidation of low-concentration methane.The results showed that the unique stair-like structure of octahedral CeO₂ was beneficial to suppress the agglomeration of Pd nanoparticles,further improved the thermal stability of the Pd-based catalyst for continuous conversion of low-concentration methane.?2?Lamellar octahedral ceria?LOC?was synthesized.Its morphology and structure was analysed,and the catalyst property and reaction kinetic were investigated;the effect of Pd loadings,types of doped metal oxides?ZrO₂,NiO and Co3O4?,ZrO₂ contents and reaction GHSV on catalytic properties were alsostudied.The result shows that the unique structural CeO₂ exposes?110?crystal plane,which is superior to?111?facet of general octahedral ceria,and exists non-stoichiometric ceria?CeO₂-x?.Meanwhile,LOC-supported Pd catalyst exhibits excellent activity and stability by optimizing Pd loadings.The catalytic property of Pd/LOC are further enhanced by introducing guest metal oxide?ZrO₂?into the LOC material,and when the loading of ZrO₂ is 5 wt.%,the catalytic property is optimum.Reaction kinetic further confirms that the catalyst has lower activation energy?85.40 KJ mol-1?.Additionally,the 0.5 wt.%Pd/LOC-5 wt.% Zr catalyst maintains high low-temperature activity and stability under various gas hourly space velocities.?3?CeO₂ materials with microspheres and nanotubes were synthesized.The influence of CeO₂ morphologies on catalytic properties of the Pd/CeO₂ catalysts were investigated by the analysis of the characterization.The results show that the CeO₂ microspheres have short porous channel,the exposed?100?plane and existence of CeO₂-x;the performance of the catalyst strongly depends on the morphology of CeO₂ carrier,namely,the property of Pd/CeO₂-microspheres catalyst is superior to that of Pd/CeO₂-nanotubes.?4?The performance of Pd/CeO₂-nanotubes catalyst for methane catalytic oxidation was further improved via the modification of alkaline earth metals,and the effect of Pd loadings,types of alkaline-earth metals and contents as well as calcination temperations of catalyst on the property of catalyst were also investigated.The results show that the methane conversion increases withincreasing the Pd loading.The introduction of alkaline-earth metal?Ca?further improves the catalytic activity,the T90% for Pd Ca/CeO₂ catalyst decreases 54 °C compared to that of Pd/CeO₂ catalyst.When Pd/Ca molar ratio is 1:0.5,Pd Ca/CeO₂ catalyst exhibits the best catalytic activity and the optimum calcination temperation is 450 °C.