The Synthesis Of Ceria Supported Palladium Catalysts For Methane Combustion

As a clean energy,methane has been widely concerned because of its high reserves.It is easy to generate pollutants such as NOx and CO,and cause the greenhou se effect when the temperature of methane combustion is high or the reaction is not enough.The fundamental way to solve this problem is to improve combustion efficiency.Compared with the traditional methane combustion mode,the catalytic combustion of methane has higher combustion efficiency and energy utilization rate,and can effectively reduce pollutant emissions.In this dissertation,the morphology and structure of Pd/CeO2 catalysts were tailored by synthesis method and parameters,and their effect on the catalytic combustion of methane were investigated.The results are shown below.(1)I-Pd/CeO2,C-Pd/CeO2 and H-Pd/CeO2 catalysts with Pd loadings of 1wt%were prepared by impregnation,co-precipitation and hydrothermal method,respectively.SEM and HAADF-STEM/EDX analysis showed that H-Pd/CeO2 particles have spherical morphology,and PdO was encapsulated by CeO2 and well dispersed.XPS and TPR measurements showed that differ from I-Pd/CeO2 and C-Pd/CeO2,the main Pd species on the surface of H-Pd/Ce02 is PdO,rather than PdOx(1<x<2)or PdO2;and it has better redox.These may contribute to the higher catalytic activity and stability for methane combustion over H-Pd/CeO2 catalyst.Under the reaction condition(feedgas of 2 vol%CH4,4 vol%O2,20 vol%CO2,balance with N2,GHSV of 100,000 mL h-1·gcat-1),the T90 of methane was 467 ? in the presence of H-Pd/CeO2-1000 catalyst(calcined at 1000?),which is almost equal to the value in the presence of H-Pd/CeO2-600 catalyst(calcined at 600?).(2)The H-Pd/CeO2-t series catalysts were prepared by adjusting the hydrothermal synthesis time.The results show that when the hydrothermal duration was fixed at 12 h or 24 h,the synthesized catalyst has better low-temperature activity.T50 for methane over the t-12 and t-24 sample is 405 ? and 407?,respectively.The t-24-1000 catalyst(calcined at 1000?)still maintained a conversion about 99%after continuously reacting at 500 ? for 50 h.SEM characterization results show that when hydrothermal synthesis time is too short,the catalyst does not form a stable morphology structure,and the active component is easy to sinter.However,when the hydrothermal synthesis time is too long,sintering occurs between the catalyst particles.From the aspects of activity and stability,the optimal hydrothermal reaction time is 24 h.(3)H-Pd/CeO2-H2O2 and H-Pd/CeO2-CA series catalysts were obtained by modulating the amount of hydrogen peroxide and citric acid in hydrothermal synthesis process.In our investigation,the activity of the catalyst increases first and than decreases as increasing the amount of hydrogen peroxide and citric acid.When the molar ratios of H2O2 and citric acid to cerium salt are 0.6 and 1,respectively,Pd/CeO2 catalyst of regular spherical morphology can be synthesized,leading to their excellent activity in the catalytic combustion of methane.