Preparation And Performance Of Pd-based Catalysts For Catalytic Combustion Of Methane With High CO₂ Concentration In The Feed Gas

Low in methane concentration,oil field exhaust gas is directly released without further purification,resulting in great energy waste and severe greenhouse effect.However,the flame temperature of direct combustion of methane reaches as high as 1500 ?,which would cause the combination of nitrogen with oxygen to nitrogen oxides,thus leading to secondary environment pollution.The catalytic combustion of methane offers an alternative means for the exploitation of oil field exhaust gas.In this thesis,the Pd/MgAlO catalyst is selected as the research object,and the way of improving its low temperature catalytic activity and stability were investigated in detailed.X-ray Diffraction(XRD),Thermal Gravity Analysis(TGA),N2-physisorption,H2-Temperature Programmed Reduction(H2-TPR),O2-Temperature Programmed Desorption(O2-TPO),H2-O2 Titration,CH4-Temperature Programmed Reduction(CH4-TPR),CO2-Temperature Programmed Desorption(CO2-TPD)and X-ray photoelectron Spectrum(XPS)techniques were employed to investigate their phase composition,Pd-dispersion,reduction-oxidation property,and so on.The effect of the composition and structure of the Pd/MgAlO catalysts on their performance for catalytic combustion of methane with high CO2 concentration were studied.The results are shown below.Firstly,the structures and catalytic activities of Pd/MgAlO catalysts,prepared by a parallel co-precipitation method with different Mg/Al molar ratios,were investigated.It is found that the Pd/Mg1Al3O catalyst with Mg/Al molar ratio equal to 1:3 shows the highest catalytic activity for catalytic combustion of methane.When the GHSV is 100,000 mL·h-1·gcat-1,the CH4 conversion reaches up to 95.3%at 550 ?.And these distinguished performances are attributed to a larger Pd particle size,a narrower relaxation temperature range between the release and generation of lattice oxygen in Pd species,and a comparable percentage of metallic Pd and PdO in the Pd/Mg1Al3O catalyst.Besides,MgO can inhibit the decomposition of PdO into metallic Pd,which leads to improve the stability of active PdO species at high temperature.The XPS characterization result indicates that the catalyst deactivation may ascribe to the reduction or partially embedded of the surface PdO species.Secondly,the effect of Pd content on Pd/Mg1Al3O catalysts prepared by impregnation method for catalytic combustion of methane was also investigated.As the increase of Pd loading,the improvement of the catalytic activity of IM-xPd/Mg1Al3O catalysts with Pd ranging from 0.6 wt.%to 1.2 wt.%declines gradually.The characterization results of N2-physical adsorption,TPR and XRD of the fresh and used IM-1.0 wt.%Pd/Mg1Al3O catalyst show that the main reason for the decrease of catalyst activity is the decomposition of PdO,which results in the decrease of the number of PdO-Pd*active cites.Finally,the rare earth(Y,La,Ce or Pr)modified IM-1.0 wt%Pd/Mg1Al3O catalysts were studied.The results show that the introduction of rare earth additive has a significant promoted effect on the catalytic activity and stability of IM-Pd-M/Mg1Al3O catalyst.After being exposed to the feed gas of 2 vol.%CH4,4 vol.%O2,20 vol.%C02,balance with N2(GHSV of 100,000 mL·h-1·gcat-1)at 500 ? for 50 h,the methane conversion over IM-Pd-Pr/Mg1Al3O trends to stable at a value of 75%,while the one without Pr doping(IM-Pd/Mg1Al3O)reduces to a level of 59%.