Design Of Manganese-Based Composite Metal Oxides As Catalysts For Methane Catalytic Combustion

In recent years,natural gas is widely used,which on the another hand brings about serious environmental problems.Methane is the main component of natural gas.The greenhouse effect of methane is 22 times more than CO2.The emission of methane exhaust gas will aggravate the greenhouse effect.Catalytic combustion can completely convert low concentration methane.Noble metal catalysts are expensive and easy to sinter in the reaction.Metal oxides are more promising as combustion catalysts because they are cheap and stable.Manganese oxide has many stable oxide forms,which shows high catalytic potential.In this paper,manganese-based composite metal oxide catalysts were prepared by combining manganese oxide with cerium oxide or cobalt oxide.Firstly,?-MnO2 and ?-MnO2 were prepared,and then Ce was introduced to obtain Ce-MnO2 composite oxide catalysts.Secondly,CoxMny-LDH hydrotalcite materials were prepared as precursors,and then the CoxMny-CMO composite oxide catalysts were obtained after calcination processes.The effects of preparation conditions on catalytic performance were studied.The structure of catalysts was systematically characterized by X-ray diffraction(XRD),N2 adsorption,transmission electron microscope(TEM),hydrogen temperature-programmed reduction(H2-TPR)and X-ray photoelectron spectroscopy(XPS).The research results are as follows:1.Ce-?-MnO2 and Ce-?-MnO2 catalysts were prepared by hydrothermal synthesis method.The effect of Ce:Mn ratio on catalytic activity was investigated.Ce-?-MnO2 and Ce-?-MnO2 catalysts exhibited highest activity in the respective system when the molar ratio of Ce to Mn was 1:10.XRD and TEM results revealed that Ce doping remarkably influenced the crystal structure and morphology of MnO2.The crystallinity of ?-MnO2 decreased and the nanorods disappeared after Ce modification.The crystal form of?-MnO2 partially changed from y phase to a phase and the morphology changed from urchin-like to microsphere after Ce modification.N2 adsorption and H2-TPR results indicated that Ce-MnO2 had larger specific surface area and stronger reducibility than MnO2,which improved the catalytic activity.2.CoxMny-LDH hydrotalcite precursors with special layered structure were prepared by co-precipitation method.Then,the hydrotalcite-derived CoxMny-CMO catalysts were obtained after thermal calcination processes.The sequence of catalytic activity was Co4Mn1-CMO>Co5Mn1-CMO>Co3O4>Co3Mn1-CMO>MnOx.Subsequently,the effect of calcination temperature of Co4Mn1--CMO on catalytic performance was investigated.The optimum calcination temperature was 350?.T10,T50 and T90 was 260,310 and 374?,respectively.XRD results showed that Mn cations entered into Co3O4 lattice,forming solid solution.H2-TPR results indicated strong interaction between the two active components in Co4Mn1-CMO.XPS results revealed that the high surface-concentrations of Mn4+,Co3+ and lattice oxygen on Co4Mn1-CMO were beneficial for catalytic performance.This work not only demonstrated the high catalytic potential of manganese oxides,but also showed that Ce doping and hydrotalcite derivation are effective strategies to improve the catalytic performance of manganese oxide.It provided new ideas for the design of manganese-based composite metal oxide catalysts for methane combustion.