The Research On Catalytic Oxidation Of Low-concentration Methane By Cerium-based Catalysts

Methane is the main component of fossil energy sources such as natural gas,coalbed methane and shale gas,and can be used as a fuel or feedstock for the preparation of high value-added chemicals.But methane is also a more active greenhouse gas than carbon dioxide,causing 20 to 25 times the greenhouse effect per unit volume.In the process of fossil fuel exploitation and use,a large amount of low-concentration methane will be formed.The molecular structure of methane is very stable,and the traditional direct combustion method consumes a lot of energy to deal with low concentration methane.Catalytic oxidation technology can significantly reduce the total oxidation temperature of methane,greatly save energy consumption,but also reduce the production of NOx,CO and other secondary pollution gases.In addition,the high value conversion of methane has attracted more and more attention,especially the catalytic oxidation of low concentration methane to methanol technology is known as a crown jewel in the field of energy conversion.Cerium oxide is a rare earth carrier commonly used in catalytic oxidation of methane with low concentration.Although the supported noble metal is expensive,and it is easy to agglomerate and sintering the active site in the process of use,which reduces the performance and life of the catalyst,the cerium based catalyst supported noble metal shows excellent catalytic activity at low temperature for the catalytic oxidation of methane.In view of the above difficulties,the cerium based catalysts with high dispersion,high stability and high activity were developed in this paper,and the reaction mechanism of catalytic oxidation of methane with low concentration was further studied.In addition,the application of cerium-based catalyst in the catalytic oxidation of low-concentration methane to methanol in the gas phase was investigated.The main research contents and conclusions are as follows:(1)In order to study the effect of catalyst surface properties on the catalytic lean methane oxidation activity of Pd/O-CeO2,this paper prepared an octa-hedral O-CeO2 carrier by hydrothermal method,and then loaded Pd as an active metal onto its surface by re-dox precipitation method to obtain Pd/O-CeO2 catalyst.The effects of crystal structure,particle size and oxidation state of Pd on the complete oxidation of methane catalyzed by Pd/O-CeO2 were investigated.The results showed that when Pd species were highly dispersed on the catalyst surface and the ratio of Pd0 to PdO was close,the catalyst showed the best catalytic activity of low concentration methane oxidation.In situ DRIFTS spectral results show that the Pd0 and PdO species in Pd/O-CeO2 catalysts can synergistically catalyze the complete oxidation of methane,in which Pd0 preferentially adsorbs and dissociates methane molecules,and the resulting intermediates are further oxidized by PdO.(2)Previous studies have shown that the activity of Pd/CeO2 catalyst depends on the high dispersity of Pd species.The one-dimensional structure of CeO2 is beneficial to improve the dispersion of metal on the catalyst surface,thus exposing more active sites.In this paper,MOF-derived one-dimensional Pd@CeO2 catalyst(Pd@CeO2-BDC)was prepared by assembly-pyrolysis method.It is found that Pd@CeO2-BDC surface is rich in oxygen vacancy and reactive oxygen species,and shows strong re-dox capacity.Due to the strong interaction between metals in the MOF skeleton,some Pd atoms replaced Ce4+and entered the CeO2 lattice,uniformly distributed on the surface of CeO2(110)in the form of PdO clusters or PdxCe1-xO2-σ,which significantly enhanced the metalsupport interaction.At a high space velocity of 60,000 mL·g-1h-1,the Pd@CeO2-BDC catalyst with 1 wt%Pd load achieved a methane conversion rate of 90%at 342 ℃.The structure-activity relationship of Pd@CeO2-BDC catalytic oxidation of low concentration methane is studied by using In situ DRIFTS and density functional theory(DFT).The results show that the energy barrier(0.39 eV)of the first C-H bond activated by PdxCe1-xO2-σ on the catalyst surface is much lower than that of Pd(0.83 eV)and Pd4(0.97 eV).(3)Obvious PdO clusters can be observed on the surface of MOF-derived onedimensional Pd@CeO2 catalyst,and due to the inherited MOF structure,the water resistance is poor.In order to improve the dispersibility and hydrothermal stability of the metal supported on the cerium catalyst surface,one-dimensional CeO2NWs support was prepared directly by hydrothermal method,and then organosilane was used to modify the surface of the support.The active species were dispersed by means of limited domain to reduce the size of precious metal palladium particles and improve the utilization rate of metal atoms.The highly dispersed Pd/CeO2NWs@SiO2 catalyst prepared by this method showed excellent catalytic performance of complete oxidation of methane with low concentration.Pd/CeO2NWs@SiO2 with Pd loading of 0.5wt%could achieve 90%methane conversion at 337℃,and showed excellent water resistance and high temperature stability.(4)The oxygen vacancy on the surface of cerium catalyst plays an important role in the catalytic oxidation of low concentration methane.Although the surface modification of CeO2 with organosilane can improve the dispersion of Pd,palladium species cannot be accurately anchored to the oxygen vacancy on the surface of CeO2.Therefore,in this paper,the surface environment of the carrier was reconstructed,the number of oxygen holes on the catalyst surface was increased by introducing the coordination unsaturated Al3+ penta site,and the Pd metal atom was anchored by the surface defect of CeO2 to prepare the highly dispersed and highly active Pd/RAl2O3-CeO2 catalyst.This defect anchoring strategy further improves the dispersion of metal supported on the catalyst surface and the catalytic complete oxidation of low concentration methane.At 60,000 mL·g-1h-1,Pd/RAl2O3-CeO2 catalyst with Pd loading of 1 wt%could achieve 90%methane conversion at 328℃,and showed excellent hydrothermal stability.(5)Compared with the complete oxidation of methane to CO2,direct selective catalytic oxidation of low concentration methane to methanol is a more promising methane utilization technology.In this paper,a CeO2-supported Cu-Ir two-site catalyst was prepared by solution-gel method.More-OH groups and surface adsorbed oxygen were exposed on Cu-Ir@CeO2 catalyst surface.Through the synergistic action of IrO2,CuO and CeO2 on the selective catalytic oxidation of methane,5Cu0.5Ir@CeO2 catalyst obtained 26.2 μmol/gcat methanol yield and 68%methanol selectivity at 550℃,and showed excellent cyclic stability.