| With the breakthrough in the mining technology of shale gas,natural gas is expected to substitute coal and become the second largest fossil resources.As a kind of clean and affluent resource,natural gas can be used as fuel to provide heat and electricity.Besides,natural gas not only can substitute petroleum as vehicle fuel but also can substitute coal as C1 chemical feedstock for syngas to reduce the emission of NOx,dust and CO2.At present,there are two main problems in these two applications of natural gas:1)the residual low-level CH4 and CO in the natural gas engine exhaust must be removed by catalytic combustion;2)a more economic and environment friendly route should be developed to produce syngas from methane.The combustion of CH4 and CO and dry reforming of methane are the key reactions for solving the above problems.While,the hot spots produced during combustion and the long-time high-temperature reaction in reforming of methane make the catalysts are easy to sinter and deactivate,which are the common scientific problems for these processes.The development of highly active and thermally-stable catalysts is thus the most challenging task in this field.Considering the requirements of the catalysts on the thermal stability and activity in the high-temperature reaction process,we enhanced the interaction between precious metals and oxide supports and the thermal stability of catalysts through rationally selecting and designing on the composition and morphology of the metals and supports and constructing the epitaxial interface structure between precious metals and oxide supports.Furthermore,the catalytic activities of the catalysts for different reactions were promoted by optimizing the compositions of precious metals and oxide additives.Finally,we have prepared highly active and long-lived catalysts for the combustion of CH4 and CO and the dry reforming of methane to syngas,and studied the stabilization mechanisms as well.The main contents are as following:(1)Effect of the morphology of mesoporous ceria as catalyst directly or catalyst support on the catalytic combustion of CH4 and CO.With a facile solvothermal method,we prepared mesoporous ceria in hollow naonocone,nanosheet and nanofiber shapes by controlling the factors such as surfactant,precipitator,oxidizing agent and solvothermal reaction times.It is found that all of these mesoporous ceria with special morphology are more active than the commercial ceria for the combustion of CO,in which the ceria with hollow naonocone shows the lowest light-off temperature(T50)as about 200 ℃.The transition-metal ions(Cu、Mn、Co、Ni)can be doped into mesoporous ceria with hollow nanocone shape during the preparation.Thus prepared Cu doped sample exhibited higher catalytic activity for CO combustion whose light-off temperature decreased to 127 ℃,presumbly because of the excellent redox performance.For the CH4 combustion reaction,the ligh-off temperatures of Pd catalysts supported on different mesoporous ceria are all between 550 and 580 ℃,which are even less active than the commercial ceria supported Pd catalyst.The reason may lie in the readily collapse of the special morphological structures at high temperatures which leads to the quick sintering of Pd.(2)Effect of thermally-stable Pt(Pd)/MgAl2O4 and ceria additives on the catalytic combustion of CH4 and CO.Based on the excellent stabilizing effects of MgAl2O4 on Pt and Pd,we prepared MgAl2O4 supported Pt and PtPd catalysts(metal loading:lwt.%)using incipient-wetness impregnation method.Moveover,the ceria additives were introduced into above catalysts using the same method.After aging at 800 ℃ in air for 7 days,comparing with the traditional Pt/Al2O3 catalyst in which Pt was seriously sintered,Pt/MgAl2O4 and Pt-Ce/MgAl2O4 can partially and totally retain the small precious metal nanoparticles(2-3 nm)respectively.Specially,for the Pt-Ce/MgAl2O4 catalyst,Pt can be stabilized as single atomic species with the help of ceria! Accordingly,Pt/MgAl2O4 and Pt-Ce/MgAl2O4 still kept high catalytic acitivies for CO combustion at low temperatures after aging whose light-off temperatures for CO combustion respectively are 190 ℃ and 160 ℃ with the GHSV as high as 500000 mL/g-s,exhibiting the excellent catalytic properties of highly dispersed precious metals.However,when these thermally-stable and highly dispersed Pt catalysts were applied for CH4 combustion(GHSV = 50000 mL/g-s),the light-off temperatures are 645 ℃ and 526 ℃ respectively,which are far above the light-off temperatures of conventional Pt catalysts with large sizes,implying that small-sized Pt is not good for CH4 combustion.The light-off temperature of aged bimetallic Pt-Pd/MgAl2O4 for CH4 combustion decreased to 495 ℃,while that of(Pt-Pd)-Ce/MgAl2O4 further decreased to 475 ℃.A highly active catalyst for CH4 combustion requires the precious metal nanoparticles in a relatively larger size.(3)Highly dispersed and thermally-stable Ru/MgAl2O4 of low Ru loading for the dry reforming of methane.Based on the superior ability of MgAl2P4 on stabilizing Ru particles,we prepared atomically dispersed Ru/MgAl2O4 catalyst with a newly developed physical vapor deposition(PVD)method by taking the advantage of the high volatility of ruthenium oxide.The mean size of Ru nanoparticles is about 1.1 nm after reduction with H2,totally avoiding the formation of large Ru nanoparticles(>100 nm)as happened during the traditional incipient-wetness impregnation process.Under reducing atmosphere of syngas produced by dry reforming of methane,metallic Ru will not volatilize and and leach any more.At 850 ℃,the reaction rate of the Ru/MgAl2O4(Ru loading:0.15 wt.%)prepared by PVD method was 279.50 mol/molRu-s,which is 10 times that of the Ru/MgAl2O4(Ru loading:1 wt.%,28.61 mol/molRu-s))prepared via incipient-wetness impregnation method and 457 times that of the highly active Ni based catalyst NiCoMg/Al2O3(0.61 mol/molNi-s)reported in the literature.At 850 ℃ and GHSV = 400000 mL/g-h with CH4:CO2 =1,this catalyst exhibit excellent catalytic stability and resistance to carbon deposition.In a stability test of 600 h,the conversions of CH4 and CO(96.7%and 98.6%)both reached the thermodynamic equilibrium conversions and the H2/CO ratio is close to the theoretical value of 1.After stability experiment,the average size of Ru nanoparticles increased to 2.7 nm.Instead of decrease,the reaction rate increased to 1.5 times that of the initial rate and no coke can be detected on the catalyst.On the contrary,Ru/MgAl2O4(Ru loading:1 wt.%)prepared by incipient-wetness impregnation method deactivated quichly with a large amount of coke under the same condition within 10 h.With decreasing the Ru loading to 0.07 wt.%,smaller Ru particles can be obtained while the reaction rate decreased to 35.72 mol/molRu-s.These results indicate that increasing the size of Ru nanoparticles can significantly increase the apparent activity at least within 2.7 nm,and in the meanwhile,no coke produced on the catalyst.The large Ru particles of Ru/MgAl2O4 prepared by incipient-wetness impregnation method may be the major reason for carbon deposition.Such low Ru loading,highly active and long-lived Riu/MgAl2O4 catalyst has a significant potential in industrial application.In summary,considering the requirements of the catalysts used for the high-temperature reaction processes on the thermal stability and activity,we have successfully prepared MgAl2O4 spinel supported Pt,Pd and Ru catalysts in which the metals are stabilized at nanometer,sub-nanometer and even single atomic scale.Furthermore,we have demonstrated the significant promotion effect of ceria on the activity by taking the advantage of its high redox property.By studying the catalytic behaviors of the above catalysts for CH4 and CO combustion and dry reforming of methane,we found that the larger precious metal nanoparticles have better apparent activity among the stabilized precious metal species in different sizes.Thus,it is promising to enhance the catalytic activity by properly increasing the mean size of stabilized precious metal particles,especially for the catalytic combustion of CO and CH4 at low temperature. |
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