Study On Preparation And Catalytic Performance Of Nanoscale Non-Noble Metal Oxides For Methane Combustion

Catalytic combustion of methane is of great practical significance for improving methane utilization and reducing the generation of atmospheric pollutants.Non-noble metal oxides,especially complex transition metal oxides,have higher activity and lower cost,and have better development prospects than noble metal catalysts.However,the low temperature catalytic activity and high temperature thermal stability of non-noble metal oxide catalysts are usually difficult to take into account at the same time.Therefore,the preparation of non-noble metal oxide catalysts with high activity and stability in a certain temperature range has important application and academic value.In this thesis,the effects of metal element ratio,quantity of synthesis auxiliaries,pH and calcination conditions on the physicochemical properties of the nanoscale perovskite-type catalyst La?Mn,Fe?O_3+?and its catalytic combustion performance of methane were studied.In addition,the effects of calcination temperature,volume fraction of CH₄?vol%?and weight hour space velocity?WHSV?on the catalytic combustion performance of the Co-based nanoscale complex metal oxide catalyst Fe₃Co₁₆O_x for methane were investigated.The catalysts were characterized by TG,XRD,XPS,BET,H₂-TPR,CH₄-TPD,O₂-TPD,SEM and TEM.The main results are as follows:?1?When the molar ratio of La to Mn and Fe with various oxidation states is 2:1:1,La?Mn,Fe?O_3+?with a perovskite structure can be formed.Compared with urea combustion method and glucose sol-gel method,the perovskite type nanocatalyst La?Mn,Fe?O_3+?prepared by citric acid?CA?method has a relative higher catalytic activity for methane combustion.When CA/M=1:2,EG/M=1:2 and no pH adjustment is performed,the perovskite catalyst exhibits the best catalytic methane combustion activity.?2?The effects of different calcination conditions?heating rate,time and temperature?on the catalytic performance of the perovskite-type catalyst La?Mn,Fe?O_3+?were studied.The results show that with the increase of the calcination temperature,the methane catalytic combustion performance shows a"volcanic"trend,and the optimum calcination temperature is⁶00°C.The calcination rate and duration have little effect on the catalytic performance,and in this work are set at 3°C/min and 3 h,respectively.The perovskite catalyst La?Mn,Fe?O_3+?calcined at 600°C possesses the characteristics of small average particle size?17nm?,high specific surface area?29.2 m²/g?,high Mn⁴⁺/Mn³⁺?1.46?,high Fe³⁺/Fe_total?0.55?and high O_abs/O_lat?0.74?,and thus exhibits excellent catalytic combustion activity of methane,with T₅₀0 and T₉₀0 at 439°C and 493°C,respectively.The catalytic activity remains substantially unchanged after 8 cycles,and can be maintained at 550°C for more than 500 h.?3?The catalytic properties and structural characteristics of the complex oxide catalyst Fe₃Co₁₆O_x calcined at different temperatures were compared.The results show that the catalytic combustion activity of methane shows a"volcanic"trend with the increase of the calcination temperature.When the calcination rate and time were the same?3°C/min and 4 h respectively?,the catalyst calcined at 230°C exhibited higher methane catalytic combustion activity(216°C,286°C and 345°C,respectively,for T₁₀,T₅₀0 and T₉₀).The test conditions were as follows:volume content of CH₄ was 1 vol%,WHSV=15000 mL/gcat.h.The apparent activation energies?E_a?of the catalyst calcined at 230,250 and 350°C were 49.4kJ/mol,60.3 kJ/mol and 85.4 kJ/mol,respectively.The catalyst calcined at 230°C has typical mesoporous structure and small particle size?the presence of polymer membranes inhibits the growth of nanoparticles?.?4?The effects of CH₄ volume fraction and WHSV on the methane catalytic combustion performance of Fe₃Co₁₆O_x were studied.When the volume fraction of CH₄ is 0.2 vol%and WHSV=10000 mL/gcat.h,the catalytic combustion activity is the best,and T₉₀0 is reduced to315°C.The excellent methane catalytic combustion performance of the catalyst is mainly attributed to its small particle size?8.1nm?,high specific surface area?171.8 m²/g?,high Co³⁺/Co²⁺ratio?2.20?,high oxygen vacancies?21.3%?and rich reactive oxygen species on its surface.The catalyst can completely convert CH₄ to CO₂ at 400°C and run steadily for more than 160 hours.The activity of the catalyst can still be maintained at not less than 80%and the selectivity of CO₂ can be maintained at 100%throughout the reaction.The catalytic activity of methane remained basically unchanged after repeating four cycles.