Study On Perovskite Monolithic Catalysts For Catalytic Lean Burn Of Methane

Recovery of energy by catalytic combustion of the huge amount of air diluted methane can get rid of the strong green house effect,meanwhile avoid the NO_x pollution accompanied by flame combustion at a temperature over 1300℃.The La Fe O₃ pervoskites are medium active,cheap in price and largely available catalysts for combustion of dilute methane,but the activity and thermal stability have to be improved for real commercial applications.In this thesis,the coprecipitation method was modified to make the La_0.8Ca_0.2Fe O₃（LCF）powder catalyst with high activity and better resistance to sintering.The spinel Mg Al₂O₄（Mg Al）and LCF powder were successively washcoated on surface of the alumina ceramic honeycomb substrates to prepare the Mg Al stabilized LCF honeycombs.The proper thinkness and calcination temperature of Mg Al washcoat were optimized to find that the best activity was obtained when the 40μm thick Mg Al washcoat calcined at 850℃was applied.The activity and thermal stability of LCF honeycomb catalysts are greatly improved once supported on the Mg Al washcoat,in comparison to that supported on Mg O washcoat or on bare honeycomb substrate.SEM analysis has revealed that there is no boundary between the Mg Al and LCF washcoat and the LCF particles have penetrated into the interstice of the Mg Al washcoat and distributed evenly on surface of the Mg Al slices.The change of activiation energy Ea with the mass ratio of LCF/Mg AL suggested the existence of interaction between LCF and Mg Al.To probe the mechanism of interaction between LCF and Mg Al,the LCF/4Mg Al composite powder with the molar ratio of LCF to Mg Al as1:4 was prepared by deposition-precipitation method.By comparion of the phases,crystalline size and morphology,redox properties,surface species and activity of the LCF and LCF/4Mg Al composite powder,it is concluded that the chemical environment is greatly modified once LCF is supported on Mg Al surface which is responsible for the improvement in activity and stability.Therefore,Mg Al is a chemical modifier rather than a mere texture stabilizer of LCF.At last,the mass process technique of the wire-mesh honeycomb substrate was developed for purpose to substitute the traditional ceramic honeycomb.The prelimilary investigation proved that the wire-mesh honeycomb substrate can raise the catalysis efficiency up to 25%under the similar reaction conditions,meanwhile be less limited by catalyst loadings as in ceramic honeycomb.