Study On LaMnO₃-based Catalyst For Ventilation Air Methane Oxidation

According to the principle of“draining first and then mining”for coal-mining safety,a large amount of ventilation air methane(VAM)is generated during coal-mining process.Although the methane concentration is very low,the amount of ventilation air is huge.Emission of VAM into the air will cause serious greenhouse effect and energy waste.Therefore,rational use of VAM can reduce methane emission and protect the ecological environment.At present,the cyclic reverse flow oxidation technology has been successfully used for the heat utilization of VAM.However,there are many shortcomings for this technology,such as temperature for methane oxidation,high energy consumption,low efficiency and secondary pollution.Catalytic combustion technology is one of the effective methods to deal with VAM.It has been widely studied because of its low ignition temperature,no secondary pollution and good environmental benefits.The key to this technology is to develop a catalyst with high activity and excellent stability for methane oxidation.Perovskite-type oxides are a type of catalysts that has been studied extensively at present.However,the methane oxidation temperature is still high,the low-temperature catalytic efficiency is low.Doping the precious metal Pd in perovskite can improve the dispersivity of Pd and improve its catalytic performance.In this paper,a Pd-doped La Mn O₃-based perovskite material was prepared.The surface modification was carried out by the method of acid etching.The effect of the catalyst before and after the surface modification on the catalytic performance of methane oxidation was systematically studied.First,La_0.8Sr_0.2Mn_0.95Pd_0.05O₃(LSMP)and LSMP+D catalysts with different porosity diameters were prepared by the sol-gel method.After etched with 5%HNO₃for 1 hour,LSMP-H and LSMP(D)-H were obtained.Using l vol%CH₄-99 vol%air mixture to simulate VAM,the catalytic activity of different catalysts was studied under the condition of space velocity of 30000 m L·(g^-1·h^-1).The results show that the catalyst LSMP(D)-H with more pores has the best catalytic activity.With T_50%of395?,and T_90%of 502?,which is 160? and 196? lower than LSMP(T_50%is 555?,T_90%is 698?).The effects of different space velocities,different methane concentrations and different mass fractions of nitric acid on the catalytic activity were also studied.It was found that with the space velocity increased,the methane conversion rate decreases.Even though a high space velocity of 75000 m L·(g^-1·h^-1)at 550?,the CH₄conversion rate still reached 80.4%.As the methane concentration and the mass fraction of nitric acid increase,the methane conversion rate also gradually decreases.The catalyst maintained a high CH₄conversion rate in the 50-hour stability test.Different materials were characterized and analyzed using XRD,BET,SEM,XPS,H₂-TPR and O₂-TPD.The results show that the A-site cations in the perovskite structure can be partially removed by acid etching,resulting in more exterior defects is formed,and with the original perovskite framework retained,thus a material with high specific surface and high surface defects is formed,which is beneficial to the higher catalytic performance.On the basis of the above research,the catalyst LSMP(D)-H was selected,to be molded in large scale.The perovskite loaded on the wire mesh substrate to prepare the monolithic catalyst.The whole molding procedure of the catalyst was introduced in detail,which lays the foundation for the industrial application of the perovskite monolithic catalyst in the catalytic combustion of VAM.