Research On The Catalytic Oxidation Of NO By Mn-Ca-O_x-?CO₃?_y Catalysts

More and more attention had paid to the emission of nitrous oxides all over the world.Coal-fired power-generation technology is the traditional power supply mode in China.SCR(Selective catalytic reduction)technology is widely used in the denitrification of coal-fired power-generation technology,but some problems still exist in SCR such as ammonia escape which lead to the formation of PM2 5.China's Ministry of Environmental Protection had published list of ammonia source which includes SCR technology.In addition,because of high reaction temperature needed for catalytic reaction,the coal-fired exhaust untreated which containing dust could shorten the life of catalyst.How to develop a technique for denitrification with low-cost,non-pollution and efficiently is a hot topic.SCO(Selective catalytic oxidation)is a competitive technology for the removal of NOx.Most of the NOx in coal-fired flue gas is NO(>90%),which is difficult to be absorbed by water.If the oxidation ratio of NOx(i.e.,NO/NO2 value)can achieve a suitable ratio(50%-60%),NOx can be almost entirely absorbed by water or an alkaline solution with SO2,no other chemical needed compared to SCR.This research is aimed to enhance MnOx(manganese oxide)catalyst's activity of NO oxidation through low-cost calcium doping.Research work had been divided into three parts:the preparation of Mn-Ca-Ox-(CO3)y catalyst,catalytic activity measurement;influence of H2O/SO2;the optimization of reaction conditions,reaction mechanism and inhibition mechanism study.The detailed results are as follow:(l)Preparation of Mn-Ca-Ox-(CO3)y catalysts and catalytic activity measurement Different conditions of manganese precursors,calcium precursors,the ratio of Mn to Ca and calcination temperatures were studied.When manganese acetate,calcium nitrate was selected as the precursor,Mn/Ca=10/1,350? calcination for 2h in air,the catalyst showed high activity.More than 50%NO conversion was achieved at 180 ?,which meets the criteria for a suitable NO2/NOx ratio(50%-60%)for SCO.The best NO conversion was 79.4%at 270 ?.XRD,XPS,FTIR,BET,H2-TPR were used for characterizations of catalysts.Making a comprehensive analysis of the results of activity test and characterizations,we get the following conclusions:The NO oxidizability of the catalysts was promoted by the addition of calcium in low-temperature regions(<240 ?),but no increase in high-temperature regions.This is because Ca doping promoted more MnO2 generated which is favourable for the catalytic oxidation of NO.Ca existed as CaO or CaCO3.The addition of calcium improved the surface area of the catalyst and changed the pore structure of the catalysts.All above was favourable for the catalytic oxidation of NO.(2)Influence of H2O/SO2 on selective catalytic oxidation of NO over Mn10Ca1Ox(CO3)y catalystGases containing different concentration of SO2,H2O vapour were uesd to test in the NO catalytic oxidation reaction to research the Mn10Ca1Ox(CO3)y catalyst's resistance of poisoning.The tests was divided into three parts(only SO2,only H2O vapour and SO2 with H2O vapour).It indicated that the influence of H2O vapour on the NO oxidation reaction was tiny.SO2 had an irreversible poisoning effect for Mn10Ca1Ox(CO3)y no matter whether coexist with H2O vapour or not.MnS04 was generated on the catalyst's surface in the condition of SO2 introducing.(3)The optimizing of reaction conditions,mechanism study of NO oxidationand poisoning reactionFirstly,the reaction conditions were optimized.The results showed that when the O2 content was 5%,NO concentration was 938 mg/m3,reaction temperature at 270 0C,the NO conversion was highest.And the NO conversion was decreased with the increase of space velocity.The DRIFTS results indicated that the nitrato was generated more easily on Mn10Ca1Ox(CO3)y at low temperature,which led to more NO was oxidized to NO2.This phenomenon is due to the following reasons:Calcium doping made more surface area produced which was favorable for the adsorption of NO and O2.More MnO2 contained in Mn10Ca1Ox(Co3)y-350 ? made more NO oxidized.So sum up above were the reason why the Mn-Ca-Ox-(CO3)y catalyst exhibited high activity at a low temperature compared to MnOx.The participation of O2 had no influence on the SO2 poisoning process,SO2 reacted with the catalyst directly,MnSO4 produced.H2O vapour promoted lots of nitro produced,which is detrimental to NO oxidation.