| Nitrous oxide(N2O)is not only a greenhouse gas with high warming potential,but also a long-lasting consumer of ozone in the stratosphere.At present,direct catalytic N2O decomposition is the most promising method in the reported methods for eliminating N2O from the industrial exhausts.However,the temperature is too low for some industrial exhausts containing N2O,for instance,the exhaust of nitrate manufacture,to support the catalysts to exert their catalytic activity;some exhausts contain the impurity gases,such as CO2,H2O,O2,SO2,for instance,the automobile exhaust,which seriously suppress the activity,leading to the catalysts hardly working.Hence,this thesis focused on developing the high active catalysts with strong tolerance to the impurity gases at lower temperature.The primary contents and corresponding results are as follows:(1)The catalyst of Co3O4 modified by PbIt was found that Pb is a much effective promoter to Co3O4 for improving the activity of the catalyst.The Pb0.04Co exhibited outstanding catalytic activity in the PbxCo catalysts(x = 0-0.2)prepared by coprecipitation for the N2O decomposition.It was found that the introduction of Pb into the catalyst not only greatly decreased the average size of Co304 crystallites,increased the surface area and the population of active sites of the catalyst,but also significantly weakened the Co-O bond.As a result,it made the activation energy decrease from 74.0 to 28.9 kJ·mol-1 and TOF of the reaction at 300? increase from 3.1×10-3 s-1 to 23.1×10-3 s-1.Because of the promotion effect of Pb on the textural structure and surface properties of the catalyst,the Pb0.04Co catalyst could completely decompose the N2O in 2000 ppmv N2O/Ar in the conditions of 350 ? and 20000 h-1.Even for 5 vol%O2 2 vol%H20,500 ppmv NO,10 vol%CO2,50 ppmv SO2,or some of them existing in the feed gas stream,the Pb0.04Co catalyst also exhibited excellent catalytic stability.In the case of NO,O2 and H2O coexisting in the feed gas stream,the N2O conversion over the Pb0.04Co catalyst could be stably maintained at?60%at 350 ?and 20000 h-1.(2)Ag improving the activity of Co3O4It was found that Ag being added to Co3O4 by the coprecipitation method has the same function as that of Pb.Different from the Pb,Ag improved the activity of Co3O4 mainly by weakening the Co-0 bond and decreasing activation energy,though it also had a less role of decreasing the average size of the Co3O4 crystallites.In the AgxCo(x = 0-0.08)catalysts,the largest N2O conversion was given by the Ag0.04Co.Compared with K0.01Co that displayed the highest activity among the catalysts of Co3O4 doped by K,the Ag0.04Co catalyst exhibited better tolerance to 5 vol%O2,2 vol%H2O and 100 ppmv NO.In the presence of O2,H2O and NO in the feed gas mixture,the N2O conversion over the Ag0.04Co catalyst could be maintained at about 80%when temperature was 400 ? and GHSV = 20000 h-1.Moreover,Ag0.04Co catalyst exhibited strong resistance to So2 and Co2 as well.Under the conditions of the two impurity gases as well as o2 coexisting in the feed gas mixture,the N2o conversion over the Ag0.04Co catalyst achieved 100%at 400 ?.(3)CO3O4/CaCO3 and Co3O4/BaCO3 catalystsTo obtain the catalysts with the advantages of having both high activity and low cost,for Co3O4/CaCO3 and Co3O4/BaCO3 catalysts,the relationship between their preparation(methods/conditions)and structure,and that between their structure and activity were studied.The CaCo2.5 catalyst prepared by stepwise precipitation was much active than those prepared by impregnation or coprecipitation.The stepwise precipitation led to small Co3O4 crystallites tightly bound to the CaCO3 particles,which endues the Co3O4 not only an ideal accessibility to the reactant,but also better interaction with the CaCO3 support.Just due to the special structure,the CaCo2.5 catalyst was much more active than the catalysts prepared in the traditional methods.Under the reaction conditions of 300 ? and 20000 h-1,the CaCo2.5 catalyst made the N2O in the feed gas 2000 ppmv N2O/Ar completely converted.Moreover,the catalyst exhibited better resistance to sintering at 800 ?.The core-shell type of Co0.24Ba catalyst prepared by the adsorption-deposition has the structure that Co mainly exists in the outer layer of the catalyst particles,forming a Co-rich porous shell doped by Ba.It gave much larger specific reaction rates in unit mass of Co3O4 and in unit mass of catalyst for the N2O decomposition than the catalysts reported in the literature.(4)Apparent activation energies and reaction rates of N2O decomposition over Co3O4 via L-H and E-R routes were calculated for the first time,with the account of active sites on the catalysts increasing with reaction temperature.It was found that the apparent activation energy was 32.9 kJ/mol for the L-H route and 44.8 kJ/mol for the E-R route,while the order of reaction with respect to O2 was-0.367 at 350 ? and-0.338 at 450 ?.The contribution of the E-R route to the total reaction rate was found to be considerably increased with the reaction temperature and which could be well interpreted by the apparent activation energies of the two reaction routes. |
PDF Full Text Request