Denitrification Performance And Reaction Mechanism Study Of Ce-M-Sb Catalysts

At present,WO₃ or MoO₃ modified V₂O₅/TiO₂ catalyst is widely used to remove NO_x from the coal power plants,but there are some shortcomings: easy to sublimate and drop,denitrification temperature window is relatively narrow?300⁴00 oC?,large amounts of N2 O was produced at higher temperature,and weaker thermal stability.Therefore,the medium-low temperature denitrification catalyst with high efficiency,low pollution,high resistance to SO₂ and H2 O,and easy to popularized are urgently needed.Compared with the traditional V-W catalyst series,Ce-Sb-based catalysts have the advantages such as the better thermal stability,surface acidity,reducibility,oxygen storage capacity,and the synergistic effect between different metal components.In addition,the high conductivity of Sb has promotes effect to the acidity and reducibility of the catalyst.All these advantages led to an excellent denitrification performance.Therefore,the denitrification performance and surface structure characteristics of Ce-M-Sb O_x?M=W,Nb,Ta?catalyst were investigated,and the denitrification mechanism was revealed.The impact of surface active metal,active oxygen,acidity and the reduction degree on denitrification process were studied;Meanwhile,the denitrification performance of Ce-M-SbO_x supported on different carriers?mesoporous TiO₂ and self-developed low-cost non pitched activated carbon?NPAC??was studied,the surface structural characteristics and denitrification kinetics of Ce-W-SbO_x/meso-TiO₂ were studied.The impact of different pyrolysis atmosphere to the surface properties of non-asphalt based coal activated coke,and the denitrification catalyst performance of Ce₃W₂SbO_x/NPAC?CO₂?were studied.The main conclusions are as follows:1.The Ce-SbO_x Catalyst prepared by coprecipitation method: when the proportion of Ce:Sb was 3:1,the maximum deNO_x efficiency could reached 82%,however,the deNO_x temperature window is narrow?300⁴00 oC?.The deNO_x efficiency?>95%?and temperature window?200-400 oC?were promoted efficiently by adding W,Nb and Ta.The optimum preparation conditions and methods of Ce-M-SbO_x were as follows : Ce:W:Sb=3:2:1?Coprecipitation method,Calcinated at 500 oC?,Ce:Nb:Sb=3:3:1?Hydrothermal method,Calcinated at 350 oC?,Ce:Ta:Sb=3:3:1?Homogeneous precipitation method,Calcinated at 350 oC?.2.The effect of operational condition on the denitrification performance of Ce-M-Sb O_x catalyst were studied.The results indicated that when the ratio of n?NH3?/n?NO?<1,the deNO_x efficency is increase obviously with the ratio increasing,and the deNO_x efficency is almost constant when the ratio of n?NH3?/n?NO?>1;The denitrification temperature window is become wider with the decrease of gas velocity,when the temperature is lower than 250 oC,the deNO_x efficiency was greatly affected by the gas velocity.the deNO_x efficiency was little affected by the gas external diffusion when temperature is more than 250 oC;It exists a lot of active lattice oxygen and adsorbed oxygen on the surface of Ce-M-SbO_x catalyst,which can promote the NH3-SCR process,and the deNO_x efficiency is more than 75% when there is no oxygen exist in the flue gas.With the increasing concentration of oxygen,the deNO_x efficiency increased to more than 95% rapidly and remained constant;When the particle size of Ce₃W₂SbO_x?CP?,Ce3Nb3 Sb O_x?HT?and Ce3Ta3SbO_x?HP?is lower than 0.45 mm,the deNO_x efficiency was little affected by the gas interdiffusion;Ce-M-Sb O_x catalysts showed a better thermal stability and resistance to water and sulfur,when 150 ppm SO₂+5% H2O?g?was pumped into the NH3-SCR system,the stability deNO_x efficiency is almost 75%.3.The deNO_x efficiency and N2 selectivity of Ce₃W₂SbO_x?CP?,Ce3Nb3SbO_x?HT?and Ce3Ta3SbO_x?HP?catalyst were all higher than Ce3 SbO_x and Ce3 MO_x,which prepared by the same method.The Ce-M-SbO_x catalyst possesses the smallest lattice size and the largest specific surface area due to the inhibition crystallization role of CeO₂ between M,Sb and Ce components.The Ce-M-Sb O_x catalyst has a better reduction performance,the conversion process of Ce4+ ?Ce3+ was promoted by the redox cycle of interior metal components,and this conversion in turn promoted the production of reactive lattice oxygen and oxygen vacancy.The adsorption and activation process of NH3 and NO were promoted by the synergistic effect between Ce,M and Sb,therefore,the deNO_x performance of Ce-M-SbO_x is superior to that of Ce-MO_x and Ce-Sb O_x.4.Both the NH4+-Br?nsted and NH3-Lewis species are involved in the NH3-SCR process.Due to the bidentate and bridging nitrates are more actively,they all involved in the NH3-SCR process,while the monodentate species is relative stable.On the surface of Ce₃W₂SbO_x?CP?and Ce3Ta3SbO_x?HP?catalyst,when the temperature is below 300 oC,the bridging nitrate and bidentate nitrate could conduct NH3-SCR reaction with the adsorbed NH4+-Br?nsted species following the Langmuir-Hinshelwood mechanism.When the temperature is above 300 oC,it will conduct NH3-SCR reaction following the Eley-Rideal mechanism.However,on the surface of Ce3Nb3SbO_x?HT?catalyst,when the temperature is in 200⁴00 oC,the bridging nitrate and bidentate nitrate could conduct NH3-SCR reaction with the adsorbed NH3-Lewis species following the Langmuir-Hinshelwood mechanism.5.Compared with Ce-M-SbO_x catalysts,the mesoporous Ce-M-SbO_x/Ti O₂?M= W,Ta,Nb?catalyst prepared by sol-gel method shows superior deNO_x performance,wider activity temperature window and the higher thermal stability;The deNO_x performance of Ce₃W₂SbO_x/TiO₂ is the best among these catalysts,which has the largest specific surface area,the minimum pore diameter and a developed pore structure.And the metal active component on the catalyst surface is highly dispersed,at the same time,the percentage of Ce and O is more than the other two kinds of catalysts.6.The kinetic model of NH3-SCR deNO_x reaction over Ce₃W₂SbO_x/TiO₂ catalyst were established under the condition: 180²80 oC,O₂? 5%,NH3/NO<500 ppm,0.30⁰.45 mm particle size,and 250,000 h-1 gas velocity.The average reaction order of O₂,NO and NH3 are 0.33,0.94 and 0.47,the activation energy of Ce₃W₂SbO_x/TiO₂ catalyst is 29 KJ/mol,which indicate that it has a better activity.7.The surface of the non pitched activated carbon?NPAC?prepared in CO₂ atmosphere possess abundant functional groups such as pyridine nitrogen?N-6?,pyrrole nitrogen?N-5?,C=O and C-O.The alkaline functional groups of N-6 etal and the acid functional groups of C=O etal could improve the adsorption or oxidation of NO.NPAC prepared in CO₂ atmosphere has the superior microporous structure and the excellent compressive mechanical strength,which was activated by H2 O steam and loaded by Ce₃W₂ SbO_x,ultimately obtained the Ce₃W₂SbO_x/NPAC?CO₂?.At the temperature of 140 oC,the deNO_x efficiency is 100% at the early stage of 0²0 min,then it gradually decreased to 75% and keep stable at the second stage of 20⁶0 min,the ablation rate for the catalyst is relatively lower?2.1%?;When the temperature is above 200 oC,the deNO_x efficiency is relatively higher due to the synergistic effect of the selective catalytic reduction of metal active component and the adsorption of surface functional group;At the temperature of 240 oC,the deNO_x efficiency increased from 76% to 98% in the early stage of 0⁴0min,then it keeps stable,the ablation rate for the catalyst is relatively higher?19.2%?.