Investigation On Synthesis Of Metal-doped Cu-SSZ-13 Catalysts And Their Catalytic Performance For NH₃-SCR

With the rapid development of the global economy in recent years,the problem of air pollution has become increasingly severe and the environment has become increasingly deteriorating,especially in emerging developing countries like China.With the gradual consumption of fuel resources and increase of environmental pollution,the use of large-scale diesel vehicles with excellent fuel economy,low CO2 emissions and strong power has gradually received attention.Diesel vehicles use lean combustion technology and the main pollutants in the exhaust gas are nitrogen oxides?NO.?and solid particles?PM?.Currently,the method for purifying NO.is NH3 selective catalytic reduction?NH3-SCR?.Among these catalysts,Cu-SSZ-13 catalyst has received extensive attention due to its excellent low temperature reactivity,excellent N2 selectivity and wide reaction temperature window.The country's motor vehicle emission regulations are becoming more and more stringent and the development of NH3-SCR catalysts with good low temperature activity,wide temperature window,excellent hydrothermal stability and resistance to sulfur is of great significance for improving urban air quality.In the present dissertation,the investigations were carried out in the following aspects:1.Firstly,the structure and NH3-SCR catalytic performance of Cu-SSZ-13 catalysts with different copper content synthesized by one-pot method were studied.The results showed that Cu3.25-SSZ-13 catalyst with the copper content of 3.25%exhibited excellent NH3-SCR catalytic performance.Its NO.conversion temperature window was 145?490 0C and the N2 selectivity was above 98%.There were three different isolated Cu2+ species in Cu-SSZ-13 catalysts:Cul,Cu2 and Cu3 species.They occupied the center of the octagonal ring structure near the eight-membered ring window,inside the supercage deviates from the double six-membered ring and the center of the hexagonal prism.Besides,Cu2 species had the highest content.The isolated Cu2+ species were the active sites of the NH3-SCR reaction.The Cu2+ species located in different positions in the SSZ-13 molecular sieve cage had different redox properties and stability.Thus,they had different effects on the catalytic performance of the NH3-SCR reaction.2.Based on the above results,by adding an appropriate amount of Fe complex?Ti complex?during the synthesis of Cu-SSZ-13 catalyst,the one-pot synthesis method was also used to synthesize different Fe?Ti?/Cu-SSZ-13 catalysts with different iron?titanium?content.The results showed that the NH3-SCR reaction temperature window of Fe0.63/Cui.50-SSZ-13 catalyst and Ti0.8i/Cu2.i5-SSZ-13 catalyst was significantly wider than Cu3.25-SSZ₁3 catalyst?145?490 0C?.Their active temperature windows were 160?580 ? and 140?540?,respectively.The doping of Fe or Ti significantly increased high temperature NH3-SCR reactivity of Cu-SSZ-13 catalysts.In addition,doping of proper content of Fe or Ti also improved the hydrothermal stability and resistance to sulfur to some extent.Especially Ti0.8i/Cu2.i5-SSZ-13 catalyst,the sulfur resistance was significantly better than Cu3.25-SSZ-13catalyst.If the content of Fe was too high,the structure of SSZ-13 molecular sieve would be destroyed,resulting in the loss of isolated Cu2+ species and the deterioration of NH3-SCR reactivity.The presence of mononuclear Fe3+ species in Fe/Cu-SSZ-13 catalyst was the main reason for the improvement in high temperature NH3-SCR reactivity.However,the doping of Ti species had little effect on the structure of Cu-SSZ-13 catalyst.The presence of mononuclear Ti4+ species was beneficial to the NH3-SCR activity at high temperature and the presence of TiOx species led to the high temperature reaction window of the catalyst narrower.Similar to Cu-SSZ-13 catalyst,the hydrothermal aging treatment would destroy the SSZ-13 molecular sieve structure to different extent and the active Cu2+,Fe3+ and Ti4+ species would migrate,agglomerate or form CuOx,oligomeric FexOy and TiOx species,which made the NH3-SCR reaction window of the catalyst narrower.The SO2 pretreatment did not affect the molecular sieve structure in the catalyst,but the active Cu2+ species also migrated,which made the low temperature NH3-SCR reaction activity of the catalyst decrease.3.In order to compare with the traditional synthesis method,we had synthesized Cu-SSZ-13 catalysts doped with different metals?Fe,Ti,Ce and Mn?by ion exchange method.The results showed that the doped-metal ions may be exchanged with the isolated Cu2+ species?Cul?near the eight-membered ring window.The disappearance of this Cu2+ species was the main reason for significant decrease in the low temperature activity of the metal-doped Cu-SSZ-13 catalyst.The NH3-SCR reaction window for Ti,Ce and Mn-doped Cu-SSZ-13 catalysts was narrower than Cu3.25-SSZ-13 catalyst except Fe0.86/Cu2.14-SSZ-13-IE catalyst?175?530 ??.Compared with the one-pot synthesis Fe-doped Fe/Cu-S SZ-13 catalyst,its hydrothermal stability and resistance to sulfur were significantly lower than the former.