Palladium-supported Hydrotalcites Catalysts For Lean Burn NO_x Storage And Reduction

Lean-burn technology is presented as a kind of very effective internal purification technology in recent decades.Lean-burn engine has the characteristics of good fuel economy and high combustion efficiency,but high air-fuel ratio release more NO_x and it is not easy to remove,NO_x has become the main pollutants in the motor vehicle exhaust.NO_x storage and reduction?NSR?technology is considered to be the one of the most effective ways to remove the NO_x,high-activity NSR catalyst is the key for exhaust purification in the NSR technology.Traditional NSR catalysts Pt-BaO/Al2O3 cost high and have poorer water/sulfur resistance and thermal stability.It is necessary to develop a economic and efficient NSR catalyst.In this paper,NO_x storage and reduction catalysts based on Pt and K supported on CuMgAlFe mixed oxides obtained from hydrotalcite-like type compounds were prepared.The crystal structure,surface properties,microtopography,pore structure characteristics and redox properties of catalysts were characterized by XRD,FT-IR,SEM,BET,H₂-TPR techniques.NO_x storage capability was investigated by NO_x adsorption/desorption and lean-rich burn cycle experiment.Further more,the reaction mechanism of NO_x storage was characterized in detailed by the situ FT-IT spectroscopy.The main works and findings are as following.Mesoporous mixed oxide catalysts?CuMgAl,MgAlFe and CuMgAlFe?with high specific surface area(160¹80 m²·g^-1)were prepared by co-precipitation method.CuMgAl and CuMgAlFe performed higher redox properties than MgAl Fe.The strong interaction between Cu and Fe species improved the NO_x storage capacity of CuMgAl Fe?216 ?mol·g-1?.NO_x stored in the form of nitrate and nitrite on catalysts.There were two kinds of NO_x storage paths on CuMgAlFe: ?nitrite path? and ?nitrate? path.?Nitrite path?: NO react alkaline oxygen of Mg?Al?O carrier generate nitrites,then nitrites were further oxidized to nitrates.?Nitrate path?: NO was oxidized to NO₂ in transition metal active sites,then NO₂ overflow to the nearby Mg site to form nitrates.Mesoporous mixed oxide catalysts?CuMgAl,PdMgAl and PdCuMgAl?that loaded noble metal Pd were prepared by co-precipitation method.Pd and Cu doped catalysts performed high pore volume and specific surface area(160¹90 m²·g^-1)compare with MgO sample,this feature contributes to the diffusion of gas molecular during the reaction and the dispersion of PdO and CuO species.The interaction between Pd and Cu species improved the redox properties of PdCuMgAl.The high NO_x storage capacity of Pd doped catalyst may be due to the improvement of the pore structure characteristics and redox properties of catalyst.The in situ FTIR spectrum showed that all NO_x stored in the form of nitrite on the MgAl at 300 oC while nitrate was dominant on the PdMgAl and PdCuMgAl.On the PdMgAl,NO_x stored follow the nitrite path while on the PdCuMgAl,NO_x stored follow the nitrate path.It showed that the interaction of Pd and Cu improved the redox properties and NO_x storage capability of catalyst.A series of Pd and K supported new NSR catalysts?Pd/CuMgAlFe,K/CuMgAlFe,Pd-K/CuMgAlFe?were prepared by impregnation of CuMgAl Fe supports obtained from hydrotalcite-like type precursor compounds calcined at 600 oC.The study found that the interaction between Pd and K species improved the redox properties of Pd-K/CuMgAl.K loaded catalyst showed high NO_x storage capacity.Particularly,the high NO_x storage capacity?1766 ?mol·g-1?of Pd-K/CuMgAl may be due to the improvement of the redox properties and alkali of catalyst.The in situ FTIR spectrum showed that NO_x stored in the form of nitrite and nitrate on the K/CuMgAl Fe while nitrate on the other catalyst.In addition,the conversion rate of nitrite to nitrate on Pd-K/CuMgAlFe was slower than the Pd/CuMgAlFe,this feature can contribute to the inhibiting effect of K to Pd.Lern-rich burn cycle experiment showed that the NO_x removal rate of K/CuMgAlFe and Pd-K/CuMgAlFe at 150 oC has reached to 70 % while more than 85 % at 300 oC and 450 oC.