Study On The NO_x Storage And Reduction Performance Of Pt/BaO/M_xMg_1-xAl₂O₄(M=Co,Mn,Cu) Catalysts At Moderate And High Temperatures

NO_x is one of the main pollutants in engine exhaust,which can severely harm the human health and environment.Lean NO_xTrap(LNT),also named as NO_x storage and reduction(NSR),is one of the most effective approaches to NO_x removal under the lean-burn conditions.Nowadays,the developments of advanced engines increase the fuel economy,whose exhaust is always above 400oC,however,at high temperatures,there is a lack of the NSC and stability for the conventional NSR catalysts,its application is greatly limited.In terms of this,we adopted spinel as the support to replace the commercial Al₂O₃ support,for the aim of enhancing the middle-and high-temperature performance of NSR catalyst.The study has significant social meaning and potentially practical value.We doped transition metals(Co,Mn,Cu)into the hydrotalcite structure and investigated the impacts of different metals on the hydrotalcite structure and catalytic performance.The doping ratio was also optimized in this study.Furthermore,we studied the NO_x storage species and possible storage path by using FT-IR.The M(M=Co,Mn,Cu)doped hydrotalcite precursors were prepared at the constant p H via the coprecipitation process,which was then calcined to form structurally stable Mn_xMg_1-xAl₂O₄ spinel.We used wet impregnation method to load the storage component Ba and active component Pt onto the support(Ba O loading:15 wt%,Pt loading:1 wt%),and prepared a series of Pt/Ba O/M_xMg_1-xAl₂O₄ catalysts.With the addition of Co and Mn,the XRD diffraction peak shifted to lower diffraction degrees,indicating the successful partial substitution of Mg by M(M=Co,Mn).The addition of transition metals significantly increased the specific surface area of the catalysts,which is conducive to the dispersion of the active center,also increased the amount of the surface adsorbed oxygen species and created more oxygen vacancies on the surface.Meanwhile,it promoted the dispersion of Ba CO₃,i.e.NO_x storage sites,and inhibited the formation of Ba CO₃ grains which had low storage activity.All of these strongly enhanced the NO_x storage capacity(NSC)of the catalysts.We conducted the NSR activity tests using the C₃H₆ as the model reducing agent.The activity of the Pt/Ba O/M_xMg_1-xAl₂O₄(M=Co,Mn,Cu)catalysts varied in a volcanic type in the temperature range of 250-500oC.Different from the conventional NSR catalyst,the Pt/Ba O/M_xMg_1-xAl₂O₄(M=Co,Mn,Cu)catalysts showed not only NO_x removal activity at the middle temperatures(350-400oC),but also prominent NSR performance at high temperatures(400-500oC).The Co-,Mn-doped spinel catalysts had the maximum activity at 400oC.Among them,the Pt/Ba O/Co_0.3Mg_0.7Al₂O₄ and Pt/Ba O/Mn_0.3Mg_0.7Al₂O₄ catalyst showed the highest activity of 93.2%and 95.8%,respectively.For the Pt/Ba O/Cu_xMg_1-xAl₂O₄ catalysts,the best doping ratio is 10 mol%.The Pt/Ba O/Cu_0.1Mg_0.9Al₂O₄ catalysts exhibited the NO_x removal activity of 89.7%at 450oC.Compared with Pt/Ba O/Mg Al₂O₄,the doped catalysts exhibited the enhanced high-temperature activity.The NO_x-TPD results showed that the doping of Co,Mn and Cu enhanced the NO_x trapping ability and the thermal stability of the NO_x storage species in the catalyst,therefore increasing the NO_x removal efficiency at high temperatures.To investigate the change of catalyst during the NSR reaction,we also conducted several characterizations over the spent catalyst.After the reaction,the catalysts maintained the nanowire morphology.No obvious change could be observed for the crystal structure.Meanwhile,the Pt⁰ content increased as a result of partial reduction.The Pt species was still well dispersed after the reaction although the particle size increased.NO_x was mainly stored in the form of ionic nitrate in the catalysts,and then reacted with the C₃H₆ in the subsequent fuel-rich period.The regenerated storage sites combined with the CO₂,i.e.reduction production,and forms Ba CO₃ again.The XRD results of the spent catalysts showed the weakened Ba CO₃ diffraction peak,indicating the redispersion of Ba CO₃on the support during the NSR reaction.