Preparation Of M (M=Mg,Co,Ni) Al Hydrotalcite-Like Catalysts And Exploration Of Catalytic Mechanism For Soot Combustion

In recent years,with the improvement of people’s living standard,the number of motor vehicles in China has grown rapidly.Motor vehicles not only bring convenience to people,but also cause a series of environmental pollution problems which have to be concerned.For example,the frequent haze weather has seriously affected people’s normal life.One of important reasons for the frequent occurrence of haze weather is the excessive emission of soot particles（PM）in vehicle exhaust.Therefore,it is important to reduce the impact of vehicle exhaust emissions on the environment to protect the people’s health.The current emission control technology of diesel vehicles is mainly divided into:internal purification and external purification.Due to the limitations of internal purification technology,catalytic oxidation based post-treatment technology has become an important method to control diesel vehicle emissions.The key factor of this technology is to design catalysts for efficient diesel vehicle exhaust purification.The elimination efficiency of soot particles is closely related to the catalytic oxidation performance of the catalyst.The combustion reaction of soot particles is a typical deep oxidation reaction derived from gas（NO,O₂）-solid（soot particles）-solid（catalyst）three-phase interface contact.According to the characteristics of this reaction,the catalyst with specific morphology is constructed to increase the contact efficiency between soot particles and the catalyst,and the adsorption and activation ability of gas reactants are enhanced by the regulation and modification of catalyst.In this paper,Co Ni Al hydrotalcite-like catalysts with porous structure and Mg MAl-LDO catalysts with three-dimensional ordered macroporous structure were synthesized by co-precipitation method,hydrothermal method and continuous impregnation method,respectively.A series of soot combustion catalysts with specific structure and adjustable Pt nano-paticles are prepared by GBMR method.The characterization methods such as XRD,SEM,XPS and XAFS,were used to analyze and test the morphology and redox properties of the catalyst.The relationship between the morphology and structure of the catalyst and the composition of the active sites Pt during the catalytic soot combustion reaction was discussed.The reaction mechanism for promoting soot oxidation is proposed.The main contents and results of this paper were as follows:（1）Co_2-xNi_xAlO hydrotalcite-derived oxides were prepared by co-precipitation method.The characteristic nanosheet structure of Co Al hydrotalcite oxides and the pore structures formed by stacking nanosheets can promote the mass transfer efficiency between soot particles and gas reactants.The doping of the Ni in the Co Al hydrotalcite oxide replaces partially the Co component,and synergistic effect occurs between binary Co and Ni cations,which can induce the formation of coordinated unsaturated cations and surface anion defects（oxygen vacancies）,thus increasing the specific surface area and number of active sites.The generation of oxygen vacancy enhances the adsorption and activation ability to O₂,which enables the adsorption of more active oxygen species,and thus can react with NO to generate NO₂ with stronger oxidation ability.Finally,the carbon smoke reacts with NO₂ and is removed.The Co_1.5Ni_0.5Al O hydrotalcite-derived oxides exhibit the best catalytic performance for soot combustion,T₅₀=333℃,TOF=1.01×10^-2 s^-1.In conclusion,the synergistic effect of binary Co and Ni ions and the structure of stacked nanosheets together make Co_2-xNi_xAl O hydrotalcite catalysts exhibit excellent catalytic performance for PM combustion.（2）The catalysts of hydrotalcite-derived Ni_1.5Co_0.5Al O nanosheets-supported highly dispersed Pt nanoparticles(Pt_n/Ni_1.5Co_0.5Al O,n%is the weigh percent of Pt element in the catalysts)were elaborately fabricated by the gas bubble assisted membrane reduction method.The characterization of the catalyst showed that Pt nanoparticles were highly dispersed on the surface of Ni_1.5Co_0.5Al O.Pt_n/Ni_1.5Co_0.5Al O catalyst showed excellent catalytic activity and stability.The specific porous structure formed by the stack of hydrotalcite-derived Ni_1.5Co_0.5Al O nanosheets can increase the transfer mass efficiency of the reactants（O₂,NO and soot）.Besides,the strong Pt-Ni_1.5Co_0.5Al O interaction can weaken the Ni/Co-O bond so as to promote the mobility of lattice oxygen and the formation of surface oxygen vacancies,thus benefiting the combustion and removal of soot particles.（3）Based on the structural characteristics and formation principle of layered double-metal hydrotalcite-like oxides and the formation conditions of three-dimensional ordered macroporous structure precursors,Mg MAl-LDO（M=Fe,Co,Ni）hydrotalcite-like catalysts with ordered and closely arranged periodic macroporous structure were prepared.The regular macroporous structure facilitates the entry of soot particles into the pores and improves the contact efficiency between the catalyst and soot particles.The special pore structure also enhances the mass transfer efficiency between the catalyst,soot particles and gas reactants（NO,O₂）within the catalyst.The synergistic effect of Mg and M（Fe,Co,Ni）transition metal ions in Mg MAl-LDO catalyst promotes the formation of more oxidizing NO₂ and plays a positive role in the oxidation elimination of soot particles.In short,the macroporous structure and the synergistic action between binary metal cations together contributes to the excellent catalytic performance.