Preparation Of Cu/MgAl₂O₄ Catalyst And Study On The Catalytic Reduction Property Of NO

For automobile exhaust purification catalysts could be more widely used to better control the pollution of NO in automobile exhaust,we chose non-precious metal element Cu whose resource is also abundant on earth as the replacement of noble metal elements Pt,Rh and Pd that has usually been used in commercial automobile exhaust purification catalysts to catalyze the reduction of NO by CO.Thus NO can be effectively purified without addition of additional reducing agent,and some of the CO pollutant in automobile exhaust can be removed either.Due to the wide operating temperature range of automobile exhaust purification catalyst and the high and low temperature cycle and frequently changing,the automobile exhaust purification catalysts are required to have good high temperature stability and thermal shock resistance,so MgAl2O4 spinel was choosed by us as the carrier to prepare the catalyst.The crystal lattice of MgAl2O4 spinel has both acid-base amphoteric activity center on the surface.Actually NO reduction by CO can be catalyzed by MgAl₂O₄ spinel at high temperature that may effectively enhance the catalytic activity and high temperature performance of catalyst.By binding Cu atoms or nano-particles on the surface lattice of MgAl₂O₄ spinel,the stability of active components of the catalyst was greatly enhanced.After CuO being reducted,oxygen vacancy on the lattice surface can easily convert adsorbed NO into N₂O thus greatly accelerated the reaction process and improved the selectivity of the target product N₂.A series tests showed that Cu-MgAl₂O₄ catalysts made excellent catalytic performance in NO reduction by CO.In order to reveal the source of high catalytic activity and thermal stability of the catalyst,methods of XRD,XRF,XPS,TEM,HAADF-STEM,Isotope labeling,in situ FT-IR and DFT calculation et al.was used by us to analyze the composition,morphology,structure and valence state of the samples.Based on the characterization and analysis results,we studied the steps and reaction mechanism of NO reduction by CO on Cu-MgAl2O4 catalysts.The main research contents and results of this paper are as follows:1.Sol-gel method was used to prepare a single Cu atom catalyst.By simply adjusting the pH Mg²⁺in MgAl₂O₄ spinel precursor was partly repalaced by Cu²⁺.After being calcined in air at 800℃for 2 h,a monodisperse Cu₁O₁-MgAl₂O₄ sample was acquired from the precipitation which has been dried in an oven at 120℃.Single Cu atom catalyst Cu₁-MgAl₂O₄ sample was prepared by pretreating Cu₁O₁-MgAl₂O₄ in the feed gas（2.6vol%NO-2.9vol%CO-He）at 700℃.Cu₁-MgAl₂O₄ showed higher catalytic efficiency than Pt/CeO2 in the catalytic reduction of NO by CO in a fixed-bed catalyst evaluation unit.In addition,the catalytic stability of Cu₁-MgAl₂O₄ was also proved by the results.Based on characterization analysis and DFT calculation,we found that the high catalytic activity and N₂ selectivity of Cu₁-MgAl₂O₄ was owing to two aspacts.On one hand,the O bonding with Cu in the catalyst was involved in the oxidation of adsorption CO,which reduced the activation energy of the reaction.On the other hand with the oxygen vacancy left by CO oxidation,the adsorbed NO could easily convert into N₂O and then into N₂.The stability of the catalyst is mainly attributed to the constraint effect of the stable MgAl₂O₄ spinel lattice on Cu atoms,which made it difficult for the monodisperse Cu atoms that in a high energy state to migrate and grow up,thus avoiding the deactivation of the catalyst caused by grain growth or sintering.2.In order to further improve the catalytic activity of Cu₁-MgAl₂O₄ multiphase catalyst for reduction of NO by CO,a series of Cu-MgAl₂O₄ composite catalysts with Cu monatomic structure and Cu nanoparticle structure on the surface were prepared by increasing the content of Cu in the catalysts.Among these samples 10%Cu-MgAl₂O₄（theoretical molar ratio Cu/Mg/Al=1:5:10）catalyst showed the highest activity and N₂ selectivity at low temperature.10%Cu-MgAl₂O₄ showed excellent catalytic stability for catalytic reduction of NO by CO as well.According to the characterization,we found that the content of Cu on the surface of 10%Cu-MgAl₂O₄catalyst was significantly increased compared with that of Cu on the surface of Cu₁-MgAl2O4 which further promoted the oxidation of adsorbed CO and the conversion of adsorbed NO into N₂O at low temperature.Thus the activity and selectivity of the catalyst was significantly improved.The presence of surface monodisperse Cu atoms can effectively increase the dispersion of Cu nanoparticles and restrict the migration and growth of Cu grains so that the stability of catalyst was extremely enhanced.