Mechanism Of Active Oxygen Effect On The Reaction Of Removal Of PM And NO_x

Large amounts of particulate materials （PM） and nitric oxides （NO_x） exhausted fromdiesel vehicle are harmful for the environment and human health, and their removal isdifficult. In this study, two system catalysts （ceria-zrirconia solid solution andperovskite-type catalyst） were modified by different elements. The influence of themodification elements on the catalytic activity was study. The relationship of surface activeoxygen, oxygen vacancy and catalytic activity was investigated preliminarily.The main contents of this study are as follow.1) Ceria-zrirconia catalyst system: Li,Na and K were used to modify Ce_0.7Zr_0.3O₂solid solution.2) Lanthanum-cobaltperovskite-type system: LaCoO₃and LaCo_0.5Ni_0.5O₃catalysts were partically substitutedby Li, Na, K and Mn. The catalytic activity of the modification catalyst was detected bytemperature-program oxidation （TPO）. The influence of the modification element on thesurface oxygen species was investigated by temperature-programmed reduction of H2（H2-TPR）, temperature-programmed desorption of O₂（O₂-TPD）, X-ray diffraction （XRD）,Fourier transform infrared spectrometer （FT-IR）, Brunauer-Emmett-Teller （BET） andX-ray photoelectron spectra （XPS）. The relationship of surface active oxygen, oxygenvacancy and catalytic activity was studied preliminarily. The results of this study are asfollow.1) It indicates that the best preparation method sol-gel and the best calcinationcondition of Ce_0.7Zr_0.3O₂catalyst are sol-gel method and600oC calcination for6h. Dopingby Li, Na and K on Ce_0.7Zr_0.3O₂, the catalytic activity is influenced by the type and thecontent of the modification element. The sequence of the catalytic activity of0.15M-CZ（M=Li, Na, K） catalysts doped by different elements is0.15K-CZ>0.15Na-CZ> CZ>0.15Li-CZ; the sequence of the catalytic activity of xK-CZ catalysts doped by different contents of K is0.15K-CZ>0.25K-C Z>> CZ. Among these,0.15K-CZ has the highestactivity, and the ignition temperature （Ti）, the maximum PM oxidation rate temperature （Tm）and the conversion of NO_x（C[NO_x]） are310oC,480oC and49%. It is found that theenhancement of the catalytic performance is attributed to the interaction of the increasecontent of superoxide ion and the mobility of lattice oxygen ion.2) It demmenstrates that LaCoO₃catalyst prepared by critic complex combustion andcalcinated at700oC for5h has the best catalytic activity. The catalytic activity of LaCoO₃and LaCo_0.5Ni_0.5O₃catalysts are promoted after practically substituted by K, when LaCoO₃and LaCo_0.5Ni_0.5O₃are practically replaced by Li, Na or K. The catalytic activity becomesbetter when LaCoO₃is replaced by K praticially. Ti, Tmand C[NO_x] of La_0.8K_0.2CoO₃catalyst are345oC,440oC and83.12%; the catalytic activity of La1-xLixCo0.5Ni0.5O3isworse than LaCo_0.5Ni_0.5O₃; when LaCo_0.5Ni_0.5O₃are replaced by Na and K, the catalyticactivity of La1-xMxCo0.5Ni0.5O3could be improved. La_0.7K_0.3Co_0.5Ni_0.5O₃has the bestcatalytic activity, and its Ti, Tmand C[NO_x] are259oC,341oC and99.14%.3) The catalytic activity of La_0.8K_0.2Co_yMn_1-yO₃is improved obviously aftersubstituted by K and Mn simultaneously. La_0.8K_0.2Co_0.7Mn_0.3O₃has not only the lowest Ti（297oC） and Tm（343oC）, but also higher NO_xconversion （97%）. La_0.8K_0.2Co_0.7Mn_0.3O₃,La_0.8K_0.2CoO₃and LaCoO₃were chosen to characterize by H2-TPR and XPS to study therelationship between active oxygen and the catalytic activity. The results show that theactivity of simultaneous removal PM and NO_xover peroviskte-type catalyst relates to thecontent of surperoxide.4) To analyze the relationship of active oxygen species, the content of oxygenvacancy and the catalytic activity, xK-CZ and La1-xKxCo1-y Mny O₃catalysts were detectedby TPO, H₂-TPR and XPS. The results show that much more oxygen vacancies form aftermodified. The exchange of the oxygen species can be achieved in the mobility process.The enhancement of the catalytic activity is the result of the interaction of the activeoxygen and oxygen vacancy.