Study On Soot Oxidation Reaction With 3DOM M_xCo_3-xO₄（M=Cu,Zn,Ni）cobalt-based Spinel Catalysts

Soot particles and NOx emissions from diesel engines are two main pollutants.Soot particles are the main cause of PM2.5 in the haze weather,and NOx are liable to cause acid rain and photo-chemical smog.It is of great significance to control the emissions of soot particles and NOx from diesel engines exhaust.A combination of catalytic oxidation technique and diesel particulate filters（CDPF）is considered as one of the most efficient after-treatment techniques for soot oxidation.The principle of operation is that catalyst is located in the washcoat of the filter to accelerate the oxidation of NO to NO2.The generated NO2 can promote soot oxidation at exhaust gas temperatures（200-500 ^oC）due to that it is a better oxidant than O2.The catalytic soot oxidation is a typical heterogeneous catalytic reaction occurring at the three-phase boundary of solid（soot）-solid（catalyst）-gas（O2,NO）.Under loose contact,two aspects ought to be considered for excellect catalytic performance of soot oxidation:one is to improve the contact efficiency between soot particles and catalyst;the other is to improve the redox ability of catalyst.Under the above two guiding principles,the design of three-dimensionally ordered macroporous structure（3DOM）can improve the contact efficiency between soot particles and catalyst.Among the transition metal oxides,Co3O4 exhibits excellent activity for oxidation reactions owing to its strong redox ability,thus Co3O4 is chosen as the research subject.Herein,the effect of fine-tuning of catalyst composition like doping heteroatom to Co3O4on catalytic soot oxidation was investigated.3DOM Mx Co3-x O4（M=Cu,Zn,Ni）catalysts were successfully fabricated through the carboxy-modified colloidal crystal templating（CMCCT）method,and its physical and chemical properties were analyzed by various characterizations.A series of in-situ experiments were used to explore the mechanism and active sites during the reaction.The main contents and results are as follows:（1）3DOM Cua Cob Oδcatalysts were prepared CMCCT method.The mass transfer of soot particles during the reaction process is benefited from the periodic arrangement of macropores,and thus the contact efficiency between soot particles and catalyst is improved.3DOM Cu-Co-2 exhibits the best catalytic activity of NO oxidation,and the conversion of NO is 66.5%at 332 ^oC.The strong oxidation ability of NO corresponds to the fact that the catalyst has higher oxidation activity of soot particles,among which,the T50 of 3DOM Cu-Co-2 in the oxidation reaction of soot particles is 376 ^oC.The doping of Cu to Co3O4 will promote the formation of oxygen vacancies and enhance lattice oxygen mobility.（2）3DOM Mx Co3-x O4（M=Zn and Ni）catalysts were prepared by CMCCT method.The catalytic activity of soot oxidation is benefited from the doping of Cu.In order to verify whether the metal ions in tetrahedral position or octahedral position in Co3O4 are the active sites for catalytic oxidation,Zn²⁺and Ni³⁺with similar ionic radius were used to replace the metal cations in tetrahedral(Co²⁺)and octahedral(Co³⁺)positions of Co3O4,respectively.By means of H2-TPR,Soot-TPR,isothermal anaerobic titration and in-situ DRIFTs,it was found that the doping of Ni to octahedral position of Co3O4 could decrease the bond strength of Co-O,promote the formation of oxygen vacancies and surface active oxygen species.The electronic structure of Co³⁺is modified that can decrease the desorption energy of NO2 from catalyst surface,which is the rete-determining step of NO oxidation.Among them,3DOM Ni Co2O4 exhibits the strongest oxidation ability of NO.The conversion of NO is 69.6%at 320 ^oC,and the T50 and TOF values of soot oxidation is 379 ^oC and 1.36×10^-3 s^-1,respectively.A possible vacancy-mediated pathway of NO oxidation on spinel catalyst by the experimental results of in situ DRIFT spectra,in situ Raman spectra and the theoretical knowledge of coordination chemistry of metal-NO.It is found that octahedral coordination sites play a bigger role in O2 activation and NO oxidation than tetrahedral coordination sites do,and the oxidation capacity of NO to NO2is highly related to surface active oxygen species.Furthermore,NO2 level is closely related to the catalytic performance of soot oxidation.On the basis of above results,NO2-assisted mechanism is proposed for soot oxidation on 3DOM Mx Co3-x O4 catalysts.