Study On The Preparation Of Honeycomb Ceramics-supported Co-based Monolith Catalysts And Their Catalytic Performance For The Combustion Of Diesel Soot

The formation of urban haze weather is closely related to the emission of vehicle exhaust pollutants.The China Mobile Source Environmental Management Annual Report,which was published by the Ministry of Environmental Protection in 2021,shows that a large number of motor vehicles are still held in our country and motor vehicle exhaust pollution is still an important source of air pollution.In particular,soot particles released from diesel vehicle exhaust can cause serious harm to environment and human body.Therefore,study and development of high-efficiency catalysts to eliminate soot particles is significant for protection of environment.In this thesis work,several series of low-cost and high-activity monolithic catalysts were designed and prepared,such as honeycomb ceramics-supported manganese cobalt spinel particles and three-dimensional macroporous-coated honeycomb ceramics-supported different transition metal oxides.The relationships among catalytic performance,redox properties,structural defects,reactive oxygen species content were studied by means of XRD,BET,SEM,H₂-TPR,Soot-TPR,O₂-TPD,NO-TPD and in-situ Raman under various atmospheres.The main research results are as follows:（1）A series of Mn_1-nK_nCo₂O₄ spinel-coated honeycomb ceramic monolithic catalysts were prepared by a simple citric acid complexation method.The as-prepared catalysts showed good catalytic performance for soot combustion,and the Mn_0.7K_0.3Co₂O₄-HC catalyst showed the best catalytic performance among the series of as-prepared samples.The initiation temperature(T₁₀)and maximum CO₂ concentration peak temperature（T_m）of the catalyst for soot combustion were 310 and 439°C,respectively.The physicochemical properties of the catalysts were characterized by SEM,XPS,H₂-TPR,Raman and other techniques.The characterization results show that K substitution is beneficial to the formation of oxygen vacancies,enhancing the mobility of reactive oxygen species,and improving the redox performance.The in-situ Raman results demonstrated that the strength of the Co-O bond in the catalyst become weaker during the high temperature reaction,which is favorable for the activation of oxygen.In addition,SEM and ultrasonic test results showed that the peeling rate of the Mn_1-nK_nCo₂O₄ spinel coating is less than 5%.The as-prepared monolithic catalyst has the advantages of simple synthesis,low cost and high performance,which is expected to be applied in soot combustion.（2）The hydrothermal preparation conditions were optimized by orthogonal screening（OS）,and Co₄Ni₁O_δsolid solution was successfully deposited in the macropores of three-dimensional macroporous Al_0.5Ce_0.5Oδsolid solution（3DM ACO）coating on honeycomb ceramics.Co₄Ni₁O_δ/3DM ACO/HC（OS）monolith catalyst was prepared.It showed high activity for soot combustion and the T₁₀ and T_m were 259°C and 356°C,respectively.CO₂ selectivity was greater than 99%.SEM and BET results showed that spherical Co₄Ni₁O_δsolid solutions with diameter of 50-80 nm were attached or filled in the 3DMACO framework.The existence of a large number of accumulated pores in the particles was beneficial to the adsorption of gas-phase reactants.XPS and in-situ Raman spectroscopy results showed that the interaction of Co-Ni enhances the vibration of the Co-O-Ni bond at the octahedral active site and promotes the increase of the proportion of high-valent cations.The presence of a large number of oxygen vacancies on Co₄Ni₁O_δ/3DM ACO/HC（OS）surface is beneficial for the catalyst to adsorb and activate gas-phase oxygen at low temperature and product superoxygen acive species.（3）Four kinds of Co-Ce O_δ/3DM ACO/HC monolith catalysts were prepared by impregnation method（IM）,microwave-assisted dry-impregnation method（IM-MW）,microwave-assisted precipitation method（CP-MW）and microwave-assisted hydrothermal method（HY-MW）.The catalytic performances of as-prepared catalysts with different methods are obviously different.Among the four methods,microwave-assisted heating can improve the dispersion of active components on the surface of honeycomb ceramic coating.The T_m for the combustion of diesel soot catalyzed by four catalysts follows the order:Co-Ce O_δ/3DM ACO/HC（CP-MW）（422°C）<Co-Ce O_δ/3DM ACO/HC（HY-MW）（441°C）<Co-Ce O_δ/3DM ACO/HC（IM-MW）（450°C）<Co-Ce O_δ/3DM ACO/HC（IM）（459°C）.SEM results showed that the as-prepared catalyst under microwave conditions had obvious three-dimensional macroporous structure and Co-Ce O_δwith diameter of 50-70nm were filled the macropores of 3DM ACO.However,too high deposition amount of Co-Ce O_δactive components resulted in the coverage of three-dimensional structure for Co-Ce O_δ/3DM ACO/HC（HY-MW）.The amounts of active oxygen species and H₂consumption in the reduction reaction over Co-Ce O_δ/3DM ACO/HC（IM-MW）were about 2 times as large as that of Co-Ce O_δ/3DM ACO/HC（IM）.In the range of200～500°C,the amount of O₂^-participating in Soot-TPR for Co-Ce O_δ/3DM ACO/HC（CP-MW）and Co-Ce O_δ/3DM ACO/HC（HY-MW）catalysts was 1.4 and 1.13times as large as that of Co-Ce O_δ/3DM ACO/HC（IM-MW）,respectively.The characterization results of H₂-TPR,XPS and Raman showed that the interaction between Ce and Co changed the charge balance of Co-Ce O_δand increased the content of high-valent cations.Meanwhile,the lattice structure of Co-Ce O_δwas distorted and a large number of oxygen vacancies were generated.（4）A series of Co₁Ce_0.1O_δ/3DM ACO/HC monolith catalysts were prepared by microwave-assisted heating of urea decomposition and high temperature atmospheric pressure-deposition crystallization method.This method could promote the deposition and growth of Co₁Ce_0.1O_δactive components in the pores of 3DM ACO coatings.The effects of reaction temperature（T）,active component dosage（V）,precipitant urea concentration（U）and reaction time（t）on the performance for soot combustion were investigated.Studies have shown that Co₁Ce_0.1Oδ/3DM ACO/HC（OS）catalyst has the best catalytic performance for soot combustion when T=120°C,V=70 ml,U=3 mol/L,t=4 h.The temperatures of T₁₀ and T₅₀ were 246°C and 370°C,respectively.The catalytic performance was slightly decreased after cycle test.SEM results showed that the small particles of Co₁Ce_0.1O_δsolid solution with 30-60 nm were interconnected to form nanowires with lengths of 300-400 nm,and three-dimensional flower-like structure was formed in the 3DM ACO framework.The interaction between Co and Ce in Co₁Ce_0.1Oδleads to the formation of a large number of oxygen vacancies on the oxides surface,which improves the adsorption and activation of gas phase oxygen and the ability for replenishment of reactive oxygen species.The above advantages are beneficial to enhancing the catalytic performance for soot combustion.The as-prepared catalysts have the advantages of simple synthesis,low cost,high catalytic activity and good application prospects.