Study On The Catalytic Performance For Soot Oxidation Over Manganese-Based Mullite Composite Oxide Catalysts

Diesel engines have the advantages of low fuel consumption and high thermal efficiency,and are widely used in automobiles,heavy trucks and light ships,etc.However,the particulate matter（PM）and nitrogen oxides（NO_x）are the main pollutants of diesel engine exhaust,which can cause adverse effects on human health,the environment,vegetation,and climate.At present,catalytic post-treatment technology is considered as an effective means to reduce exhaust emissions.The normal temperature range of diesel exhaust is 200～400℃,and the temperature required for the combustion of carbon smoke particles is about 600℃.Therefore,it is necessary to develop catalysts with higher activity at lower temperatures,otherwise the soot combustion efficiency will be lower due to the back pressure generated by the clogging of the filter.Manganese-based mullite oxides have excellent NO oxidation activity,low cost,good thermal stability,and has potential application value for soot catalytic combustion reaction.In this thesis work,manganese-based mullite Sm Mn₂O₅oxide with mesoporous structure was synthesized by the method of co-precipitation and the addition of F127 mesoporous template.On this basis,two series of catalysts,i.e.,mesoporous Sm Mn₂O₅-supported transition metal oxide Co₃O₄and rare earth metal oxide Ce O₂were designed and prepared.The performance of Sm Mn₂O₅oxide catalyst for soot combustion was further improved.The physical and chemical properties of the catalysts were characterized by various methods,and the catalytic performances of the catalysts for soot oxidation were evaluated.The main research contents and results are as follows:（1）Manganese-based mullite catalysts with different microstructures were synthesized by three methods of sol-gel（CA）,hydrothermal（HT）and co-precipitation（CP）.They were used to catalyze the soot combustion,respectively,and all resulted in a significant reduction of the peak temperature of soot combustion.Among them,the mesoporous Sm Mn₂O₅catalyst prepared by F127 co-precipitation method gave the highest catalytic soot combustion activity with T₅₀=369℃.Its surface has the lowest temperature interval of active oxygen species and a high number of lattice oxygen species,with better redox properties and higher mobility of surface adsorbed oxygen species,and NO_2-assisted catalysis also plays an important role.（2）The catalytic performance was further enhanced by loading different contents of Co₃O₄on the surface of mesoporous SMO support.With the increase of loading amount,the specific surface area of the catalyst increased,and the internal structure of Sm Mn₂O₅changed to some extent,and the Mn-O-Mn bond angle and bond length changed,which improved the mobility of oxygen species on the catalyst surface and lattice.At the same time,the number of high-valent Mn⁴⁺increases,the number of Mn³⁺decreases.Moreover,the surface of the catalyst is enriched with high-valent Co³⁺.The oxidizing property,reaction rate,catalytic efficiency,and active oxygen density of the supported catalysts are all higher than those of the Sm Mn₂O₅catalyst.Among the series catalysts of Co₃O₄/SMO,50%Co₃O₄/SMO sample was most favorable for the oxidation reaction of soot particles.Its T₅₀was 338℃,S_CO2^mwas 98.5%.Compared with the SMO catalyst,T₅₀of 50%Co₃O₄/SMO reduced by 30℃and S_CO2^mincreased by 6.3%,so the catalytic performance for soot oxidation was greatly improved.（3）Different amounts of Ce O₂were loaded on the surface of mesoporous SMO by using the sodium carbonate precipitation method and a series of x%Ce O₂/SMO catalysts were obtained.It was found that x%Ce O₂/SMO played a good role in promoting PM elimination,and T₅₀was further reduced.Among this series of catalysts,30%Ce O₂/SMO showed the optimal catalytic performance for soot combustion,and its T₅₀=335℃is much lower than that of Sm Mn₂O₅(T₅₀=369℃).The test results of BET,H₂-TPR,Soot-TPR and XPS showed that the specific surface area,the number of active oxygen,the number of high-valent Mn⁴⁺and Ce⁴⁺of the catalyst all increased after loading Ce O₂,and the oxidizing property of the catalysts enhanced.Activation energy test results showed that 30%Ce O₂/SMO had the lowest activation energy and the lowest reaction energy barrier,which is most conducive to the catalytic soot combustion reaction.This corresponds to the results of catalytic activity for soot oxidation reaction.