Modification Of MnO_x And Its Catalytic Performance For Soot Oxidation

The soot particles generated by the incomplete combustion of diesel internal fuel are one of the main reasons for heavy pollution weather such as haze.The catalytic oxidation of soot is the key control technology.In this work,the reactive oxygen species affecting the catalytic performance of the materials were studied.The generation,migration and transformation rates of the reactive oxygen species were optimized by morphology control,surface modification and crystal surface engineering.The reaction mechanism was discussed by experimental and theoretical methods.α-MnO2 with nanoflower(NF)and nanowire(NW)morphologies was synthesized by hydrothermal method.Silver with different mass fractions(5-15wt.%)(Ag(x)-NF)was loaded on the surface of nanoflower-like α-MnO2 to explore its effect on the generation rate of active oxygen species.The results of activity evaluation showed that the T90 of Ag(10)-NF was only 371℃ under tight contact condition,exhibiting excellent catalytic activity.The characterization results show that the amount of surface oxygen vacancies varies in different α-MnO2,resulting in different active oxygen species generation ability,and NF has the highest active superoxide(O2-)ratio.In addition,the optical amount of Ag-modified NF has excellent metal-support interaction,which can improve the concentration of Mn3+ and the adsorbed oxygen species,weaken the strength of Mn-O bond and increase the generation rate of reactive oxygen species.The calculated results show that the surface of Ag-modified α-MnO2 has higher surface energy,lower formation energy of oxygen vacancy and strong affinity to the reactant molecules,which revealed the reason for its high activity from the perspective of energy.Heterogeneous metal-doped MnOx-based soot oxidation catalysts were also synthesized by hydrothermal method,and the morphology and the exposed crystal plane were optimized by controlling the doping amount.W,Cu-codoped samples have excellent catalytic performance,and the value of T90 is 395℃ under tight contact condition.The results can be attributed to the high specific surface area and the reconstruction of local electronic structure on the surface.The characterization results demonstrate that the introduction of W and Cu leads to the change of lattice array.The introduction of heteroatoms can increas the concentration of Mn3+,form more oxygen vacancies,greatly improve the content of chemical adsorbed oxygen,which increased the number of active oxygen species.The incorporation of W and Cu also significantly ameliorate the low-temperature reduction performance of MnOx.Through theoretical calculations,it was found that different types of oxygen vacancies on the co-modified Mn2O3(400)surface possess different activation capacities for O2 and H2O,indicating that there was a non-competitive adsorption and activation process on the surface of Mn2O3.This work deepens the understanding of the surface active sites of co-decorated MnOx,and provids a strategy to improve the catalytic performance through morphology control and crystal surface engineering.