Study On Modification Of Mn-mullite Oxide And Its Catalytic Elimination Of Exhaust Pollutants

Catalytic materials are the core of diesel engine exhaust aftertreatment systems.China has made considerable progress in this field after nearly 20 years of development.However,the existing catalytic materials are difficult to support sustainable development of Chinese exhaust catalytic industry,for a late start and weak foundation.Hence,the development of new catalytic materials has become a hot research field to match new emission regulation requirements for the types and limits of exhaust emissions.In this dissertation,the research,based on our team's previous work of theoretical calculations and experimental research on mullite material systems,expended new catalytic material systems of exhaust elimination by controlling structure and surface chemical state of mullite.The internal relationship amoung structure,surface chemical state and reaction mechanism of mullite was clarified,which can provide new idea for the development of new exhaust elimination catalysts.The main re-search works are as follows:A method of constructing internal cross-linked hydrogen bond to increase the decom-position temperature of colloidal crystal template was developed.This method was used to overcome the mismatch between mullite sintering temperature and template heat-resisting temperature,and firstly achieved the structural design and preparation of three-dimensional ordered macroporous mullite catalyst.The new structure mullite catalyst shows higher NO_x-assisted soot combustion activity than the traditional nano-particle structure mullite catalyst.A strategy was proposed to enhance the amount and intensity of NO_x adsorption on the surface of mullite to improve the activity of NO_x-assisted soot combustion.The chemical state of mullite surface was changed by K₂Mn₄O₈ modification,and the amount of NO ad-sorption and NO₂ adsorption strength of the catalyst were improved.Then,the K modified catalyst with better catalytic activity than pure mullite was obtained.By verifying that NO₂^*has stronger oxidation activity than O^*in NO_x-assisted soot combustion,the mechanism of NO_x-assisted soot combustion reaction on mullite is revealed.It was found that mullite catalysts showed excellent oxidation activity in selective cata-lytic oxidation of ammonia,and Sm Mn₂O₅ catalyst showed the best ammonia oxidation ac-tivity.The reaction path of mullite catalyst for NH₃-SCO was clarified based on theoretical calculation and in situ DRIFTS,and the reaction mechanism was confirmed to be imide mechanism by confirming the intermediate species of-HNO.Based on the analysis of the potential barrier relationship between N₂O and N₂ products in selective catalytic oxidation of ammonia in density functional theory calculation,a strategy to improve the catalyst selectivity for N₂ by enhancing the adsorption quantity and intensity of-NH was proposed.Then,this strategy was verified by the surface chemical state of mullite modified by solid acid of Nb₂O₅,and a mullite catalyst with high N₂ selectivity was obtained.It is clarified that the essence of this modification is to improve the electron absorption effect of the catalyst on the lone pair electrons of N atoms,which provides a feasible idea for the development of new catalysts.