Study On Preparation And Performance Of Automobile Exhaust Denitration Catalyst

With the increasing of China’s comprehensive national strength,people’s living standards are gradually improved.The increase of car ownership has led to a series of environmental problems.With the full implementation of the latest emission requirements,it is urgent to solve the problem of nitrogen oxide pollution in automobile exhaust.In recent years,zeolite as catalyst carrier of NH3-SCR technology is the focus of scholars.Zeolite has a large specific surface area,regular pore structure and good ion exchange capacity,which are the necessary conditions for automobile exhaust denitration catalyst.Using fly ash as raw material to synthesize SSZ-13 zeolite with CHA structure,and applying the zeolite product to the treatment of nitrogen oxides in automobile exhaust,is a new environmental protection concept of"waste treatment",which is in line with the national sustainable development goal.In this project,a method of preparing SSZ-13 zeolite from industrial solid waste fly ash was proposed,and it was modified by metal cations to remove nitrogen oxides in automobile exhaust.The research content of this project can be divided into three aspects:the synthesis of fly ash based SSZ-13 zeolite,the metal ion loading modification of fly ash based SSZ-13 zeolite,and the NH3-SCR vehicle exhaust denitration performance test of metal ion loading SSZ-13 catalyst.The results show that the silicon dioxide and aluminum oxide which are beneficial to zeolite synthesis in fly ash account for about 81%of the total composition.Most of the micro surface of fly ash is spherical with regular shape,and its activity can be fully released by alkali melting method.The fly ash can provide abundant silicon and aluminum sources for the preparation of fly ash based zeolite with high silicon aluminum ratio.Calcination temperature,calcination time,crystallization temperature and crystallization time all seriously affect the final quality of zeolite products.When the temperature is 600℃,the purity of SSZ-13 zeolite is low,and the crystallinity is 74%.When the temperature is 700℃,the XRD pattern of the zeolite sample is mainly the diffraction peak of SSZ-13 zeolite,and the crystallinity reaches 81%.When the calcination time is 1 h,the purity of SSZ-13 zeolite decreases due to insufficient alkali melting pretreatment;If the calcination time is too long,the crystal phase transformation of fly ash clinker will occur,resulting in the reduction of final product purity.When the crystallization temperature is low,the environmental temperature is low,which inhibits the formation of crystal nucleus,and the crystallinity of zeolite products is low;When the crystallization temperature is high,the type of zeolite nucleation is not required for the experimental purpose,and 120℃ is the ideal crystallization temperature.The nucleation time of different kinds of zeolite is different,and 6d is the best crystallization time for the synthesis of SSZ-13 zeolite based on fly ash.The zeolite synthesized under the optimum conditions was analyzed by infrared spectroscopy,ion exchange capacity,pore structure and scanning electron microscopy.There are Si-O tetrahedron and Si-O-Si structural units in the infrared spectrum,which are the basic framework of SSZ-13 zeolite.It can be determined that SSZ-13 zeolite based on fly ash was synthesized through the preparation process of this experiment.The total specific surface area of zeolite is 624.38 m2·g-1,of which the micropore specific surface area is 598.76 m2·g-1.The micropore specific surface area of zeolite accounts for 95.89%of the total specific surface area.The ion exchange capacity of zeolite reaches 412 cmol/kg,indicating that the zeolite product has strong ion exchange capacity,The ion exchange method is an ideal method to prepare Cu/SSZ-13 denitration catalyst by modifying zeolite.The catalyst samples were characterized by three groups of experiments under different ion exchange concentration,ion exchange temperature and ion exchange time.The results of XRF show that the Si/Al ratio of fresh catalyst samples is between 15 and 17.XRD results show that the catalyst still keeps CHA structure.The total pore volume decreases with the increase of ion exchange capacity,and the maximum is 0.24 cm3·g-1,which is the same as that of pre modified zeolite,and the minimum is 0.22 cm3·g-1.The pore size of the catalyst ranged from 1.67 nm to 1.71 nm.With the increase of Cu2+loading,the pore size of the catalyst decreased slightly.The microporous surface area of the catalyst accounts for more than 92%of the total surface area,which indicates that the catalyst has high surface activity.The performance of catalysts prepared under different conditions was tested.The results show that the maximum N2O formation concentrations are 28ppm,25ppm,20ppm and 22ppm under different ion exchange concentrations.The denitrification efficiency of the catalysts prepared under this condition is 85%,87%,92.5%and 91%respectively.The optimum preparation ion exchange concentration of Cu/SSZ-13 catalyst is 0.045mol/l according to the width of temperature window and its aging performance.According to the experimental data and by comparing the width of the temperature window and the performance after aging,the optimal preparation ion exchange temperature of Cu/SSZ-13 catalyst is 60℃ and the optimal preparation ion exchange time is 4h.