Modification Of Cu-SAPO-34/Cordierite By HF And Its Application On The Removal Of NOx From Diesel Vehicle Exhausts

With the development of global economy and the improvement of p living standard, the auto industry has been rapidly developed. However, monoxide (CO), hydrocarbons (CH), nitrogen oxides (NOx) and part matter (PM) emitted along with the vehicle exhaust have caused more pollution to the ecological environment, so that lead to the extensive atten people. Compared to gasoline engine, diesel engine has the advantages of power, high thermal efficiency, low oil consumption, low greenhous emission, longer service life and cheaper fuel price, so its application automotive field is increasingly popular, and the worldwide using diesel f automobile has became a trend. But the traditional three-way catalysts (1only show a lower de-NOx activity in the oxygen-rich environment of (vehicle exhausts, and thus can not achieve the requirements of the rel(?) emission regulations. Two major techniques, NOX storage and reduction as as the selective catalytic reduction (SCR), are being employed to remove N1the oxygen-rich atmosphere and hydrocarbon, urea or ammonia are used as reducing agent. In these de-NOx techniques, the preparation of catalyst high activity for the selective catalytic reduction of NOx as well as high ther stability and hydrothermal stability is still the key problem needed to be solv(?)The monolithic catalyst composed of carrier, coating, and act component can be used to purify diesel vehicle exhaust. Among which, catalyst carrier properties have a great effect on the activity and service-life of the catalyst. Cordierite (2MgO·2Al2O3·5SiO2) monoliths, which possesses the advantages of high thermal stability, stable mechanical property, small thermal expansion coefficient and low cost, is the proper candidate of catalyst carrier. Compared to the traditional coating technology as well as the immersion and ion exchange methods used in the conventional load of the active components, molecular sieve/cordierite monolithic catalyst prepared via in situ synthesis technology not only has a higher specific surface area, but also the inside and outside surfaces of the molecular sieves in the catalyst can be fully utilized, moreover, the active components can be more easily dispersed on the surfaces of the molecular sieve crystals or its skeleton structure. The active components in the skeleton can be better used as the active centers of the oxidation or reduction reactions. HF can not only play the role of mineralization, but also be similar to the templates to guide and stabilize the molecular sieve structure in the process of molecular sieve crystallization.Cu-SAPO-34/cordierite catalyst has been successfully prepared via in situ hydrothermal synthesis technology in our group. Based on this, HF was added in the process of situ hydrothermal synthesis Cu-SAPO-34/cordierite catalyst via one-step method and the effects of HF on the de-NOx performances of catalyst samples were researched in this paper. The morphological structures and surface properties of the as-synthesized samples were characterized by the means of X-ray diffraction (XRD), scanning electron microscopy (SEM), N2-adsorption and desorption, XPS and ICP techniques. The de-NOx performance and anti-aging properties of the catalyst samples were evaluated at the simulated diesel exhaust atmosphere. The following conclusions have been obtained by detailedly analyzing the experimental data.(1) In the process of preparing Cu-SAPO-34/cordierite, the presence of HF at a certain amount can significantly promote the growth and dispersion of the molecular sieve grains on the carrier surfaces, improve the structure regularity of the grains and its arrangement tightness, and increase Cu-SAPO-34loading amount on the carrier surfaces. When the crystallization time is12h, Cu-SAPO-34loading amount can come up to more than11%, which is obviously higher than7.0%of catalyst without HF. The NOx conversion rate of the catalyst prepared after crystallization12h with HF can be kept at about95%and the temperature range of340℃to600℃is wider, comparing to460℃to520℃from catalyst prepared without HF, and it is stable as the crystallization time continuing to increase. In addition, the catalysts prepared with HF have a stronger anti-aging properties, and when the crystallization time reaches to12h or above, the NOx conversion rate of the aged catalysts can still be up to about95%or so in the temperature range from340℃to520℃, which showes that its low temperature activities are not affected by the aging treatment process. However, the catalyst prepared without HF can only kept at about80%in a narrow temperature window.(2) The optimization experiment results of the catalyst prepared in the presence of HF with a certain amount show that its body phase structures, external form, Cu-SAPO-34loadings as well as de-NOx performance and anti-aging properties are affected by the amount of HF in the molecular sieve mother liquor. This means that the samples with a neat structure, a larger Cu-SAPO-34loading, and a excellent catalytic activity can be obtained in the presence of HF with a right amount, but it is bad for the growth of molecular sieve grains on the carrier surfaces, or may damage the structures of grains which have been generated under the condition of the excessive HF existence. The HF amount in the molecular sieve mother liquor is optimum during the process of preparation catalysts when the ranges of n(HF)/n(Al2O3) and n(SiO2)/n(Al2O3) are0.01-0.03and0.3-0.7. The active temperature windows of the fresh, aging10h and aging20h samples, in which the conversion rate of NOx is above95%, are320-580℃,340-480℃and380-440℃, respectively. Meanwhile, the crystallization time of the Cu-SAPO-34/cordierite sample can be selected in the range from12h to36h, comparing to7d without HF.(3) The effects of HF added in the process of preparation Cu-SAPO-34/cordierite monolithic catalyst are to promote the grow of molecular sieve grains and its dispersion on the carrier surfaces of the samples, steady the structures of molecular sieve and the ratio of Cu+/Cu2+in the surface Cu component of the catalyst samples, and increase the specific surface area and specific pore volume of the samples and so the catalysts before and after the aging treatment all show a excellent catalytic activity in NOx-SCR reactions.