【作者】 王新葵；

【导师】 杨学锋； 张涛；

【作者基本信息】 大连理工大学 ， 应用化学， 2008， 博士

【摘要】 担载型纳米金催化剂在很多反应中具有优异的低温催化活性,这使其成为多相催化领域的研究热点之一。金催化剂的这一性质也使得其在发动机冷启动时尾气的净化中具有应用潜力。因此,研究金催化剂在富氧气氛下催化烃类选择还原NO_x是一个很有意义的课题。本文考察了可还原性复合氧化物担载的纳米金催化剂在催化丙烯选择还原NO_x反应中的性能,并将金催化剂的结构与其活性相关联,取得了一些有意义的结果。1.采用沉积-沉淀方法制备的Au/CeO₂/Al₂O₃催化剂在丙烯选择还原NO反应中表现出优异的低温催化活性。在250℃下,当使用0.1%NO/0.1%C₃H₆/5%O₂/He为原料气,空速为30,000h^-1的条件下,NO被选择还原为N₂的转化率达到46%,且NO转化为N₂O的转化率可以忽略。此催化性能优于在同等条件下公认的具最佳低温活性的Pt/Al₂O₃催化剂,Pt/Al₂O₃催化剂在此温度下NO转化为N₂O的转化率高达30%。在反应气氛中加入2%水蒸气可使催化活性进一步提高,而加入20 ppm SO₂,低温催化活性有所降低,但H₂O和SO₂的同时加入却可提高催化活性并拓宽了催化反应温度区间。在300℃,反应气组成为0.1%NO/0.1%C₃H₆/5%O2/2%H₂O/20 ppm SO₂/He,空速为30,000 h^-1的条件下,NO选择还原为N₂的转化率达到50%,且NO转化为N₂O的转化率可以忽略。2.采用多种技术对Au/CeO₂/Al₂O₃催化剂进行了表征和催化反应机制研究:（a）TPD研究结果表明,NO和过量的O₂共吸附于催化剂表面形成NO_y（y≥2）吸附物种。SO₂的加入抑制了催化剂表面NO_y（y≥2）吸附物种的形成,从而降低了其催化活性。而H2_O和SO₂的同时加入可能通过增加催化剂表面酸性中心的数目,且抑制了丙烯的完全燃烧,从而提高催化活性并拓宽了催化反应温度区间。（b）XRD、HRTEM、H₂-TPR和XPS表征结果表明:经过400℃的高温焙烧,金的存在形式为金属态,粒径大小在3-5 nm之间。CeO₂的加入不仅可以提供反应所需的活性氧,而且还起到了稳定金粒子的作用。纳米金粒子与高度分散的CeO₂之间存在的强相互作用促进了CeO₂的还原,这可能对Au/CeO₂/Al2O₃催化剂的低温催化活性起着重要的作用。（c）Au/CeO₂/Al₂O₃催化剂上原位DRIFTS研究结果表明,在NO和O₂共存条件下,有NO_y（y≥2）物种检出;在C₃H₆和O₂共存条件下,有甲酸盐和乙酸盐物种检出;在反应气氛下出现-NCO和-CN物种。推测Au/CeO₂/Al₂O₃上丙烯选择还原NO反应的可能机理为:在氧气作用下,C₃H₆和NO首先经历部分氧化,分别形成包括甲酸盐、乙酸盐在内的C_xH_yO_z和NO_y（y≥2）等表面吸附物种,然后C_xH_yO_z物种再与NO_y物种形成含-NCO,-CN等基团的C_aH_bO_cN_d中间体,此类中间体再与NO₂或（NO+O₂）反应生成最终产物N₂,H₂O和CO₂/CO。3.匀相沉积沉淀方法制备的Au/Fe₂O₃/Al₂O₃催化剂在丙烯选择还原NO反应中同样也表现出良好的低温催化活性,在300℃时,当使用0.1%NO/0.1%C₃H₆/5%O₂/He为原料气,空速为30,000 h^-1的条件下,NO被选择还原为N₂的转化率可达43%,且NO转化为N₂O的转化率可以忽略。反应气氛中添加2%水蒸气可使催化活性略有提高,而20ppm SO₂的加入使低温催化活性有所降低。采用XRD、HRTEM、H₂-TPR、XPS等技术对Au/Fe₂O₃/Al₂O₃催化剂进行了表征,我们得出了与Au/CeO₂/Al₂O₃催化剂相似的结论:400℃的高温焙烧使得金的存在形式为金属态,粒径大小在3-5 nm之间。纳米金粒子与高度分散的Fe₂O₃之间存在的强相互作用促进了Fe₂O₃的还原。Au和Fe₂O₃之间的协同作用可能是Au/Fe₂O₃/Al₂O₃催化剂在丙烯选择还原NO反应中具有较高的低温催化活的原因之一。更多还原

【Abstract】 Supported gold catalysts have been the hot spot in heterogeneous catalysis due to their exceptionally high activities at low temperatures.Such a unique feature of gold gives it the potential to solve the cold-start emission problem of engines.Therefore,it is desirable to examine the catalytic activity of supported gold catalysts for NO_x reduction.In this work,the selective catalytic reduction（SCR） of NO_x by C₃H₆ in the excess of oxygen was investigated over Au/MO_x/Al₂O₃（M:Ce,Fe） catalysts.The correlation between the structure of catalysts and their catalytic behaviors was studied.The main results presented in the dissertation have been summarized as follows:1.Au/CeO₂/Al₂O₃ catalysts were prepared by deposition-precipitation method and exhibited good low-temperatures activity in C₃H₆-SCR of NO in the excess of oxygen.The conversion of NO to N₂ was 46%and the conversion to N₂O was negligible under the condition of 0.1%NO,0.1%C₃H₆,5%O₂ in He at 250℃and GHSV of 30,000 h^-1.Such a catalytic performance is superior to that of Pt/Al₂O₃,on which a significant amount of N₂O （about 30%） was formed under the same conditions.It was also found that adding 2%water vapour to the feed stream enhanced the NO conversion while the presence of 20 ppm SO₂ decreased NO conversion at low temperatures.It was interesting that in the simultaneous presence of 2%water vapour and 20 ppm SO₂,the conversion of NO to N₂ was increased and the temperature window was widened.The conversion of NO to N₂ reached 50%and the conversion to N₂O was negligible under the condition of 0.1%NO,0.1%C₃H₆,5%O₂,2%H₂O, 20 ppm SO₂ in He at 300℃and GHSV of 30,000 h^-1.2.The Au/CeO₂/Al₂O₃ catalysts were characterized by different methods:（a） TPD results demonstrated that surface NO_y（y≥2） species were formed from co-adsorption of NO and O₂. The presence of SO₂ inhibited the formation of surface NO_y（y≥2） species,which results in the loss of catalytic activity at low temperatures.The introduction of water vapour and SO₂ created more acid sites and suppressed the combustion of C₃H₆.Therefore,the conversion of NO to N₂ was increased and the temperature window was widened.（b） The results of XRD, HRTEM,H₂-TPR and XPS revealed that gold existed in the form of metallic gold by calcination at 400℃and the size of gold nanoparticles was in the range of 3-5 nm.CeO₂ can not only provide the active oxygen for the reaction,but also stabilize the gold particles against sintering.There was strong interaction between Au and CeO₂,which facilitated the reduction of CeO₂.The synergistic effect between Au and CeO₂ was probably responsible for the superior catalytic performance of the Au/CeO₂/Al₂O₃.（c） In situ reflectance Fourier infrared transform spectroscopy（DRIFTS） results showed that NO_y（y≥2） species were observed on the surface of Au/CeO₂/Al₂O₃ after exposure to the gas mixture of NO and O₂.Formate and acetate species were presented after exposure to the gas mixture of C₃H₆ and O₂.Cyanide （-CN） and isocyanate（-NCO） species were observed as important intermediates in C₃H₆-SCR reaction.A reaction mechanism of the C₃H₆-SCR of NO over Au/CeO₂/Al₂O₃ catalyst has been proposed:NO_y and C_xH_yO_z species are formed due to the oxidation of NO and partial oxidation of C₃H₆ in the presence of O₂,and then C_xH_yO_z species react with NO_y species to generate C_aH_bO_cN_d intermediates including -CN and -NCO species,and then the intermediates react with NO₂ or（NO+O₂） to produce N₂,H₂O and CO₂/CO.3.Au/Fe₂O₃/Al₂O₃ catalysts were prepared by the homogeneous deposition-precipitation method and exhibited good low-temperature activity in C₃H₆-SCR of NO in the excess of oxygen.The conversion of NO to N₂ was 43%and the conversion to N₂O was negligible under the condition of 0.1%NO,0.1%C₃H₆,5%O₂ in He at 300℃and GHSV of 30,000 h^-1. The addition of 2%water vapor to the feed stream enhanced slightly the NO conversion, while the presence of 20 ppm SO₂ decreased NO conversion at low temperatures.The catalysts were characterized by XRD,TEM,H₂-TPR and XPS techniques.The results indicated that that gold existed in the form of metallic gold by calcination at 400℃and the size of gold nanoparticles was in the range of 3-5 nm.There was strong interaction between Au and Fe₂O₃,which facilitated the reduction of Fe₂O₃.The synergistic effect between Au and Fe₂O₃ was probably responsible for the good catalytic performance of the Au/Fe₂O₃/Al₂O₃ catalyst at low temperatures.更多还原