摘要
汽车尾气中CO,HC,NO_x,硫化物及其颗粒粉尘严重危害人们身体健康和大气环境,是大气环境的主要污染源之一.目前,尾气净化是其减排的最主要方式.汽车尾气催化剂的发展经历了几代的研究,一直以来广泛采用Pt,Pd和Rh等贵金属,但因其资源匮乏,价格昂贵,容易被S和P中毒,因此人们逐渐将目光投向非贵金属催化剂的研发.钙钛矿复合氧化物因具有独特的物理化学性质以及灵活的"化学剪裁"特性而在材料研究等领域颇受青睐,有望成为贵金属催化剂的替代品.一般而言,催化剂的比表面积越大,表面活性位点越多,其催化活性越高,且会明显降低起燃温度.目前,一些制备工艺,如水热法、共沉淀法、微乳液法和硬模板法,虽可在一定程度上提高催化剂的比表面积,但却存在费时、耗能及制备工艺复杂等缺点.因此,如何简单有效地制备出大比表面积的钙钛矿型催化剂依然是一个难题.本文以合成的分级多孔δ-MnO_2微球为模板,采用熔盐法制备出球状多孔La_(1-x)Sr_xMn_(0.8)Fe_(0.2)O_3(0≤x≤0.6)钙钛矿氧化物,研究了球状多孔钙钛矿氧化物的形成过程和合适的制备温度,以及B位Fe_3+掺杂量为20%时A位Sr^(2+)掺杂量对钙钛矿催化剂结构和催化活性的影响.采用X射线粉末衍射、扫描电子显微镜、透射电子显微镜、N^2吸附-脱附、傅里叶红外光谱(FT-IR)和X射线能谱(XPS)等方法对催化剂进行了表征.在固定床石英管反应器上评价了催化剂催化CO氧化活性及稳定性,采用气相色谱联接氢火焰离子化检测器检测了产物和反应物的组成.结果表明,以分级多孔δ-MnO_2微球为模板,采用熔盐法在450oC反应4h制备出的球状多孔La_(1-x)Sr_xMn_(0.8)Fe_(0.2)O_3(0≤x≤0.6)钙钛矿氧化物具有良好的结晶性、较大的比表面积(55.73m^2/g)和孔体积(0.37cm^3/g).其球状多孔结构的形成可分为两个阶段:原位形成钙钛矿相和纳片表面析出钙钛矿晶粒及钙钛矿晶粒的再生长.另外,FT-IR光谱表明,Fe^(3+)和Sr^(2+)成功进入A,B位.同时,CO转化曲线表明,B位Fe^(3+)的掺杂量为20%时,A位Sr^(2+)的掺杂量高于30%时可以明显改善催化剂催化CO氧化活性:La_(1-x)Sr_xMn_(0.8)Fe_(0.2)O_3(0≤x≤0.3)的T_(50)和T_(90)分别在180和198℃左右;而La_(0.55)Sr_(0.45)Mn_(0.8)Fe_(0.2)O_3和La_(0.4)Sr_(0.6)Mn_(0.8)Fe_(0.2)O_3的T_(50)均低于125℃;La_(0.55)Sr_(0.45)Mn_(0.8)Fe_(0.2)O_3的T_(90)为181℃,而La_(0.4)Sr_(0.6)Mn_(0.8)Fe_(0.2)O_3却仍低于125℃.XPS结果则证明,较高的催化活性得益于La_(0.4)Sr_(0.6)Mn_(0.8)Fe_(0.2)O_3表面存在较多的Mn+、氧空位及吸附氧.最后,La_(0.55)Sr_(0.45)Mn_(0.8)Fe_(0.2)O_3和La_(0.4)Sr_(0.6)Mn_(0.8)Fe_(0.2)O_3的稳定性测试结果表明,采用熔盐法以δ-MnO_2为模板在450℃焙烧4h制备的多孔球状钙钛矿具有较好的催化稳定性.虽然催化剂制备工艺简单,周期短,但比表面积最大只有55.73m^2/g,为硬模板法的1/2,因此提高比表面积将是今后研究的方向.
A molten salt method was developed to prepare porous La1‐xSrxMn0.8Fe0.2O3 （0≤ x ≤ 0.6） micro‐spheres using hierarchical porous δ‐MnO2 microspheres as a template in eutectic NaNO3‐KNO3. X‐ray diffraction patterns showed that single phase LaMn0.8Fe0.2O3 with good crystallinity was syn‐thesized at 450℃ after 4 h. Transmission electron microscope images exhibited that the LaMn0.8Fe0.2O3 sample obtained at 450？？ after 4 h possessed a porous spherical morphology com‐posed of aggregated nanocrystallites. Field emission scanning electron microscope images indicated that the growth of the porous LaMn0.8Fe0.2O3 microspheres has two stages. SEM pictures showed that a higher calcination temperature than 450？？ had an adverse effect on the formation of a po‐rous spherical structure. The LaMn0.8Fe0.2O3 sample obtained at 450？？ after 4 h displayed a high BET surface area of 55.73 m2/g with a pore size of 9.38 nm. Fourier transform infrared spectra suggested that Sr2＋ions entered the A sites and induced a decrease of the binding energy between Mn and O. The CO conversion with the La1‐xSrxMn0.8Fe0.2O3 （0≤x≤0.6） samples indicated that the La0.4Sr0.6Mn0.8Fe0.2O3 sample had the best catalytic activity and stability. Further analysis by X‐ray photoelectron spectroscopy demonstrated that Sr2＋doping altered the content of Mn4＋ions, oxygen vacancies and adsorbed oxygen species on the surface, which affected the catalytic performance for CO oxidation.
基金
supported by the National Science Foundation for Young Scientists of China (51202171)~~
