Characterization and FTIR studies of MnOx-CeO2 catalyst for low-temperature selective catalytic reduction of NO with NH3
Characterization and FTIR studies of MnOx-CeO2 catalyst for low-temperature selective catalytic reduction of NO with NH3
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DOI：
10.1021/jp048431h
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发表时间：
2004-10-07
影响因子：
3.3
通讯作者：
Yang, RT
中科院分区：
文献类型：
--
作者：
Qi, GS;Yang, RT
制备了一系列用于低温（373-453 K）氨选择性催化还原（SCR）NO的高活性锰铈氧化物催化剂。分别采用柠檬酸法（CA）、共沉淀法（CP）和浸渍法（IM）制备了纳米TiO 2，并用XRD、ESR、XPS和FTIR等技术对其进行了表征。通过CA方法制备的样品MnOx（0.3）-CeO 2（923）显示出最高的活性。XRD结果表明，CA法制备的催化剂粒径最小，XRD峰强度最弱。利用ESR和XPS，Mn 4+，Mn 3+，Mn 2+的氧化物物种发现后，在空气中煅烧。CA法制备的MnOx-CeO 2催化剂中存在3种Mn相：（1）在CeO 2载体上聚集的Mn 2 O3;（2）与CeO 2相互作用强的高分散的Mn 2 O3;（3）Mn原子嵌入CeO 2晶格中。Mn物种的分布取决于制备方法。由于Mn原子的混入而在CeO 2晶格中形成的氧空位吸附并活化分子氧，形成活性氧物种。因此，MnOx（0.3）-CeO 2（923）的高活性归因于高度分散的Mn物质和形成的更活泼的氧物质。氨分子吸附在MnOx（0.3）-CeO 2（923）上形成NH 4+和配位NH3。同时，由于H-提取，观察到NH 2。NO2，亚硝酸盐，硝酸盐的氧化NO。此反应的机理途径提出了早期的研究结果和FTIR结果的基础上，在这项工作中得到的。反应的第一步是NH3吸附在刘易斯酸中心上，然后生成NH 2，NH 2与NO反应生成N2-和H2O。提出了可能的中间体，所有中间体都能转化为NH_2NO，NH_2NO能进一步反应生成N_2-和H_2O。
A series of high-activity manganese-cerium oxide catalysts for the low-temperature (373-453 K) selective catalytic reduction (SCR) of NO, with ammonia were prepared. They were prepared by using the citric acid method (CA), coprecipitation method (CP), and impregnation method (IM) and were characterized by XRD, ESR, XPS, and FTIR techniques. A sample prepared by the CA method, MnOx(0.3)-CeO2(923), showed the highest activity. XRD results showed that the catalyst prepared by the CA method had the smallest particle size and the weakest XRD peak intensity. Using ESR and XPS, Mn4+, Mn3+, and Mn2+ oxide species were found after calcination in air. Three kinds of Mn phases existed in the MnOx-CeO2 catalysts that were prepared by the CA method: (1) aggregated Mn2O3 on the CeO2 support, (2) highly dispersed Mn2O3 with strong interactions with CeO2, and (3) Mn atoms incorporated into the CeO2 lattice. The distribution of Mn species depends on the preparation methods. Any oxygen vacancy formed in the CeO2 lattice caused by incorporation of Mn atoms adsorbs and activates molecular oxygen to form active oxygen species. Thus, the high activity of MnOx(0.3)-CeO2(923) is attributed to the highly dispersed Mn species and the more active oxygen species that is formed. Ammonia molecules adsorbed onto MnOx(0.3)-CeO2(923) to form NH4+ and coordinated NH3. At the same time, NH2 was observed because of H-abstraction. NO2, nitrite, and nitrate were formed by oxidation of NO. A mechanistic pathway for this reaction was proposed on the basis of earlier findings and the FTIR results obtained in this work. The initial step was adsorption of NH3 onto the Lewis acid sites, and then, the NH2 species was formed, followed by reaction between NH2 and NO to produce N-2 and H2O. Possible intermediates are proposed, and all the intermediates could transform into NH2NO, which could further react to produce N-2 and H2O.