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银锰分子筛的制备、结构以及对甲醛和氮氧化物的催化性能研究
Structure and Catalytic Performance of Silver-Manganese Oxide Sieves
【作者】 曹青青；
【作者基本信息】 复旦大学 ， 环境工程， 2011， 硕士
【摘要】 本论文主要研究了银锰分子筛(Ag-OMS-2)的结构,及其在治理空气污染物甲醛和氮氧化物的催化性能研究。本论文采用回流法成功合成了氧化锰八面体分子筛纳米棒,并在分子筛孔道内掺杂了银原子和银离子(Ag0-OMS-2和Ag+-OMS-2),利用同步辐射X射线吸收精细结构谱(XAFS)和X射线衍射的全谱拟合对银锰分子筛的结构进行了精细解析。TEM和XRD研究结果表明Ago成功掺杂于OMS-2分子筛的孔道中,形成了高分散的单原子Ag结构,这一结果的发现对金属单原子分散的研究上具有创新意义。ICP表征发现Ag0-OMS-2中K+相对载体K-OMS-2中K含量并未减少,说明Ag进入孔道并非通过离子交换的形式,而是通过占据OMS-2空的孔道。比较Ag0-OMS-2, Ag+-OMS-2, K-OMS-2三种催化剂催化甲醛完全氧化的性能,发现Ag0-OMS-2具有最高的催化活性。H2-TPR结果同时表明,Ago具有Ag0-OMS-2较好的低温还原性能。比表面积BET测试表明三种催化剂具有相似的外表面。因此,银锰分子筛催化氧化甲醛的吸附位和活性位点主要发生在孔口。Ag0-OMS-2具有较高催化氧化能力的原因：位于孔口的Ag0可极大地促进氧的活化,进一步将吸附在孔口的甲醛氧化,最终达到提高K-OMS-2催化完全氧化甲醛的目的。此外,本论文还研究了银锰分子筛在NH3-SCR反应上的构效关系。研究表明,NH3-SCR反应主要发生在银锰分子筛催化剂的外表面半孔道结构上,孔道中掺杂的金属原子／离子对NH3-SCR反应的影响不大。NH3-SCR反应机理在银锰氧分子筛的反应机理为：半孔道上的高活性晶格氧是锰氧分子筛具有优异NH3-SCR反应性能的主要原因。
【Abstract】 In this work, the fine structures of Ag-doped manganese oxide octahedral molecular sieves (Ag-OMS-2) were studied. Using an fluidized bed reactor system, we investigated the structure-activity relationships of the catalysts for complete oxidation of formaldehyde and NH3-SCR reactions.Manganese oxide octahedral molecular sieve nanorods were synthesized by a refluxing method. Silver atoms or silver ions were inserted in the tunnels of the molecular sieve (noted as Ag0-OMS-2 and Ag+-OMS-2 respectively). The fine structures of silver manganese molecular sieves were analyzed by synchrotron radiation X-ray absorption fine structure (XAFS) and X-ray diffraction whole-pattern fitting method, which showed that both silver atoms and silver ions were successfully inserted in the molecular sieve tunnels to form single atom arranged. Moreover, the silver atoms migrated gradually from silver particles on the outer surface of K-OMS-2 nanorods into the tunnels of molecular sieve after calcination. Whole-parttern fitting results also showed that there were two kinds of crystal structures in Ag0-OMS-2, K1.33Mn8O16 and Ag1.8Mn8O16 (41.8% and 58.2% respectively, mass percentage). ICP analysis showed that the K content in Ag0-OMS-2 did not reduce with the addition of Ag, indicating that the process of Ag nanoparticles into the channels was not ion exchange. Silver nanoparticles migarate into the molecular sieve tunnels can be described as:with a slow calcination, the silver particles anchored on the outer surface of nanorods moved along the semi-tunnels of OMS-2 to the ends of nanorods, and ultimately migrated into the tunnels, and the Ag single atom is stabilized by four O atoms in the tunnels with a square-planar coordination of Ag-O.By comparing the ability of complete oxidation of formaldehyde over the Ag0-OMS-2, Ag+-OMS-2, K-OMS-2 catalysts, we found that Ag0-OMS-2 had the highest catalytic activity, despite that the catalysts have the similar outer surface structure. The oxidation of formaldehyde by Ag-OMS-2 occurrs mainly in the tunnels ports (formaldehyde molecules can be adsorbed on the pores) and the reason why Ag0-OMS-2 had a higher oxidation capacity of formaldehyde is that:the Ag0 species filled in the pore can greatly promote the activation of lattice oxygen of the pore, which can further oxidate the formaldehyde moleculars adsorbed, and ultimately achieved the higher activities for formaldehyde oxidation. H2-TPR results also have the similar results, which showed that hydrogen spillover phenomenon occured on the surface of Ag0, which indicated that Ag0 in the pore could have the higher redox property.In addition, we also investigated the structure-activity relationship of NH3-SCR reaction over silver manganese oxide octahedral molecular sieves. The results showed that, NH3-SCR reaction occured mainly on the surface semitunnl structures of Ag-OMS-2. The structure-activity relationship revealed that both efficient semitunnel structured external surfaces and high active surface lattice oxygen atoms predominantly accounted for the high catalytic activities of Ag-OMS-2. Either modified by metal atoms/ions or not do not make significant difference.