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Structure and Catalytic Performance of Silver-Manganese Oxide Sieves

Author: CaoQingQing
Tutor: TangXingFu；JiaLiWei
School: Fudan University
Course: Environmental Engineering
Keywords: manganese oxide octahedral molecular sieves (Ag-OMS-2) fine structure analysis catalytic oxidation of formaldehyde oxygen activation NH3-SCR reaction mechanism
CLC: O643.3
Type: Master's thesis
Year: 2011
Downloads: 139
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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.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic
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