Applications of XAS on Characterization of Metal Containing Molecular Sieves and Metal Oxides

• Main Authors: An Chyi Wei, 魏安祺
• Other Authors: Kuei-Jung Chao
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/17319056968248040788

博士 === 國立清華大學 === 化學系 === 89 === The object of this thesis is the applications of synchrotron on the characterization of transition metals (V, Cr, Fe, Co) and main group metal (Ga) containing molecular sieves, zeolite supported nobel metal (Pt) catalysts. Metal-containing aluminophosphate molecular sieves (MeAPOs) were observed to exhibit remarkable catalytic activity in partial oxidation of hydrocarbons under mild condition. The Co K-edge XAS of CoAPO-11 showed that cobalt ions are mainly in an oxidation state of 2+ state during redox treatments, and that cobalt might distorted from the regular tetrahedral site of as-synthesized form and posses an oxygen vacancy during redox treatments. High resolution X-ray powder and single crystal diffractions showed that dehydrated, calcined AlPO4-5 and CoAPO-5 have the commensurate c axis modulated helical structure, with modulation wave vector q, which is one dimensional and determined as 1/8c*. The commensurate modulated structure is reversible during hydration and dehydration processes. The in-situ V K-edge XAS of VAPO-5 showed that the oxidation and reduction between V4+ and V5+ are reversible, and isolated vanadium exists as di-oxo V5+O2(Of)2 on the calcined and dehydrated VAPO-5 and as mono-oxo (V4+=O)(Of)4 species on the reduced and dehydrated VAPO-5. The XAS results are further confirmed by cluster modeling calculation based on density function theory (DFT) and Raman spectroscopy. The DFT calculation results also show that the substitution of either a phosphorus or an aluminum site (TO4, T = Al or P) of AFI framework by a vanadium (IV) ion is actually not feasible. The vanadium oxide may dock onto the 12-ring of the framework. The in-situ Ga K-edge XAS is used to probe the nature of gallium species in the hydrothermally synthesized ([Si,Ga]-beta) incipient- impregnated (Ga/H[Si,Al]-beta) and those under redox treatments. Ga in [Si,Ga]-beta behaved differently from Ga in Ga/H[Si,Al]-beta under reduction treatment. The atomically dispersed Ga3+ of Ga/H[Si,Al]-beta was observed to be not stable under reduction treatment and can fall out from tetrahedral sites to form gallium hydride oligomer. Ga ions might be introduced into defect sites on a lattice (such as the silanol groups) through calcining the impregnated sample. The 71Ga-NMR and DFT calculation results are consistent with the XAS results. For [Si,Ga]-beta, the DFT calculation suggests that Ga may be located at T7, T8 and T9 sites of the BEA framework. During the aromatiztion of light paraffins, gallium zeolite catalysts undergo extensive coking. Platinum was added to Ga/H[Si,Al]-beta in order to reduce the catalyst decay as well as to accelerate the dehydrogenation of alkanes. The in-situ XAS of Ga K-edge as well as Pt LIII-edge indicated the formation of Ga-Pt bonding with a bond distance of 2.51~2.54 Å after reduction. The labile reduced Ga species can either interact with H2 to form gallium hydride oligomer or adsorb on Pt particles through the formation of Ga-Pt bimetallic bonds. Vanadium and iron-containing silicates are well-established cases of transition metal ions incorporating silicates. The local environments around V sites in calcined (hydrated and dehydrated) and reduced (dehydrated) V-MCM-41 were investigated by in-situ V K-edge XAS and EPR spectroscopies. V=O3+ species was found to bind to the intraframework wall of MCM-41 through there V-O-Si bridges with V-O bond distances of 1.78 Å and V=O bond distance of 1.65 Å as for a VO4 unit. However, after reduction, VO4 units of one long V=O bond of 1.70 Å and two V-O bond of 1.98 Å in a pseduo-octahedral coordination were found to aggregate together through two V─O×××××V bridging bonds. EXAFS further showed the variation of the coordination geometry and dispersion of V5+ species on calcined V-MCM-41 in the presence of moisture from air under ambient temperature. V5+O4 units were found to be linked together through oxygen atoms of surface Si-OH groups with V-V distance of 3.1 Å and some elongation of V-O single bonds without any increase in V=O terminal bond. The Fe K-edge XAS was used to monitor the chemical states of Fe and their coordination to neighboring atoms in Fe/ZSM-11 and Fe/FER before and after Clauses reaction. It suggests that tetrahedral Fe3+ ions of Fe/ZSM-11 are in framework before reaction and may fall out from framework to form Fe2O3 after reaction. Tetrahedral framework Fe3+ species of Fe/FER were observed to migrate towards extraframework positions after reaction, probably forming of octahedral oxide microaggregates. This octahedral oxide clusters may stay in the intrachannel void space of FER zeolite.