Berlier Gloria, Prestipino Carmelo, Rivallan Mickaël, Bordiga Silvia, Lamberti Carlo, Zecchina Adriano
Department of Inorganic, Physical and Materials Chemistry, and INSTM Research Unity of Turin University, NIS Center of Excellence, University of Torino, via P. Giuria, 7 I-10125 Torino, Italy.
J Phys Chem B. 2005 Dec 1;109(47):22377-85. doi: 10.1021/jp052210+.
We report on the characterization of an isomorphously substituted Fe-MCM-22 sample containing both Fe and Al in framework positions (Si/Fe = 44, Si/Al = 25). XANES spectroscopy was used to study the evolution of Fe sites as a consequence of thermal activation at high temperature (1073 K) and subsequent oxidation with N2O. The results were compared to those obtained in the same conditions on a well-known Fe-silicalite sample (Si/Fe = 68, Si/Al = infinity). In both samples, thermal activation causes migration of a fraction of Fe ions from framework to extraframework positions, this migration being accompanied by a reduction of Fe3+ to Fe2+. Upon oxidation with N2O at 523 K, the two samples show a different behavior. While in Fe-silicalite practically all of the Fe2+ sites formed by thermal activation are reoxidized to Fe3+, in Fe-MCM-22 only a fraction of the extraframework iron sites is involved in the reoxidation process. The accessibility of the extraframework Fe sites was also investigated by using the NO molecule as a surface probe. Upon NO dosage on the sample, the modification of the pre-edge peak and of the edge position suggests an important charge release from the extraframework Fe2+ ions to the adsorbed molecules. This could be formalized with the formation of Fe3+(NO-) complexes, compatible (on the basis of the simple molecular orbital theory) with a bent NO geometry. The formation of a complex family of Fe2+ mono-, di-, and trinitrosyl complexes was also confirmed by FTIR spectroscopy. Similarly to what was observed in the oxidation experiments, the fraction of extraframework Fe sites able to interact with NO in Fe-MCM-22 sample is smaller than that in Fe-silicalite treated in the same conditions. This trend is explained with a major clustering of extraframework Fe sites in Fe-MCM-22 sample, as was also suggested by FTIR experiments. These results suggest that the dispersion of iron in zeolitic matrixes prepared by isomorphous substitution could also depend on the zeolitic structure.
我们报道了一种通过同晶取代制备的Fe-MCM-22样品的表征，该样品的骨架位置同时含有铁和铝（硅/铁 = 44，硅/铝 = 25）。利用X射线吸收近边结构（XANES）光谱研究了高温（1073 K）热活化以及随后用N₂O氧化后铁位点的演变。将结果与在相同条件下对一种著名的铁硅沸石样品（硅/铁 = 68，硅/铝 = ∞）所获得的结果进行了比较。在这两种样品中，热活化都会导致一部分铁离子从骨架位置迁移到骨架外位置，这种迁移伴随着Fe³⁺还原为Fe²⁺。在523 K用N₂O氧化后，这两种样品表现出不同的行为。在铁硅沸石中，通过热活化形成的几乎所有Fe²⁺位点都会重新氧化为Fe³⁺，而在Fe-MCM-22中，只有一部分骨架外铁位点参与了再氧化过程。还使用NO分子作为表面探针研究了骨架外铁位点的可及性。在样品上加入NO后，预边峰和边位置的变化表明从骨架外Fe²⁺离子向吸附分子有重要的电荷释放。这可以通过形成Fe³⁺(NO⁻)配合物来形式化，根据简单分子轨道理论，该配合物与弯曲的NO几何结构相容。傅里叶变换红外光谱（FTIR）也证实了形成了一系列复杂的Fe²⁺单亚硝基、二亚硝基和三亚硝基配合物。与氧化实验中观察到的情况类似，在Fe-MCM-22样品中能够与NO相互作用的骨架外铁位点的比例小于在相同条件下处理的铁硅沸石中的比例。FTIR实验也表明，这种趋势可以用Fe-MCM-22样品中骨架外铁位点的主要聚集来解释。这些结果表明，通过同晶取代制备的沸石基质中铁的分散情况也可能取决于沸石结构。
