| 1. |
- Asahina, Shunsuke, et al.
(författare)
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A new HRSEM approach to observe fine structures of novel nanostructured materials
- 2011
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Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 146:1-3, s. 11-17
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Tidskriftsartikel (refereegranskat)abstract
- A new approach for observing fine structures of novel thin, nanostructured materials called through the employed to observe interesting features on a variety of new, catalyticallyimportant hierarchically porous rattlespheres.
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| 2. |
- Castro, Maria, et al.
(författare)
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Zeolite Beta Formation from Clear Sols : Silicate Speciation, Particle Formation and Crystallization Monitored by Complementary Analysis Methods
- 2016
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539. ; 22:43, s. 15307-15319
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Tidskriftsartikel (refereegranskat)abstract
- The formation of silicate nanoaggregates (NAs) at the very early stages of precursor sols and zeolite beta crystallization from silicate nanoparticles (NPs) are investigated in detail using a combination of different analysis methods, including liquid-state 29Si, 27Al, 14N, and 1H NMR spectroscopy, mass spectrometry (MS), small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and transmission electron microscopy at cryogenic temperatures (cryo-TEM). Prior to hydrothermal treatment, silicate NAs are observed if the Si/OH ratio in the reaction mixture is greater than 1. Condensation of oligomers within the NAs then generates NPs. Aluminum doped into the synthesis mixtures is located exclusively in the NPs, and is found exclusively in a state that is fourfold connected to silicate, favoring their condensation and aggregation. These results are in agreement with general trends observed for other systems. Silicate NAs are essential intermediates for zeolite formation and are generated by the aggregation of hydrated oligomers, aluminate, and templating cations. Subsequent further intra-nanoaggregate silicate condensation results in the formation of NPs. 1H and 14N liquid NMR as well as diffusion ordered spectroscopy (DOSY) experiments provide evidence for weakly restricted rotational and translational mobility of the organic template within NAs as a consequence of specific silicate–template interactions. NAs thus appear as key species in clear sols, and their presence in the precursor sol favors silicate condensation and further crystallization, promoted either by increasing the Si/OH ratio or by heating.
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| 3. |
- Lu, An-Hui, et al.
(författare)
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Spatially and Size Selective Synthesis of Fe-Based Nanoparticles on Ordered Mesoporous Supports as Highly Active and Stable Catalysts for Ammonia Decomposition
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:40, s. 14152-14162
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Tidskriftsartikel (refereegranskat)abstract
- Uniform and highly dispersed gamma-Fe2O3 nanoparticles with a diameter of similar to 6 nm supported on CMK-5 carbons and C/SBA-15 composites were prepared via simple impregnation and thermal treatment. The nanostructures of these materials were characterized by XRD, Mossbauer spectroscopy, XPS, SEM, TEM, and nitrogen sorption. Due to the confinement effect of the mesoporous ordered matrices, gamma-Fe2O3 nanoparticles were fully immobilized within the channels of the supports. Even at high Fe-loadings (up to about 12 wt %) on CMK-5 carbon no iron species were detected on the external surface of the carbon support by XPS analysis and electron microscopy. Fe2O3/CMK-5 showed the highest ammonia decomposition activity of all previously described Fe-based catalysts in this reaction. Complete ammonia decomposition was achieved at 700 degrees C and space velocities as high as 60 000 cm(3) g(cat)(-1) h(-1). At a space velocity of 7500 cm(3) g(cat)(-1) h(-1), complete ammonia conversion was maintained at 600 degrees C for 20 h. After the reaction, the immobilized gamma-Fe2O3 nanoparticles were found to be converted to much smaller nanoparticles (gamma-Fe2O3 and a small fraction of nitride), which were still embedded within the carbon matrix. The Fe2O3/CMK-5 catalyst is much more active than the benchmark NiO/Al2O3 catalyst at high space velocity, due to its highly developed mesoporosity. gamma-Fe2O3 nanoparticles supported on carbon-silica composites are structurally much more stable over extended periods of time but less active than those supported on carbon. TEM observation reveals that iron-based nanoparticles penetrate through the carbon layer and then are anchored on the silica walls, thus preventing them from moving and sintering. In this way, the stability of the carbon-silica catalyst is improved. Comparison with the silica supported iron oxide catalyst reveals that the presence of a thin layer of carbon is essential for increased catalytic activity.
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