| 1. |
- Simonsen, S. B., et al.
(författare)
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Direct Observations of Oxygen-induced Platinum Nanoparticle Ripening Studied by In Situ TEM
- 2010
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 132:23, s. 7968-7975
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Tidskriftsartikel (refereegranskat)abstract
- This study addresses the sintering mechanism of Pt nanoparticles dispersed on a planar, amorphous Al2O3 support as a model system for a catalyst for automotive exhaust abatement. By means of in situ transmission electron microscopy (TEM), the model catalyst was monitored during the exposure to 10 mbar air at 650 degrees C. Time-resolved image series unequivocally reveal that the sintering of Pt nanoparticles was mediated by an Ostwald ripening process. A statistical analysis of an ensemble of Pt nanoparticles shows that the particle size distributions change shape from an initial Gaussian distribution via a log-normal distribution to a Lifshitz-Slyozov-Wagner (LSW) distribution. Furthermore, the time-dependency of the ensemble-averaged particle size and particle density is determined. A mean field kinetic description captures the main trends in the observed behavior. However, at the individual nanoparticle level, deviations from the model are observed suggesting in part that the local environment influences the atom exchange process.
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| 2. |
- Simonsen, S. B., et al.
(författare)
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Ostwald ripening in a Pt/SiO2 model catalyst studied by in situ TEM
- 2011
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 281:1, s. 147-155
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Tidskriftsartikel (refereegranskat)abstract
- Sintering of Pt nanoparticles dispersed on a planar SiO(2) support was studied by in situ transmission electron microscopy (TEM). A time-lapsed TEM image series of the Pt nanoparticles, acquired during the exposure to 10 mbar synthetic air at 650 degrees C, reveal that the sintering was governed by the Ostwald ripening mechanism. The in situ TEM images also provide information about the temporal evolution of the Pt particle size distribution and of the growth or decay of the individual nanoparticles. The observed Pt nanoparticle changes compare well with predictions made by mean-field kinetic models for ripening, but deviations are revealed for the time-evolution for the individual nanoparticles. A better description of the individual nanoparticle ripening is obtained by kinetic models that include local correlations between neighboring nanoparticles in the atom-exchange process.
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| 3. |
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| 4. |
- Ek, M., et al.
(författare)
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Probing Catalyst Surfaces at the Atomic-scale
- 2023
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Ingår i: Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada. - 1435-8115. ; 29:1, s. 1291-1291
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Tidskriftsartikel (refereegranskat)
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| 5. |
- Ek, Martin, 1984, et al.
(författare)
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Redox dynamics of 2D crystalline vanadium oxide phases on high-index anatase facets
- 2023
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Ingår i: Nanoscale. - 2040-3372 .- 2040-3364. ; 15:21, s. 9503-9509
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Tidskriftsartikel (refereegranskat)abstract
- Vanadium oxides exist in a multitude of phases with varying structure and stoichiometry. This abundance of phases can be extended through the use of other oxides as supports, and through redox treatments. However, the combined effects of different supports and redox treatments can be difficult to identify, particularly when present as different terminating facets on nanoparticles. Here, we examine structural dynamics of 2D vanadium oxides supported on anatase TiO2 nanoparticles, correlated with changes in oxidation state, using in situ transmission electron microscopy imaging and electron energy loss spectroscopy. As the average oxidation state is reduced below V(iv), an ordered cubic V(ii) phase is observed exclusively at the high-index {10l} facets of the support. This local accommodation of highly reduced states is necessary for explaining the observed range of average oxidation states. In turn, the findings show that oxidation states extending from V(v)-V(iv) to V(ii) can be simultaneously stabilized by different supporting oxide surfaces during exposure to atmospheres with controlled redox potential.
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| 6. |
- Rasmussen, Morten K., et al.
(författare)
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Stable Cation Inversion at the MgAl2O4(100) Surface
- 2011
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Ingår i: Physical Review Letters. - 1079-7114. ; 107:3
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Tidskriftsartikel (refereegranskat)abstract
- From an interplay of atom-resolved noncontact atomic force microscopy, surface x-ray diffraction experiments, and density functional theory calculations, we reveal the detailed atomic-scale structure of the (100) surface of an insulating ternary metal oxide, MgAl2O4 (spinel). We surprisingly find that the MgAl2O4(100) surface is terminated by an Al and O-rich structure with a thermodynamically favored amount of Al atoms interchanged with Mg. This finding implies that so-called Mg-Al antisites, which are defects in the bulk of MgAl2O4, become a thermodynamically stable and integral part of the surface.
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| 7. |
- Simonsen, S. B., et al.
(författare)
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Coarsening of Pd nanoparticles in an oxidizing atmoshere studied by in situ TEM
- 2016
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 648, s. 278-283
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Tidskriftsartikel (refereegranskat)abstract
- The coarsening of supported palladiumnanoparticles in an oxidizing atmospherewas studied in situ by means of transmission electron microscopy (TEM). Specifically, the Pd nanoparticles were dispersed on a planar and amorphousAl2O3 support and were observed during the exposure to 10 mbar technical air at 650 °C. Time-resolved TEM image series reveal that the Pd nanoparticles were immobile and that a few percent of the nanoparticles grew or shrank, indicating a coarsening process mediated by the Ostwald ripening mechanism. The TEM image contrast suggests that the largest nanoparticles tended to wet the Al2O3 support to a higher degree than the smaller nanoparticles and that the distribution of projected particle sizes consequently broadens by the appearance of an asymmetric tail toward the larger particle sizes. A comparison with computer simulations based on a simple mean-fieldmodel for the Ostwald ripening process indicates that the observed change in the particle size distribution can be accounted for by wetting of the Al2O3 support by the larger Pd nanoparticles.
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| 8. |
- Simonsen, S. B., et al.
(författare)
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Effect of particle morphology on the ripening of supported Pt nanoparticles
- 2012
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:9, s. 5646-5653
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Tidskriftsartikel (refereegranskat)abstract
- To improve the understanding of sintering in diesel and lean-burn engine exhaust aftertreatment catalysts, we examined oxygen-induced sintering in a model catalyst consisting of Pt nanoparticles supported on a planar, amorphous Al2O3 substrate. After ageing at increasingtemperatures, transmission electron microscopy analysis reveals that highly monodisperse ensembles of nanoparticles transformed into ensembles with bimodal and subsequently Lifshitz-Slyozov-Wagner particle size distributions. Moreover, scanning transmission electron microscopy and atomic force microscopy analysis suggest that the Pt nanoparticles have size-dependent morphologies after sintering in oxidizing environment. The evolution of the particle sizes is described by a simple kinetic model for ripening and the size-dependent particle morphology is proposed as an explanation for the observed bimodal particle size distribution shapes.
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