| 1. |
- Bleken, Bjørn Tore L., et al.
(författare)
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Unit cell thick nanosheets of zeolite H-ZSM-5 : Structure and activity
- 2013
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Ingår i: Topics in catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 56:9-10, s. 558-566
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Tidskriftsartikel (refereegranskat)abstract
- nosheets of zeolite H-ZSM-5 were synthesized and characterized by X-ray diffraction, transmission electron microscopy (TEM), N2- physisorption, FT-IR spectroscopy, 27Al and 29Si MAS NMR spectroscopy in addition to catalytic testing in conversion of methanol to hydrocarbons (MTH). It was found that Rietveld analysis, involving anisotropic broadening parameters, gave average crystallite dimensions in good agreement with TEM images. The selectivities in MTH is intact in the mesoporous nanosheet H-ZSM-5 with the largest difference being a higher C3/C2 ratio compared to regular H-ZSM-5.
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| 2. |
- Chakraborty, Debasish, et al.
(författare)
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Reversible Atomization and Nano-Clustering of Pt as a Strategy for Designing Ultra-Low-Metal-Loading Catalysts
- 2022
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:38, s. 16194-16203
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Tidskriftsartikel (refereegranskat)abstract
- Noble metal-based catalysts have numerous industrial uses, and maximum utilization of the precious metals by lowering the metal loading is of significant interest in heterogeneous catalysis research. However, lowering the metal loading could lead to single-atom metal species formation, which may not be active for important reactions like propylene oxidation. We report a way to drastically reduce precious metal loading of catalysts by judiciously choosing an active metal/support pair and using the reversible atomization-nanoparticulate formation of transition metal on a high-surface area support. Here, Pt and MgAl2O4 are used as the transition metal and high-surface area support, respectively. Through catalytic testing and characterization using scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy, a reversible change between atomization and nano-cluster formation under oxidizing and reducing conditions has been found. Via density functional theory, favorable sites for reversible Pt adsorption are identified, including ionic Pt4+ sites that can serve to nucleate nanoclusters. Catalytic reaction modeling also rationalizes the catalytic inertness of atomic Pt sites. Finally, a re-activation mechanism for the atomized Pt based on gases present during reaction has been formulated and demonstrated.
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| 3. |
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| 4. |
- Ek, Martin, 1984, et al.
(författare)
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Probing surface-sensitive redox properties of VOx/TiO2 catalyst nanoparticles
- 2021
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 13:15, s. 7266-7272
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Tidskriftsartikel (refereegranskat)abstract
- Redox processes of oxide materials are fundamental in catalysis. These processes depend on the surface structure and stoichiometry of the oxide and are therefore expected to vary between surface facets. However, there is a lack of direct measurements of redox properties on the nanoscale for analysing the importance of such faceting effects in technical materials. Here, we address the facet-dependent redox properties of vanadium-oxide-covered anatase nanoparticles of relevance to, e.g., selective catalytic reduction of nitrogen oxides. The vanadium oxidation states at individual nanoscale facets are resolved in situ under catalytically relevant conditions by combining transmission electron microscopy imaging and electron energy loss spectroscopy. The measurements reveal that vanadium on {001} facets consistently retain higher oxidation states than on {10l} facets. Insight into such structure-sensitivity of surface redox processes opens prospects of tailoring oxide nanoparticles with enhanced catalytic functionalities.
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| 5. |
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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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| 9. |
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