| 1. |
- Amandusson, Helena
(author)
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Hydrogen Extraction with Palladium Based Membranes
- 2000
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Doctoral thesis (other academic/artistic)abstract
- Palladium membranes are commercially used to purify hydrogen gas and in dehydrogenation reactions. The combination of the catalytic ability of the membrane surface and the selectivity of hydrogen permeation offers a tool to extract pure hydrogen and to shift a dehydrogenation reaction towards the product side. In this thesis, hydrogen extraction over palladium and palladium-silver based membranes both from different gas mixtures and from dehydrogenated organic molecules is investigated. The aim has been to find the optimal conditions for hydrogen extraction in different environments.The hydrogen permeation rate has been shown to depend on both silver concentration on the surface and in the bulk of a palladium based membrane. The diffusion through the membrane is the rate limiting step in the permeation process of most studied membranes. For a palladium membrane with 20 Å silver deposited on the upstream surface, the surface reactions, however, become rate limiting.Co-adsorbed oxygen will inhibit hydrogen permeation by blocking hydrogen adsorption sites and by consuming already adsorbed hydrogen in the water forming reaction on Pd membrane surfaces. On Pd70Ag30 membranes, however, oxygen has no effect on the hydrogen permeation rate, mainly due to an effective hydrogen dissolution into silver and a strongly reduced water formation rate. CO blocks hydrogen adsorption sites on both Pd and PdAg membranes effectively below 150°C, but above 300°C, CO has almost no effect on hydrogen permeation.Hydrogen can also be extracted through the dehydrogenation of organic molecules. A steady and continuous dehydrogenation of methanol and ethanol, and a subsequent hydrogen permeation, can be maintained in the presence of oxygen through both Pd and PdAg membranes. Without oxygen, a blocking contaminating layer is formed from the decomposition products, which prevents alcohol adsorption and thus also the hydrogen permeation. The hydrogen yield is larger over PdAg membranes than over Pd membranes mainly due to a smaller hydrogen consumption in the water forming reaction, but also due to a larger conversion of the alcohol on PdAg.The long time objective of this research has been to develop a method to extract hydrogen from anaerobic bacteria degradation of organic waste material in a co-operation project with microbiologists at the Department of Water and Environmental Studies at Linköping University. The selectivity towards hydrogen permeation in palladium membranes offers a tool to obtain clean hydrogen, which can be used as an energy carrier. By draining the bacteria culture of hydrogen, and thereby reducing the partial pressure of hydrogen, the fermentation process is directed towards a higher production of hydrogen.
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| 2. |
- Chakraborty, Debasish, et al.
(author)
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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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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:38, s. 16194-16203
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Journal article (peer-reviewed)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. |
- Frydendal, Rasmus, et al.
(author)
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Benchmarking the Stability of Oxygen Evolution Reaction Catalysts: The Importance of Monitoring Mass Losses
- 2014
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In: ChemElectroChem. - : Wiley. - 2196-0216. ; 1:12, s. 2075-2081
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Journal article (peer-reviewed)abstract
- Because of the rising need for energy storage, potentially facilitatedby electrolyzers, improvements to the catalysis of theoxygen evolution reaction (OER) become increasingly relevant.Standardized protocols have been developed for determiningcritical figures of merit, such as the electrochemical surfacearea, mass activity and specific activity. Even so, when new andmore active catalysts are reported, the catalyst stability tendsto play a minor role. In this work, we monitor corrosion onRuO2 and MnOx by combining the electrochemical quartz crystalmicrobalance (EQCM) with inductively coupled plasma massspectrometry (ICP–MS). We show that a meaningful estimationof the stability cannot be achieved based on purely electrochemicaltests. On the catalysts tested, the anodic dissolutioncurrent was four orders of magnitude lower than the total current.We propose that even if long-term testing cannot be replaced,a useful evaluation of the stability can be achievedwith short-term tests by using EQCM or ICP–MS.
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| 4. |
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| 5. |
- Koroidov, Sergey, et al.
(author)
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Chemisorbed oxygen or surface oxides steer the selectivity in Pd electrocatalytic propene oxidation observed by operando Pd L-edge X-ray absorption spectroscopy
- 2021
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In: Catalysis Science & Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 11:10, s. 3347-3352
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Journal article (peer-reviewed)abstract
- Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8–1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. These reactions are proposed to proceed via the Langmuir–Hinshelwood mechanism. In contrast, for the potential range of 1.1–1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface.
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| 6. |
- Nierhoff, Anders, et al.
(author)
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Adsorbate induced surface alloy formation investigated by near ambient pressure X-ray photoelectron spectroscopy
- 2015
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In: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 244, s. 130-135
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Journal article (peer-reviewed)abstract
- Formation of meta-stable surface-alloys can be used as a way to tune the binding strength of reaction intermediates and could therefore be used as improved catalyst materials for heterogeneous catalysis. Understanding the role of adsorbates on such alloy surfaces can provide new insights for engineering of more active or selective catalyst materials. Dynamical surface changes on alloy surfaces due to the adsorption of reactants in high gas pressures are challenging to investigate using standard characterization tools. Here we apply synchrotron illuminated near ambient pressure X-ray photoelectron spectroscopy to probe the surface structure of a CuPt model system. We report an investigation of the formation of a CuPt surface alloy induced and stabilized by exposure to mbar pressures of CO. The location of Pt within the alloy is assessed by deconvolution of the Pt 4f core level spectra into surface Pt and bulk Pt contributions. The study provides direct evidence on how it is possible to monitor the surface structure under near operation conditions. (C) 2014 Elsevier B.V. All rights reserved.
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| 7. |
- Pedersen, Anders F., et al.
(author)
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Operando XAS Study of the Surface Oxidation State on a Monolayer IrOx, on RuOx and Ru Oxide Based Nanoparticles for Oxygen Evolution in Acidic Media
- 2018
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 122:2, s. 878-887
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Journal article (peer-reviewed)abstract
- Herein we present surface sensitive operando XAS L-edge measurements on IrOx/RuO2 thin films as well as mass-selected RuOx and Ru nanoparticles. We observed shifts of the white line XAS peak toward higher energies with applied electrochemical potential. Apart from the case of the metallic Ru nanoparticles, the observed potential dependencies were purely core-level shifts caused by a change in oxidation state, which indicates no structural changes. These findings can be explained by different binding energies of oxygenated species on the surface of IrOx and RuOx. Simulated XAS spectra show that the average Ir oxidation state change is strongly affected by the coverage of atomic O. The observed shifts in oxidation state suggest that the surface has a high coverage of O at potentials just below the potential where oxygen evolution is exergonic in free energy. This observation is consistent with the notion that the metal-oxygen bond is stronger than ideal.
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| 8. |
- Simonsen, S. B., et al.
(author)
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Coarsening of Pd nanoparticles in an oxidizing atmoshere studied by in situ TEM
- 2016
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In: Surface Science. - : Elsevier BV. - 0039-6028. ; 648, s. 278-283
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Journal article (peer-reviewed)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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| 9. |
- Simonsen, S. B., et al.
(author)
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Effect of particle morphology on the ripening of supported Pt nanoparticles
- 2012
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 116:9, s. 5646-5653
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Journal article (peer-reviewed)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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| 11. |
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