| 1. |
- Baber, Ashleigh E., et al.
(författare)
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In Situ Imaging of Cu2O under Reducing Conditions : Formation of Metallic Fronts by Mass Transfer
- 2013
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 135:45, s. 16781-16784
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Tidskriftsartikel (refereegranskat)abstract
- Active catalytic sites have traditionally been analyzed based on static representations of surface structures and characterization of materials before or after reactions. We show here by a combination of in situ microscopy and spectroscopy techniques that, in the presence of reactants, an oxide catalyst's chemical state and morphology are dynamically modified. The reduction of Cu2O films is studied under ambient pressures (AP) of CO. The use of complementary techniques allows us to identify intermediate surface oxide phases and determine how reaction fronts propagate across the surface by massive mass transfer of Cu atoms released during the reduction of the oxide phase in the presence of CO. High resolution in situ imaging by AP scanning tunneling microscopy (AP-STM) shows that the reduction of the oxide films is initiated at defects both on step edges and the center of oxide terraces.
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| 2. |
- Brown, Rosemary, 1990, et al.
(författare)
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Unraveling the Surface Chemistry and Structure in Highly Active Sputtered Pt3Y Catalyst Films for the Oxygen Reduction Reaction
- 2020
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Ingår i: ACS Applied Materials & Interfaces. - : American Chemical Society (ACS). - 1944-8252 .- 1944-8244. ; 12:4, s. 4454-4462
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Tidskriftsartikel (refereegranskat)abstract
- Platinum is the most widely used and best performing sole element for catalyzing the oxygen reduction reaction (ORR) in low-temperature fuel cells. Although recyclable, there is a need to reduce the amount used in current fuel cells for their extensive uptake in society. Alloying platinum with rare-earth elements such as yttrium can provide an increase in activity of more than seven times, reducing the amount of platinum and the total amount of catalyst material required for the ORR. As yttrium is easily oxidized, exposure of the Pt-Y catalyst layer to air causes the formation of an oxide layer that can be removed during acid treatment, leaving behind a highly active pure platinum overlayer. This paper presents an investigation of the overlayer composition and quality of Pt3Y films sputtered from an alloy target. The Pt3Y catalyst surface is investigated using synchrotron radiation X-ray photoelectron spectroscopy before and after acid treatment. A new substoichiometric oxide component is identified. The oxide layer extends into the alloy surface, and although it is not completely removed with acid treatment, the catalyst still achieves the expected high ORR activity. Other surface-sensitive techniques show that the sputtered films are smooth and bulk X-ray diffraction reveals many defects and high microstrain. Nevertheless, sputtered Pt3Y exhibits a very high activity regardless of the film's oxide content and imperfections, highlighting Pt3Y as a promising catalyst. The obtained results will help to support its integration into fuel cell systems.
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| 3. |
- Duchon, Tomas, et al.
(författare)
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Establishing structure-sensitivity of ceria reducibility : real-time observations of surface hydrogen interactions
- 2020
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Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 8:11, s. 5501-5507
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Tidskriftsartikel (refereegranskat)abstract
- The first Layer of atoms on an oxide cataLyst provides the first sites for adsorption of reactants and the Last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria cataLyst with different surface terminations under an H2 environment to unequivocally establish the effect of the Last Layer of atoms on surface reduction. (111) and (100) terminated epitaxiaL isLands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of isLands, more pronounced oxygen release, and overall higher reducibility were observed for (100) isLands compared to (111) ones. The findings are in agreement with coordination -Limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts.
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