| 1. |
- Bengtsson, Alexander, et al.
(author)
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Characterization and modeling of acousto-optic signal strengths in highly scattering media
- 2019
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In: Biomedical Optics Express. - 2156-7085. ; 10:11, s. 5565-5584
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Journal article (peer-reviewed)abstract
- Ultrasound optical tomography (UOT) is an imaging technique based on the acousto-optic effect that can perform optical imaging with ultrasound resolution inside turbid media, and is thus interesting for biomedical applications, e.g. for assessing tissue blood oxygenation. In this paper, we present near background free measurements of UOT signal strengths using slow light filter signal detection. We carefully analyze each part of our experimental setup and match measured signal strengths with calculations based on diffusion theory. This agreement between experiment and theory allows us to assert the deep tissue imaging potential of similar to 5 cm for UOT of real human tissues predicted by previous theoretical studies [Biomed. Opt. Express 8, 4523 (2017)] with greater confidence, and indicate that future theoretical analysis of optimized UOT systems can be expected to be reliable. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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| 2. |
- Bosio, Noemi, 1993, et al.
(author)
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Interface Reactions Dominate Low-Temperature CO Oxidation Activity over Pt/CeO 2
- 2022
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:38, s. 16164-16171
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Journal article (peer-reviewed)abstract
- First-principles-based kinetic Monte Carlo simulations and kinetic experiments are used to explore CO oxidation over Pt/CeO2. The simulations compare CO oxidation over a ceria-supported 1 nm particle with simulations of a free-standing particle and Pt(111). The onset of the CO oxidation over ceria supported Pt is shifted to lower temperatures compared to the unsupported systems thanks to a Mars-van Krevelen mechanism at the Pt/CeO2 interface perimeter, which is not sensitive to CO poisoning. Both the Mars-van Krevelen mechanism and the conventional Langmuir-Hinshelwood mechanism over the Pt nanoparticle are contributing to the conversion after the reaction onset. The reaction orders in CO and O2 are compared experimentally for Pt/CeO2 and Pt/Al2O3. The reaction orders over Pt/CeO2 are close to zero for both CO and O2, whereas the corresponding reaction orders are-0.75 and 0.68 over Pt/Al2O3. The measured zero orders for Pt/CeO2 show the absence of CO/O2 site competition and underline the relevance of interface reactions. The measurements for Pt/Al2O3 indicate that the main reaction path for CO oxidation over Pt is a conventional Langmuir-Hinshelwood reaction. The results elucidate the interplay between condition-dependent reaction mechanisms for CO oxidation over Pt supported on reducible oxides.
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| 3. |
- Di, Mengqiao, 1994, et al.
(author)
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Chasing PtO x species in ceria supported platinum during CO oxidation extinction with correlative operando spectroscopic techniques
- 2022
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 409, s. 1-11
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Journal article (peer-reviewed)abstract
- Industrially relevant, highly dispersed, Pt/ceria and reference Pt/alumina catalysts with narrow Pt particle size distributions have been prepared, characterised ex situ and studied for CO oxidation by operando infrared and X-ray absorption spectroscopy. At high CO conversions, spectator CO ad-species on ionic platinum are observed while the CO oxidation proceeds on Pt particles in a high oxidation state exhibiting significant Pt[sbnd]O coordination. During the protracted catalytic extinction, the CO coverage builds up gradually while the Pt oxidation state and Pt[sbnd]O coordination remain high because of interactions with ceria. The observed CO oxidation at high CO coverage is suggested to involve sites at the platinum-ceria boundary that cannot be CO self-poisoned. This behaviour is in stark contrast to that of Pt/alumina, which shows removal of platinum oxides formed during CO oxidation and the classical drop in catalytic activity caused by rapid CO self-poisoning when reaching a critical temperature.
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| 4. |
- Di, Mengqiao, 1994
(author)
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CO Oxidation over Oxide Supported Platinum Catalysts
- 2023
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Doctoral thesis (other academic/artistic)abstract
- Catalytic oxidation of carbon monoxide (CO) is one of the most studied reactions that still needs to be improved because of its practical use in the chemical industry including feedstock purification and applications such as emission control, in-door air cleaning, improvement of fuel cell efficiency, etc. Concerning CO emissions, the transportation sector is a large contributor. The development of modern powertrains and driving patterns lead to cold exhausts. Thus, catalysts must be active for CO oxidation at low temperatures, which is a challenge. Further, CO oxidation is influenced by other compounds in the exhausts that may either promote or inhibit essential catalytic functions. For combustion exhausts, water is definitely inevitable and nitrogen oxides are common components. This work scrutinizes the kinetics of CO oxidation over Pt/alumina and Pt/ceria catalysts through analysis of reaction orders obtained experimentally from flow-reactor measurements and theoretically by kinetic Monte Carlo simulations and connects this to kinetic model formulation. Further, the catalytic structure-function relationship is explored by operando infrared and X-ray absorption spectroscopy. The influence of water and nitrogen oxide on the CO oxidation kinetics is investigated with in situ infrared spectroscopy. Finally, iron oxide is explored as an active support for platinum with a focus on the structural dynamics of Pt/FeOx under reaction conditions. The results show that reaction orders depend on reaction conditions and operating mechanism, and the adsorbate-adsorbate interactions play a crucial role. Pt/ceria is active at lower temperatures than Pt/alumina thanks to lattice oxygen in the ceria support that participates according to a Mars-van Krevelen mechanism. This mechanism is promoted by water but inhibited by nitrogen oxide through nitrate formation. On Pt/alumina, the reaction proceeds via the Langmuir-Hinshelwod mechanism, which is also promoted by water and inhibited by nitrates. Finally, using iron oxide as support for Pt opens for a catalyst design with a support even more interacting with Pt than ceria in terms of redox properties at low temperatures.
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| 5. |
- Di, Mengqiao, 1994
(author)
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CO oxidation over platinum-ceria catalysts -- Structural dynamics and reaction mechanisms
- 2021
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Licentiate thesis (other academic/artistic)abstract
- Catalytic oxidation of carbon monoxide (CO) continues to be important due to its wide applicability in emission control, in-door air cleaning, fuel cell efficiency enhancement, chemical feedstocks purification etc. The transportation sector emits most of the CO emissions. This is because modern powertrains and driving patterns result in exhausts with low temperatures at which the catalyst cannot operate efficiently. Thus, catalysts for CO oxidation needs to be developed as to be more active at low temperatures, ideally, even at ambient conditions. The metal oxide supported platinum catalyst is a practical choice thanks to its high robustness, i.e., resistance to sintering and deactivation by water, carbon dioxide and sulfur species. Although it has been extensively studied, the influence of chemical and structural dynamics on the catalytic activity under reaction conditions is still debated, especially for industrial catalysts. This work aims at understanding the catalytic function of platinum highly dispersed onto ceria, which is a reducible support. The kinetic behaviours during catalytic extinction and reaction orders have been experimentally determined using a fixed-bed flow reactor. To explore the structure-function relationships, operando infrared and X-ray absorption spectroscopy have been used. Also, detailed reaction pathways have been simulated using kinetic Monte Carlo with kinetic parameters determined from ab initio calculations. The CO oxidation kinetics for Pt/ceria is qualitatively different from that of reference Pt/alumina. The extinction profile for Pt/ceria catalyst exhibits a smooth decay in CO conversion rather than a stepwise drop as for the Pt/alumina catalyst. This is due to the two supports modifying the Pt particles differently as well as complementary reaction paths towards CO2 facilitated by boundary sites only for the Pt/ceria catalyst. Furthermore, operando spectroscopy reveals that the Pt particles bind strongly with ceria showing an unaltered Pt-O bond distance of 2 Å during catalytic extinction. Although difficult to experimentally determine, charge transfer from Pt particles to ceria supplemented with reverse spillover of ceria lattice oxygen to the vicinity of Pt particles likely occur.
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| 6. |
- Di, Mengqiao, 1994, et al.
(author)
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Why nitrogen oxide inhibits CO oxidation over highly dispersed platinum ceria catalysts
- 2024
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In: Catalysis Today. - 0920-5861. ; 426
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Journal article (peer-reviewed)abstract
- The influence of nitrogen oxide on the lean CO oxidation activity of highly dispersed Pt/ceria and reference Pt/alumina catalysts has been studied by kinetic measurements and infrared spectroscopic characterization. Co-feeding of nitrogen oxide leads to the formation of nitrates on the supports that induce a highly oxidized character of the Pt sites and in the case of Pt/ceria, inhibit ceria lattice oxygens to react with CO adsorbed on Pt rim sites via a Mars-van Krevelen mechanism below the ignition temperature. The build-up of nitrates below the light-off temperatures is faster when CO is present in the feed. Above the light-off temperatures, carbonates replace the nitrates while the catalytic activity remains high.
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| 7. |
- Feng, Yanyue, 1993, et al.
(author)
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Synthesis and Characterization of Catalytically Active Au Core─Pd Shell Nanoparticles Supported on Alumina
- 2022
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In: Langmuir. - : American Chemical Society (ACS). - 1520-5827 .- 0743-7463. ; 38:42, s. 12859-12870
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Journal article (peer-reviewed)abstract
- A two-step seeded-growth method was refined to synthesize Au@Pd core@shell nanoparticles with thin Pd shells, which were then deposited onto alumina to obtain a supported Au@Pd/Al2O3 catalyst active for prototypical CO oxidation. By the strict control of temperature and Pd/Au molar ratio and the use of l-ascorbic acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is formed around the Au core, as evidenced by transmission electron microscopy and energy-dispersive spectroscopy. The core@shell structure and the Pd shell remain intact upon deposition onto alumina and after being used for CO oxidation, as revealed by additional X-ray diffraction and X-ray photoemission spectroscopy before and after the reaction. The Pd shell surface was characterized with in situ infrared (IR) spectroscopy using CO as a chemical probe during CO adsorption-desorption. The IR bands for CO ad-species on the Pd shell suggest that the shell exposes mostly low-index surfaces, likely Pd(111) as the majority facet. Generally, the IR bands are blue-shifted as compared to conventional Pd/alumina catalysts, which may be due to the different support materials for Pd, Au versus Al2O3, and/or less strain of the Pd shell. Frequencies obtained from density functional calculations suggest the latter to be significant. Further, the catalytic CO oxidation ignition-extinction processes were followed by in situ IR, which shows the common CO poisoning and kinetic behavior associated with competitive adsorption of CO and O2 that is typically observed for noble metal catalysts.
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| 8. |
- Li, Qi, 1990, et al.
(author)
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Explanation of anomalous rate capability enhancement by manganese oxide incorporation in carbon nanofiber electrodes for electrochemical capacitors
- 2020
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In: Electrochimica Acta. - : Elsevier BV. - 0013-4686. ; 340
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Journal article (peer-reviewed)abstract
- Electrochemical capacitors (ECs) can provide ultra-long cycle life and ultra-fast energy delivery, characteristics which most battery technologies lack. Making composites out of carbon and pseudocapacitive materials is a popular strategy directed on narrowing the gap in energy density with regard to batteries. Usually, the incorporation of pseudocapacitive materials leads to a decrease in power performance compared to a pure carbon matrix, due to inferior electrical conductivity. This work, however, presents significant improvement in rate capability demonstrated by a composite electrode containing carbon nanofibers (NCNF) and manganese oxides (MnO2). The NCNF/MnO2 is prepared with a common method through the reaction with permanganate. The material has excellent performance metrics, especially a 78.2% rate capability (capacitance retention at 15 A g−1 relative to 0.5 A g−1), more than 10 times that for the NCNF carbon matrix. The exceptional enhancement can be explained by the development of micropores and surface area of NCNF, thus alleviating the “pore starvation” issue, and surface functional groups variation that enhances capacitive performance. This work highlights the importance of paying attention to the modification of carbon substrate when investigating carbon composite electrodes e.g. carbon/MnO2 networks.
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| 9. |
- Wang, Aiyong, 1989, et al.
(author)
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Insight into hydrothermal aging effect on Pd sites over Pd/LTA and Pd/SSZ-13 as PNA and CO oxidation monolith catalysts
- 2020
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In: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 278
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Journal article (peer-reviewed)abstract
- In this study, Pd/LTA and Pd/SSZ-13 were prepared and then hydrothermally aged at the temperature of 750, 800, 850, and 900 °C. Multiple Pd species, including isolated Pd ions (Pd2+ and [Pd(OH)]+) and 1∼2 nm PdOx nanoparticles, were presented in two fresh samples. The Pd/LTA sample showed remarkable hydrothermal stability, but the Pd/SSZ-13 sample experienced severe damage after aging at 900 °C. The destruction of the aged Pd/SSZ-13 sample led to the migration and sintering of PdOx nanoparticles, which formed bulk PdOx particles on the surface of the zeolite crystallite. A large number of PdOx nanoparticles were retained after aging of the Pd/LTA sample. Pd/LTA contained a higher concentration of Pd2+ sites, while Pd/SSZ-13 had more [Pd(OH)]+ sites. It is found that the improvement of NO adsorption ability with CO addition onto Pd2+ was more significant than onto [Pd(OH)]+.
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