| 39421. |
- Kullgren, Jolla, et al.
(författare)
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B3LYP calculations of cerium oxides
- 2010
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics. - 0021-9606 .- 1089-7690. ; 132:5, s. 054110-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we evaluate the performance of density functional theory with the B3LYP functional for calculations on ceria (CeO2) and cerium sesquioxide (Ce2O3). We demonstrate that B3LYP is able to describe CeO2 and Ce2O3 reasonably well. When compared to other functionals, B3LYP performs slightly better than the hybrid functional PBE0 for the electronic properties but slightly worse for the structural properties, although neither performs as well as LDA+U(U = 6 eV) or PBE+U(U = 5 eV). We also make an extensive comparison of atomic basis sets suitable for periodic calculations of these cerium oxides. Here we conclude that there is currently only one type of cerium basis set available in the literature that is able to give a reasonable description of the electronic structure of both CeO2 and Ce2O3. These basis sets are based on a 28 electron effective core potential (ECP) and 30 electrons are attributed to the valence space of cerium. Basis sets based on 46 electron ECPs fail for these materials
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| 39422. |
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| 39423. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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DFT-based Monte Carlo Simulations of Impurity Clustering at CeO2(111)
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 121:28, s. 15127-15134
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Tidskriftsartikel (refereegranskat)abstract
- The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+U and lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+U data for all symmetrically inequivalent (F-)(n)(Ce3+)(n) nearest-neighbor clusters with n = 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ ions. The distributions of surface hydroxyl groups are noticeably different.
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| 39424. |
- Kullgren, Jolla, 1987-
(författare)
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Oxygen Vacancy Chemistry in Ceria
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cerium(IV) oxide (CeO2), ceria, is an active metal oxide used in solid oxide fuel cells and for the purification of exhaust gases in vehicle emissions control. Behind these technically important applications of ceria lies one overriding feature, namely ceria's exceptional reduction-oxidation properties. These are enabled by the duality of the cerium ion which easily toggles between Ce4+ and Ce3+. Here the cerium 4f electrons and oxygen vacancies (missing oxygen ions in the structure) are key players. In this thesis, the nature of ceria's f electrons and oxygen vacancies are in focus, and examined with theoretical calculations. It is shown that for single oxygen vacancies at ceria surfaces, the intimate coupling between geometrical structure and electron localisation gives a multitude of almost degenerate local energy mimima. With many vacancies, the situation becomes even more complex, and not even state-of-the-art quantum-mechanical calculations manage to predict the experimentally observed phenomenon of vacancy clustering. Instead, an alternative set of computer experiments managed to produce stable vacancy chains and trimers consistent with experimental findings from the literature and revealed a new general principle for surface vacancy clustering. The rich surface chemistry of ceria involves not only oxygen vacancies but also other active oxygen species such as superoxide ions (O2−). Experiments have shown that nanocrystalline ceria demonstrates an unusually large oxygen storage capacity (OSC) and an appreciable low-temperature redox activity, which have been ascribed to superoxide species. A mechanism explaining these phenomena is presented. The ceria surface is also known to interact with SOx molecules, which is relevant both in the context of sulfur poisoning of ceria-based catalysts and sulfur recovery from them. In this thesis, the sulfur species and key mechanisms involved are identified.
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| 39425. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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Oxygen Vacancy Clustering at the Ceria(111) surface
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Oxygen vacancy clustering at the ceria(111) surface have been studied with both force-field and density function theory methods. Two of the methods predict that stable clusters of surface oxygen vacancies should form on these surface, as seen in numerous experimental studies. We propose that vacancy clustering of both surface and sub-surface vacancies follow the simple principle of sharing their Ce(III) neighbors. For surface oxygen vacancies this leads to compact clusters separated by one surface lattice constant. On the other hand for sub-surface this leads to sparse clusters separated by two surface lattice constants.
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| 39426. |
- Kulshreshtha, Chandramouli, et al.
(författare)
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Excitation Intensity Dependent Carrier Dynamics of Chalcogen Heteroatoms in Medium-Bandgap Polymer Solar Cells
- 2017
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- The excitation intensity dependent carrier dynamics of blends with PC[70]BM of three new medium-band gap conjugated polymers with central chalcogen heteroatoms, PBDTfDTBX (X = O, T(Sulphur), Se) were studied. The PBDTfDTBX polymers (Poly[4,8-bis(5-(2-butyloctyl)thiophene-2-yl)benzo[1,2-b;4,5-b′]dithiophene-alt-4,7-bis(4-(2-ethylhexyl)-2-thienyl)-dithieno[3′,2′:3,4;2″,3″:5,6]benzo[1,2-c][1,2,5] furazan or thiadiazole or selenadiazole]) have symmetrical structures but exhibit different solar cell performances. In this study, we determined how the photogenerated charge carrrier dynamics of the PBDTfDTBX:PC[70]BM blends varies with the heteroatom by performing transient absorption measurements at various excitation intensities. It was found that the charge carrier dynamics of the PBDTfDTBX blends with X = T or Se heteroatoms are dependent on the excitation intensity whereas that of the PBDTfDTBO blend is independent of the intensity. The photogenerated charge carrier dynamics of the PBDTfDTBO:PCBM, PBDTfDTBT:PCBM, and PBDTfDTBSe:PCBM blends were all modeled globally and rates were estimated for different photophysical processes occurring on different time scales.
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| 39427. |
- Kulyk, Dmytro S., et al.
(författare)
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Charge inversion under plasma-nanodroplet reaction conditions excludes Fischer esterification for unsaturated fatty acids: a chemical approach for type II isobaric overlap
- 2024
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Ingår i: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 15:3, s. 914-922
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Tidskriftsartikel (refereegranskat)abstract
- Direct infusion ionization methods provide the highest throughput strategy for mass spectrometry (MS) analysis of low-volume samples. But the trade-off includes matrix effects, which can significantly reduce analytical performance. Herein, we present a novel chemical approach to tackle a special type of matrix effect, namely type II isobaric overlap. We focus on detailed investigation of a nanodroplet-based esterification chemistry for differentiating isotopologue [M + 2] signal due to unsaturated fatty acid (FA) from the monoisotopic signal from a saturated FA. The method developed involves the online fusion of nonthermal plasma with charged nanodroplets, enabling selective esterification of saturated FAs. We discovered that unsaturated FAs undergo spontaneous intramolecular reaction via a novel mechanism based on a carbocation intermediate to afford a protonated lactone moiety (resonance stabilized cyclic carbonium ion), whose mass is the same as the original protonated unsaturated FA. Therefore, the monoisotopic signal from any saturated FA can be selectively shifted away from the mass-to-charge position where the isobaric interference occurs to enable effective characterization by MS. The mechanism governing the spontaneous intramolecular reactions for unsaturated FAs was validated with DFT calculations, experimentation with standards, and isotope labeling. This novel insight achieved via the ultrafast plasma-nanodroplet reaction environment provides a potentially useful synthetic pathway to achieve catalyst-free lactone preparation. Analytically, we believe the performance of direct infusion MS can be greatly enhanced by combining our approach with prior sample enrichment steps for applications in biomedicine and food safety. Also, combination with portable mass spectrometers can improve the efficiency of field studies since front-end separation is not possible under such conditions. The fusion of nonthermal plasma with charged nanodroplets enables selective esterification of saturated fatty acids, which is utilized to overcome challenges associated with type II isobaric overlap in direct infusion mass spectrometry.
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| 39428. |
- Kumagai, Yuta, et al.
(författare)
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gamma-Radiation and H2O2 induced oxidative dissolution of uranium(iv) oxide in aqueous solution containing phthalic acid
- 2020
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Ingår i: Dalton Transactions. - : ROYAL SOC CHEMISTRY. - 1477-9226 .- 1477-9234. ; 49:6, s. 1907-1914
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Tidskriftsartikel (refereegranskat)abstract
- This study aims to reveal possible involvements of organic acids in the oxidative dissolution of UO2. Using phthalic acid as a model compound, we have measured adsorption on UO2 and investigated effects on the reaction between H2O2 and UO2 and on oxidative dissolution induced by gamma-irradiation. Significant adsorption of phthalic acid was observed even at neutral pH. However, the reaction between H2O2 and UO2 in phthalic acid solution induced oxidative dissolution of U(vi) similar to in aqueous bicarbonate solution. Moreover, degradation products of phthalic acid were not detected after the reaction of H2O2. These results indicate that even though phthalic acid adsorbs on the UO2 surface, it is not involved in the interfacial reaction of H2O2. In contrast, the dissolution of U by irradiation was inhibited in aqueous phthalic acid solution, whereas H2O2 generated by radiolysis was consumed by UO2. Based on these contrasting results, possible roles of radical species generated by water radiolysis were discussed.
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| 39429. |
- Kumaniaev, Ivan, et al.
(författare)
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Adsorption Isotherms of Lignin-Derived Compounds on a Palladium Catalyst
- 2019
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Ingår i: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 58:16, s. 6899-6906
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the interaction of lignin fragments obtained from catalytic fractionation with a heterogeneous palladium catalyst. By studying the adsorption of verified substrate and product molecules on the palladium surface, understanding of what governs adsorption and desorption dynamics of both substrates and products has been obtained. In addition, we have studied the kinetic isotope effect of hydrogen-transfer reactions occurring on the surface of the catalyst. These studies give insights into the thermodynamics of the process in which species from lignin-derived species adsorb to the catalyst surface, are then transformed by hydrogenation–hydrogenolysis reactions in a slow reaction step, and finally desorbed. It was found that the adsorption dynamics depended on the degree of unsaturation as well as the presence of methoxy groups on the aryl. Thereby, the adsorption is stronger for substrate molecules derived from lignin than for reduced molecules obtained after the rate-determining transfer-hydrogenation and hydrogenolysis transformations.
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| 39430. |
- Kumaniaev, Ivan, et al.
(författare)
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Conversion of birch bark to biofuels
- 2020
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Ingår i: Green Chemistry. - 1463-9262 .- 1463-9270. ; 22:7, s. 2255-2263
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Tidskriftsartikel (refereegranskat)abstract
- Substitution of fossil energy sources for bio-based ones will require development of efficient processes that can convert inedible and preferably low-value fractions that currently are not used into high-value products. It is desirable that such processes are developed so that both current logistics and infrastructure can be used. Bark, which is the outer layer of woody biomass, is currently burnt in a low-value process or left in the forests to decay and is therefore considered waste. In this work, birch (Betula pendula) bark was converted to hydrocarbons suitable for use in both road and aviation fuels in two efficient steps. Development of an efficient, recyclable, salt- and metal-free solvent-based system to solubilize birch bark under benign reaction conditions was a key outcome. The obtained gum was composed of organosolv lignin and suberin oligomers and was fully characterized. This gum had unique properties and could be directly processed in a conventional hydroprocessing unit set-up to afford hydrocarbons in the road and aviation fuel ranges. Life cycle assessment was applied to evaluate different scenarios for implementing this technology. When using bark generated as a forestry by-product and current infrastructure in a pulp mill, the process had a favorable low carbon dioxide footprint for biofuel generation.
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