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- Streich, Daniel, 1983-
(författare)
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Stepping into Catalysis : Kinetic and Mechanistic Investigations of Photo- and Electrocatalytic Hydrogen Production with Natural and Synthetic Molecular Catalysts
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In light of its rapidly growing energy demand, human society has an urgent need to become much more strongly reliant on renewable and sustainable energy carriers. Molecular hydrogen made from water with solar energy could provide an ideal case. The development of inexpensive, robust and rare element free catalysts is crucial for this technology to succeed. Enzymes in nature can give us ideas about what such catalysts could look like, but for the directed adjustment of any natural or synthetic catalyst to the requirements of large scale catalysis, its capabilities and limitations need to be understood on the level of individual reaction steps. This thesis deals with kinetic and mechanistic investigations of photo- and electrocatalytic hydrogen production with natural and synthetic molecular catalysts. Photochemical hydrogen production can be achieved with both E. coli Hyd-2 [NiFe] hydrogenase and a synthetic dinuclear [FeFe] hydrogenase active site model by ruthenium polypyridyl photosensitization. The overall quantum yields are on the order of several percent. Transient UV-Vis absorption experiments reveal that these yields are strongly controlled by the competition of charge recombination reactions with catalysis. With the hydrogenase major electron losses occur at the stage of enzyme reduction by the reduced photosensitizer. In contrast, catalyst reduction is very efficient in case of the synthetic dinuclear active site model. Here, losses presumably occur at the stage of reduced catalyst intermediates. Moreover, the synthetic catalyst is prone to structural changes induced by competing ligands such as secondary amines or DMF, which lead to catalytically active, potentially mononuclear, species. Investigations of electrocatalytic hydrogen production with a mononuclear catalyst by cyclic voltammetry provide detailed kinetic and mechanistic information on the catalyst itself. By extension of existing theory, it is possible to distinguish between alternative catalytic pathways and to extract rate constants for individual steps of catalysis. The equilibrium constant for catalyst protonation can be determined, and limits can be set on both the protonation and deprotonation rate constant. Hydrogen bond formation likely involves two catalyst molecules, and even the second order rate constant characterizing hydrogen bond formation and/or release can be determined.
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| 2. |
- Irebo, Tania, 1980-
(författare)
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Proton-Coupled Electron Transfer from Hydrogen-Bonded Phenols
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Proton-coupled electron transfer (PCET) is one of the elementary reactions occurring in many chemical and biological systems, such as photosystem II where the oxidation of tyrosine (TyrZ) is coupled to deprotonation of the phenolic proton. This reaction is here modelled by the oxidation of a phenol covalently linked to a Ru(bpy)32+-moitey, which is photo-oxidized by a laser flash-quench method. This model system is unusual as mechanism of PCET is studied in a unimolecular system in water solution. Here we address the question how the nature of the proton accepting base and its hydrogen bond to phenol influence the PCET reaction. In the first part we investigate the effect of an internal hydrogen bond PCET from. Two similar phenols are compared. For both these the proton accepting base is a carboxylate group linked to the phenol on the ortho-position directly or via a methylene group. On the basis of kinetic and thermodynamic arguments it is suggested that the PCET from these occurs via a concerted electron proton transfer (CEP). Moreover, numerical modelling of the kinetic data provides an in-depth analysis of this CEP reaction, including promoting vibrations along the O–H–O coordinate that are required to explain the data. The second part describes the study on oxidation of phenol where either water or an external base the proton acceptor. The pH-dependence of the kinetics reveals four mechanistic regions for PCET within the same molecule when water is the base. It is shown that the competition between the mechanisms can be tuned by the strength of the oxidant. Moreover, these studies reveal the conditions that may favour a buffer-assisted PCET over that with deprotonation to water solution.
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| 3. |
- Schéle, Ingrid, 1967-
(författare)
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Gendered experiences of work environment : A study of stress and ambiguity among dental students in Sweden
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores how dental students experience their education. We aim to generate ways to understand which elements relate to the students’ experience based on current theories and models regarding the quality of working life and gender (and) power relations. Methods Twelve interviews with Umeå dental students in their clinical semesters were analysed with a Grounded Theory (GT) as well as a content analysis approach. A web-survey was sent to all clinical dental students in Sweden (P ≈ 805) with a response rate of 40% (p = 322). The quantitative methods included structural equation modelling and cluster analysis. Results The GT analysis resulted in the core category “Experiencing ambiguity,” that captured the student’s role-ambiguity. Central categories focused on perceived stress and performance assessment in relation to ambiguous inner and outer demands. The content analysis resulted in three categories: “Notions of inequalities,” “Gendering,” and “The student position.” These categories present the ways groups of students are constructed in relation to the student/dentist norm and social gender relations, and how women and men of foreign descent risk subordination and stereotyping. The SEM-model contained psychosocial work environment, tolerance for ambiguity, perceived stress, and student satisfaction. Work environment influenced both perceived stress and satisfaction, and stood for almost all of the explained variance in perceived stress for women, indicating that women are constructed as co-responsible for the work environment. About half of the variance for the men was explained by tolerance for ambiguity, indicating that the feeling of uncertainty may lead to stress in men who include “being in control” in their gender identity. The cluster analysis resulted in a six-cluster solution ranging from “The fresh and positive” to “The worn critiques.” Psychosocial work environment again appeared to be the main factor. Gender also appears to be a factor as the gender distribution in the best as well as the two worst clusters differs from the population. Conclusion Work environment stands out among the factors that relate to the students wellbeing and satisfaction, but the student group is heterogeneous and the ways students perceive their work environment relate to different processes and experiences. We suggest that the ways gender and ethnicity appear to be constructed in relation to the sociocultural gender power relations and the (traditional) medical hierarchy could be of importance for how the students’ experience their psychosocial work environment.
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| 4. |
- Lissau, Jonas Sandby, 1984-
(författare)
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Non-Coherent Photon Upconversion on Dye-Sensitized Nanostructured ZrO2 Films for Efficient Solar Light Harvesting
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a photophysical process that facilitates the conversion of two low-energy photons into a single high-energy photon. A low-energy photon is absorbed by a sensitizer molecule that produces a triplet excited state which is transferred to an emitter molecule. When two emitter triplet states encounter each other, TTA can take place to produce a singlet excited state which decays by emission of a high-energy (upconverted) photon. While traditional single-threshold dye-sensitized solar cells (DSSCs) have a maximum efficiency limit of ca. 30%, it has been predicted theoretically that implementation of UC-STTA in DSSCs could increase that efficiency to more than 40%.A possible way to implement UC-STTA into DSSCs, would be to replace the standard sensi- tized nanostructured TiO2 photoanodes by upconverting ones loaded with emitter molecules. Following TTA, the excited emitter molecule would be quenched by injection of a high-energy electron into the conduction band of the TiO2. To explore the practical aspects of this strategy for a highly efficient DSSC, in this thesis UC-STTA is studied in model systems based on nanostructured ZrO2 films. These ZrO2 films are a good proxy for the TiO2 films used in DSSCs, and allow for relatively easy optimization and study of UC-STTA by allowing measurements of the upconverted photons without the complications of electron injection into the film.Herein it is experimentally proven that UC-STTA is viable on nanostructured metal oxide films under non-coherent irradiation with intensities comparable to sunlight. Two different system architectures are studied, differing in the position of the molecular components involved in the UC-STTA mechanism. Both architectures have the emitter molecules adsorbed onto the ZrO2 surface, but the sensitizers are positioned either in solution around the nanostructure, or co-adsorbed with the emitters onto the ZrO2 surface. A set of challenges in the study and optimization of the UC-STTA process is identified for each type of system. Proposals are also given for how to further improve the understanding and UC-STTA optimization of these systems toward application in DSSCs to overcome the present solar energy conversion efficiency limit.
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| 5. |
- Petersson, Jonas, 1980-
(författare)
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Ultrafast, Non-Equilibrium Electron Transfer Reactions of Molecular Complexes in Solution
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photoinduced electron transfer is a fundamentally interesting process; it occurs everywhere in the natural world. Studies on electron transfer shed light on questions about the interaction between molecules and how the dynamics of these can be utilized to steer the electron transfer processes to achieve a desired goal. The goal may be to get electrons to the electrode of a solar cell, or to make the electrons form an energy rich fuel such as hydrogen, and it may also be an input or output for molecular switches. The importance of electron transfer reactions will be highlighted in this thesis, however, the main motivation is to gain a better understanding of the fundamental processes that affect the rate and direction of the electron transfer.A study of photoinduced electron transfer (ET) in a series of metallophorphyrin/bipyridinium complexes in aqueous solution provided fresh insight concerning the intimate relationship between vibrational relaxation and electron transfer. The forward electron transfer from porphyrin to bipyridinium as well as the following back electron transfer to the ground state could be observed by femtosecond transient absorption spectroscopy. Both the reactant and the product states of the ET processes were vibrationally unrelaxed, in contrary to what is assumed for most expressions of the ET rates. This could be understood from the observation of unrelaxed ground states. The excess energy given by the initial excitation of the porphyrin does not relax completely during the two steps of electron transfer. This is an unusual observation, not reported in the literature prior the studies presented in this thesis. This study also gave the first clear evidence of electronically excited radical pairs formed as products of intramolecular electron transfer. Signs of electronically excited radical pairs were seen in transient spectra, and were further verified by the observation that the rates followed a Marcus normal region behavior for all excitation wavelengths, despite the relatively large excess energy of the second excited state.This thesis also concerns electron transfer in solar cell dyes and mixed valence complexes. In the ruthenium polypyridyl complex Ru(dcb)2(NCS)2, where dcb = 4,4’-dicarboxy-2,2’-bipyridine, inter-ligand electron transfer (ILET) in the 3MLCT state was followed by means of femtosecond transient absorption anisotropy that was probed in the mid-IR region. Unexpectedly, ILET was not observed because electron density was localized on the same bpy during the time-window allowed by the rotational lifetime.
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