| 1. |
- 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. |
- Tian, Lei, 1987-
(författare)
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Exploring Dye-Sensitized Mesoporous NiO Photocathodes : from Mechanism to Applications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing attention has been paid on solar energy conversion since the abundant solar energy possesses the potential to solve the problems on energy crisis and climate change. Dye-sensitized mesoporous NiO film was developed as one of the attractive photocathodes to fabricate p-type dye-sensitized solar cells (p-DSCs) and dye-sensitized photoelectrosynthetic cells (p-DSPECs) for electricity and chemical fuels generation, respectively. In this thesis, we designed a well-structured NiO-dye-TiO2 configuration by an atomic layer deposition (ALD) technique, with an organic dye PB6 as the photosensitizer. From kinetic studies of charge separation, ultrafast hole injection (< 200 fs) was observed from the excited state of PB6 dye into the valence band of NiO; dye regeneration (electron injection) was in t1/2 ≤ 500 fs, which is the fastest reported in any DSCs. On the basis of NiO-dye-TiO2 configuration, we successfully fabricated solid-state p-type DSCs (p-ssDSCs). Insertion of an Al2O3 layer was adopted to reduce charge recombination, i.e. NiO-dye-Al2O3-TiO2. Theoretically, such a configuration is possible to maintain efficient charge separation and depressed charge recombination. Based on NiO-dye-Al2O3-TiO2 configuration, the open-circuit voltage was improved to 0.48 V. Replacing electron conductor TiO2 with ZnO, short-circuit current density was increased to 680 μA·cm-2. The photocatalytic current density for H2 evolution was improve to 100 μA·cm-2 with a near unity of Faraday efficiency in p-DSPECs.However, to further improve the performance of p-DSCs is very challenging. In p-ssDSCs, the limitation was confirmed from the poor electronically connection of the electron conductor (TiO2 or ZnO) inside the NiO-dye films. We further investigated the electronic property of surface states on mesoporous NiO film. We found that the surface sates, not the bulk, on NiO determined the conductivity of the mesoporous NiO films. The dye regeneration in liquid p-DSCs with I-/I3- as redox couples was significantly affected by surface states. A more complete mechanism is suggested to understand a particular hole transport behavior reported in p-DSCs, where hole transport time is independent on light intensity. The independence of charge transport is ascribed to the percolation effect in the hole hopping on the surface states.
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| 3. |
- Wrede, Sina, 1995-
(författare)
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Interfaces in Dye-Sensitized Electrodes : From Fundamental Understanding to Devices
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Renewable energy solutions are vital in realising a more equitable and sustainable future. Among several solar light harvesting and storage technologies, dye-sensitized electrodes offer a cost-effective and flexible solution for solar cells or solar fuel devices. In order to enhance their solar conversion efficiency, however, the understanding of charge transfer pathways, particularly at the dye-sensitized surface, is crucial. Surface properties and interfacial processes have a great effect on the final device and are the overarching theme of this thesis.Firstly, intermolecular charge transfer across dye-sensitized surfaces was investigated, which play a role in both facilitating charge accumulation and affecting recombination rates and are therefore pivotal factors influencing solar cell efficiencies. Specifically, investigations on nickel oxide (NiO) and ZrO2 surfaces elucidate charge transfer mechanisms across the surface and their dependence on solvent properties, offering possible pathways for optimizing device performance.Due to their significance on dye-sensitized photocathodes, the chemical nature of NiO surface states was explored as they are known to affect charge recombination and the dye-regeneration processes. Spectroscopic insights during controlled atmosphere experiments highlight the influence of surface species generated by oxygen and water molecules on the electronic properties of NiO, particularly of hydroxide and oxygen-related species.Thirdly, the design and characterization of the first reported solid-state p-n tandem dye-sensitized solar cell was demonstrated. Such a cell can surpass the voltage limitations observed in liquid tandem cells and could achieve an open-circuit voltage of 1.4 V. These tandem cells hold promise for applications in solar fuel production, where high potential differences are essential for driving chemical reactions.
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| 4. |
- Bagnall, Andrew J.
(författare)
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Novel electrode and photoelectrode materials for hydrogen production based on molecular catalysts
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The PhD project focussed on the application of a cobalt tetraazamacrocyclic complex, in the literature commonly referred to as [Co(CR)Cl2]+ as a molecular catalyst for the hydrogen evolution reaction (HER). This was within the broader scope of the EU MSCA H2020 ITN ‘eSCALED’ project, which primarily aimed to create artificial leaf devices for the storage of solar energy in chemical fuels and, as part of this, sought the development of novel bio-inspired and scalable materials. This included researching molecular catalysts without platinum group metals (PGMs) currently relied upon in commercial technology.Three main projects were pursued: firstly, studies of the mechanism of the catalyst itself under organic electrocatalytic conditions. Catalytic intermediates were generated and identified using spectroscopy (UV-vis, NMR, EPR) and the catalytic behaviour was followed with electrochemical techniques. An ECEC mechanism with a rate-determining second protonation step associated with the release of H2 was identified, noting in particular an initial protonation step on the macrocycle at the Co(II) state that was hypothesised to involve the macrocycle amine group acting as a proton relay under the investigated conditions.Secondly, a new synthetic strategy towards novel derivatives of [Co(CR)Cl2]+ was developed to prepare a derivative for anchoring onto sp2-carbon surfaces by pi-stacking interactions. The immobilised catalyst was studied by electrochemical methods and compared with another derivative from collaborators at ICIQ, showing that both derivatives work as heterogenised electrocatalysts for the HER with high faradaic efficiencies and good stability over one hour at pH 2 and especially pH 7, but one derivative displays higher current densities and stability, invoking some consideration of rational design principles for modifying molecular catalysts.Thirdly, studies of a photocatalytic system made up of copper indium sulfide quantum dots (CuInS2 QDs) as a photosensitiser with either [Co(CR)Cl2]+ or its benzoic acid-functionalised derivative were carried out in ascorbate buffer, focussing on the photocatalytic performance and electron transfer (ET) processes between the CuInS2 QDs and the catalyst to explain the remarkable activity and robustness reported for closely related systems. CuInS2 QDs modified to have a ‘hybrid-passivation’ ligand system for compatibility with NiO films were used. Rapid QD-catalyst ET processes were noted for both catalysts. A static binding model with a strong binding equilibrium was adapted for the system, applying a Poisson distribution. This prompts a reconsideration of the importance of anchoring groups for QD-catalyst ET efficiency in solution.
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| 5. |
- Berggren, Gustav, 1980-
(författare)
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Mimicking Nature – Synthesis and Characterisation of Manganese Complexes of Relevance to Artificial Photosynthesis
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of efficient catalyst for water oxidation is of paramount importance to artificial photosynthesis, but before this can be achieved a deeper understanding of this reaction is essential. In nature this reaction occurs in a tetranuclear Mn-cluster which serves as the work-horse of oxygenic photosynthesis. This thesis summarises my efforts at developing molecular systems capable of mimicking this complex employing a biomimetic approach. Three different approaches towards this goal are described here-in. The first section describes a screening study, in which a number of manganese complexes were tested to see whether or not they were capable of catalysing the formation of dioxygen when treated with different oxidants (Papers I). For those reactions in which dioxygen formation was observed the reactions were repeated in labelled water and the incorporation of labelled O-atoms was studied by mass spectrometry. This allowed us to determine to what extent water was the source of the evolved dioxygen (Papers II-III). In Chapter three a reported catalyst and a derivative thereof is studied in depth. The influence of changes to the ligand on the oxygen–oxygen bond forming reaction could unfortunately not be reliably addressed, because of the instability of the complexes under “catalytic” conditions. Nevertheless, the study allowed us to revise the “carboxylate shift”-mechanism suggested in the literature (Papers IV-V). Chapter four describes the continuation of my work on ligands featuring the carboxylate ligand motif first introduced in Chapter three. In this study ligands containing multiple binding pockets were designed and synthesised (Paper VI). A better understanding of the mechanism in the natural water oxidising enzyme will facilitate the design of biomimetic complexes, this is discussed in Chapter five. In this work model complexes (Paper VII) are used to study the mechanism by which natures own water oxidising catalyst performs this reaction.
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| 6. |
- 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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| 7. |
- Mirmohades, Mohammad, 1986-
(författare)
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Insight into Catalytic Intermediates Relevant for Water Splitting
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Catalysis is an important part of chemistry. This is also reflected in the chemical industry where 85-90 % of all products are made catalytically. Also nature employs catalysts, i.e. enzymes, for its reactions.To improve on the already existing catalysts one can learn a lot from nature which often uses earth-abundant elements in the enzymes which have also been optimized and finely tuned for billions of years. To gain a deeper understanding of both enzymatic and artificial catalysis one needs to investigate the mechanism of the catalytic process. But for very efficient catalysts with turnover frequencies of several thousand per second this is not easy, since an investigation of the mechanism involves resolving intermediates in the catalytic cycle. The intermediates in these instances are short-lived corresponding to their turnover frequencies. A maximum turnover frequency of 1,000 s-1 e.g. means that each catalyst goes through the whole catalytic cycle in 1 ms. Therefore time-resolved techniques are necessary that have a faster detection speed than the turnover frequency of the catalyst.Flash photolysis is a spectroscopic technique with an instrument response function down to 10 ns. Coupling this technique with mid-infrared probing yields an excellent detection system for probing different redox and protonation states of carbonyl metal complexes. Since many catalysts as well as natural enzymes involved in water splitting are metal carbonyl complexes this is an ideal technique to monitor the intermediates of these catalysts.Chapter 3 covers the investigation of [FeFe] hydrogenases, enzymes that catalyze the reduction of protons to hydrogen in nature. Chapter 4 investigates the intermediates of biomimetic complexes, resembling the active site of natural [FeFe] hydrogenases. Chapter 5 covers the insights gained from investigating other catalysts which are also involved in water splitting and artificial photosynthesis.
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| 8. |
- Nilsen-Moe, Astrid, 1991-
(författare)
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Mechanistic Studies on Proton-Coupled Electron Transfer from Tyrosine and Tryptophan Derivatives
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Proton-coupled electron transfer (PCET) from tyrosine (Y) and tryptophan (W) is vital to many redox reactions in Nature where PCET between several Ys, or Ws, or between a mix of Ys and Ws can be used to transfer electrons, or protons, or both over large distances of several Å. Studying the PCET reaction mechanisms of Y and W is important for fundamental knowledge, and can help researchers that wish to mimic redox reactions in Nature. To this end, model proteins and small model molecules can be used to investigate PCET reactions without the complexity of large enzymes. PCET can proceed via two different types of mechanisms; the stepwise mechanism and the concerted mechanism. Detailed mechanistic studies to determine which PCET mechanism dominates are often difficult to perform on biological systems due to their size and complexity, which is why we instead study model systems. In this thesis, the PCET mechanisms and their dependence on pH and driving force for electron transfer (ET) and proton transfer (PT) are studied by determining PCET rate constants using transient absorption (TA) as a function of pH and driving force for ET and PT. In Papers I and II, Y and a Y derivative sequestered in a model protein are studied. The results show that the PCET mechanism for Y is dependent on bulk pH, with a stepwise PCET mechanism at high pH, and a concerted or stepwise mechanism at lower pH depending on the driving force for ET and PT. Interestingly, these are all parameters that can shift depending on the protein environment, which can be finetuned in Nature to promote a certain PCET mechanism. H2O is an inherently poor proton acceptor due to its low pKa = 0. Nevertheless, from TA kinetic and molecular dynamic simulation studies, we suggest that H2O is the primary proton acceptor for the CEPT reaction in the model systems. These studies also indicate that the protein structure gates intrinsically better proton acceptors (such as buffer species) from coming into sufficiently close contact with the Y or Y derivative, even when the Y derivative exhibits as much as 30 to 40% exposure to the solution. In Paper III, the PCET mechanism and primary proton acceptor for two small molecule W analogs in solution are investigated. Due to the relatively large pKa value exhibited by oxidized W (pKa = 4.3), PCET was previously thought to not proceed via the concerted mechanism when H2O was the primary proton acceptor. By studying these two W derivatives, we show that W oxidation can in fact proceed via the concerted mechanism when appropriate oxidants are used. Our results also show that both W analogs exhibit concerted rate constants with a weak pH dependence that currently lacks a theoretical explanation. These results have implications for solution exposed W in biological systems by showing that the concerted mechanism is viable for W PCET even with water as the primary proton acceptor.
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| 9. |
- Pavliuk, Mariia
(författare)
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Accumulative Charge Separation in Photocatalysis : From Molecules to Nanoparticles
- 2019
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- Photochemical energy conversion into solar fuel involves steps of light absorption, charge separation and catalysis. Nature has taught us that the effective accumulation of redox equivalents and charge separation are the key steps in sunlight conversion. The focus of this thesis is to unveil photophysical and photochemical processes that lead to accumulative charge separation. The optimization of electron transfer process will be held by minimization of losses via recombination, and extension of the lifetime of the charge separated state by usage of the electron relay.The goal is to couple light induced electron transfer process with the multi-electron catalytic process of hydrogen evolution. In this regard, light harvesters (molecules, metal nanostructures) that generate at least two electrons per absorbed photon will be studied. Additionally, semiconductors that generate long-lived charge separated states are utilized to accumulate several redox equivalents necessary for hydrogen evolution.The hybrid systems produced by the combination of the advantageous properties of molecules, semiconductors, and metal nanoparticles are under the scope of investigation. Metal nanoparticles are advantageous because of their high absorption cross-section. The molecular linkers provide control and flexibility in tuning the connection between the light absorber and the electron relay. Semiconductor nanoparticles offer the desired charge separation properties via prolonging the lifetime sufficiently to perform photocatalysis.The detailed understanding, investigation and development of the hybrid systems is at the heart of the progress of photochemical solar fuel production.
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| 10. |
- 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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