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Insight into Catalytic Intermediates Relevant for Water Splitting
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 81 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1359
Keyword [en]
Catalysis, Artificial photosynthesis, Molecular biomimetics
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry; Chemistry with specialization in Chemical Physics
Identifiers
URN: urn:nbn:se:uu:diva-281447ISBN: 978-91-554-9526-8 (print)OAI: oai:DiVA.org:uu-281447DiVA: diva2:914420
Public defence
2016-06-03, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-05-11 Created: 2016-03-24 Last updated: 2016-06-01
List of papers
1. Voltammetric and spectroscopic characterization of early intermediates in the Co(II)-polypyridyl-catalyzed reduction of water
Open this publication in new window or tab >>Voltammetric and spectroscopic characterization of early intermediates in the Co(II)-polypyridyl-catalyzed reduction of water
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2013 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 49, no 77, 8638-8640 p.Article in journal (Refereed) Published
Abstract [en]

Early intermediates of catalytic water reduction by a Co(II)-polypyridyl species have been characterized. Electrochemical detection of the Co(III)-hydride and time-resolved spectroscopic detection of the Co(I)-ligand intermediates provide an understanding of their reactivity in electrolytic or light-driven reduction of protons to hydrogen.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-208179 (URN)10.1039/c3cc44655g (DOI)000323758900017 ()
Available from: 2013-09-24 Created: 2013-09-24 Last updated: 2017-12-06
2. Direct Observation of Key Catalytic Intermediates in a Photoinduced Proton Reduction Cycle with a Diiron Carbonyl Complex
Open this publication in new window or tab >>Direct Observation of Key Catalytic Intermediates in a Photoinduced Proton Reduction Cycle with a Diiron Carbonyl Complex
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2014 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 136, no 50, 17366-17369 p.Article in journal (Refereed) Published
Abstract [en]

The structure and reactivity of intermediatesin the photocatalytic cycle of a proton reductioncatalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), wereinvestigated by time-resolved spectroscopy. The singlyreduced catalyst [Fe2(bdt)(CO)6]−, a key intermediate inphotocatalytic H2 formation, was generated by reactionwith one-electron reductants in laser flash-quench experimentsand could be observed spectroscopically on thenanoseconds to microseconds time scale. From UV/visand IR spectroscopy, [Fe2(bdt)(CO)6]− is readilydistinguished from the two-electron reduced catalyst[Fe2(bdt)(CO)6]2− that is obtained inevitably in theelectrochemical reduction of [Fe2(bdt)(CO)6]. For thedisproportionation rate constant of [Fe2(bdt)(CO)6]−, anupper limit on the order of 107 M−1 s−1 was estimated,which precludes a major role of [Fe2(bdt)(CO)6]2− inphotoinduced proton reduction cycles. Structurally [Fe2-(bdt)(CO)6]− is characterized by a rather asymmetricallydistorted geometry with one broken Fe−S bond and sixterminal CO ligands. Acids with pKa ≤ 12.7 protonate[Fe2(bdt)(CO)6]− with bimolecular rate constants of 4 ×106, 7 × 106, and 2 × 108 M−1 s−1 (trichloroacetic,trifluoroacetic, and toluenesulfonic acids, respectively).The resulting hydride complex [Fe2(bdt)(CO)6H] istherefore likely to be an intermediate in photocatalyticcycles. This intermediate resembles structurally andelectronically the parent complex [Fe2(bdt)(CO)6], withvery similar carbonyl stretching frequencies.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-240552 (URN)10.1021/ja5085817 (DOI)000346682600003 ()
Available from: 2015-01-07 Created: 2015-01-07 Last updated: 2017-12-05
3. Sensitizer-Catalyst Assemblies for Water Oxidation
Open this publication in new window or tab >>Sensitizer-Catalyst Assemblies for Water Oxidation
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2015 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 6, 2742-2751 p.Article in journal (Refereed) Published
Abstract [en]

Two molecular assemblies with one Ru(II)-polypyridine photosensitizer covalently linked to one Ru(II)(bda)L2 catalyst (1) (bda = 2,2'-bipyridine-6,6'-dicarboxylate) and two photosensitizers covalently linked to one catalyst (2) have been prepared using a simple C-C bond as the linkage. In the presence of sodium persulfate as a sacrificial electron acceptor, both of them show high activity for catalytic water oxidation driven by visible light, with a turnover number up to 200 for 2. The linked photocatalysts show a lower initial yield for light driven oxygen evolution but a much better photostability compared to the three component system with separate sensitizer, catalyst and acceptor, leading to a much greater turnover number. Photocatalytic experiments and time-resolved spectroscopy were carried out to probe the mechanism of this catalysis. The linked catalyst in its Ru(II) state rapidly quenches the sensitizer, predominantly by energy transfer. However, a higher stability under photocatalytic condition is shown for the linked sensitizer compared to the three component system, which is attributed to kinetic stabilization by rapid photosensitizer regeneration. Strategies for employment of the sensitizer-catalyst molecules in more efficient photocatalytic systems are discussed.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-251799 (URN)10.1021/ic502915r (DOI)000351325200028 ()25700086 (PubMedID)
Available from: 2015-04-28 Created: 2015-04-24 Last updated: 2017-12-04
4. Following [FeFe] Hydrogenase Active Site Intermediates by Flash Photolysis/Mid-IR Probing
Open this publication in new window or tab >>Following [FeFe] Hydrogenase Active Site Intermediates by Flash Photolysis/Mid-IR Probing
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-281443 (URN)
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2016-06-01
5. Direct Experimental Observation of Interfacial Photo-Reduction of a Molecular Proton Reduction Catalyst Incorporated in a Metal-Organic Framework
Open this publication in new window or tab >>Direct Experimental Observation of Interfacial Photo-Reduction of a Molecular Proton Reduction Catalyst Incorporated in a Metal-Organic Framework
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-281444 (URN)
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2016-06-01
6. Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface
Open this publication in new window or tab >>Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface
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2016 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 26, 8060-8063 p.Article in journal (Other academic) Published
Abstract [en]

The combination of molecular dyes and catalysts with semiconductors into dye-sensitized solar fuel devices (DSSFDs) requires control of efficient interfacial and surface charge transfer between the components. The present study reports on the light-induced electron transfer processes of p-type NiO films cosensitized with coumarin C343 and a bioinspired proton reduction catalyst, [FeFe](mcbdt)(CO)(6) (mcbdt = 3-carboxybenzene-1,2-dithiolate). By transient optical spectroscopy we find that ultrafast interfacial electron transfer (tau approximate to 200 fs) from NiO to the excited C343 ("hole injection") is followed by rapid (t(1/2) approximate to 10 ps) and efficient surface electron transfer from C343 to the coadsorbed [FeFe] (mcbdt)(CO)(6). The reduced catalyst has a clear spectroscopic signature that persists for several tens of microseconds, before charge recombination with NiO holes occurs. The demonstration of rapid surface electron transfer from dye to catalyst on NiO, and the relatively long lifetime of the resulting charge separated state, suggests the possibility to use these systems for photocathodes on. DSSFDs.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-281445 (URN)10.1021/jacs.6b03889 (DOI)000379455600005 ()27314570 (PubMedID)
Funder
Knut and Alice Wallenberg FoundationSwedish Energy AgencyCarl Tryggers foundation
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2017-11-30Bibliographically approved
7. IR and UV-Vis Spectroscopic Characterization of a One-Electron Reduced Intermediate of an FeFe Hydrogenase Mimic on the Nano- to Microsecond Time Scale
Open this publication in new window or tab >>IR and UV-Vis Spectroscopic Characterization of a One-Electron Reduced Intermediate of an FeFe Hydrogenase Mimic on the Nano- to Microsecond Time Scale
(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-281446 (URN)
Available from: 2016-03-24 Created: 2016-03-24 Last updated: 2016-06-01

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