| 1. |
|
|
| 2. |
- Boscolo Bibi, Sara, 1993-
(författare)
-
Going for gold : A spectroelectrochemical and catalytic study of gold materials
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the increase in demand for renewable energy, understanding chemical processes is essential for improving the design of catalysts in order to achieve better performance. This thesis summarises the experimental investigation of three types of catalytic gold materials: gold oxide formed from gold films, oxide-derived gold (OD-Au) produced from gold films, and gold nanoparticles supported on metal oxides. Different spectroscopic techniques were employed, such as operando sum frequency generation (SFG) and in situ and ex situ X-ray spectroscopies. These methods allowed the probing of the electronic and chemical states of gold after oxidising electrochemical treatments. The results indicate the presence of subsurface gold oxide remnants after formation of OD-Au, which may help explain its improved catalytic properties with respect to pure gold. In addition, a mathematical model to couple the early stages of gold oxide formation with the nonlinear optical response of gold during this process is presented. This model suggests that the growth proceeds from small oxide islands to 3D oxide growth, while SFG oxidation variation is due to the suppression of the free electron density by negatively-charged adsorbing oxygen atoms. Gold oxide was also studied with both in situ and operando X-ray spectroscopies, showing the importance of a continuous electrochemical treatment during measurements to avoid beam induced effects. Furthermore, gold nanoparticles supported on metal oxides (TiO2 and γ-Fe2O3) were investigated mainly with mass spectrometry. The results indicate two different reaction pathways for oxidation of CO to CO2 depending on the type of metal oxide support. These findings could be used to help design future gold-based catalysts.
|
|
| 3. |
- Boscolo Bibi, Sara, 1993-, et al.
(författare)
-
Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes
- 2024
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 26:3, s. 2332-2340
-
Tidskriftsartikel (refereegranskat)abstract
- Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Diesen, Elias, et al.
(författare)
-
Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy
- 2021
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 127:1
-
Tidskriftsartikel (refereegranskat)abstract
- We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.
|
|
| 7. |
- Fan, Junpeng, et al.
(författare)
-
Solid-state synthesis of few-layer cobalt-doped MoS2 with CoMoS phase on nitrogen-doped graphene driven by microwave irradiation for hydrogen electrocatalysis
- 2020
-
Ingår i: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 10:56, s. 34323-34332
-
Tidskriftsartikel (refereegranskat)abstract
- The high catalytic activity of cobalt-doped MoS2 (Co–MoS2) observed in several chemical reactions such as hydrogen evolution and hydrodesulfurization, among others, is mainly attributed to the formation of the CoMoS phase, in which Co occupies the edge-sites of MoS2. Unfortunately, its production represents a challenge due to limited cobalt incorporation and considerable segregation into sulfides and sulfates. We, therefore, developed a fast and efficient solid-state microwave irradiation synthesis process suitable for producing thin Co–MoS2 flakes (∼3–8 layers) attached on nitrogen-doped reduced graphene oxide. The CoMoS phase is predominant in samples with up to 15 at% of cobalt, and only a slight segregation into cobalt sulfides/sulfates is noticed at larger Co content. The Co–MoS2 flakes exhibit a large number of defects resulting in wavy sheets with significant variations in interlayer distance. The catalytic performance was investigated by evaluating the activity towards the hydrogen evolution reaction (HER), and a gradual improvement with increased amount of Co was observed, reaching a maximum at 15 at% with an overpotential of 197 mV at −10 mA cm−2, and a Tafel slope of 61 mV dec−1. The Co doping had little effect on the HER mechanism, but a reduced onset potential and charge transfer resistance contributed to the improved activity. Our results demonstrate the feasibility of using a rapid microwave irradiation process to produce highly doped Co–MoS2 with predominant CoMoS phase, excellent HER activity, and operational stability.
|
|
| 8. |
- Fan, Junpeng, et al.
(författare)
-
β-Mo2C Nanoparticles Produced by Carburization of Molybdenum Oxides with Carbon Black under Microwave Irradiation for Electrocatalytic Hydrogen Evolution Reaction
- 2021
-
Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 4:11, s. 12270-12277
-
Tidskriftsartikel (refereegranskat)abstract
- The synthesis of electrochemically active β-Mo2C nanoparticles for hydrogen production was achieved by a fast and energy-efficient microwave-assisted carburization process from molybdenum oxides and carbon black. With the use of microwave-based production methods, we aim to reduce the long-time high-temperature treatments and the use of hazardous gases often seen in traditional molybdenum carbide synthesis processes. In our process, carbon black not only serves as a carbon source but also as a susceptor (microwave absorber) and conductive substrate. The irradiation power, reaction time, and Mo:C ratio were optimized to achieve the highest electrocatalytic performance toward hydrogen production in an acidic electrolyte. A complete transformation of MoO3 to β-Mo2C nanoparticles and an additional graphitization of the carbon black matrix were achieved at 1000 W, 600 s, and Mo:C ratio above 1:7.5. Under these conditions, the optimized composite exhibited an excellent HER performance (η10 = 156 mV, Tafel slope of 53 mV·dec–1) and large turnover frequency per active site (3.09 H2·s–1 at an overpotential of 200 mV), making it among the most efficient non-noble-metal catalysts. The excellent activity was achieved thanks to the abundance of β-Mo2C nanoparticles, the intimate nanoparticle-substrate interface, and enhanced electron transport toward the carbon black matrix. We also investigated the flexibility of the synthesis method by adding additional Fe or V as secondary transition metals, as well as the effect of the substrate.
|
|
| 9. |
- Fuller, Franklin D, et al.
(författare)
-
Drop-on-demand sample delivery for studying biocatalysts in action at X-ray free-electron lasers
- 2017
-
Ingår i: Nature Methods. - : Macmillan Publishers Ltd.. - 1548-7091 .- 1548-7105. ; 14, s. 443-449
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray crystallography at X-ray free-electron laser sources is a powerful method for studying macromolecules at biologically relevant temperatures. Moreover, when combined with complementary techniques like X-ray emission spectroscopy, both global structures and chemical properties of metalloenzymes can be obtained concurrently, providing insights into the interplay between the protein structure and dynamics and the chemistry at an active site. The implementation of such a multimodal approach can be compromised by conflicting requirements to optimize each individual method. In particular, the method used for sample delivery greatly affects the data quality. We present here a robust way of delivering controlled sample amounts on demand using acoustic droplet ejection coupled with a conveyor belt drive that is optimized for crystallography and spectroscopy measurements of photochemical and chemical reactions over a wide range of time scales. Studies with photosystem II, the phytochrome photoreceptor, and ribonucleotide reductase R2 illustrate the power and versatility of this method.
|
|
| 10. |
- Goodwin, Christopher M., 1989-, et al.
(författare)
-
Operando Probing of the Surface Chemistry During the Haber-Bosch Process
- 2024
-
Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 625:7994, s. 282-286
-
Tidskriftsartikel (refereegranskat)abstract
- The Haber-Bosch process produces NH3 from N2 and H21,2, typically with Fe and Ru3. HB has been proposed as the most important scientific invention in the 20th century4. The chemical state during reaction has been proposed as oxides5, nitrides2, metallic, or surface nitride6. The proposed rate-limiting step has been the dissociation of N27–9, reaction of adsorbed nitrogen10, or desorption of NH311. Due to the vacuum requirement for surface-sensitive techniques, studies at reaction conditions are limited to theory computations12–14. We determined the surface composition, during NH3 production, at pressures up to 1 bar and temperatures as high as 723 K on flat, stepped Fe, and stepped Ru single crystal surfaces using operando X-ray Photoelectron Spectroscopy15. We found that all surfaces remain metallic. On Fe only a small amount of adsorbed N remains, yet Ru’s surface is almost adsorbate free. At 523 K, high amines (NHx) coverages appear on the stepped Fe surface. The results show that the rate-limiting step on Ru is always N2 dissociation. Still, on Fe the hydrogenation step involving adsorbed N atoms is essential for the total rate, as predicted by theory13. If the temperature is lowered on Fe, the rate-limiting steps switch and become surface species’ hydrogenation.
|
|
| 11. |
- Görlin, Mikaela, et al.
(författare)
-
Key activity descriptors of nickel-iron oxygen evolution electrocatalysts in the presence of alkali metal cations
- 2020
-
Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni2+/(3+delta)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH>NaOH approximate to KOH>RbOH>LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect. It is commonly accepted that electrolyte alkali metal cations modify the catalytic activity for oxygen evolution reaction. Here the authors challenge this assumption, showing that the activity is actually affected by a change in the electrolyte pH rather than a specific alkali cation.
|
|
| 12. |
|
|
| 13. |
- Holm, Alexander, 1983-, et al.
(författare)
-
A Water-Promoted Mars-van Krevelen Reaction Dominates Low-Temperature CO Oxidation over Au-Fe2O3 but Not over Au-TiO2
- 2024
-
Ingår i: ACS Catalysis. - 2155-5435. ; 14:5, s. 3191-3197
-
Tidskriftsartikel (refereegranskat)abstract
- We provide experimental evidence that is inconsistent with often proposed Langmuir−Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au–Fe2O3. Passing CO and H2O, but no O2, over Au-γ-Fe2O3 at 25 °C, we observe significant CO2 production, inconsistent with LH mechanistic hypotheses. Experiments with H218O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe2O3. Guided by density functional theory, we instead postulate a water-promoted Mars–van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO2 production in the absence of O2 and with CO oxidation in the presence of H218O and 16O2. In contrast, for Au-TiO2, our data is consistent with previous LH mechanistic hypotheses.
|
|
| 14. |
- Kern, Jan, et al.
(författare)
-
Simultaneous Femtosecond X-ray Spectroscopy and Diffraction of Photosystem II at Room Temperature
- 2013
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 340:6131, s. 491-495
-
Tidskriftsartikel (refereegranskat)abstract
- Intense femtosecond x-ray pulses produced at the Linac Coherent Light Source (LCLS) were used for simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) of microcrystals of photosystem II (PS II) at room temperature. This method probes the overall protein structure and the electronic structure of the Mn4CaO5 cluster in the oxygen-evolving complex of PS II. XRD data are presented from both the dark state (S-1) and the first illuminated state (S-2) of PS II. Our simultaneous XRD-XES study shows that the PS II crystals are intact during our measurements at the LCLS, not only with respect to the structure of PS II, but also with regard to the electronic structure of the highly radiation-sensitive Mn4CaO5 cluster, opening new directions for future dynamics studies.
|
|
| 15. |
- Kern, Jan, et al.
(författare)
-
Taking snapshots of photosynthetic water oxidation using femtosecond X-ray diffraction and spectroscopy
- 2014
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5, s. 4371-
-
Tidskriftsartikel (refereegranskat)abstract
- The dioxygen we breathe is formed by light-induced oxidation of water in photosystem II. O-2 formation takes place at a catalytic manganese cluster within milliseconds after the photosystem II reaction centre is excited by three single-turnover flashes. Here we present combined X-ray emission spectra and diffraction data of 2-flash (2F) and 3-flash (3F) photosystem II samples, and of a transient 3F' state (250 mu s after the third flash), collected under functional conditions using an X-ray free electron laser. The spectra show that the initial O-O bond formation, coupled to Mn reduction, does not yet occur within 250 mu s after the third flash. Diffraction data of all states studied exhibit an anomalous scattering signal from Mn but show no significant structural changes at the present resolution of 4.5 angstrom. This study represents the initial frames in a molecular movie of the structural changes during the catalytic reaction in photosystem II.
|
|
| 16. |
- Kjær, Kasper S., et al.
(författare)
-
Finding intersections between electronic excited state potential energy surfaces with simultaneous ultrafast X-ray scattering and spectroscopy
- 2019
-
Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 10:22, s. 5749-5760
-
Tidskriftsartikel (refereegranskat)abstract
- Light-driven molecular reactions are dictated by the excited state potential energy landscape, depending critically on the location of conical intersections and intersystem crossing points between potential surfaces where non-adiabatic effects govern transition probabilities between distinct electronic states. While ultrafast studies have provided significant insight into electronic excited state reaction dynamics, experimental approaches for identifying and characterizing intersections and seams between electronic states remain highly system dependent. Here we show that for 3d transition metal systems simultaneously recorded X-ray diffuse scattering and X-ray emission spectroscopy at sub-70 femtosecond time-resolution provide a solid experimental foundation for determining the mechanistic details of excited state reactions. In modeling the mechanistic information retrieved from such experiments, it becomes possible to identify the dominant trajectory followed during the excited state cascade and to determine the relevant loci of intersections between states. We illustrate our approach by explicitly mapping parts of the potential energy landscape dictating the light driven low-to-high spin-state transition (spin crossover) of [Fe(2,2′-bipyridine)3]2+, where the strongly coupled nuclear and electronic dynamics have been a source of interest and controversy. We anticipate that simultaneous X-ray diffuse scattering and X-ray emission spectroscopy will provide a valuable approach for mapping the reactive trajectories of light-triggered molecular systems involving 3d transition metals.
|
|
| 17. |
- Kjær, Kasper S., et al.
(författare)
-
Solvent control of charge transfer excited state relaxation pathways in [Fe(2,2′-bipyridine)(CN)4]2-
- 2018
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 20:6, s. 4238-4249
-
Tidskriftsartikel (refereegranskat)abstract
- The excited state dynamics of solvated [Fe(bpy)(CN)4]2-, where bpy = 2,2′-bipyridine, show significant sensitivity to the solvent Lewis acidity. Using a combination of optical absorption and X-ray emission transient spectroscopies, we have previously shown that the metal to ligand charge transfer (MLCT) excited state of [Fe(bpy)(CN)4]2- has a 19 picosecond lifetime and no discernable contribution from metal centered (MC) states in weak Lewis acid solvents, such as dimethyl sulfoxide and acetonitrile.1,2 In the present work, we use the same combination of spectroscopic techniques to measure the MLCT excited state relaxation dynamics of [Fe(bpy)(CN)4]2- in water, a strong Lewis acid solvent. The charge-transfer excited state is now found to decay in less than 100 femtoseconds, forming a quasi-stable metal centered excited state with a 13 picosecond lifetime. We find that this MC excited state has triplet (3MC) character, unlike other reported six-coordinate Fe(ii)-centered coordination compounds, which form MC quintet (5MC) states. The solvent dependent changes in excited state non-radiative relaxation for [Fe(bpy)(CN)4]2- allows us to infer the influence of the solvent on the electronic structure of the complex. Furthermore, the robust characterization of the dynamics and optical spectral signatures of the isolated 3MC intermediate provides a strong foundation for identifying 3MC intermediates in the electronic excited state relaxation mechanisms of similar Fe-centered systems being developed for solar applications.
|
|
| 18. |
- Koroidov, Sergey, et al.
(författare)
-
Chemisorbed oxygen or surface oxides steer the selectivity in Pd electrocatalytic propene oxidation observed by operando Pd L-edge X-ray absorption spectroscopy
- 2021
-
Ingår i: Catalysis Science & Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 11:10, s. 3347-3352
-
Tidskriftsartikel (refereegranskat)abstract
- Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8–1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. These reactions are proposed to proceed via the Langmuir–Hinshelwood mechanism. In contrast, for the potential range of 1.1–1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface.
|
|
| 19. |
- Koroidov, Sergey, et al.
(författare)
-
First turnover analysis of water-oxidation catalyzed by Co-oxide nanoparticles
- 2015
-
Ingår i: Energy & Environmental Science. - : Royal Society of Chemistry (RSC). - 1754-5692 .- 1754-5706. ; 8:8, s. 2492-2503
-
Tidskriftsartikel (refereegranskat)abstract
- Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Presently, several different proposals exist for how they catalyze O2 formation from water. Knowledge about this process at molecular detail will be required for their further improvement. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet mass spectrometry (MIMS) experiments to study the mechanism of water oxidation in Co/methylenediphosphonate (Co/M2P) oxide nanoparticles using [Ru(bpy)3]3+ (bpy = 2,2'-bipyridine) as chemical oxidant. We show that 16O–Co/M2P-oxide nanoparticles produce 16O2 during their first turnover after simultaneous addition of H218O and [Ru(bpy)3]3+, while sequential addition with a delay of 3 s yields oxygen reflecting bulk water 18O-enrichment. This result is interpreted to show that the O–O bond formation in Co/M2P-oxide nanoparticles occurs via intramolecular oxygen coupling between two terminal Co–OHn ligands that are readily exchangeable with bulk water in the resting state of the catalyst. Importantly, our data allow the determination of the number of catalytic sites within this amorphous nanoparticular material, to calculate the TOF per catalytic site and to derive the number of holes needed for the production of the first O2 molecule per catalytic site. We propose that the mechanism of O–O bond formation during bulk catalysis in amorphous Co-oxides may differ from that taking place at the surface of crystalline materials.
|
|
| 20. |
- Koroidov, Sergey, et al.
(författare)
-
Mechanism of water oxidation catalyzed by Co/M2P-oxides studied by isotope-ratio membrane inlet mass spectrometry
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Co-oxides are promising water oxidation catalysts for artificial photosynthesis devices. Their catalytic mechanism was studied previously both experimentally and theoretically, but there is presently no agreement whether the O-O bond formation occurs via nucleophilic attack or by direct coupling, and whether or not bridging oxygens participate as substrate during O-O bondformation. Here we present time-resolved 18O-labelling isotope-ratio membrane-inlet massspectrometry experiments employing the previously introduced Co/methylenediphosphonate (Co/M2P) system in combination with [Ru(bpy)3]3+ (bpy = 2,2’-bipyridine) as chemical oxidant. Our data demonstrate that for Co/M2P-oxide O-O bond formation occurs between two pre-bound, fast exchanging oxygen species, i.e. likely via direct coupling between two terminal water-derived oxygen ligands. Detailed modeling of the dependence of the O2-isotope ratios on the [Ru(bpy)3]3+ concentration revealed that in the Co/M2P-oxidenanoparticles almost all Co ions are catalytically active (2.35 Co per catalytic site) and that, starting from a ‘resting state‘, 3.5 electrons need to be removed from each catalytic site for thefirst O2 formation. Since it was previously demonstrated that in the resting state most Co ions are in the oxidation state Co(III), we conclude that the coupling mechanism involves at least one Co(IV)-O• radical.
|
|
| 21. |
- Koroidov, Sergey, et al.
(författare)
-
Mobile hydrogen carbonate acts as proton acceptor in photosynthetic water oxidation
- 2014
-
Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 11:17, s. 6299-6304
-
Tidskriftsartikel (refereegranskat)abstract
- Cyanobacteria, algae and plants oxidize water to the O2 we breathe, and consume CO2 during the synthesis of biomass. Although these vital processes are functionally and structurally well separated in photosynthetic organisms, there is a long-debated role for CO2/HCO3− in water oxidation. Using membrane-inlet mass spectrometry we demonstrate that HCO3− acts as a mobile proton acceptor that helps to transport the protons produced inside of photosystem II by water-oxidation out into the chloroplast's lumen, resulting in a light-driven production of O2 and CO2. Depletion of HCO3− from the media leads, in the absence of added buffers, to a reversible down-regulation of O2 production by about 20%. These findings add a previously unidentified component to the regulatory network of oxygenic photosynthesis, and conclude the more than 50-y-long quest for the function of CO2/ HCO3− in photosynthetic water oxidation.
|
|
| 22. |
- Koroidov, Sergey, et al.
(författare)
-
Probing the Electron Accepting Orbitals of Ni-Centered Hydrogen Evolution Catalysts with Noninnocent Ligands by Ni L-Edge and S K-Edge X-ray Absorption
- 2018
-
Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:21, s. 13167-13175
-
Tidskriftsartikel (refereegranskat)abstract
- The valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal-ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt)(2) (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H)(2) intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H)(2) intermediate and Ni(mpo)(2) (mpo = 2-mercaptopyridyl-N-oxide) catalyst are found to have considerably larger Ni 3d (46-47%) and S 3p (17-18% per S) character, consistent with an orbital localized on the metal-ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni-H bond formation for the reduced Ni(abt-H)(2) intermediate and Ni(mpo)(2) catalyst, a critical reaction intermediate in H-2 generation.
|
|
| 23. |
- Koroidov, Sergey, 1982-
(författare)
-
Water splitting in natural and artificial photosynthetic systems
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photosynthesis is the unique biological process that converts carbon dioxide into organic compounds, for example sugars, using the energy of sunlight. Thereby solar energy is converted into chemical energy. Nearly all life depends on this reaction, either directly, or indirectly as the ultimate source of their food. Oxygenic photosynthesis occurs in plants, algae and cyanobacteria. This process created the present level of oxygen in the atmosphere, which allowed the formation of higher life, since respiration allows extracting up to 15-times more energy from organic matter than anaerobic fermentation. Oxygenic photosynthesis uses as substrate for the ubiquitous water. The light-induced oxidation of water to molecular oxygen (O2), catalyzed by the Mn4CaO5 cluster associated with the photosystem II (PS II) complex, is thus one of the most important and wide spread chemical processes occurring in the biosphere. Understanding the mechanism of water-oxidation by the Mn4CaO5 cluster is one of today’s great challenges in science. It is believed that one can extract basic principles of catalyst design from the natural system that than can be applied to artificial systems. Such systems can be used in the future for the generation of fuel from sunlight.In this thesis the light-induced production of molecular oxygen and carbon dioxide (CO2) by PSII was observed by membrane-inlet mass spectrometry. By analyzing this observation is shown that CO2 not only is the substrate in photosynthesis for the production of sugars, but that it also regulates the efficiency of the initial steps of the electron transport chain of oxygenic photosynthesis by acting, in form of HCO3-, as acceptor for protons produced during water-splitting. This finding concludes the 50-years old search for the function of CO2/HCO3− in photosynthetic water oxidation.For understanding the mechanism of water oxidation it is crucial to resolve the structures of all oxidation states, including transient once, of the Mn4CaO5 cluster. With this application in mind a new illumination setup was developed and characterized that allowed to bring the Mn4CaO5 cluster of PSII microcrystals into known oxidation states while they flow through a narrow capillary. The optimized illumination conditions were employed at the X-ray free electron laser at the Linac Coherent Light Source (LCLS) to obtain simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) at room temperature. This two methods probe the overall protein structure and the electronic structure of the Mn4CaO5 cluster, respectively. Data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. This approach opens new directions for studying structural changes during the catalytic cycle of the Mn4CaO5 cluster, and for resolving the mechanism of O-O bond formation.In two other projects the mechanism of molecular oxygen formation by artificial water oxidation catalysts containing inexpensive and abundant elements were studied. Oxygen evolution catalyzed by calcium manganese and manganese only oxides was studied in 18O-enriched water. It was concluded that molecular oxygen is formed by entirely different pathways depending on what chemical oxidant was used. Only strong non-oxygen donating oxidants were found to support ‘true’ water-oxidation. For cobalt oxides a study was designed to understand the mechanistic details of how the O-O bond forms. The data demonstrate that O-O bond formation occurs by direct coupling between two terminal water-derived ligands. Moreover, by detailed theoretical modelling of the data the number of cobalt atoms per catalytic site was derived.
|
|
| 24. |
- Kubin, Markus, et al.
(författare)
-
Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers
- 2017
-
Ingår i: Structural Dynamics. - : AMER INST PHYSICS. - 2329-7778. ; 4:5
-
Tidskriftsartikel (refereegranskat)abstract
- X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn similar to 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions. (C) 2017 Author(s).
|
|
| 25. |
- Kunnus, Kristjan, et al.
(författare)
-
Chemical control of competing electron transfer pathways in iron tetracyano-polypyridyl photosensitizers
- 2020
-
Ingår i: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 11:17, s. 4360-4373
-
Tidskriftsartikel (refereegranskat)abstract
- Photoinduced intramolecular electron transfer dynamics following metal-to-ligand charge-transfer (MLCT) excitation of [Fe(CN)(4)(2,2 '-bipyridine)](2-) (1), [Fe(CN)(4)(2,3-bis(2-pyridyl)pyrazine)](2-) (2) and [Fe(CN)(4)(2,2 '-bipyrimidine)](2-) (3) were investigated in various solvents with static and time-resolved UV-Visible absorption spectroscopy and Fe 2p3d resonant inelastic X-ray scattering (RIXS). This series of polypyridyl ligands, combined with the strong solvatochromism of the complexes, enables the (MLCT)-M-1 vertical energy to be varied from 1.64 eV to 2.64 eV and the (MLCT)-M-3 lifetime to range from 180 fs to 67 ps. The (MLCT)-M-3 lifetimes in 1 and 2 decrease exponentially as the MLCT energy increases, consistent with electron transfer to the lowest energy triplet metal-centred ((MC)-M-3) excited state, as established by the Tanabe-Sugano analysis of the Fe 2p3d RIXS data. In contrast, the (MLCT)-M-3 lifetime in 3 changes non-monotonically with MLCT energy, exhibiting a maximum. This qualitatively distinct behaviour results from a competing (MLCT)-M-3 -> ground state (GS) electron transfer pathway that exhibits energy gap law behaviour. The (MLCT)-M-3 -> GS pathway involves nuclear tunnelling for the high-frequency polypyridyl breathing mode (h nu = 1530 cm(-1)), which is most displaced for complex 3, making this pathway significantly more efficient. Our study demonstrates that the excited state relaxation mechanism of Fe polypyridyl photosensitizers can be readily tuned by ligand and solvent environment. Furthermore, our study reveals that extending charge transfer lifetimes requires control of the relative energies of the (MLCT)-M-3 and the (MC)-M-3 states and suppression of the intramolecular distortion of the acceptor ligand in the (MLCT)-M-3 excited state.
|
|
| 26. |
- Kunnus, Kristjan, et al.
(författare)
-
Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering
- 2020
-
Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.
|
|
| 27. |
- La Hera, Vladimir Miranda, et al.
(författare)
-
Controlled Synthesis of Tellurium Nanowires
- 2022
-
Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 12:23
-
Tidskriftsartikel (refereegranskat)abstract
- One-dimensional tellurium nanostructures can exhibit distinct electronic properties from those seen in bulk Te. The electronic properties of nanostructured Te are highly dependent on their morphology, and thus controlled synthesis processes are required. Here, highly crystalline tellurium nanowires were produced via physical vapour deposition. We used growth temperature, heating rate, flow of the carrier gas, and growth time to control the degree of supersaturation in the region where Te nanostructures are grown. The latter leads to a control in the nucleation and morphology of Te nanostructures. We observed that Te nanowires grow via the vapour–solid mechanism where a Te particle acts as a seed. Transmission electron microscopy (TEM) and electron diffraction studies revealed that Te nanowires have a trigonal crystal structure and grow along the (0001) direction. Their diameter can be tuned from 26 to 200 nm with lengths from 8.5 to 22 μm, where the highest aspect ratio of 327 was obtained for wires measuring 26 nm in diameter and 8.5 μm in length. We investigated the use of bismuth as an additive to reduce the formation of tellurium oxides, and we discuss the effect of other growth parameters.
|
|
| 28. |
- LaRue, Jerry, et al.
(författare)
-
Symmetry-resolved CO desorption and oxidation dynamics on O/Ru(0001) probed at the C K-edge by ultrafast x-ray spectroscopy
- 2022
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 157:16
-
Tidskriftsartikel (refereegranskat)abstract
- We report on carbon monoxide desorption and oxidation induced by 400 nm femtosecond laser excitation on the O/Ru(0001) surface probed by time-resolved x-ray absorption spectroscopy (TR-XAS) at the carbon K-edge. The experiments were performed under constant background pressures of CO (6 × 10−8 Torr) and O2 (3 × 10−8 Torr). Under these conditions, we detect two transient CO species with narrow 2π* peaks, suggesting little 2π* interaction with the surface. Based on polarization measurements, we find that these two species have opposing orientations: (1) CO favoring a more perpendicular orientation and (2) CO favoring a more parallel orientation with respect to the surface. We also directly detect gas-phase CO2 using a mass spectrometer and observe weak signatures of bent adsorbed CO2 at slightly higher x-ray energies than the 2π* region. These results are compared to previously reported TR-XAS results at the O K-edge, where the CO background pressure was three times lower (2 × 10−8 Torr) while maintaining the same O2 pressure. At the lower CO pressure, in the CO 2π* region, we observed adsorbed CO and a distribution of OC–O bond lengths close to the CO oxidation transition state, with little indication of gas-like CO. The shift toward “gas-like” CO species may be explained by the higher CO exposure, which blocks O adsorption, decreasing O coverage and increasing CO coverage. These effects decrease the CO desorption barrier through dipole–dipole interaction while simultaneously increasing the CO oxidation barrier.
|
|
| 29. |
- Miranda la Hera, Vladimir, et al.
(författare)
-
Controlled synthesis of tellurium nanowires by physical vapor deposition
- 2022
-
Ingår i: Nanomaterials. - : MDPI. - 2079-4991. ; 12:23
-
Tidskriftsartikel (refereegranskat)abstract
- One-dimensional tellurium nanostructures can exhibit distinct electronic properties from those seen in bulk Te. The electronic properties of nanostructured Te are highly dependent on their morphology, and thus controlled synthesis processes are required. Here, highly crystalline tellurium nanowires were produced via physical vapour deposition. We used growth temperature, heating rate, flow of the carrier gas, and growth time to control the degree of supersaturation in the region where Te nanostructures are grown. The latter leads to a control in the nucleation and morphology of Te nanostructures. We observed that Te nanowires grow via the vapour–solid mechanism where a Te particle acts as a seed. Transmission electron microscopy (TEM) and electron diffraction studies revealed that Te nanowires have a trigonal crystal structure and grow along the (0001) direction. Their diameter can be tuned from 26 to 200 nm with lengths from 8.5 to 22 μm, where the highest aspect ratio of 327 was obtained for wires measuring 26 nm in diameter and 8.5 μm in length. We investigated the use of bismuth as an additive to reduce the formation of tellurium oxides, and we discuss the effect of other growth parameters.
|
|
| 30. |
- Mitzner, Rolf, et al.
(författare)
-
L-Edge X-ray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using an X-ray Free-Electron Laser
- 2013
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:21, s. 3641-3647
-
Tidskriftsartikel (refereegranskat)abstract
- L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming 0 K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples.
|
|
| 31. |
- Sauter, Nicholas K., et al.
(författare)
-
No observable conformational changes in PSII
- 2016
-
Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 533:7603, s. E1-E2
-
Tidskriftsartikel (refereegranskat)
|
|
| 32. |
- Schreck, Simon, et al.
(författare)
-
Atom-Specific Probing of Electron Dynamics in an Atomic Adsorbate by Time-Resolved X-Ray Spectroscopy
- 2022
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 129:27
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic excitation occurring on adsorbates at ultrafast timescales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) of a simple well-known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel [Ni(100)] surface, following intense laser optical pumping at 400 nm. We observe ultrafast (∼100 fs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed by slower changes on a few picoseconds timescale, shown to be consistent with thermalization of the complete C/Ni system. Density functional theory spectrum simulations support this interpretation.
|
|
| 33. |
- Shevela, Dmitriy, et al.
(författare)
-
Biogenesis of water splitting by photosystem II during de-etiolation of barley (Hordeum vulgare L.)
- 2016
-
Ingår i: Plant, Cell and Environment. - : John Wiley & Sons. - 0140-7791 .- 1365-3040. ; 39:7, s. 1524-1536
-
Tidskriftsartikel (refereegranskat)abstract
- Etioplasts lack thylakoid membranes and photosystem complexes. Light triggers differentiation of etioplasts into mature chloroplasts, and photosystem complexes assemble in parallel with thylakoid membrane development. Plastids isolated at various time points of de-etiolation are ideal to study the kinetic biogenesis of photosystem complexes during chloroplast development. Here, we investigated the chronology of photosystem II (PSII) biogenesis by monitoring assembly status of chlorophyll-binding protein complexes and development of water splitting via O2 production in plastids (etiochloroplasts) isolated during de-etiolation of barley (Hordeum vulgare L.). Assembly of PSII monomers, dimers and complexes binding outer light-harvesting antenna [PSII-light-harvesting complex II (LHCII) supercomplexes] was identified after 1, 2 and 4 h of de-etiolation, respectively. Water splitting was detected in parallel with assembly of PSII monomers, and its development correlated with an increase of bound Mn in the samples. After 4 h of de-etiolation, etiochloroplasts revealed the same water-splitting efficiency as mature chloroplasts. We conclude that the capability of PSII to split water during de-etiolation precedes assembly of the PSII-LHCII supercomplexes. Taken together, data show a rapid establishment of water-splitting activity during etioplast-to-chloroplast transition and emphasize that assembly of the functional water-splitting site of PSII is not the rate-limiting step in the formation of photoactive thylakoid membranes.
|
|
| 34. |
- Shevela, Dmitriy, et al.
(författare)
-
Calcium manganese oxides as oxygen evolution catalysts : o(2) formation pathways indicated by (18) o-labelling studies
- 2011
-
Ingår i: Chemistry - A European Journal. - Weinheim : Wiley-Blackwell. - 0947-6539 .- 1521-3765. ; 17:19, s. 5415-5423
-
Tidskriftsartikel (refereegranskat)abstract
- Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in (18) O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O(2) isotopologues (16) O(2) , (16) O(18) O and (18) O(2) in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H(2) O(2) ), hydrogen persulfate (HSO(5) (-) ) or single-electron oxidants such as Ce(IV) and [Ru(III) (bipy)(3) ](3+) . Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of μ-oxido manganese or μ-oxido calcium manganese sites. The studied processes show very similar (18) O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.
|
|
| 35. |
- Shevela, Dmitriy, et al.
(författare)
-
Efficiency of photosynthetic water oxidation at ambient and depleted levels of inorganic carbon
- 2013
-
Ingår i: Photosynthesis Research. - : Springer. - 0166-8595 .- 1573-5079. ; 117:1-3, s. 401-412
-
Tidskriftsartikel (refereegranskat)abstract
- Over 40 years ago, Joliot et al. (Photochem Photobiol 10:309-329, 1969) designed and employed an elegant and highly sensitive electrochemical technique capable of measuring O2 evolved by photosystem II (PSII) in response to trains of single turn-over light flashes. The measurement and analysis of flash-induced oxygen evolution patterns (FIOPs) has since proven to be a powerful method for probing the turnover efficiency of PSII. Stemler et al. (Proc Natl Acad Sci USA 71(12):4679-4683, 1974), in Govindjee's lab, were the first to study the effect of "bicarbonate" on FIOPs by adding the competitive inhibitor acetate. Here, we extend this earlier work by performing FIOPs experiments at various, strictly controlled inorganic carbon (Ci) levels without addition of any inhibitors. For this, we placed a Joliot-type bare platinum electrode inside a N2-filled glove-box (containing 10-20 ppm CO2) and reduced the Ci concentration simply by washing the samples in Ci-depleted media. FIOPs of spinach thylakoids were recorded either at 20-times reduced levels of Ci or at ambient Ci conditions (390 ppm CO2). Numerical analysis of the FIOPs within an extended Kok model reveals that under Ci-depleted conditions the miss probability is discernibly larger (by 2-3 %) than at ambient conditions, and that the addition of 5 mM HCO3 (-) to the Ci-depleted thylakoids largely restores the original miss parameter. Since a "mild" Ci-depletion procedure was employed, we discuss our data with respect to a possible function of free or weakly bound HCO3 (-) at the water-splitting side of PSII.
|
|
| 36. |
- Tatsuno, Hideyuki, et al.
(författare)
-
Hot Branching Dynamics in a Light-Harvesting Iron Carbene Complex Revealed by Ultrafast X-ray Emission Spectroscopy
- 2020
-
Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 59:1, s. 364-372
-
Tidskriftsartikel (refereegranskat)abstract
- Iron N-heterocyclic carbene (NHC) complexes have received a great deal of attention recently because of their growing potential as light sensitizers or photocatalysts. We present a sub-ps X-ray spectroscopy study of an FeIINHC complex that identifies and quantifies the states involved in the deactivation cascade after light absorption. Excited molecules relax back to the ground state along two pathways: After population of a hot 3MLCT state, from the initially excited 1MLCT state, 30 % of the molecules undergo ultrafast (150 fs) relaxation to the 3MC state, in competition with vibrational relaxation and cooling to the relaxed 3MLCT state. The relaxed 3MLCT state then decays much more slowly (7.6 ps) to the 3MC state. The 3MC state is rapidly (2.2 ps) deactivated to the ground state. The 5MC state is not involved in the deactivation pathway. The ultrafast partial deactivation of the 3MLCT state constitutes a loss channel from the point of view of photochemical efficiency and highlights the necessity to screen transition-metal complexes for similar ultrafast decays to optimize photochemical performance.
|
|
| 37. |
- Tran, Rosalie, et al.
(författare)
-
The Mn4Ca photosynthetic water-oxidation catalyst studied by simultaneous X-ray spectroscopy and crystallography using an X-ray free-electron laser
- 2014
-
Ingår i: Philosophical Transactions of the Royal Society of London. Biological Sciences. - : The Royal Society. - 0962-8436 .- 1471-2970. ; 369:1647, s. 20130324-
-
Forskningsöversikt (refereegranskat)abstract
- The structure of photosystem II and the catalytic intermediate states of the Mn4CaO5 cluster involved in water oxidation have been studied intensively over the past several years. An understanding of the sequential chemistry of light absorption and the mechanism of water oxidation, however, requires a new approach beyond the conventional steady-state crystallography and X-ray spectroscopy at cryogenic temperatures. In this report, we present the preliminary progress using an X-ray free-electron laser to determine simultaneously the light-induced protein dynamics via crystallography and the local chemistry that occurs at the catalytic centre using X-ray spectroscopy under functional conditions at room temperature.
|
|
| 38. |
- Wang, Hsin-Yi, et al.
(författare)
-
Time-resolved observation of transient precursor state of CO on Ru(0001) using carbon K-edge spectroscopy
- 2020
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 22:5, s. 2677-2684
-
Tidskriftsartikel (refereegranskat)abstract
- The transient dynamics of carbon monoxide (CO) molecules on a Ru(0001) surface following femtosecond optical laser pump excitation has been studied by monitoring changes in the unoccupied electronic structure using an ultrafast X-ray free-electron laser (FEL) probe. The particular symmetry of perpendicularly chemisorbed CO on the surface is exploited to investigate how the molecular orientation changes with time by varying the polarization of the FEL pulses. The time evolution of spectral features corresponding to the desorption precursor state was well distinguished due to the narrow line-width of the C K-edge in the X-ray absorption (XA) spectrum, illustrating that CO molecules in the precursor state rotated freely and resided on the surface for several picoseconds. Most of the CO molecules trapped in the precursor state ultimately cooled back down to the chemisorbed state, while we estimate that ∼14.5 ± 4.9% of the molecules in the precursor state desorbed into the gas phase. It was also observed that chemisorbed CO molecules diffused over the metal surface from on-top sites toward highly coordinated sites. In addition, a new “vibrationally hot precursor” state was identified in the polarization-dependent XA spectra.
|
|
| 39. |
- Yano, Junko, et al.
(författare)
-
Light-dependent production of dioxygen in photosynthesis
- 2015
-
Ingår i: Sustaining life on planet earth. - Cham : Springer. - 9783319124148 - 9783319124155 ; , s. 13-43
-
Bokkapitel (refereegranskat)abstract
- Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation, which is catalyzed by a Mn4CaO5 cluster in photosystem II (PS II), a multi subunit membrane protein complex. X-ray and other spectroscopy studies of the electronic and geometric structure of the Mn4CaO5 cluster as it advances through the intermediate states have been important for understanding the mechanism of water oxidation. The results and interpretations, especially from X-ray spectroscopy studies, regarding the geometric and electronic structure and the changes as the system proceeds through the catalytic cycle will be summarized in this review. This review will also include newer methodologies in time-resolved X-ray diffraction and spectroscopy that have become available since the commissioning of the X-ray free electron laser (XFEL) and are being applied to study the oxygen-evolving complex (OEC). The femtosecond X-ray pulses of the XFEL allows us to outrun X-ray damage at room temperature, and the time-evolution of the photo-induced reaction can be probed using a visible laser-pump followed by the X-ray-probe pulse. XFELs can be used to simultaneously determine the light-induced protein dynamics using crystallography and the local chemistry that occurs at the catalytic center using X-ray spectroscopy under functional conditions. Membrane inlet mass spectrometry has been important for providing direct information about the exchange of substrate water molecules, which has a direct bearing on the mechanism of water oxidation. Moreover, it has been indispensable for the time-resolved X-ray diffraction and spectroscopy studies and will be briefly reviewed in this chapter. Given the role of PS II in maintaining life in the biosphere and the future vision of a renewable energy economy, understanding the structure and mechanism of the photosynthetic water oxidation catalyst is an important goal for the future.
|
|
| 40. |
- Young, Iris D., et al.
(författare)
-
Structure of photosystem II and substrate binding at room temperature
- 2016
-
Ingår i: Nature. - : Macmillan Publishers Ltd.. - 0028-0836 .- 1476-4687. ; 540:7633, s. 453-457
-
Tidskriftsartikel (refereegranskat)abstract
- Light-induced oxidation of water by photosystem II (PS II) in plants, algae and cyanobacteria has generated most of the dioxygen in the atmosphere. PS II, a membrane-bound multi-subunit pigment protein complex, couples the one-electron photochemistry at the reaction centre with the four-electron redox chemistry of water oxidation at the Mn4CaO5 cluster in the oxygen-evolving complex (OEC). Under illumination, the OEC cycles through five intermediate S-states (S0 to S4)1, in which S1 is the dark-stable state and S3 is the last semi-stable state before O–O bond formation and O2 evolution2,3. A detailed understanding of the O–O bond formation mechanism remains a challenge, and will require elucidation of both the structures of the OEC in the different S-states and the binding of the two substrate waters to the catalytic site4–6. Here we report the use of femtosecond pulses from an X-ray free electron laser (XFEL) to obtain damage-free, room temperature structures of dark-adapted (S1), two-flash illuminated (2F; S3-enriched), and ammonia-bound two-flash illuminated (2F-NH3; S3-enriched) PS II. Although the recent 1.95 Å resolution structure of PS II at cryogenic temperature using an XFEL7 provided a damage-free view of the S1 state, measurements at room temperature are required to study the structural landscape of proteins under functional conditions8,9, and also for in situ advancement of the S-states. To investigate the water-binding site(s), ammonia, a water analogue, has been used as a marker, as it binds to the Mn4CaO5 cluster in the S2 and S3 states10. Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site10–13. This approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O–O bond formation mechanisms.
|
|
