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1.	Abrashev, Miroslav V., et al. (författare) Origin of the heat-induced improvement of catalytic activity and stability of MnOx electrocatalysts for water oxidation 2019 Ingår i: Journal of Materials Chemistry A. - : ROYAL SOC CHEMISTRY. - 2050-7488 .- 2050-7496. ; 7:28, s. 17022-17036 Tidskriftsartikel (refereegranskat)abstract Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure-activity relations, we investigate the influence of moderate annealing (100-300 degrees C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure-reactivity relations. Oxide structures featuring both di-mu-oxo bridged Mn ions and (close to) linear mono-mu-oxo Mn3+-O-Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O-2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like zeta-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 degrees C) and this is the first report where it was stable under ambient conditions.
2.	Behrouzi, Leila, et al. (författare) Electrochemical alcohols oxidation mediated by N-hydroxyphthalimide on nickel foam surface 2020 Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 10:1 Tidskriftsartikel (refereegranskat)abstract Alcohol to aldehyde conversion is a critical reaction in the industry. Herein, a new electrochemical method is introduced that converts 1 mmol of alcohols to aldehydes and ketones in the presence of N-hydroxyphthalimide (NHPI, 20 mol%) as a mediator; this conversion is achieved after 8.5 h at room temperature using a piece of Ni foam (1.0 cm2) and without adding an extra-base or a need for high temperature. Using this method, 10 mmol (1.08 g) of benzyl alcohol was also successfully oxidized to benzaldehyde (91%) without any by-products. This method was also used to oxidize other alcohols with high yield and selectivity. In the absence of a mediator, the surface of the nickel foam provided oxidation products at the lower yield. After the reaction was complete, nickel foam (anode) was characterized by a combination of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectroelectrochemistry, which pointed to the formation of nickel oxide on the surface of the electrode. On the other hand, using other electrodes such as Pt, Cu, Fe, and graphite resulted in a low yield for the alcohol to aldehyde conversion.
3.	Bhowmick, Asmit, et al. (författare) Going around the Kok cycle of the water oxidation reaction with femtosecond X-ray crystallography 2023 Ingår i: IUCrJ. - : International Union Of Crystallography. - 2052-2525. ; 10:6, s. 642-655 Forskningsöversikt (refereegranskat)abstract The water oxidation reaction in photosystem II (PS II) produces most of the molecular oxygen in the atmosphere, which sustains life on Earth, and in this process releases four electrons and four protons that drive the downstream process of CO2 fixation in the photosynthetic apparatus. The catalytic center of PS II is an oxygen-bridged Mn4Ca complex (Mn4CaO5) which is progressively oxidized upon the absorption of light by the chlorophyll of the PS II reaction center, and the accumulation of four oxidative equivalents in the catalytic center results in the oxidation of two waters to dioxygen in the last step. The recent emergence of X-ray free-electron lasers (XFELs) with intense femtosecond X-ray pulses has opened up opportunities to visualize this reaction in PS II as it proceeds through the catalytic cycle. In this review, we summarize our recent studies of the catalytic reaction in PS II by following the structural changes along the reaction pathway via room-temperature X-ray crystallography using XFELs. The evolution of the electron density changes at the Mn complex reveals notable structural changes, including the insertion of OX from a new water molecule, which disappears on completion of the reaction, implicating it in the O-O bond formation reaction. We were also able to follow the structural dynamics of the protein coordinating with the catalytic complex and of channels within the protein that are important for substrate and product transport, revealing well orchestrated conformational changes in response to the electronic changes at the Mn4Ca cluster.
4.	Bhowmick, Asmit, et al. (författare) Structural evidence for intermediates during O2 formation in photosystem II 2023 Ingår i: Nature. - : Springer Nature. - 0028-0836 .- 1476-4687. ; 617:7961, s. 629-636 Tidskriftsartikel (refereegranskat)abstract In natural photosynthesis, the light-driven splitting of water into electrons, protons and molecular oxygen forms the first step of the solar-to-chemical energy conversion process. The reaction takes place in photosystem II, where the Mn4CaO5 cluster first stores four oxidizing equivalents, the S0 to S4 intermediate states in the Kok cycle, sequentially generated by photochemical charge separations in the reaction center and then catalyzes the O–O bond formation chemistry. Here, we report room temperature snapshots by serial femtosecond X-ray crystallography to provide structural insights into the final reaction step of Kok’s photosynthetic water oxidation cycle, the S3→[S4]→S0 transition where O2 is formed and Kok’s water oxidation clock is reset. Our data reveal a complex sequence of events, which occur over micro- to milliseconds, comprising changes at the Mn4CaO5 cluster, its ligands and water pathways as well as controlled proton release through the hydrogen-bonding network of the Cl1 channel. Importantly, the extra O atom Ox, which was introduced as a bridging ligand between Ca and Mn1 during the S2→S3 transition, disappears or relocates in parallel with Yz reduction starting at approximately 700 μs after the third flash. The onset of O2 evolution, as indicated by the shortening of the Mn1–Mn4 distance, occurs at around 1,200 μs, signifying the presence of a reduced intermediate, possibly a bound peroxide.
5.	Boniolo, Manuel, et al. (författare) Electronic and geometric structure effects on one-electron oxidation of first-row transition metals in the same ligand framework 2021 Ingår i: Dalton Transactions. - : Royal Society of Chemistry. - 1477-9226 .- 1477-9234. ; 50:2, s. 660-674 Tidskriftsartikel (refereegranskat)abstract Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl] pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (M-II) and the one-electron oxidized (M-III) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal-ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.
6.	Boniolo, Manuel, 1990-, et al. (författare) Pentapyridyl Base Metal Complexes: Apical Ligand Exchange, Redox Potential, Stability and Water Oxidation Annan publikation (övrigt vetenskapligt/konstnärligt)
7.	Boniolo, Manuel, et al. (författare) Spin transition in a ferrous chloride complex supported by a pentapyridine ligand 2020 Ingår i: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 56:18, s. 2703-2706 Tidskriftsartikel (refereegranskat)abstract Ferrous chloride complexes [FeIILxCl] commonly attain a high-spin state independently of the supporting ligand(s) and temperature. Herein, we present the first report of a complete spin crossover with T1/2 = 80 K in [FeII(Py5OH)Cl]+ (Py5OH = pyridine-2,6-diylbis[di(pyridin-2-yl)methanol]). Both spin forms of the complex are analyzed by X-ray spectroscopy and DFT calculations.
8.	Boniolo, Manuel, et al. (författare) Water Oxidation by Pentapyridyl Base Metal Complexes? : A Case Study 2022 Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:24, s. 9104-9118 Tidskriftsartikel (refereegranskat)abstract The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.
9.	Cheah, Mun Hon, et al. (författare) Electrochemical oxidation of ferricyanide 2021 Ingår i: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11 Tidskriftsartikel (refereegranskat)abstract We report the electrochemical oxidation of ferricyanide, [Fe-III(CN)(6)](3-) and characterised the oxidation product by in-situ FTIR and XAS spectroelectrochemistry methods. Oxidation of [Fe-III(CN)(6)](3-) is proposed to proceed via a tentative Fe(IV) intermediate that undergoes reduction elimination to give cis-[Fe-III(CN)(4)(CH3CN)(2)](1-) as stable product in acetonitrile. Speciation of the oxidation product by DFT calculations is underpinned by good agreement to experimental data.
10.	Chernev, Petko, et al. (författare) Light-driven formation of manganese oxide by today's photosystem II supports evolutionarily ancient manganese-oxidizing photosynthesis 2020 Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 11:1 Tidskriftsartikel (refereegranskat)abstract Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III, IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today's photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by downsizing of protein-bound manganese-oxide nanoparticles to finally yield today's catalyst of photosynthetic water oxidation.
11.	de Lichtenberg, Casper, et al. (författare) The exchange of the fast substrate water in the S-2 state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism 2021 Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 12:38, s. 12763-12775 Tidskriftsartikel (refereegranskat)abstract The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn4CaO5 cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow exchanging substrate water (W-s)', while, in the S-2 state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (W-f)'. A critical point for the assignment of W-f is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn4CaO5 cluster. In this study, we measured the rates of (H2O)-O-16/(H2O)-O-18 substrate water exchange in the S-2 and S-3 states of PSII core complexes from wild-type (WT) Synechocystis sp. PCC 6803, and from two mutants, D1-D61A and D1-E189Q, that are expected to alter water access via the Cl1/O4 channels and the O1 channel, respectively. We found that the exchange rates of W-f and W-s were unaffected by the E189Q mutation (O1 channel), but strongly perturbed by the D61A mutation (Cl1/O4 channel). It is concluded that all channels have restrictions limiting the isotopic equilibration of the inner water pool near the Mn4CaO5 cluster, and that D61 participates in one such barrier. In the D61A mutant this barrier is lowered so that W-f exchange occurs more rapidly. This finding removes the main argument against Ca-bound W3 as fast substrate water in the S-2 state, namely the indifference of the rate of W-f exchange towards Ca/Sr substitution.
12.	Graça, André T., et al. (författare) CryoEM insight into hydrogen positions and water networks in photosystem II Annan publikation (övrigt vetenskapligt/konstnärligt)
13.	Görlin, Mikaela, et al. (författare) Formation of unexpectedly active Ni-Fe oxygen evolution electrocatalysts by physically mixing Ni and Fe oxyhydroxides 2019 Ingår i: Chemical Communications. - : Royal Society of Chemistry (RSC). - 1359-7345 .- 1364-548X. ; 55:6, s. 818-821 Tidskriftsartikel (refereegranskat)abstract We present an unusual, yet facile, strategy towards formation of physically mixed Ni-Fe(OxHy) oxygen evolution electrocatalysts. We use in situ X-ray absorption and UV-vis spectroscopy, and high-resolution imaging to demonstrate that physical contact between two inferior Ni(OH)(2) and Fe(OOH) catalysts self-assemble into atomically intermixed Ni-Fe catalysts with unexpectedly high activity.
14.	Han, Guangye, et al. (författare) Molecular basis for turnover inefficiencies (misses) during water oxidation in photosystem II 2022 Ingår i: Chemical Science. - : Royal Society of Chemistry. - 2041-6520 .- 2041-6539. ; 13:29, s. 8667-8678 Tidskriftsartikel (refereegranskat)abstract Photosynthesis stores solar light as chemical energy and efficiency of this process isv highly important. The electrons required for CO2 reduction are extracted from water in a reaction driven by light-induced charge separations in the Photosystem II reaction center and catalyzed by the CaMn4O5-cluster. This cyclic process involves five redox intermediates known as the S-0-S-4 states. In this study, we quantify the flash-induced turnover efficiency of each S state by electron paramagnetic resonance spectroscopy. Measurements were performed in photosystem II membrane preparations from spinach in the presence of an exogenous electron acceptor at selected temperatures between -10 degrees C and +20 degrees C and at flash frequencies of 1.25, 5 and 10 Hz. The results show that at optimal conditions the turnover efficiencies are limited by reactions occurring in the water oxidizing complex, allowing the extraction of their S state dependence and correlating low efficiencies to structural changes and chemical events during the reaction cycle. At temperatures 10 degrees C and below, the highest efficiency (i.e. lowest miss parameter) was found for the S-1 -> S-2 transition, while the S-2 -> S-3 transition was least efficient (highest miss parameter) over the whole temperature range. These electron paramagnetic resonance results were confirmed by measurements of flash-induced oxygen release patterns in thylakoid membranes and are explained on the basis of S state dependent structural changes at the CaMn4O5-cluster that were determined recently by femtosecond X-ray crystallography. Thereby, possible "molecular errors" connected to the e(-) transfer, H+ transfer, H2O binding and O-2 release are identified.
15.	Hussein, Rana, et al. (författare) Cryo-electron microscopy reveals hydrogen positions and water networks in photosystem II 2024 Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 384:6702, s. 1349-1355 Tidskriftsartikel (refereegranskat)abstract Photosystem II starts the photosynthetic electron transport chain that converts solar energy into chemical energy and thus sustains life on Earth. It catalyzes two chemical reactions: water oxidation to molecular oxygen and plastoquinone reduction. Coupling of electron and proton transfer is crucial for efficiency; however, the molecular basis of these processes remains speculative owing to uncertain water binding sites and the lack of experimentally determined hydrogen positions. We thus collected high-resolution cryo-electron microscopy data of fully hydrated photosystem II from the thermophilic cyanobacterium Thermosynechococcus vestitus to a final resolution of 1.71 angstroms. The structure reveals several previously undetected partially occupied water binding sites and more than half of the hydrogen and proton positions. This clarifies the pathways of substrate water binding and plastoquinone B protonation.
16.	Hussein, Rana, et al. (författare) Structural dynamics in the water and proton channels of photosystem II during the S2 to S3 transition 2021 Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 12:1 Tidskriftsartikel (refereegranskat)abstract Light-driven oxidation of water to molecular oxygen is catalyzed by the oxygen-evolving complex (OEC) in Photosystem II (PS II). This multi-electron, multi-proton catalysis requires the transport of two water molecules to and four protons from the OEC. A high-resolution 1.89 Å structure obtained by averaging all the S states and refining the data of various time points during the S2 to S3 transition has provided better visualization of the potential pathways for substrate water insertion and proton release. Our results indicate that the O1 channel is the likely water intake pathway, and the Cl1 channel is the likely proton release pathway based on the structural rearrangements of water molecules and amino acid side chains along these channels. In particular in the Cl1 channel, we suggest that residue D1-E65 serves as a gate for proton transport by minimizing the back reaction. The results show that the water oxidation reaction at the OEC is well coordinated with the amino acid side chains and the H-bonding network over the entire length of the channels, which is essential in shuttling substrate waters and protons.
17.	Ibrahim, Mohamed, et al. (författare) Untangling the sequence of events during the S-2 -> S-3 transition in photosystem II and implications for the water oxidation mechanism 2020 Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : NATL ACAD SCIENCES. - 0027-8424 .- 1091-6490. ; 117:23, s. 12624-12635 Tidskriftsartikel (refereegranskat)abstract In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S-1, S-2, S-3, and S-0, showing that a water molecule is inserted during the S-2 -> S-3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O-2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S-2 -> S-3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, QA and QB, are observed. At the donor site, tyrosine YZ and His190 H-bonded to it move by 50 mu s after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of OX(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a "water wheel"-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (t of similar to 350 mu s) during the S-2 -> S-3 transition mirrors the appearance of OX electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.
18.	Ibrahim, Mohamed, et al. (författare) Untangling the sequence of events during the S2 -> S3 transition in photosystem II and implications for the water oxidation mechanism 2020 Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 117:23, s. 12624-12635 Tidskriftsartikel (refereegranskat)abstract In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S1, S2, S3, and S0, showing that a water molecule is inserted during the S2 -> S3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S2 -> S3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, QA and QB, are observed. At the donor site, tyrosine YZ and His190 H-bonded to it move by 50 μs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of OX(H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a "water wheel"-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 μs) during the S2 -> S3 transition mirrors the appearance of OX electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.
19.	Keable, Stephen M., et al. (författare) Room temperature XFEL crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem I 2021 Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 11:1 Tidskriftsartikel (refereegranskat)abstract Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.
20.	Leidel, Nils, et al. (författare) Electronic Structure of an [FeFe] Hydrogenase Model Complex in Solution Revealed by X-ray Absorption Spectroscopy Using Narrow-Band Emission Detection 2012 Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:34, s. 14142-14157 Tidskriftsartikel (refereegranskat)abstract High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe-2(mu-adt)(CO)(4)(PMe3)(2)] (1, adt = S-CH2-(NCH2Ph)-CH2-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1(powder)), in MeCN solution (1'), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy protonation of the Fe-Fe bond to form a bridging metal hydride), relations between the molecular structures and the electronic configurations were determined. EXAFS analysis and DFT geometry optimization suggested prevailing rotational isomers in MeCN, which were similar to the crystal structure or exhibited rotation of the (CO) ligands at Fe1 (1(CO), 1Hy(CO)) and in addition of the phenyl ring (1H(CO,ph), 1HHy(CO,ph)), leading to an elongated solvent-exposed Fe-Fe bond. Isomer formation, adt-N protonation, and hydride binding caused spectral changes of core-to-valence (pre-edge of the Fe K-shell absorption) and of valence-to-core (K beta(2,5) emission) electronic transitions, and of K alpha RIXS data, which were quantitatively reproduced by DFT. The study reveals (1) the composition of molecular orbitals, for example, with dominant Fe-d character, showing variations in symmetry and apparent oxidation state at the two Fe ions and a drop in MO energies by similar to 1 eV upon each protonation step, (2) the HOMO-LUMO energy gaps, of similar to 2.3 eV for 1(powder) and similar to 2.0 eV for 1', and (3) the splitting between iron d(z(2)) and d(x(2-)y(2)) levels of similar to 0.5 eV for the nonhydride and similar to 0.9 eV for the hydride states. Good correlations of reduction potentials to LUMO energies and oxidation potentials to HOMO energies were obtained. Two routes of facilitated bridging hydride binding thereby are suggested, involving ligand rotation at Fe1 for 1Hy(CO) or adt-N protonation for 1HHy(CO,ph). XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.
21.	Leidel, Nils, et al. (författare) High-valent [MnFe] and [FeFe] cofactors in ribonucleotide reductases 2012 Ingår i: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1817:3, s. 430-444 Tidskriftsartikel (refereegranskat)abstract Ribonucleotide reductases (RNRs) are essential for DNA synthesis in most organisms. In class-Ic RNR from Chlamydia trachomatis (Ct), a MnFe cofactor in subunit R2 forms the site required for enzyme activity, instead of an FeFe cofactor plus a redox-active tyrosine in class-la RNRs, for example in mouse (Mus musculus, Mm). For R2 proteins from Ct and Mm, either grown in the presence of, or reconstituted with Mn and Fe ions, structural and electronic properties of higher valence MnFe and FeFe sites were determined by X-ray absorption spectroscopy and complementary techniques, in combination with bond-valence-sum and density functional theory calculations. At least ten different cofactor species could be tentatively distinguished. In Cr R2, two different Mn(IV)Fe(III) site configurations were assigned either L4MnIV(mu O)(2)(FeL4)-L-III (metal-metal distance of similar to 2.75 angstrom, L = ligand) prevailing in metal-grown R2, or L4MnIV(mu O)(mu OH)(FeL4)-L-III (similar to 2.90 angstrom) dominating in metal-reconstituted R2. Specific spectroscopic features were attributed to an Fe(IV)Fe(III) site (similar to 2.55 angstrom) with a L4FeIV(mu O)(2)(FeL3)-L-III core structure. Several Mn,Fe(III)Fe(III) (similar to 2.9-3.1 angstrom) and Mn,Fe(III)Fe(II) species (similar to 3.3-3.4 angstrom) likely showed 5-coordinated Mn(III) or Fe(III). Rapid X-ray photoreduction of iron and shorter metal-metal distances in the high-valent states suggested radiation-induced modifications in most crystal structures of R2. The actual configuration of the MnFe and FeFe cofactors seems to depend on assembly sequences, bound metal type, valence state, and previous catalytic activity involving subunit RI. In Ct R2, the protonation of a bridging oxide in the Mn-IV(mu O)(mu OH)Fe-III core may be important for preventing premature site reduction and initiation of the radical chemistry in R1.
22.	Leidel, Nils, et al. (författare) Site-Selective X-ray Spectroscopy on an Asymmetric Model Complex of the [FeFe] Hydrogenase Active Site 2012 Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 51:8, s. 4546-4559 Tidskriftsartikel (refereegranskat)abstract The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe-2(I)(CO)(Ph2P-CH2-NCH3-CH2-PPh2)] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. K beta emission spectra (K beta(1,3), K beta') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the K beta(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (K beta(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d -> 1s emission and 1s -> 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. K beta-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal F(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various K beta emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.
23.	Loos, Stefan, et al. (författare) Electromodified NiFe Alloys as Electrocatalysts for Water Oxidation : Mechanistic Implications of Time-Resolved UV/Vis Tracking of Oxidation State Changes 2019 Ingår i: ChemSusChem. - : Wiley-VCH Verlagsgesellschaft. - 1864-5631 .- 1864-564X. ; 12:9, s. 1966-1976 Tidskriftsartikel (refereegranskat)abstract Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. During cyclic voltammetry and after sudden potential jumps between noncatalytic and catalytic potentials, Ni oxidation/reduction was tracked with millisecond time resolution by a UV/Vis reflectance signal. Optimal catalysis at intermediate Ni/Fe ratios is explained by two opposing trends for increasing Fe content: a)pronounced slowdown of the Ni2+/Ni3+ oxidation step and b)increased reactivity of the most oxidized catalyst state detectable at catalytic potentials. This state may involve an equilibrium between Ni4+ ions and Ni2+ ions with neighboring ligand holes, possibly in the form of bound peroxides.
24.	Moghaddam, Navid Jameei, et al. (författare) A Chemical Evolution-Like Method to Synthesize a Water-Oxidizing Catalyst 2021 Ingår i: ChemElectroChem. - : John Wiley & Sons. - 2196-0216. ; 8:23, s. 4612-4617 Tidskriftsartikel (refereegranskat)abstract The development of new procedures for synthesizing new, low-cost, and stable metal oxides for the oxygen-evolution reaction (OER) by water oxidation is critical. Since the 2000s, there has been a rapid rise in the use of first-row transition metal (hydr)oxides for the OER. However, there is still a need to design and synthesize an efficient and stable catalyst for the OER. The present paper aims to design and synthesize an OER catalyst based on "a look at nature" strategy. In a simple and chemical evolution-like experiment, for the first time, a solution composed of various perchlorate cations, namely, Li(I), Mg(II), Ca(II), Al(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ba(II), and Na2SiO3, at a concentration of 1.0 mM at pH=4 and a potential of 1.40 V vs. NHE, which was under stirring for 15 days, was investigated. The stable OER catalyst on the electrode was characterized by X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Based on the extrapolation of the Tafel plot, the onset of the OER in the presence of this catalyst was 130 mV lower than a bare electrode without it. This approach could be a roadmap to design and synthesize new and stable catalysts.
25.	Mousazade, Younes, et al. (författare) A synthetic manganese-calcium cluster similar to the catalyst of Photosystem II : challenges for biomimetic water oxidation 2020 Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 49:17, s. 5597-5605 Tidskriftsartikel (refereegranskat)abstract Herein, we report the synthesis, characterization, crystal structure, density functional theory calculations, and water-oxidizing activity of a pivalate Mn-Ca cluster. All of the manganese atoms in the cluster are Mn(iv) ions and have a distorted MnO6 octahedral geometry. Three Mn(iv) ions together with a Ca(ii) ion and four-oxido groups form a cubic Mn3CaO4 unit which is similar to the Mn3CaO4 cluster in the water-oxidizing complex of Photosystem II. Using scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, extended X-ray absorption spectroscopy, chronoamperometry, and electrochemical methods, a conversion into nano-sized Mn-oxide is observed for the cluster in the water-oxidation reaction.
26.	Mousazade, Younes, et al. (författare) Revisiting Metal-Organic Frameworks for Oxygen Evolution : A Case Study 2020 Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 59:20, s. 15335-15342 Tidskriftsartikel (refereegranskat)abstract Water splitting is a promising reaction for storing sustainable but intermittent energies. In water splitting, water oxidation is a bottleneck, and thus different catalysts have been synthesized for water oxidation. Metal-organic frameworks (MOFs) are among the highly efficient catalysts for water oxidation, and so far, MOF-based catalysts have been divided into two categories: MOF-derived catalysts and direct MOF catalysts. In particular, a nickel/cobalt MOF is reported to be one of the best direct catalysts for water oxidation. For the first-row transition MOF structures in general, a hypothesis is that the harsh conditions of OER could cause the decomposition of organic ligands and the formation of water-oxidizing oxide-based structures. By electrochemical methods, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy, a nickel/cobalt MOF known to be a highly efficient catalyst for water oxidation is shown to form Ni/Co oxide, making it a candidate catalyst for oxygen evolution. MOFs are interesting precatalysts for metal oxide water-oxidizing catalysts, but control experiments are necessary for determining whether a certain MOF or other MOFs are true catalysts for OER. Thus, finding a true and direct MOF electrocatalyst for OER is a challenge.
27.	Mousazade, Younes, et al. (författare) Water oxidation by a manganese-potassium cluster : Mn oxide as a kinetically dominant "true" catalyst for water oxidation 2018 Ingår i: Catalysis Science & Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:17, s. 4390-4398 Tidskriftsartikel (refereegranskat)abstract Nature uses an Mn cluster for water oxidation, and thus, water oxidation using Mn clusters is interesting when used in artificial water-splitting systems. An important question is whether an Mn cluster is a true catalyst for water oxidation or not. Herein, an Mn-K cluster was investigated for electrochemical water oxidation to find the true and the kinetically dominant catalyst using X-ray absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electrochemical methods. The experiments showed that conversion into nanosized Mn oxide occurred for the cluster, and the nanosized Mn oxides are the true catalyst for water oxidation.
28.	Pasquini, Chiara, et al. (författare) Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film 2021 Ingår i: Analytical and Bioanalytical Chemistry. - : Springer Nature. - 1618-2642 .- 1618-2650. ; 413:21, s. 5395-5408 Tidskriftsartikel (refereegranskat)abstract Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.
29.	Pham, Long Vo, et al. (författare) Unequal misses during the flash-induced advancement of photosystem II : effects of the S state and acceptor side cycles 2019 Ingår i: Photosynthesis Research. - : Springer. - 0166-8595 .- 1573-5079. ; 139:1-3, s. 93-106 Tidskriftsartikel (refereegranskat)abstract Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (S-i states, i=0-4) resulting in O-2 evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the S-i-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S-2, S-3 and S-0 state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of Q(A)/Q(A)(-) in PSII is significantly less negative in C. merolae (E-m=-104mV) than in spinach (E-m=-163mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the E-m of Q(A)(-)/Q(A) plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from Q(A)(-) to Pheo slows down the charge recombination reaction with the S-3 and S-2 states considerably. In addition, our data support the concept that the S-2 S-3 transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.
30.	Safdari, Rasoul, et al. (författare) A mononuclear cobalt complex for water oxidation : new controversies and puzzles 2018 Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 47:46, s. 16668-16673 Tidskriftsartikel (refereegranskat)abstract Herein the role of a mononuclear cobalt(iii) complex, [Co-III(DPKOH)(2)]ClO4 (DPK = di(2-pyridyl)ketone), in the water electrooxidation process is investigated with scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction studies, NMR, chronoamperometry, cyclic voltammetry, extended X-ray absorption fine structure and X-ray absorption near edge structure determination. Our experiments show that, in comparison to the reported literature, other cobalt-containing structures on the surface of the FTO electrode could also be the true catalyst for water oxidation.
31.	Schuth, Nils, et al. (författare) K alpha X-ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S-3 State 2018 Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 57:16, s. 10424-10430 Tidskriftsartikel (refereegranskat)abstract The unique manganese calcium-catalyst in photosystem II (PSII) is the natural paragon for efficient light driven water oxidation to yield O-2. The oxygen-evolving complex (OEC) in the dark-stable state (S-1) comprises a Mn4CaO4 core with five metal-bound water species. Binding and modification of the water molecules that are substrates of the water-oxidation reaction is mechanistically crucial but controversially debated. Two recent crystal structures of the OEC in its highest oxidation state (S-3) show either a vacant Mn coordination site or a bound peroxide species. For purified PSII at room temperature, we collected Mn K alpha X-ray emission spectra of the S-0, S-1, S-2, and S-3 intermediates in the OEC cycle, which were analyzed by comparison to synthetic Mn compounds, spectral simulations, and OEC models from density functional theory. Our results contrast both crystallographic structures. They indicate Mn oxidation in three S-transitions and suggest additional water binding at a previously open Mn coordination site. These findings exclude Mn reduction and render peroxide formation in S-3 unlikely.
32.	Sigfridsson Clauss, Kajsa, et al. (författare) Rapid X-ray photoreduction of dimetal-oxygen cofactors in ribonucleotide reductase. 2013 Ingår i: Journal of Biological Chemistry. - 1083-351X .- 0021-9258. Tidskriftsartikel (refereegranskat)abstract Prototypic dinuclear metal cofactors with varying metallation constitute a class of O2-activating catalysts in numerous enzymes such as ribonucleotide reductase (RNR1). Reliable structures are required to unravel the reaction mechanisms. However, protein crystallography data may be compromised by X-ray photoreduction (XPR). We studied XPR of Fe(III)Fe(III) and Mn(III)Fe(III) sites in the R2 subunit of Chlamydia trachomatis RNR using X-ray absorption spectroscopy. Rapid and biphasic XPR kinetics at 20 K and 80 K for both cofactor types suggested sequential formation of (III,II) and (II,II) species and similar redox potentials of Fe and Mn sites. Comparing with typical X-ray doses in crystallography implies that (II,II) states are reached in <1 s in such studies. First-sphere metal coordinations and metal-metal distances differed after chemical reduction at room temperature and after XPR at cryogenic temperatures, as corroborated by model structures from density functional theory calculations. The inter-metal distances in the (II,II) states, however, are similar to R2 crystal structures. Therefore, crystal data of initially oxidized R2-type proteins mostly contain photoreduced (II,II) cofactors, which deviate from the native structures functional in O2-activation, explaining observed variable metal ligation motifs. This situation may be remedied by novel femtosecond free-electron-laser protein crystallography techniques.
33.	Simon, Philipp S., et al. (författare) Capturing the sequence of events during the water oxidation reaction in photosynthesis using XFELs 2023 Ingår i: FEBS Letters. - : John Wiley & Sons. - 0014-5793 .- 1873-3468. ; 597:1, s. 30-37 Tidskriftsartikel (refereegranskat)abstract Ever since the discovery that Mn was required for oxygen evolution in plants by Pirson in 1937 and the period-four oscillation in flash-induced oxygen evolution by Joliot and Kok in the 1970s, understanding of this process has advanced enormously using state-of-the-art methods. The most recent in this series of innovative techniques was the introduction of X-ray free-electron lasers (XFELs) a decade ago, which led to another quantum leap in the understanding in this field, by enabling operando X-ray structural and X-ray spectroscopy studies at room temperature. This review summarizes the current understanding of the structure of Photosystem II (PS II) and its catalytic centre, the Mn4CaO5 complex, in the intermediate Si (i = 0–4)-states of the Kok cycle, obtained using XFELs.
34.	Zaharieva, Ivelina, et al. (författare) Room-Temperature Energy-Sampling K beta X-ray Emission Spectroscopy of the Mn4Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O-2 Formation 2016 Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 55:30, s. 4197-4211 Tidskriftsartikel (refereegranskat)abstract In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn K beta(1,3) line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The K beta(1,3) line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)(3)Mn(IV) in S-0, Mn(III)(2)Mn(IV)(2) in S-1, Mn(III)Mn(IV)(3) in S-2, and Mn(IV)(4) in S-3 and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S-4 state unlikely. The difference spectra of both manganese K beta(1,3) emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S-2 -> S-3 transition when compared to Mn(III) oxidation in the S-1 -> S-2 transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S-2 -> S-3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal ligand bonding patterns at the Mn4Ca complex in the S-2 -> S-3 transition are discussed.
35.	Zand, Zahra, et al. (författare) Nickel-Vanadium Layered Double Hydroxide under Water-Oxidation Reaction : New Findings and Challenges 2019 Ingår i: ACS Sustainable Chemistry and Engineering. - : AMER CHEMICAL SOC. - 2168-0485. ; 7:20, s. 17252-17262 Tidskriftsartikel (refereegranskat)abstract Nickel-vanadium layered double hydroxide has recently been considered as a highly active, low-cost electrocatalyst and as a benchmark non-noble metal-based electrocatalyst for water oxidation. The material showed a current density of 27 mA/cm(2) at an overpotential of 350 mV, which is comparable to the best-performing nickel-iron-layered double hydroxides for water oxidation in alkaline media. The enhanced conductivity and facile electron transfer were suggested among important factors for the high activity of nickel-vanadium layered double hydroxide. In the present study, the stability of an Ni-V catalyst was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and electrochemical characterization methods. These methods show that the initial Ni-V catalyst during water oxidation in alkaline conditions is converted from an initial alpha-Ni(OH)(2) phase to a partially oxidized alpha-Ni(OH)(2/)NiOOH phase and VO(4)(3-)ions. We carefully evaluate the stability of the catalysts and analyze the compositional changes during prolonged water-oxidation conditions using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The experiments using both Fe-free electrolyte and Fe-free nickel-vanadium layered double hydroxide reveal that vanadium do not affect the water-oxidizing activity of alpha-Ni(OH)(2).
36.	Zand, Zahra, et al. (författare) Role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex 2023 Ingår i: Communications Chemistry. - : Springer Nature. - 2399-3669. ; 6:1 Tidskriftsartikel (refereegranskat)abstract Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)(2)). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water. Metal complexes are often used as catalysts for oxidation reactions, however, there are open questions about the role of the decomposition products in the catalytic process. Here, the authors explore the potential role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex catalyst adsorbed on an SBA-15 substrate.

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