Oxidation State Of Iron Research Articles

Fe3+ -hosting carbon phases in the deep Earth

Iron-bearing carbonates play an important role in Earth's carbon cycle. Owing to their stability at mantle conditions, recently discovered iron carbonates with tetrahedrally coordinated carbon atoms are candidates for carbon storage in the deep Earth. The carbonates' iron oxidation and spin state at extreme pressure and temperature conditions contribute to the redox conditions and element partitioning in the deep mantle. By laser heating ${\mathrm{FeCO}}_{3}$ at pressures of about 83 GPa, ${\mathrm{Fe}}_{4}^{3+}{\mathrm{C}}_{3}{\mathrm{O}}_{12}$ and ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ were synthesized and then investigated by x-ray emission spectroscopy to elucidate their spin state, both in situ and temperature quenched. Our experimental results show both phases in a high-spin state at all pressures and over the entire temperature range investigated, i.e., up to 3000 K. The spin state is conserved after temperature quenching. A formation path is favored where ${\mathrm{Fe}}_{4}^{3+}{\mathrm{C}}_{3}{\mathrm{O}}_{12}$ forms first and then reacts to ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$, most likely accompanied by the formation of oxides. Density functional theory calculations of ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ at 80 GPa confirm the experimental findings with both ferric and ferrous iron in high-spin state with antiferromagnetic order at 80 GPa. As the intercrystalline cation partitioning between the Fe-bearing carbonates and the surrounding perovskite and ferropericlase depends on the spin state of the iron, an understanding of the redox conditions prevalent in subducted slab regions in the lower mantle has to take the latter into account. Especially, ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ may play a key role in subducted material in the lower mantle, potentially with a similar role as silicate perovskite.

Tektite glasses from Belize, Central America: Petrography, geochemistry, and search for a possible meteoritic component

The presence of tektite-like glasses from a geographically restricted area in Belize (Central America) has been known for several decades. We comprehensively studied 18 such Belize glasses by a variety of petrographic and geochemical methods, including major and trace element analysis, radiogenic isotopic composition (Rb-Sr, Sm-Nd, and Re-Os), water content, oxidation state, and cosmogenic radionuclides. The aims were to determine their compositional variation, their mode of formation and possible source rocks, and their relation to known tektites, and to search for evidence of an extraterrestrial component.In terms of petrography, the samples are similar to tektites from the four “classical” strewn fields, with the presence of lechatelierites, schlieren, and vesicles; these are also widely accepted indicators of an impact origin. No close similarities to volcanic glasses are evident. Water contents are very low, and iron oxidation states are mostly reduced, in both cases similar to observations for other tektites. The geochemical and isotopic data presented, such as Cr, Co and Ni elemental abundances and interelement ratios, as well as trace element patterns are typical for local and regional volcanics from the active Central American Arc. Particular similarities to material comparable to volcanic rocks from Honduras or Guatemala are noted. This is confirmed by Sr-Nd isotope signatures of the Belize glasses, showing close similarities to Central American volcanics in general, and Honduran and Guatemalan volcanic, in particular. Osmium concentrations and 187Os/188Os ratios are comparable to arc volcanics from world-wide locations, but - in a few of the samples - elevated Ir concentrations, near-chondritic Pt/Ir and 187Os/188Os ratios can also be interpreted with the admixture of a minor meteoritic component to some of the Belize samples. 10Be concentrations are consistent with values typical of both, young or deeply buried soils and with values for Central American volcanics, which carry subducted 10Be.Geochemical data clearly indicate a source different from that of the Australasian tektites. Both isotope data sets for the Belize glasses indicate a close relationship to local arc lavas, especially those from Guatemala and Honduras, suggesting that the glasses were not deposited very far from their source. The main evidence that the Belize glasses are of impact origin are their petrographic characteristics and low water content. The evidence from 10Be is consistent with, but does not require, a model of formation for the Belize glasses by an impact on loosely consolidated surface sediments exposed to rain. A probable meteoritic component is low and heterogeneously distributed.

Aerosol iron speciation and seasonal variation of iron oxidation state over the western Antarctic Peninsula

The iron (Fe) speciation and oxidation state have been considered critical factors affecting Fe solubility in the atmosphere and bioavailability in the surface ocean. In this study, elemental composition and Fe speciation in aerosol samples collected at the Palmer Station in the West Antarctic Peninsula were determined using synchrotron-based X-ray fluorescence (XRF) and X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The elemental composition of coarse-mode (>1 μm) Fe-containing particles suggests that the region's crustal emission is the primary source of aerosol Fe. The Fe minerals in these aerosol particles were predominantly hematite and biotite, but minor fractions of pyrite and ilmenite were observed as well. The Fe oxidation state showed an evident seasonal variation. The Fe(II) content accounted for 71% of the total Fe in the austral summer, while this fraction dropped to 60% in the austral winter. Multivariate linear models involving meteorological parameters suggested that the wind speed, relative humidity, and solar irradiance were the factors that significantly controlled the percentage of Fe(II) in the austral summer. On the contrary, no relationship was found between these factors and the Fe(II) percentage in the austral winter, suggesting that atmospheric photoreduction and regional dust emission were limited. Moreover, the snow depth was significantly (p < 0.05) correlated with the aerosol Fe concentration, confirming the limiting effect of snow/ice cover on the regional dust emission. Given that the Antarctic Peninsula has experienced rapid warming during recent decades, the ice-free areas in the Antarctic Peninsula may act as potential dust sources.

Differentiating Defects and Their Influence on Hematite Photoanodes Using X-ray Absorption Spectroscopy and Raman Microscopy.

A high degree of variability in behavior and performance of hematite as photoanodes for the oxygen evolution reaction signifies a need to improve our understanding of the interplay between defects and photoelectrochemical performance. We approach this problem by applying structure-property analysis to a series of hematite samples synthesized under either O2 or N2 environments such that they exhibit highly variable performance for photoelectrocatalytic oxygen evolution. X-ray absorption fine-structure spectroscopy and Raman spectroscopy provide parameters describing the structure of samples across the series. Systematic comparisons of these parameters to those describing photoelectrochemical performance reveal different defects in samples prepared under N2 or O2. Distinct correlations between both the iron oxidation state and charge carrier density with photoelectrocatalytic performance lead to assignment of the primary defects as oxygen vacancies (N2) and iron vacancies (O2). Differences in the structural distortions caused by these defects are seen in correlations between short-range structural parameters and photoelectrochemical behavior. These distortions are readily observed by Raman spectroscopy, suggesting that it may be possible to calibrate the width, energy, and intensity of peaks in Raman spectra to enable direct analysis of defects in hematite photoanodes.

A note on Mössbauer analysis of white oak surfaces colored with aqueous iron salt solutions

Aqueous solutions of iron salts can be applied to a wood surface to modify its color. When applied on wood, iron ions are chelated by the wood’s natural phenolic compounds. The resulting color of the wood surface is due to the type of reaction products formed, such as mono-, bis- and tris- complexes of polyphenols with iron cations. In order to identify the different complexes formed on Quercus alba L. wood’s surface and the oxidation state of iron after application of different iron salts on the same wood species which influence the resulting color of wood’s surface, Mössbauer spectroscopy was performed directly on iron-stained wood samples. Colors of the stained wood samples, measured by a spectrophotometer, were analyzed in relation to the differences between the reaction products. The results showed that for a given wood species, the oxidation and reduction behavior of the iron depended on both the type of counterion and the oxidation state of the chosen iron salt.

Proposition of a Thermogravimetric Method to Measure the Ferrous Iron Content in Metallurgical-Grade Chromite

The oxidation state of iron in minerals is an important part of analysis. Especially for minerals used as a raw material for metallurgical processes, the oxidation state has a significant impact on the process. One crucial impact is the varying carbon requirement in smelting furnaces, which can be significantly different if the oxidation state is not assessed correctly. Compared to methods usually used to determine the oxidation state, a relatively simple and fast thermogravimetric method is proposed in this article. As a sample, a detailed analyzed chromite sample from Turkey is used. Bulk chemical analysis, Raman spectroscopy, X-ray diffraction, and QEMSCAN® are used to determine the preconditions of the sample. Mössbauer spectroscopy is used as a reference method to determine the oxidation state of iron in the sample. Uncertified wet chemical methods are investigated as well in this paper and the results are compared with the reference measurement. Using a thermochemical simulation tool, parameters for the thermogravimetric method are investigated and the limitation of this method is examined. The mean ferrous ratio in the sample determined by the proposed method is 75.205%, which is only slightly lower than the ferrous ratio of 76% determined by Mössbauer spectroscopy.

Iron oxide thin films grown on (00l) sapphire substrate by pulsed-laser deposition

Magnetite (Fe3O4) and wustite (FeO) are well-known oxides for their promising magnetic properties. However, growing stable FeOx thin films with controlled iron oxidation states could be of interest for further applications. In this work, we highlight the formation of different FeOx-based thin films grown by pulsed-laser deposition (PLD) from Fe or α-Fe2O3 targets onto sapphire substrates. Based on X-ray diffraction and Rutherford backscattering spectroscopy analyses, we report on films with various O/Fe ratios obtained by controlling both the substrate temperature and the oxygen pressure in the 10−5 – 10 Pa range during the PLD growth. Thus, the different crystalline phases of the Fe-O system can be grown: metallic iron, α-Fe2O3, Fe3O4, and the thermodynamically metastable phase FeO. On one hand, we demonstrate that at low pressures (< 10−4 Pa) metal-oxide composite thin films with both metallic Fe and Fe3O4 phase are epitaxially grown on the sapphire substrates even at room temperature. On the other hand, we highlight that using a α-Fe2O3 target, films composed with a mixture of wustite and spinel phases have been obtained at room temperature. The precise growth conditions leading to the formation of FeO-based films or metal-oxide composite films are discussed.

Oxidation or cation re-arrangement? Distinct behavior of riebeckite at high temperature

Abstract In this work we address the stability of riebeckite at high temperatures and compare the different behaviors observed under various oxidation conditions. For this purpose, we annealed powders of a sample from Mt. Malosa (Malawi), which is compositionally close to the end-member; the run products obtained after annealing in air vs. in vacuum were studied by Mössbauer spectroscopy and powder X-ray diffraction. The results show that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, it is stable in the hydrous form up to relatively low temperatures (400–450 °C), then it undergoes a rapid (within ~50 °C) dehydrogenation, forming oxo-riebeckite, which is stable up to ~900 °C. The final breakdown products of the oxo-amphibole include aegirine + cristobalite + hematite. Based on the relative intensity of the (310) Bragg reflection, the activation energy (Ea) for the riebeckite to oxo-riebeckite transition is 166 ± 6 kJ/mol. Under vacuum conditions, no Fe oxidation is observed, and riebeckite is stable up to much higher temperatures (750–800 °C); however, in the 550 &amp;lt; T &amp;lt; 700 °C range, it undergoes a significant re-arrangement of the C cations (those hosted in the strip of octahedra). Indeed, the amphibole stable in the 700–800 °C range has the same chemical formula as riebeckite but has a disordered and nonstandard cation distribution at the octahedra, i.e., M(1)(Fe3+Fe2+)M(2)(Fe3+Fe2+)M(3)Fe2+; we call this phase “CR3+ disordered riebeckite”. For T ≥ 800 °C, it decomposes to aegirine + fayalite + cristobalite + H2O. External oxygen is required for the release of water into the surrounding system, being a prerequisite for the Fe-amphiboles to be a carrier of H2O in the lower crust and upper mantle. One important implication of our results is that characterization of the overall oxidation state of iron does not necessarily provide the redox conditions of the environment of formation because a crystal-chemical re-arrangement under reducing conditions allows riebeckite to maintain its Fe3+/Fe2+ composition up to higher temperatures.

Interfacial electronic properties of FeTPP-Cl on HOPG

Conjugated tetrapyrrole complexes have potential for novel spintronic and optoelectronic devices. Detailed understanding of electronic properties at molecule–substrate interface is essential for their potential applications. In this report, electronic properties of iron (III) chloride tetraphenylporphyrin (FeTPP-Cl, C44H28ClFeN4) thin films on highly oriented pyrolytic graphite (HOPG) have been investigated using photoemission and X-ray absorption spectroscopy. Photoemission analysis shows that no significant charge transfer takes between FeTPP-Cl and HOPG. Fe 2P3/2 core level analysis indicates towards dechlorination of FeTPP-Cl on HOPG in the monolayer regime. Fe L2,3 edge X-ray absorption spectroscopy reveal that iron oxidation state changes from +2 to +3 due to adsorption on to HOPG, suggesting a substrate driven dechlorination of FeTPP-Cl. Curve fitting analysis of XPS Fe 2p3/2 spectrum for the deposition of FeTPP-Cl on HOPG in the monolayer regime confirms +2 oxidation state of central metal atom. An interface dipole of 0.2 eV has been found at FeTPP-Cl/HOPG interface suggesting weaker molecule–substrate interactions.

Ice nucleation imaged with X-ray spectro-microscopy.

Ice nucleation is one of the most uncertain microphysical processes, as it occurs in various ways and on many types of particles. To overcome this challenge, we present a heterogeneous ice nucleation study on deposition ice nucleation and immersion freezing in a novel cryogenic X-ray experiment with the capability to spectroscopically probe individual ice nucleating and non-ice nucleating particles. Mineral dust type particles composed of either ferrihydrite or feldspar were used and mixed with organic matter of either citric acid or xanthan gum. We observed in situ ice nucleation using scanning transmission X-ray microscopy (STXM) and identified unique organic carbon functionalities and iron oxidation state using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the new in situ environmental ice cell, termed the ice nucleation X-ray cell (INXCell). Deposition ice nucleation of ferrihydrite occurred at a relative humidity with respect to ice, RHi, between ∼120–138% and temperatures, T ∼ 232 K. However, we also observed water uptake on ferrihydrite at the same T when deposition ice nucleation did not occur. Although, immersion freezing of ferrihydrite both in pure water droplets and in aqueous citric acid occurred at or slightly below conditions for homogeneous freezing, i.e. the effect of ferrihydrite particles acting as a heterogeneous ice nucleus for immersion freezing was small. Microcline K-rich feldspar mixed with xanthan gum was also used in INXCell experiments. Deposition ice nucleation occurred at conditions when xanthan gum was expected to be highly viscous (glassy). At less viscous conditions, immersion freezing was observed. We extended a model for heterogeneous and homogeneous ice nucleation, named the stochastic freezing model (SFM). It was used to quantify heterogeneous ice nucleation rate coefficients, mimic the competition between homogeneous ice nucleation; water uptake; deposition ice nucleation and immersion freezing, and predict the T and RHi at which ice was observed. The importance of ferrihydrite to act as a heterogeneous ice nucleating particle in the atmosphere using the SFM is discussed.

On the nature of durable active sites in Fe–N–C fuel cell catalysts

Fe–N–C catalysts have the potential to replace the costly platinum catalysts in fuel cells but face the challenge of instability. It is of vital importance to identify the chemical nature of durable active sites in Fe–N–C. In this perspective, we analyze the geometric and electronic factors that affect the intrinsic durability of the FeNxCy moieties and propose that iron–oxygen binding energy is most relevant. A weak Fe–O binding is beneficial to mitigate the oxidation attack to the iron center by reaction intermediates thus enhancing its resistance to demetalation. We then propose the iron oxidation (valence) state as an apparent descriptor of the Fe–O binding strength. A valence state of +2 indicates a high anti-oxidation ability and promises superior stability. Our proposal will deepen the understanding of the activity–stability trade-off for Fe–N–C catalysts and guide future active site optimization.

Explorations of the nonheme high-valent iron-oxo landscape: crystal structure of a synthetic complex with an [FeIV2(μ-O)2] diamond core relevant to the chemistry of sMMOH.

Methanotrophic bacteria utilize methane monooxygenase (MMO) to carry out the first step in metabolizing methane. The soluble enzymes employ a hydroxylase component (sMMOH) with a nonheme diiron active site that activates O2 and generates a powerful oxidant capable of converting methane to methanol. It is proposed that the diiron(II) center in the reduced enzyme reacts with O2 to generate a diferric-peroxo intermediate called P that then undergoes O-O cleavage to convert into a diiron(IV) derivative called Q, which carries out methane hydroxylation. Most (but not all) of the spectroscopic data of Q accumulated by various groups to date favor the presence of an FeIV2(μ-O)2 unit with a diamond core. The Que lab has had a long-term interest in making synthetic analogs of iron enzyme intermediates. To this end, the first crystal structure of a complex with a FeIIIFeIV(μ-O)2 diamond core was reported in 1999, which exhibited an Fe⋯Fe distance of 2.683(1) Å. Now more than 20 years later, a complex with an FeIV2(μ-O)2 diamond core has been synthesized in sufficient purity to allow diffraction-quality crystals to be grown. Its crystal structure has been solved, revealing an Fe⋯Fe distance of 2.711(4) Å for comparison with structural data for related complexes with lower iron oxidation states.

Iron Oxalate Humboldtine Crystallization by Fungus Aspergillus niger

Microfungi were able to alternate solid substrate in various environments and play a noticeable role in the formation of insoluble calcium oxalate crystals in subaerial biofilms on rock surfaces. The present work describes how iron oxalate dihydrate humboldtine is acquired under the influence of the acid-producing microscopic fungus Aspergillus niger on the surface of two iron- bearing mineral substrates in vitro. Pyrrhotite and siderite rocks, as well as the products of their alteration, were investigated using a complex of analytical methods, including powder X-ray diffraction, optical microscopy, scanning electron microscopy and EDX spectroscopy. The effect of the underlying rocks with different composition and solubility and different oxidation states of iron on Fe-oxalate crystallization and on the morphology of humboldtine crystals was shown. The mechanisms of humboldtine formation were discussed. The results obtained in vitro seem promising for using fungi in bioleaching iron and other metals from processed ores and for the development of environmentally friendly biotechnologies.

Efficient removal of methylene blue by turbostratic carbon/Fe3C/Fe composite synthesized by catalytic graphitization of sucrose

Turbostratic carbon/Fe3C/Fe composite was prepared by catalytic graphitization of sucrose by α-Fe2O3. The synthesized composites were characterized by X-ray diffractometry (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD patterns revealed the formation of turbostratic carbon with a d spacing of 0.3412 nm and the presence of Fe3C and Fe nanoparticles. Raman spectrum exhibited the D, G and G' peaks. XPS analysis shows the presence of graphitic carbon and +3 oxidation state of iron. Synthesized composite can remove methylene blue with an adsorption capacity of 17.8 mg/g at pH 7. Adsorption follows pseudo second order kinetics and the Langmuir isotherm model. As the synthesized composite is magnetic it could be easily separated from the mixture and could be reused effectively without significant activity loss for five cycles.

An ab initio study of the oxygen defect formation and oxide ion migration in (Sr1-xPrx)2FeO4±δ

The Sr n+1 Fe n O 3n+1 Ruddlesden-Popper (RP) perovskite family displays promising oxygen permeability and serves as a host stoichiometry for the design of solid oxide electrode materials. A strategy to tune electronic and ionic properties is the introduction of substitutional dopants like Pr 3+ to the A-site. In this study, we investigate the bulk structural, electronic, and oxygen migration properties for the n = 1 RP perovskite (Sr 1-x Pr x ) 2 FeO 4± δ (x = 0, 0.125, 0.25, 0.375, and 0.5). The oxygen partial pressure is adjusted to elucidate how anodic and cathodic operating conditions influence the formation of oxygen defects. Under anodic conditions, the oxygen vacancy is the dominant oxide defect for all dopant-configurations. Under cathodic conditions, oxygen vacancy defects dominate for configurations from x = 0 to x = 0.375 while the oxygen peroxide interstitial defect becomes the primary defect for x = 0.5. Next, we examine the relationship between Pr 3+ concentration, iron oxidation state, and charge compensation with defect formation to explain the trends in vacancy and peroxide interstitial formation energies. Results clarify the role of lanthanide A-site substitutional dopants on the electronic conductivity and oxide defect formation and migration in Sr-based RP perovskites. These atomic-scale insights suggest design directions for RP perovskites in SOFCs. • Doping strategy of Sr 2 FeO 4 with Pr 3+ for targeted electronic and ionic properties. • Mechanism of vacancy defect formation outlined in terms of charge delocalization. • Mechanism of interstitial defect formation outlined in terms of A-site identity. • Studied all non-symmetrical vacancy-mediated migration paths for given Pr 3+ value.

Resonant X-ray photo-oxidation of light-harvesting iron (II/III) N-heterocyclic carbene complexes

Two photoactive iron N-heterocyclic carbene complexes {[hbox {Fe}^{{{rm{II}}}}(hbox {btz})_2(hbox {bpy})]^{2+}} and {[hbox {Fe}^{{rm{III}}}(hbox {btz})_3]^{3+}}, where btz is 3,3’-dimethyl-1,1’-bis(p-tolyl)-4,4’-bis(1,2,3-triazol-5-ylidene) and bpy is 2,2’-bipyridine, have been investigated by Resonant Photoelectron Spectroscopy (RPES). Tuning the incident X-ray photon energy to match core-valence excitations provides a site specific probe of the electronic structure properties and ligand-field interactions, as well as information about the resonantly photo-oxidised final states. Comparing measurements of the Fe centre and the surrounding ligands demonstrate strong mixing of the Fe {hbox {t}_{{rm{2g}}}} levels with occupied ligand pi orbitals but weak mixing with the corresponding unoccupied ligand orbitals. This highlights the importance of pi-accepting and -donating considerations in ligand design strategies for photofunctional iron carbene complexes. Spin-propensity is also observed as a final-state effect in the RPES measurements of the open-shell hbox {Fe}^{{rm{III}}} complex. Vibronic coupling is evident in both complexes, where the energy dispersion hints at a vibrationally hot final state. The results demonstrate the significant impact of the iron oxidation state on the frontier electronic structure and highlights the differences between the emerging class of hbox {Fe}^{{rm{III}}} photosensitizers from those of more traditional hbox {Fe}^{{rm{II}}} complexes.

Effective degradation of cyclohexanecarboxylic acid by visible LED driven photo-Fenton

This work explores the feasibility of visible LEDs (λ: 400–800 nm) used in photo-Fenton for the degradation of a 50 mg/L cyclohexanecarboxylic acid (CHA) as model naphthenic acid present in process wastewater from the oil sands industry. The influence of initial pH, catalyst concentration and iron oxidation state, H2O2 dose and radiant flux were studied. In all trials, an initial lag phase in both the H2O2 decomposition and TOC evolution was observed. This seems to be related to the generation of a complex between CHA, hydrogen peroxide and iron (CHA-H2O2-Fe). This iron scavenging decreases the H2O2 decomposition rate, leading to a lagged mineralization kinetics. Nonetheless, process intensification using visible radiation (λmax: 460 nm) favors the CHA-H2O2-Fe photoactivation and breakdown when using an H2O2 concentration higher than 33% of the stoichiometric dose for complete CHA mineralization. Under the optimum conditions pH0 3, Φ: 280 W/m2, Fe2+:20 mg/L and H2O2: 144 mg/L, which corresponds to 60% of the stoichiometric amount, 75% of TOC removal was achieved.

Hydrogen Production from Alteration of Chicxulub Crater Impact Breccias: Potential Energy Source for a Subsurface Microbial Ecosystem.

A sulfate-reducing population of thermophiles grew in porous, permeable niches within glass-bearing impact breccias of the Chicxulub impact crater. The microbial community grew in an impact-generated hydrothermal system that vented on the seafloor several hundred meters beneath the sea surface. Potential electron donors for that metabolism are hydrocarbons, although a strong C-isotope signature of that source does not exist. Model calculations explored here suggest that alteration of glass within the impact breccias may have produced H2 in sufficient quantities for population growth as the hydrothermal system cooled through thermophilic temperatures, although it is sensitive to the oxidation state of iron in the melt rock prior to hydrothermal alteration and the secondary mineral assemblage. At high water-to-rock ratios and temperatures below 45°C, H2 yields are insufficient to maintain a population of hydrogenotrophic sulfate-reducing bacteria, but yields double with a higher proportion of ferrous iron between 45 and 65°C. The most reduced rocks (i.e., highest proportion of ferrous iron) that are allowed to form andradite, which is observed in core samples, produce copious amounts of H2 in the temperature window for thermophiles and hyperthermophiles. Mixtures of melt rock and carbonate, which is observed in breccia matrices, produce somewhat less H2, and the onset of massive H2 production is shifted to higher temperatures (i.e., lower W/R).

Quantum chemical insight into the effects of the local electron environment on T2*-based MRI

T2* relaxation is an intrinsic magnetic resonance imaging (MRI) parameter that is sensitive to local magnetic field inhomogeneities created by the deposition of endogenous paramagnetic material (e.g. iron). Recent studies suggest that T2* mapping is sensitive to iron oxidation state. In this study, we evaluate the spin state-dependence of T2* relaxation using T2* mapping. We experimentally tested this physical principle using a series of phantom experiments showing that T2* relaxation times are directly proportional to the spin magnetic moment of different transition metals along with their associated magnetic susceptibility. We previously showed that T2* relaxation time can detect the oxidation of Fe2+. In this paper, we demonstrate that T2* relaxation times are significantly longer for the diamagnetic, d10 metal Ga3+, compared to the paramagnetic, d5 metal Fe3+. We also show in a cell culture model that cells supplemented with Ga3+ (S = 0) have a significantly longer relaxation time compared to cells supplemented with Fe3+ (S = 5/2). These data support the hypothesis that dipole–dipole interactions between protons and electrons are driven by the strength of the electron spin magnetic moment in the surrounding environment giving rise to T2* relaxation.

Fe-N-C Catalysts for Oxygen Reduction Reaction: A Computational Analysis of Electronic Structure and Spectroscopic Properties

In proton exchange fuel cells (PEFCs), the electrochemical reduction of oxygen (ORR) plays a key role as the complementary half-cell reaction to the oxidation of hydrogen. It requires the transport of four electrons per oxygen molecule associated with high activation barriers and slow kinetics and thus requires high amounts of catalyst. [1] State-of-the art catalysts for both reactions are based on platinum, an expensive metal.Iron and nitrogen doped carbon (Fe-N-C) catalysts show high ORR activity and are thus a very promising alternative to Pt catalysts. It is believed that in Fe-N-C catalysts, iron is atomically dispersed as pseudo-molecular active centres with four- or fivefold nitrogen coordination spheres which are embedded in a graphene matrix. However, the exact nature of the active site in terms of Fe spin and oxidation states, precise coordination environment and spatial extent of the graphene sheets are still debated, as is the influence of these factors on the ORR mechanism in Fe-N-C catalysts. [2-4]Unlike other spectroscopies, Mössbauer spectroscopy, which provides direct insights on iron spin and oxidation states, is used successfully to characterise the amorphous FeNC catalysts. Until recently, the interpretation of Mössbauer spectra was limited to comparisons with small reference complexes [2] which lack the extended π-systems of Fe-N-C catalysts. Recently, with increasing use of in situ and operando experiments on these catalysts, the interest in computational models for the interpretation of experimental Mössbauer spectra has grown. [4-6]. Using density functional theory, we show for different FeN4 environments how the electronic structure is influenced by the size and symmetry of the π-system. The influence is quantified in terms of structural parameters, relative spin state energies, spin distributions and spectroscopic properties.