Iron-containing metal–organic frameworks are promising Fenton catalysts. However, the absence of additional modifiers has proven difficult due to the low reaction rates and the inability to manipulate the catalysts. We hypothesize that the production of iron oxide NPs in the presence of a metal-organic framework will increase the rate of the Fenton reaction and lead to the production of particles that can be magnetically manipulated without changing the structure of the components. A comprehensive approach lead to a metal organic framework using the example of MIL-88b (Materials of Institute Lavoisier) modified with iron oxides NPs: formulation of iron oxide in the presence of MIL-88b and vice versa. The synthesis of MIL-88b consists of preparing a complexation compound with the respective structure and addition of terephthalic acid. The synthesis of MIL-88b facilitates to control the topology of the resulting material. Both methods for composite formulation lead to the preservation of the structure of iron oxide, however, a more technologically complex approach to obtaining MIL-88b in the presence of Fe3O4 suddenly turned out to be the more efficient for the release of iron ions.

The main advantage of neutron diffraction over X-ray diffraction, arises from the fact that the interaction of neutrons with material is relatively weak and not related to the number of electrons, and consequently the penetration depth of neutrons is about 102-103 larger than that of laboratory X-ray diffraction. This is particular essential for the non-destructive texture analysis of archaeological objects as no additional surface treatments of the samples (e.g. polishing) are necessary. STRESS-SPEC at MLZ is designed as a state of the art multi-purpose diffractometer for strain and texture analysis. Besides the optimized high neutron flux the available large variability in gauge volume definition systems together with the robotic sample handling option offer high flexibility for bulk or gradient texture measurements. Since 2014, local and bulk textures of iron and gold artefacts collected by Bavarian State Archaeological Collection (Munich, Germany) have been thoroughly investigated at STRESS-SPEC. Results showed that heat treatment of iron artefacts at high temperatures can re-orientate the inner crystallites. In the gold foil artefacts, the texture represented by the measured pole figures shows a high symmetry – the so-called Cube component, which is commonly found in annealed fcc materials. For comparison, laboratory samples were produced by rolling, flat hammering, and pin / round hammering and also measured in order to elucidate possible manufacturing and processing routes. In turned out that both rolling and pin / round hammering followed by a high temperature annealing can produce similar pole figures to those of the gold artefacts foils.

The stability of Fe−N−C oxygen reduction reaction (ORR) electrocatalysts has been considered a primary challenge for their practical application in proton exchange membrane fuel cells (PEMFCs). While several studies have attempted to reveal the possible degradation mechanism of Fe−N−C ORR catalysts, there are few research results reporting on their stability as well as the possible Fe species formed under different voltages in real PEMFC operation. In this work, we employ in‐situ X‐ray absorption near‐edge structure (XANES) to monitor the active‐site degradation byproducts of an atomically dispersed Fe−N−C ORR catalyst under a H2/O2‐operating PEMFC at 90 % relative humidity and 80 °C. For this, stability tests were carried out at two constant cell voltages, namely 0.4 and at 0.8 V. Even though the ORR activity of the Fe−N−C catalyst decreased significantly and was almost identical at the end of the tests for the two voltages employed, the analysis of the XANES recorded under H2/N2 configuration at 0.6 and 0.9 V within the stability test suggests that two different degradation mechanisms occur. They are demetalation of iron cations followed by their precipitation into Fe oxides upon operation at 0.8 V, versus a chemical carbon oxidation close to the active sites, likely triggered by reactive oxygen species (ROS) originated from the H2O2 formation, during the operation at 0.4 V.

Haltern 70 amphorae sherds from Castro do Vieito, a Roman settlement from the NW of Portugal occupied during the early imperial period, were studied by Mössbauer spectroscopy at room temperature and 4.2 K, XRD, and XRF, aiming to understand the firing conditions of their production. Firing in air at 750 and 800 °C were performed in a sherd that was carefully studied. Also, a handle with part of the neck attached and with the potter’s stamp “LH …” was studied. In general, it can be deduced that the amphorae were fired under reducing conditions between 800 and 950 °C, having been subjected to an oxidation process only when already cooling. It was also inferred that the provenance of all the Haltern 70 amphorae found in Castro do Vieito is probably the same and that the stamped amphora also seems to come from the same locality.

The degradation of a single-site atomically dispersed, model Fe-N-C powder catalyst with high activity is investigated using cryo-Mössbauer spectroscopy. The results indicate a degradation initiated by an Fe2+ to Fe3+ oxidation due to coordination of oxygen to tetrapyrrolic Fe-N4 sites at atmospheric conditions (change between characteristic doublets) before iron(III) oxide is formed (sextet). Thermal reactivation can be used to restore substantial catalytic activity of aged Fe-N-C powders.

Mononuclear Fe ions ligated by nitrogen (FeNx) dispersed on nitrogen-doped carbon (Fe-N-C) serve as active centers for electrocatalytic O2 reduction and thermocatalytic aerobic oxidations. Despite their promise as replacements for precious metals in a variety of practical applications, such as fuel cells, the discovery of new Fe-N-C catalysts has relied primarily on empirical approaches. In this context, the development of quantitative structure-reactivity relationships and benchmarking of catalysts prepared by different synthetic routes and by different laboratories would be facilitated by the broader adoption of methods to quantify atomically dispersed FeNx active centers. In this study, we develop a kinetic probe reaction method that uses the aerobic oxidation of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic method is compared with low-temperature Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical reductive stripping of NO derived from NO2- on a suite of Fe-N-C catalysts prepared by diverse routes and featuring either the exclusive presence of Fe as FeNx sites or the coexistence of aggregated Fe species in addition to FeNx. The FeNx site densities derived from the kinetic method correlate well with those obtained from CO pulse chemisorption and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification approach. The kinetic method developed here does not require pretreatments that may alter active-site distributions or specialized equipment beyond reaction vessels and standard analytical instrumentation.

Abstract M−N−C electrocatalysts are considered pivotal to replace expensive precious group metal‐based materials in electrocatalytic conversions. However, their development is hampered by the limited availability of methods for the evaluation of the intrinsic activity of different active sites, like pyrrolic FeN4 sites within Fe−N−Cs. Currently, new synthetic procedures based on active‐site imprinting followed by an ion exchange reaction, e.g. Zn‐to‐Fe, are producing single‐site M−N−Cs with outstanding activity. Based on the same replacement principle, we employed a conservative iron extraction to partially remove the Fe ions from the N4 cavities in Fe−N−Cs. Having catalysts with the same morphological properties and Fe ligation that differ solely in Fe content allows for the facile determination of the decrease in density of active sites and their turn‐over frequency. In this way, insight into the specific activity of M−N−Cs is obtained and for single‐site catalysts the intrinsic activity of the site is accessible. This new approach surpasses limitations of methods that rely on probe molecules and, together with those techniques, offers a novel tool to unfold the complexity of Fe−N−C catalyst and M−N−Cs in general.

A scalable synthesis of magnesium ion imprinted nitrogen-doped carbon allows for facile preparation of large quantities of Fe–N–C, for large-scale fuel cell research.

Kidney disease is one of the main non-communicable diseases. Every year millions of people worldwide die from kidney dysfunction. One cause is disturbances in the mineral metabolism, such as abnormally high phosphate concentrations in the blood, medically referred to as hyperphosphatemia. A new active ingredient based on nanoscale iron(oxyhydr)oxide with particle sizes below 3 nm surrounded by an organic coating has been developed for a more effective treatment. The examination of the structural properties of these particles within this study promises to gain further insights into this improved effectiveness. More than half of the active ingredient consists of organic substances, the rest is mostly iron(oxyhydr)oxide. Analyzes by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) show that the organic molecules act as stabilizers and lead to ultrasmall iron(oxyhydr)oxide cores with a size of 1.0-2.8 nm. The nanoparticles coated with the organic molecules have an average size of 11.7 nm. At 4.2 K, the nanoparticles display a magnetic hyperfine field of 45.5 T in the Mössbauer spectrum, which is unusually low for iron(oxyhydr)oxide. The material is also not ferrimagnetic. Combining these results and taking into account the composition of the nanoparticles, we identify low crystalline ferrihydrite as the most likely phase in the iron(oxyhydr)oxide nuclei. At the same time, we want to emphasize that a final identification of the crystal structure in iron(oxyhydr)oxides can be impeded by ultrasmall particle sizes. In summary, by a combinatorial characterization, we are able to observe extraordinary properties of the ultrasmall nanomaterial, which is the basis for the investigation of the high phosphate-binding efficacy of this active ingredient.

Combining the abundance and inexpensiveness of their constituent elements with their atomic dispersion, atomically dispersed Fe-N-C catalysts represent the most promising alternative to precious-metal-based materials in proton exchange membrane (PEM) fuel cells. Due to the high temperatures involved in their synthesis and the sensitivity of Fe ions toward carbothermal reduction, current synthetic methods are intrinsically limited in type and amount of the desired, catalytically active Fe-N-4 sites, and high active site densities have been out of reach (dilemma of Fe-N-C catalysts). We herein identify a paradigm change in the synthesis of Fe-N-C catalysts arising from the developments of other M-N-C single-atom catalysts. Supported by DFT calculations we propose fundamental principles for the synthesis of M-N-C materials. We further exploit the proposed principles in a novel synthetic strategy to surpass the dilemma of Fe-N-C catalysts. The selective formation of tetrapyrrolic Zn-N-4 sites in a tailor-made Zn-N-C material is utilized as an active-site imprint for the preparation of a corresponding Fe-N-C catalyst. By successive low- and high-temperature ion exchange reactions, we obtain a phase-pure Fe-N-C catalyst, with a high loading of atomically dispersed Fe (>3 wt %). Moreover, the catalyst is entirely composed of tetrapyrrolic Fe-N-4 sites. The density of tetrapyrrolic Fe-N-4 sites is more than six times as high as for previously reported tetrapyrrolic single-site Fe-N-C fuel cell catalysts.

Aeration ofwetland soils containing iron (Fe) sulfides can cause strong acidification due to the generation of large amounts of sulfuric acid and formation of Fe oxyhydroxy sulfate phases such as jarosite. Remediation by re-establishment of anoxic conditions promotes jarosite transformation to Fe oxyhydroxides and/or Fe sulfides, but the driving conditions and mechanisms are largely unresolved. We investigated a sandy, jarosite-containing soil (initial pH = 3.0, Eh similar to 600 mV) in a laboratory incubation experiment under submerged conditions, either with or without wheat straw addition. Additionally, a model soil composed of synthesized jarosite mixed with quartz sand was used. Eh and pH values were monitored weekly. Solution concentrations of total dissolved organic carbon, Fe, S, and K as well as proportions of Fe2+ and SO42- were analysed at the end of the experiment. Sequential Fe extraction, X-ray diffraction, and Mossbauer spectroscopy were used to characterize the mineral composition of the soils. Only when straw was added to natural and artificial sulfuric soils, the pH increased up to 6.5, and Eh decreased to approx. 0 mV. The release of Fe (mainly Fe2+), K, and S (mainly SO42-) into the soil solution indicated redox- and pH-induced dissolution of jarosite. Mineralogical analyses confirmed jarosite losses in both soils. While lepidocrocite formed in the natural sulfuric soil, goethite was formed in the artificial sulfuric soil. Both soils showed also increases in non-sulfidized, probably organically associated Fe2+/Fe3+, but no (re-)formation of Fe sulfides. Unlike Fe sulfides, the formed Fe oxyhydroxides are not prone to support re-acidification in the case of future aeration. Thus, inducing moderately reductive conditions by controlled supply of organic matter could be a promising way for remediation of soils and sediments acidified by oxidation of sulfuric materials. (C) 2021 Elsevier B.V. All rights reserved.

Fe2+ ions in Fe7(P2O7)4 were substituted by Zn2+and the magnetic properties and Mössbauer spectra of the new material were studied. The obtained sample of Zn5Fe2(P2O7)4 is isostructural with the parent Fe7(P2O7)4, which is orthorhombic with a C2221 space group. Magnetometry shows that the Néel temperature is TN = 15.4(1) K, which is in accordance with Mössbauer measurements performed at RT and 4.2 K.

Commercial application of the CO2 methanation reaction demands for the development of catalysts that feature an enhanced stability to increase catalyst lifetime. By time-resolved aging studies, we show that the improved deactivation resistance of co-precipitated NiFeAlOx catalysts compared to NiAlOx is obtained by a temporal increase of the intrinsic catalytic activity of NiFeAlOx, provoked by aging at elevated temperature and pressure in thermodynamic equilibrium. Detailed structural characterization of reduced and aged catalysts resolves that aging triggers the partial segregation of (gamma Fe,Ni) nanoparticles initially formed during activation, accompanied by the oxidation of Fe. Thereby, the intrinsic catalytic activity increases, which can be explained by the generation of redox-active Fe2+ sites that offer an additional reaction pathway for CO2 activation. The deactivation behavior of NiFeAlOx can be described by a superimposition of activity increase related to Fe2+ site formation and decrease due to the loss of active sites by sintering processes.

Cationic cylindrical polymer brushes based on polybutadiene-block-poly(2-vinylpyridine) were applied as structure-directing agent for mesostructuring Fe2O3 nanoparticles into nanotubes. After temperature-controlled template removal, the obtained non-woven catalysts were tested for the photodegradation of ciprofloxacin under terrestrial solar radiation. At a slightly basic pH value, as typically encountered in clinical wastewaters, the mesostructured Fe2O3 shows a 4.5 times faster degradation of ciprofloxacin than commercial Aeroxide® TiO2 P25. Even wide-bandgap ZnO, mesostructured in the same way, is 1.6 times slower. Moreover, the non-woven-like structure of the catalyst allows for easy recovery of the catalyst and operation in a continuous flow reactor.

The high cost and the restricted availability of Platinum-Group-Metals (PGM) used in current catalysts is one of the major hurdles for the large-scale commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs). In the last decade, great efforts have been made to develop efficient PGM-free catalysts for the oxygen reduction reaction (ORR), especially metal-nitrogen-doped carbons (M-N-Cs, with M = Fe, Co). The activity gap towards Pt has successfully been narrowed, now reaching the activity requirements for practical applications. 1-3 For this class of catalysts, the active site is a MN 4 moiety, as known from molecules such as phthalocyanines and porphyrins. 4 Due to the metastability of the MN 4 sites at the temperature of their pyrolytic formation, the final transition metal loading is currently limited and significant amounts of inorganic byproducts are formed. Although synthesis protocols have been successfully optimized, multiple processing steps are required, making the preparation time-consuming. In this contribution we will show that Zn 2+ ions can be utilized in our novel concept of active-site imprinting, where Zn is used as template-ion in a pyrolytic process to form Zn-N-C precursor materials. 5 The Zn-N-C materials presented in this work are nitrogen-doped carbons comprising ZnN 4 sites, obtained with high yield and from inexpensive precursors. The active-site imprinted carbon supports possess high surface area and hierarchical porosity, which makes them structurally advantageous for catalytic applications in terms of mass transport and high accessibility of the active sites. Through a zinc-to-iron ion exchange reaction, Fe-N-C catalysts with high Fe loading are obtained at only 170 °C. Since the active-site formation by the Zn-to-Fe exchange reaction can be conducted at low temperatures, the structural properties of the Zn-N-C precursor material are retained. 6 Moreover, this synthetic procedure assures the absence of the otherwise obtained harmful side-phases (e.g., iron carbide). Cryo-Mössbauer and X-ray adsorption spectroscopy, supported by calculations of the extended X-ray absorption fine structure, reveal an exclusive presence of Fe as single atoms coordinated to four nitrogen atoms, i.e., in form of the desired FeN 4 moieties. Identical-location scanning transmission electron microscopy with atomic resolution is further employed to visualize the trans-metalation event. The herein obtained catalysts match the state-of-the-art electrocatalytic activity for Fe-N-C catalyts, both in a rotating disk electrode and in single cell PEMFC measurements. The novel synthesis method will be discussed regarding its advantages and disadvantages compared to the conventional pyrolytic M-N-C catalyst syntheses, with a focus on the potential to surpass the current limitations of restricted active-site density and catalyst stability. ACKNOWLEDGEMENTS: The German Federal Ministry of Economic Affairs and Energy (BMWi) is acknowledged for funding within the Verbundproject innoKA (Project No.: 03ET6096A) REFERENCES: M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science , 2009, 324 , 71-74. R. Bashyam and P. Zelenay, Nature , 2006, 443 , 63-66. H. A. Gasteiger, S. S. Kocha, B. Sompalli and F. T. Wagner, Applied Catalysis B , 2005, 56 , 9-35. Q. Jia, N. Ramaswamy, U. Tylus, K. Strickland, J. Li, A. Serov, K. Artyushkova, P. Atanassov, J. Anibal, C. Gumeci, S. C. Barton, M.-T. Sougrati, F. Jaouen, B. Halevi and S. Mukerjee, Nano Energy , 2016, 29 , 65-82. A. Mehmood, J. Pampel, G. Ali, H. Y. Ha, F. Ruiz-Zepeda and T.-P. Fellinger, Advanced Energy Materials , 2018, 8 . D. Menga, F. Ruiz-Zepeda, L. Moriau, M. Šala, F. Wagner, B. Koyutürk, M. Bele, U. Petek, N. Hodnik, M. Gaberšček and T.-P. Fellinger, Advanced Energy Materials , 2019, 9 , 1902412. Figure 1

Fe 2+ ions in Fe 7 (P 2 O 7 ) 4 were substituted by Zn 2+ and the magnetic properties and Mössbauer spectra of the new material were studied. The obtained sample of Zn 5 Fe 2 (P 2 O 7 ) 4 is isostructural with the parent Fe 7 (P 2 O 7 ) 4 , which is orthorhombic with a C222 1 space group. Magnetometry shows that the Néel temperature is T N = 15.4(1) K, which is in accordance with Mössbauer measurements performed at RT and 4.2 K.

Maghemite (γ‐Fe2O3) is a metastable iron oxide phase and usually undergoes fast phase transition to hematite at elevated temperatures (>350 °C). Maghemite nanoparticles were synthesized by the polyol method and then intercalated into a highly swollen (>100 nm separation) nematic phase of hectorite. A composite of maghemite nanoparticles sandwiched between nanosheets of synthetic hectorite was obtained. The confinement of the nanoparticles hampered Ostwald ripening up to 700 °C and consequently the phase transition to hematite is suppressed. Only above 700 °C γ‐Fe2O3 nanoparticles started to grow and undergo phase transition to α‐F2O3. The structure and the phase transition of the composite was evaluated using X‐ray diffraction, TEM, SEM, physisorption, TGA/DSC, and Mößbauer spectroscopy.

Atomically dispersed Fe-N-C catalysts are considered the most promising precious-metal-free alternative to state-of-the-art Pt-based oxygen reduction electrocatalysts for proton-exchange membrane fuel cells. The exceptional progress in the field of research in the last approximate to 30 years is currently limited by the moderate active site density that can be obtained. Behind this stands the dilemma of metastability of the desired FeN4 sites at the high temperatures that are believed to be a requirement for their formation. It is herein shown that Zn2+ ions can be utilized in the novel concept of active-site imprinting based on a pyrolytic template ion reaction throughout the formation of nitrogen-doped carbons. As obtained atomically dispersed Zn-N-Cs comprising ZnN4 sites as well as metal-free N-4 sites can be utilized for the coordination of Fe2+ and Fe3+ ions to form atomically dispersed Fe-N-C with Fe loadings as high as 3.12 wt%. The Fe-N-Cs are active electocatalysts for the oxygen reduction reaction in acidic media with an onset potential of E-0 = 0.85 V versus RHE in 0.1 m HClO4. Identical location atomic resolution transmission electron microscopy imaging, as well as in situ electrochemical flow cell coupled to inductively coupled plasma mass spectrometry measurements, is employed to directly prove the concept of the active-site imprinting approach.

Samples of páramo soil for Mössbauer studies down to a temperature of 4.2 K were collected from a sampling site in the municipality of Ventaquemada, district of Montoya, sector of Matanegra. This location is close to Tunja, capital of Boyacá Province, Colombia. Páramo soils often result from weathering of volcanic ash and are rich in allophane and organic matter. The Mössbauer spectra show the iron in all samples to be present mainly as a ferric component that splits magnetically at temperatures below about 20 K into a sextet with a mean hyperfine field of about 44 T and broad resonance lines that indicate a distribution of hyperfine fields. Additionally, weak contributions of hematite, magnetite, goethite, and ferric and ferrous iron in clays, are present as well as a structure-less pattern (collapsed sextet) that appears at low temperatures. The broad magnetically split component represents up to 70% of the iron in the Páramo soils. It is quite stable towards heating the soils, converting to hematite mainly between 600 and 800 °C. This rules out that the component is ferrihydrite, which decomposes already below 400 °C. Presumably, it is attributable to the allophane component in the Páramo soils. Leaching by the DCB method removes this component as well as most of the goethite, but only part of the hematite, whereas oxalate leaching removes mainly the broad magnetically split component attributed to allophane, but leaves hematite, magnetite and goethite largely unchanged. Leaching with H2O2, which oxidises and removes the organic matter, has no effect on the Mössbauer spectra, showing that iron bound to organic substances is practically absent.

Controlled laboratory experiments were combined with field measurements to better understand the interactions between dissolved organic matter (DOM) and reduced iron in organic-rich peatlands. Addition of peat-derived humic acid extract (HA) to Sideroxydans lithotrophicus ES-1 liquid cultures led to higher cell numbers and up to 1.4 times higher Fe(II) oxidation rates compared to chemical controls. This effect was positively correlated with increasing HA concentrations. Similar Fe(III) (oxyhydr)oxide mineralogies were formed both abiotically and biotically irrespective of HA amendment, but minerals formed in the presence of ES-1 and HA were smaller. ES-1 growth with HA promoted aggregation of Fe(III) products in agarose-stabilized gradient tubes as shown by voltammetric profiling. In situ voltammetry in an acidic, iron-rich peatland revealed a gap between oxygen penetration and iron reduction that may reflect active Fe(II)-oxidizing microorganisms. The highest abundance of Fe(II)oxidizers Sideroxydans (4.9 x 10(7) gene copies gww(-1)) and Gallionella (1.5 x 10(7) gene copies gww(-1)) in the upper peat layer coincided with small-sized minerals resembling nanoparticulate ferrihydrite or goethite. Our results suggest that microbially mediated Fe(II) oxidation dominates in the presence of DOM leading to the formation of nano-sized biogenic Fe(III) (oxyhydr)oxides that might be readily bioavailable and likely important to iron and carbon cycling.