Direct conversion of CH₄ into value-added chemicals is impeded by the inert C-H bonds and inefficient C-C coupling. We report a spatially separated Rh-O-Fe active-site architecture that decouples CH₄ and H₂O activation through a high-valent-metal mediated radical mechanism, enabling selective CH₃COOH synthesis. In-situ infrared, operando Mössbauer spectroscopy, and quasi in-situ high-field EPR reveal that O₂ oxidizes Rh and Fe to high valence states. Rh^(III) activates CH₄ to •CH₃, while Fe^(IV) = O dissociates H₂O into •OH through a truncated water-gas shift pathway. •OH rapidly reacts with CO to form •COOH intermediates, which couples with •CH₃ within the zeolite to yield CH₃COOH. This dual-site strategy circumvents kinetic limits of conventional water-gas shift and CO insertion steps. The catalyst achieves 18.2 mmol g_cat^-1 h^-1 CH₃COOH with 92% selectivity and 100-hour stability in continuous operation. This study establishes radical decoupling enabled by high-valent metal sites as a design principle for selective alkane oxidation.

Direct hydrogenation of CO₂ to synthetic fuels and olefins using renewable hydrogen remains challenging owing to the scarcity of low-cost, active, and highly selective catalysts. Current catalytic systems often exhibit either excessive CO selectivity or generate substantial alkanes to cause a low olefin-to-paraffin (O/P) ratio that limits hydrogen utilization efficiency. Herein, we report an unsaturated ZrO_x-modified iron catalyst (Na–Fe/a-ZrO₂) that delivers an olefin selectivity exceeding 80% among organic products, while suppressing the combined CO+CH₄ selectivity to below 25% and raising the O/P ratio to as high as ∼28. Multimodal in situ/operando spectroscopies and synchrotron-based vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) demonstrate that unsaturated ZrO_x sites can modulate the activation modes of surface-bound *CO intermediates and selectively promote *CO insertion into alkyl species. This process consumes reactive intermediates that would otherwise desorb to form undesirable byproducts, thereby enabling selective olefin synthesis compatible with hydrogen-efficient CO₂ conversion.

This study evaluates the operational impacts and vivianite formation potential of converting a full-scale wastewater treatment plant from Enhanced Biological Phosphorus Removal (EBPR) to Chemical Phosphorus Removal (CPR) with the aim of producing vivianite-rich sludge as a prerequisite for downstream phosphorus recovery in the form of vivianite. By increasing the Fe/P molar ratio in digested sludge from 0.5 to 1.6, up to 90 % of sludge phosphorus was bound as vivianite. Although the additional iron was dosed into the primary sludge line to target vivianite formation in the digester, substantial vivianite formation may have occurred mainly in the secondary system before anaerobic digestion. The transition to CPR improved effluent quality (86 % lower P and 34 % lower COD), reduced biogas H₂S levels from 332 ppm to 1 ppm, and increased dewatered cake dry matter from 24 % to 30 %. Mass balances indicated that dosing iron into the primary thickener triggered substantial reduction of Fe³⁺ to Fe²⁺, causing iron recirculation to the influent and possibly impairing primary settling. Microbial analysis showed pronounced but reversible shifts in community structure, while Polyphosphate-Accumulating Organisms (PAOs) remained stable before and after the return to EBPR operation. For future implementation, maintaining waterline iron dosing for effluent P control while dosing only the additional iron required for vivianite formation directly into the digester may improve process stability, but this strategy requires site-specific testing.

We herein provide a combined experimental investigation and theoretical calculations on the impact of Mn doping and Fe off-stoichiometry on the magnetoelastic transition and the magnetocaloric properties of Laves phase Hf_0.82Ta_0.18Fe₂ alloys. Mn substitution led to an increase in unit-cell volume while Fe vacancies induced lattice contraction. By adjusting the Mn and Fe content, we achieved a table-like magnetocaloric response with a magnetic entropy change of 1.7–2.2 J/(kg K) at a magnetic field change of 2 T over a wide temperature range from 190 to 260 K. Mössbauer spectroscopy, neutron powder diffraction and density functional theory calculations all reveal that both Mn atoms and Fe vacancies preferentially occupy the 6h crystallographic site of the lattice structure with space group P6₃/mmc, and that the shortest intralayer Fe-6h interatomic distance governs the magnetoelastic transition in (Hf, Ta)Fe₂ Laves phases. The tunable magnetic transition is ascribed to the slight change of the electronic state of the Fe-6h site and limited hybridization between Mn and Fe atoms. These findings offer new insight into the site-specific control for optimizing the magnetocaloric properties of Fe-based Laves phase alloys and inspire the design of other promising magnetocaloric materials with magnetoelastic transitions.

Iron is an attractive binding partner for phosphorus removal from digested sludge, with recovery as the Fe(II) phosphate mineral vivianite (Fe₃(PO₄)₂·8H₂O) representing a promising resource recovery pathway. However, sulphides compete strongly with phosphate for Fe(II), limiting vivianite formation and often requiring higher iron dosages. Because both iron and sulphur undergo redox transformations, controlled microaeration may alter the balance between iron sulphides, iron phosphates, and oxidized iron and sulphur species. This study investigated the effect of oxidation–reduction potential (ORP)-controlled microaeration (−220 to 0 mV) on Fe–S–P transformations in digested sludge. Microaeration showed contrasting effects. Mössbauer spectroscopy indicated an approximately 20% decrease in the Fe(II) associated with the vivianite fraction under all ORP conditions, while a pyrite-like iron sulphide phase remained largely unchanged. In contrast, the soluble phase showed increasing sulphur concentrations (ultimately recovered as sulphate) and a ∼70% decrease in soluble phosphorus, suggesting enhanced iron–phosphate binding or adsorption during microaeration. Although the applied aeration conditions ultimately did not lead to vivianite formation, the proposed mechanism identifies an early microaeration window that favours phosphorus removal, providing a basis for further optimization of microaeration strategies for phosphorus recovery from anaerobic sludges.

Affordable, non-critical metal-enriched stable polyanionic cathode chemistries are appealing for scalable energy storage applications. Tavorite LiFePO₄OH forms one such environmentally benign cathode for Li-ion batteries. Optimizing their electrochemical kinetics and cycling performance relies on elucidating the interplay between structural transformation and electrochemical redox activity during battery operation. In this study revisiting LiFePO₄OH hydroxyphosphate, we elaborate the crystal/magnetic structure and the mechanistic origin of structural transitions encountered during electrochemical cycling. Low-temperature magnetometry, Mössbauer/Raman spectroscopy and neutron diffraction revealed a long-range G-type antiferromagnetic ordering with propagation vector k = (1/2, 0, 1/2). Upon cathode optimization, it delivered the highest discharge capacity ∼133 mAh g⁻¹ with excellent cycling stability involving an Fe³⁺/Fe²⁺ redox potential centered ∼2.5 V (vs. Li/Li⁺). LiFePO₄OH exhibits a reversible biphasic transformation involving the evolution of metastable and partially amorphous Li₂FePO₄OH with local distortions around Fe atoms upon the first discharge. A permanent structural reconstruction driven by local Li rearrangements during the first cycle was confirmed by in situ and ex situ diffraction and Mössbauer spectroscopy. Tavorite-type LiFePO₄OH enriches the critical-metal free Fe-based cathode database for Li-ion batteries.

Potassium (K) has been widely employed as an alkali promoter for Fe-based catalysts in the reverse water gas shift (RWGS) reaction, Fischer-Trospch synthesis (FTS) and the integration of RWGS and FTS for direct CO₂ hydrogenation to higher hydrocarbons, commonly referred to as the CO₂-FTS process. K is recognized as both a structural promoter of the Fe phase and an electronic promoter that modifies the adsorption and activation behavior of reactants on the catalyst surface. However, its role in reaction pathways and the effect of K loading in each reaction remains largely unexplored. In this study, a series of K-promoted carbon-supported Fe-based catalysts was prepared to investigate the effect of K/Fe molar ratios from 0.02 to 0.5 on the RWGS and FTS reactions in a fixed-bed reactor at 300 °C, 11 bar, H₂/CO₂/Ar=3/1/1, 1000–375000 mL·g_cat⁻¹·h⁻¹. Quasi in situ Mössbauer spectroscopy shows that sufficient K promotion (K/Fe ≥ 0.1) on carbon-supported Fe-based catalysts led to complete carburization following a reduction-carburization activation procedure. The resulting Fe carbides remained stable after both RWGS and FTS conditions, suggesting that they are responsible for catalyzing both reactions. This finding contrasts with literature reports that attribute RWGS and FTS activity to Fe oxides and Fe carbides, respectively. Increasing the K/Fe ratio progressively suppressed CO₂ methanation as a primary reaction, ultimately rendering primary methanation insignificant and leaving RWGS as the only primary reaction. Both RWGS and FTS activities reached their optimum at a K/Fe ratio of 0.1. However, a higher K/Fe ratio led to higher CO selectivity in the RWGS reaction, whereas an optimal K/Fe ratio of 0.1 was found for FTS to promote the production of higher hydrocarbons.

In recent years growing interest has been placed on the role of dopant concentrations of tertiary precious and base metals in modifying the performance of supported AuPd nanoalloys towards the direct synthesis of H ₂O ₂. Within this contribution, we expand on these earlier studies, with a focus on Fe-containing systems. Through rational catalyst design, an optimal 0.5%Au-0.5%Pd-0.02%Fe/TiO ₂ formulation has been developed, which not only outperforms the parent bimetallic analogue but also offers increased reactivity compared to alternative trimetallic formulations previously reported, including those which incorporate Pt. Such observations may be surprising given the propensity for Fe to decompose H ₂O ₂via Fenton pathways. However, detailed analysis by CO-DRFITS and XPS reveals that the enhanced activity can be attributed to the electronic modification of Pd and the formation of domains of mixed Pd ²⁺/Pd ⁰ oxidation state, upon Fe introduction.

Phosphorus runoff from agricultural land is a major driver of eutrophication, with manure serving as a significant source of phosphorus input. In regions such as the Netherlands, high livestock densities and limited land availability pose challenges for manure management, particularly in pig farming. Recovering phosphorus from manure and redistributing it to phosphorus-deficient areas offers a sustainable solution. This study explores phosphate recovery via vivianite (Fe₃(PO₄)₂·8H₂O) precipitation—a method previously demonstrated in municipal wastewater treatment plant sludge—and evaluates its applicability to pig manure. Vivianite formation was investigated in fresh, 4-month-aged, and digested pig manure, as well as in Thermal Hydrolysis Plant (THP) derived digested sewage sludge. A key finding is that high dissolved inorganic carbon (DIC) concentrations inhibit vivianite formation by promoting siderite (FeCO₃) precipitation. In digested manure, a DIC threshold of approximately 3 g/L HCO₃⁻ was identified, below which vivianite formation is favored. THP sludge, characterized by elevated DIC, exhibited similar inhibitory effects. More generally, vivianite was shown to form without significant competition with siderite if the DIC concentration is <2.5 times the iron concentration. Experimental results were compared with thermodynamic predictions using Visual MINTEQ and experiments in ultrapure water, revealing discrepancies which may be attributed to the ionic composition in environmental matrices. Strategies such as combining ammonia and DIC stripping or targeting fresh manure were shown to enhance vivianite formation. These findings can be used to propose the integration of vivianite-based phosphorus recovery into broader resource recovery frameworks, including biomethane production, ammonium recovery, and carbon capture.

Dark etching regions (DERs) are a widely studied phenomenon in the context of sub-surface microstructure decay in rolling bearings. These regions result from dislocation motion (plasticity) and carbon migration due to rolling contact fatigue (RCF). We conducted a systematic study using Mössbauer spectroscopy to identify phase evolution during tempering and DER formation. Our findings resolve a long-standing debate in the literature by demonstrating that fine temper carbides dissolve during DER formation. A slight decrease in non-stoichiometric carbides was observed in the DERs, indicating the dissolution of fine carbides. The released carbon is believed to back-diffuse into ferrite and/or martensite.

CO₂hydrogenation into long-chain hydrocarbons offers a potential contribution towards achieving a sustainable carbon cycle. The reverse water gas shift (RWGS) process converts CO₂and H₂to CO and H₂O, enabling the use of CO as a carbon feedstock by utilizing existing syngas (CO and H₂) conversion technologies. Most RWGS processes operate at high temperatures (>600 °C) and ambient pressure due to favorable thermodynamics, whereas lower temperatures and higher pressures are preferred for subsequent syngas conversion via Fischer–Tropsch synthesis (FTS). H₂O is an inherent by-product of both processes with highly oxidizing properties and may influence the catalytic performance. This study investigates the effects of hydrophobic and hydrophilic carbon-supported Fe-based catalysts on RWGS and FTS. HNO₃reflux treatment of the pristine hydrophobic carbon support is performed to introduce hydrophilicity. The overall hydrophilicity of the catalysts depends on both the carbon support and the Fe loading, as Fe-based nanoparticles also exhibit hydrophilic characteristics. H₂O vapor sorption and contact angle measurements are employed to assess the catalysts’ H₂O affinity, which is linked to the catalytic properties, giving consistent results. Catalytic performance is evaluated at 300 °C, 11 bar, H₂/CO₂/Ar = 3/1/1, 600–500 000 mL g_cat⁻¹h⁻¹. RWGS is investigated at CO₂conversions below the equilibrium limit of 23%, and the more hydrophobic catalyst exhibits higher activity and CO selectivity compared to the more hydrophilic catalysts. Notably, the Sabatier reaction emerges as a competing pathway for 5 wt% Fe-based catalysts supported on more hydrophilic carbon. This higher CO₂methanation is likely facilitated by hydrogen transfer from the carbon support, and it can be suppressed by larger Fe nanoparticle size and higher Fe loading. No significant influence of support hydrophilicity on either the RWGS or FTS reactions is observed for 20 wt% Fe/C catalysts, likely due to their overall hydrophilic nature resulting from the high Fe loading.

The influence of chromium and aluminium doping on the over-reduction during activation of iron-oxide-based water-gas shift catalysts was investigated using Mössbauer spectroscopy for the first time. In situ Mössbauer spectra of catalysts exposed to industrially relevant gas compositions were recorded with increasingly reducing R factors R = [CO]*[H₂]/[CO₂]*[H₂O]. Whereas α-Fe and cementite formed during exposure of a non-doped iron-oxide catalyst to process conditions with an R factor of 2.09, such phases were only observed at R = 4.60 for a chromium-doped catalyst, showing that chromium stabilizes the catalyst. Over-reduction was enhanced to R = 2.88 in a chromium-copper co-doped catalyst. α-Fe was already observed at R = 1.64 in an aluminium-doped catalyst, while cementite formation occurred at R = 2.09, showing that over-reduction was enhanced, the presence of aluminium delaying carburization. Co-doping copper in the aluminium-doped catalyst showed cementite formation at R = 2.09, the same as a non-doped catalyst.

While Eu²⁺ → Eu³⁺ energy transfer is well known, in this study the energy transfer from Eu³⁺ to Eu²⁺ is reported for the first time. The predominant condition for Eu³⁺ → Eu²⁺ energy transfer is a Eu²⁺ 4f⁵5d band at lower energy than the position of the Eu³⁺ 4f⁶[⁵D₀] level, which is fulfilled in Eu-doped CaO. X-ray powder diffraction, Eu Mössbauer spectroscopy and optical absorption measurements are employed to determine the Eu³⁺ and Eu²⁺ concentrations in the prepared CaO:1at.%Eu samples. Synthesis in an H₂/N₂ atmosphere and addition of graphite powder as a reducing agent to the starting mixture are found to result in respective Eu³⁺ and Eu²⁺ concentrations of 0.6–0.7% and 0.3–0.4%. For this sample, the Eu³⁺ → Eu²⁺ energy transfer efficiency is estimated to be high (> 90%). This is explained by the high oscillator strength of the 4f⁷ → 4f⁶5d excitation transition of the Eu²⁺ ion to which energy is transferred. As the Eu²⁺ 4f⁵5d band lies below the Eu³⁺ 4f⁶[⁵D₀] level, Eu³⁺ does not act as a killer center for the near-infrared (NIR) Eu²⁺ emission at about 720 nm. Therefore, a full reduction of Eu³⁺ is not required to attain a high quantum efficiency. Implications of the demonstrated Eu³⁺ → Eu²⁺ energy transfer for application of long wavelength Eu²⁺ phosphors are discussed.

Zero thermal expansion (ZTE) materials with the advantage of an invariable length with varying temperatures are in high demand for modern industry but are relatively rare for metals. Fe-based Laves phases attract significant attention due to the rich and intriguing physical properties resulting from the coupling between crystal, electric, and magnetic structures. In this work, the structural, magnetic transition, thermal expansion, and magnetocaloric effect of single-phase Fe_2-xHf_0.80Nb_0.20 Laves phase alloys were investigated by means of macroscopic magnetic measurements, Mössbauer spectroscopy, and X-ray diffraction at the temperature range of 4.2-400 K. With the introduction of Fe vacancies, the ZTE coefficient of −1.2 ppm/K is smaller than that (1.7 ppm/K) of stoichiometric Fe₂Hf_0.80Nb_0.20 alloy. Meanwhile, the magnetic entropy change experiences an enhancement from 0.39 to 0.50 J/kg K at a magnetic field change of 2 T. These improved properties are attributed to the vacancy-induced coexistence of ferromagnetic and antiferromagnetic phases, as evidenced by variable-temperature X-ray diffraction and Mössbauer spectroscopy. This work unveils a promising avenue for new zero thermal expansion materials by controlling the vacancies at magnetic atom positions in Fe-based Laves phase alloys.

Phosphorus recovery via vivianite extraction from digested sludge has recently gained considerable interest. The separation of vivianite was demonstrated earlier at the pilot scale, and operational parameters were optimized. In this study, we tested the robustness of this technology by changing the sludge characteristics, such as dry matter, and via that, sludge viscosity, and vivianite particle size. It was proven that the main factor influencing recovery was the concentration of vivianite in the feed. The technology can extract vivianite even when the sludge has higher dry matter (1.8% - 3.3%) and, therefore, higher viscosity. Smaller vivianite sizes (< 10 µm) can still be recovered but at a lower rate. This made magnetic separation applicable to a wide range of wastewater treatment plants.