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.

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Vivianite presents a significant phosphorus pool in iron-rich, reducing environments, necessitating the development of an affordable and routine quantification method. A novel extraction protocol using 2,2′-bipyridine (Bipy) was proposed as a promising approach. However, the efficacy of this protocol in achieving complete vivianite extraction remains uncertain and lacks robust analytical validation.

This study systematically assessed the Bipy extraction protocol on known amount of synthetic and two magnetically recovered environmental vivianite samples, with varying oxidation levels, impurity content, and particle size. Extraction efficiencies of iron and phosphorus were 13–44 % and 13–55 %, respectively, indicating incomplete extraction under the original protocol conditions (0.2 % Bipy, 24 h). An initially rapid release of iron and phosphorus slowed down across all samples, indicating non-constant reaction rates, and suggesting that the extraction is governed by mechanisms beyond kinetic control. Extending the extraction time to 48 h and increasing the Bipy concentration to 1 % yielded marginal improvements, with efficiency gains of 14 % or less. Grinding, which reduced particle size, nearly doubled extraction efficiency. Conversely, the sample with the highest Fe2+ content showed the overall lowest overall extraction efficiencies. Similarly, recovered vivianite samples containing impurities, namely magnesium and calcium, were extracted less efficiently than synthetic vivianite. Additionally, the extracted iron-to‑phosphorus ratio exceeded the theoretical value of 0.67, indicating non-stoichiometric extraction.

To establish Bipy extraction as a reliable analytical method, it is crucial to address incomplete extraction and determine whether vivianite can be fully extracted or only to a certain extent. A potential strategy involves reducing extraction time and repeating the extraction step.@en

The removal of carbon deposits from carburized Fe-based Fischer–Tropsch catalysts is a critical aspect of their performance. In this study, a method is presented to remove carbon deposits from freshly prepared χ-Fe5C2. The method involves successive passivation and reduction steps, which do not affect the bulk structure of the χ-Fe5C2 catalyst. The passivation step transforms the carbonaceous deposits from a graphitic structure to a disordered oxygen-functionalized structure, facilitating its removal by a reduction step in hydrogen. This results in a higher initial activity of the catalyst and substantially shortens the induction period observed without such pretreatment. The findings underscore the possibility of improving catalytic performance of Fe-carbides by changing the structure and reactivity of carbonaceous deposits.@en

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.

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Iron-modified Al-ZSM-5 increases selectivity to propene, a key petrochemical resulting from fluid catalytic cracking (FCC). However, the type and role of active iron species remain unclear, hindering efforts to streamline the design of selective FCC additives. Here, we investigated Al-free Fe-ZSM-5 catalysts containing iron species in the form of framework Fe3+, extra-framework Fe3+, oxidic clusters, and oxide micro aggregates in n-octane cracking (FCC model) to assess their effect on catalytic cracking. DR-UV–Vis spectroscopy, 57Fe Mössbauer Spectroscopy, FTIR studies of pyridine adsorption, and n-octane cracking tests at 500 °C revealed that framework-associated coordinatively unsaturated Fe3+ species, which induce strong Lewis acidity, are responsible for paraffin cracking initiation, whereas bulk iron oxides on the zeolite surface are inactive. In comparison with Al-ZSM-5, Fe-ZSM-5 increases the olefinicity of the valuable C3-C4 fractions (selectivity to propene and butenes) and promotes aromatization reactions due to the lower relative strength of Fe-induced Brønsted acid sites and dehydrogenation properties. As shown by our 57Fe Mössbauer study (performed at −269 °C) of the catalyst in calcined, spent, and regenerated states, Fe-ZSM-5 deactivation is associated with the loss of tetrahedrally coordinated Fe3+ species. Therefore, tuning Fe-ZSM-5 C3-C4 selective FCC additives requires stabilizing framework Brønsted and framework-associated Lewis acid sites while decreasing the concentration of iron oxide species. Ultimately, these findings may enable us to meet the demand for propene derived from FCC cracking, which is expected to grow in the foreseeable future.@en

CO₂ hydrogenation to chemicals and fuels has the potential to alleviate CO₂ emissions and displace fossil resources simultaneously via consecutive RWGS and FTS reactions, also known as CO₂-FTS. As Fe-based catalysts are active and selective for both reactions, their bifunctionality requires a delicate balance between the RWGS and FTS. In this work, we investigated the thermodynamic constraints of RWGS and CO₂-FTS, the influence of CO₂ conversion on selectivity and the influence of Fe nanoparticle size within the range of 4.7 to 10.3 nm. An inert carbon support was selected to rule out metal-support interaction and promoting effects of the support. Catalytic performance was evaluated at 300 °C, 11 bar, H₂/CO₂/Ar = 3/1/1, 600 to 72000 mL·g_cat^-1·h^-1. At a CO₂ conversion level below RWGS equilibrium conversion of 23 %, RWGS was found to be the primary and dominant reaction. No primary Sabatier reaction was observed. At higher CO₂ conversion till the CO₂-FTS threshold of 42 %, the secondary FTS reaction became dominant. Notably, a non-linear relation between CO₂ conversion and CO selectivity was discovered. Comparing two catalysts with identical 5 wt% Fe loading but different average Fe nanoparticle size (6.6 and 8.4 nm), the 8.4 nm Fe catalyst was at least two times more active than the 6.6 nm Fe catalyst. In situ Mössbauer spectroscopy suggested a positive correlation between particle size, carburization and selectivity towards long-chain hydrocarbons. For these potassium-promoted carbon-supported Fe-based catalysts, nanoparticles of at least 8 nm are required for the formation of Fe carbides and improved reactivity.

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Cobalt oxidation is a relevant deactivation pathway of titania-supported cobalt catalysts used in Fischer-Tropsch synthesis (FTS). To work towards more stable catalysts, we studied the effect of the surface area of the titania support and noble metal promotion on cobalt oxidation under simulated high conversion conditions. Mössbauer spectroscopy was used to follow the evolution of cobalt during reduction and FTS operation as a function of the steam pressure. The reduction of the oxidic cobalt precursor becomes more difficult due to stronger metal-support interactions when the titania surface area is increased. The reducibility was so low for cobalt on GP350 titania (surface area 283 m²/g) that the catalytical activity was negligible. Although cobalt was more difficult to reduce on P90 titania (94 m²/g) than on commonly used P25 titania (50 m²/g), the Co/P90 catalyst showed increased resistance against cobalt sintering and higher FTS performance than Co/P25. The addition of platinum to Co/P90 led to a higher reduction degree of cobalt and a higher cobalt dispersion, representing a catalyst with promising performance at relatively low steam pressure. Nevertheless, the stronger cobalt-titania interactions result in more extensive deactivation at high steam pressure due to oxidation.

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Oil has long been the dominant feedstock for producing fuels and chemicals, but coal, natural gas and biomass are increasingly explored alternatives. Their conversion first generates syngas, a mixture of CO and H2, which is then processed further using Fischer–Tropsch (FT) chemistry. However, although commercial FT technology for fuel production is established, using it to access valuable chemicals remains challenging. A case in point is linear α-olefins (LAOs), which are important chemical intermediates obtained by ethylene oligomerization at present. The commercial high-temperature FT process and the FT-to-olefin process under development at present both convert syngas directly to LAOs, but also generate much CO2 waste that leads to a low carbon utilization efficiency. The efficiency is further compromised by substantially fewer of the converted carbon atoms ending up as valuable C5–C10 LAOs than are found in the C2–C4 olefins that dominate the product mixtures. Here we show that the use of the original phase-pure χ-iron carbide can minimize these syngas conversion problems: tailored and optimized for the process of FT to LAOs, this catalyst exhibits an activity at 290 °C that is 1–2 orders higher than dedicated FT-to-olefin catalysts can achieve above 320 °C, is stable for 200 h, and produces desired C2–C10 LAOs and unwanted CO2 with carbon-based selectivities of 51% and 9% under industrially relevant conditions. This higher catalytic performance, persisting over a wide temperature range (250–320 °C), demonstrates the potential of the system for developing a practically relevant technology.@en

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.

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Multifunctional, biocompatible magnetic materials, such as iron oxide nanoparticles (IONPs), hold great potential for biomedical applications including diagnostics (e.g., MRI) and cancer therapy. In particular, they can play a crucial role in advancing cancer thermotherapy by generating heat when administered intratumorally and when exposed to an alternating magnetic field. This heat application is often combined with radio- (chemo)therapy and/or imaging. Consequently, the design of materials for such a multimodal approach requires hybrid nanoparticles that retain their magnetic properties while integrating additional functionalities. This work introduces synthesis and investigation of magnetically enhanced nanoparticles with a palladium core (envisioned for future radiolabeling with therapeutic 103Pd) and a magnetic iron oxide shell containing paramagnetic manganese (Pd/Fe|(nMn)-oxide, n = 0.25 and 0.5). Doping the iron oxide lattice with Mn significantly increases magnetic saturation, boosting specific loss power up to 1.7 times compared to that of undoped analogs. Interestingly, higher Mn-content in Pd/Fe|(0.5Mn)-oxide leads to a pronounced Mn outer rim, enhancing the heating efficiency at 346 kHz and 23 mT and contributing to the water exchange on the surface of the paramagnetically doped nanoparticles, resulting in additional T1 MRI contrast. The enhanced magnetic properties of the hybrid Pd/Fe|Mn-oxide nanoparticles enable effective therapeutic outcomes with injection of only small quantities of the material, offering great potential for effective cancer treatment strategies that combine hyperthermia/thermal ablation with radiotherapy while allowing for real-time monitoring via MRI.@en

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.

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Iron oxide-based adsorbents showed potential to reach ultra-low phosphorus (P) concentrations to prevent eutrophication and recover P. High affinity, high capacity at low P concentrations (<1 mg L⁻¹), good stability, and reusability of the adsorbent are key factors for economic viability. In this study, nanoparticles of goethite (α-FeOOH), a highly stable phase, have been synthesized with increasing Zn²⁺-doping, 0–20 %at. Zn/Fe, to manipulate the surface properties, following the results of a previous work. Mössbauer spectroscopy showed preserved goethite phase and increased point of zero charge (pzc) at low Zn-doping percentages, while at higher percentages (>5%at.) co-existing phases with increased specific surface area formed. Low concentrations (0.1–10 mg L⁻¹) batch adsorption tests showed increased P removal per unit mass with increasing doping. However, the highest pzc, affinity and P removal per unit area were observed for the 5%at. doped sample, suggesting this dopant concentration to provide the most effective surface. A regeneration test, performed at a lower pH than usual, showed preserved, even improved P desorption with increasing doping. Mössbauer spectroscopy showed that the nanoparticle phase and composition, up to 5%at., doping was preserved throughout the process. These results are promising to develop a stable effective Zn-doped goethite-based adsorbent for P recovery at ultra-low concentrations.

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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.

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Magnetic refrigeration (MR) is a cutting-edge technology that promises high energy efficiency and eco-friendliness, making it an exciting alternative to traditional refrigeration systems. However, the main challenge to its widespread adoption is cost competitiveness. In this context, the use of liquid metals as heat transfer liquids in the MR has been proposed as a game-changing solution. Unfortunately, the toxicity and flammability of these liquid metals have raised serious concerns, limiting their practical use. In this study, we investigate the compatibility of a nontoxic and nonflammable GaInSn-based liquid metal with a magnetocaloric material, La(Fe,Mn,Si)₁₃H_z, over a 1.5 year period. Our findings reveal nearly a 14% reduction in specific cooling energy and peak-specific isothermal magnetic entropy change for the considered magnetocaloric material. Our study provides valuable insights into the long-term stability of magnetocaloric materials and their compatibility with liquid metals, facilitating the development of more cost-effective and sustainable MR systems.

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The study of titania-supported cobalt nanoparticles is relevant for industrial Fischer-Tropsch synthesis (FTS). Herein, we report about various deactivation pathways of cobalt supported on P25 titania (cobalt loading 2–8 wt%) under simulated high conversion conditions using in situ Mössbauer spectroscopy. A fraction of metallic cobalt was oxidized under humid FTS conditions. The absolute amount of oxidized cobalt was ∼ 1.2 wt% independent of the cobalt loading, indicating that specific cobalt-titanol interactions are involved in the oxidation process. The formation of cobalt-titanate-like compounds was only observed under very high water-to-hydrogen ratios in the absence of carbon monoxide. Steam considerably enhances cobalt sintering under FTS conditions. As such, deactivation under humid FTS conditions is not only caused by cobalt oxidation but also by enhancing sintering of the active phase.

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Understanding the deactivation mechanism of cobalt-based Fischer-Tropsch catalysts is of significant practical importance. Herein, we explored the role of manganese as a structural promoter on silica-supported cobalt nanoparticles under simulated high CO conversion conditions, i.e., high relative humidity. The structural changes in cobalt dispersion and oxidation state were followed by in situ Mössbauer emission spectroscopy. Adding manganese oxide to silica-supported cobalt enhanced the dispersion of metallic cobalt in the reduced catalysts. This higher cobalt dispersion, however, led to a stronger tendency of cobalt silicate formation under humid conditions. Without manganese, the cobalt particles sintered, and the larger ones were prone to transformation into cobalt carbide under high conversion conditions. As such, silica is not preferred as a support for practical FTS.

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Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe³⁺ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe²⁺ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products.

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Phosphorus (P) removal from freshwater bodies to ultra-low concentrations is fundamental to prevent eutrophication, while its recovery is necessary to close the P usage cycle. Iron oxide-based adsorbents seem promising candidates, being abundant, cheap, and easy to synthesize compounds, with good affinity for P. Affinity is the key parameter when targeting ultra-low concentrations. Also, adsorbent regeneration and re-use is fundamental for the economic viability, hence the adsorbent stability is important. Goethite, (α-FeOOH), is one of the most stable iron (Fe³⁺) (hydr)oxide species, with higher affinity, but lower adsorption capacity (per kg) compared to other species. Doping could change goethite surface properties, to boost the adsorption capacity, while preserving the high stability and affinity for P. In this work, pure goethite was compared to goethite doped (5%at.) with different elements of different preferential oxidation states: Zn²⁺, Mn³⁺, and Zr⁴⁺. Doping was successfully achieved for all elements, albeit Zr showed a lower Fe substitution than targeted. Zn doping increased the goethite point of zero charge and adsorption capacity (per mass and per surface area), preserving the high affinity, while Mn- and Zr- doping displayed a decrease in all the parameters. These could be explained with surface protonation as a charge compensation mechanism in Zn²⁺-for-Fe³⁺ substitution. The regeneration test showed improved P recovery for Zr- and Zn-doped goethite. All samples remained stable throughout the whole process. This work provides promising insights on doping as a strategy to manipulate iron oxides surface properties and for developing a highly performing and long-lasting goethite-based adsorbent.

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