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CO2 conversion via hydrogenation over iron‐based catalysts on non‐carbon supports produces mainly CO or methane by the Sabatier reaction, while the formation of C2+ hydrocarbons is of greatest interest. CxHy production from CO2 may be considered as a two‐step process with the initial formation of carbon monoxide by the reverse water gas shift reaction followed by the Fischer‐Tropsch synthesis (FTS). In the present work CO2 hydrogenation over iron‐based catalysts (Fe, FeCr, FeK) deposited on a carbon carrier has been studied. The catalyst structure has been investigated by XRD, TEM, XPS, Mössbauer spectroscopy and in situ magnetometry. Spinel‐type oxide phases (magnetite Fe3O4; maggemite γ‐Fe2O3, and, in the case of FeCr/C catalyst, iron chromite Fe1+xCr2‐xO4) are formed on the catalysts, and they contribute exclusively to the CO production. Iron carbides, active in FTS, are formed on Fe‐ and FeK‐catalysts during pre‐activation in reducing environment and then during the reaction. The reaction over the 20Fe1K/C catalyst in supercritical high‐density CO2/H2 substrate (400 °C, 8.5 MPa) leads to 72 % selectivity for C1–C12+ hydrocarbons (alkanes and alkenes). Under the same conditions, iron carbides do not form on the FeCr/C catalysts, and CO2 hydrogenation results in the CO formation with the selectivity of 90–100 %. [ABSTRACT FROM AUTHOR]

Sintering dust is a typical refractory secondary iron resource. A technology‐based utilization of sintering dust as iron carbide by applying chlorination, carburization, and magnetic separation is proposed. Under optimized conditions, an electric furnace burden comprised of 83.51% Fe and 6.52% C and with a corresponding iron recovery rate of 81.21% is prepared. Meanwhile, 96.97% Pb can be removed by chlorination and magnetic separation. Furthermore, the separation mechanism is revealed using scanning electron microscopy, X‐ray powder diffraction, and optical microscopy. The results show that sodium sulfate can promote the carburizing efficiency of sintering dust, strengthen the growth of iron carbide particles, and improve the embedding relationship between iron carbide and gangue minerals, which significantly promotes the separation efficiency. The study demonstrates that the preparation of iron carbide from sintering dust using the proposed technology is a feasible method. [ABSTRACT FROM AUTHOR]

The CO2 hydrogenation to olefins process over Fe‐based catalysts offer a promising route for the production of high‐value chemicals from non‐fossil route. Herein, we present a Na promoted ZnFe2O4 spinel catalyst and their structural evolution from fresh state to reduction and to reaction period was elucidated. The clear phase segregation between Fe and Zn occurred after reduction/reaction because of crystalline phase transformation in the order of ZnFe2O4→ZnO+Fe7C3+χ‐Fe5C2→FeO+ZnO+Fe7C3+χ‐Fe5C2. The formation of iron carbide phases (Fe7C3 and χ‐Fe5C2) was enhanced dramatically by Na promoter. The adjacent Na promoter increase the electron density in Fe−Zn interfaces and enhance its ability to dissociate CO* to form adsorbed CHx* species, thus promoting the formation of olefins. This Na promoted rich electronic Fe−Zn interfaces alters the balance between iron oxides and iron carbides on the catalyst surface, which accelerate the chain growth reaction and facilitate the coupling efficiency of tandem reaction. The various catalysts were characterized by N2 physisorption, SEM, HR‐TEM, HAADF‐STEM, ex‐situ /in situ XRD, Mössbauer spectra, ICP, in situ XPS, and operando DRIFTs. [ABSTRACT FROM AUTHOR]

Fe–Ce–Zr-based catalysts (FCZ) were promoted with potassium and evaluated in the CO 2 hydrogenation reaction, carried out at atmospheric pressure, to produce light olefins, as ethene and propene. The FCZ catalyst was prepared using the coprecipitation technique, followed by a hydrothermal step. Potassium was added to the catalyst considering four nominal weight contents (0.5, 1, 2, and 4% wt) through wet impregnation. They were characterized using the following techniques: XRF, XRD, XPS, TPR-H 2 , TPD-H 2 , TPD-CO 2, and in situ DRIFTS along the CO 2 desorption and CO 2 hydrogenation reaction. Selectivity towards ethene and propene reached the highest values with catalysts containing 1% and 2% wt of K. With this last one, the selectivity towards CO was around 95%, as potassium promotes the reverse water-gas shift reaction. Nonetheless, with this catalyst, the selectivity towards CH 4 , on a CO-free basis, was lower than 40%, and the olefins' concentration in the C 2 –C 3 range was superior to 90%, which can facilitate the light olefins separation in downstream processes. The catalysts were also in situ activated using the same feed flow, i.e., H 2 and CO 2 simultaneous flow, with the H 2 : CO 2 ratio of 3:1, at 550 °C, for 1 h. CO 2 conversion showed opposite trends for the case of H 2 and H 2 +CO 2 activation. With the former condition, the CO 2 conversion increased with the incorporation of potassium, and it decreased for the second case. As the capability of the H 2 adsorption diminished with the potassium incorporation to the catalysts, there was not enough hydrogen available to react with the CO 2. Hence, for the activation using H 2 +CO 2 flow, CO 2 conversion diminished with potassium incorporation. [Display omitted] • Addition of potassium increased selectivity towards light olefins. • Ethene and propene production were maximized with both FCZ-K1 and FCZ-K2 catalysts. • Potassium attenuates the H 2 adsorption and stabilizes iron carbides phases. • HCOO− are responsible for the light olefin's formation, mainly in the K-containing catalysts. [ABSTRACT FROM AUTHOR]

Scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) results for cementite in carbon steel suggest that the alloying elements containing Cr and Mn. In order to explore the influence of Cr and Mn on Fe3C (FC), FC and Fe6Cr4Mn2C4 (FCMC) atomic structures and their energies and properties were calculated and compared by first-principles method. The results show that Cr and Mn can easily dissolve in FC and form stable FCMC. The structures of FC and FCMC are electrically neutral, the C atoms in them can get electrons, while the metal atoms (Mn, Fe, Cr) lose electrons, it is worth noting that Cr atoms close to Mn will get a small number of electrons due to Mn atoms have the strongest ability to lose electrons. The magnetic moment results suggest that FC is ferromagnetic and FCMC is sub-ferromagnetic, their magnetic moments of them are 22.18 and 5.09 μB/cell, respectively. The C−M (M = Fe, Cr, and Mn) bonds in FC and FCMC are covalent, the covalent electrons mainly provide by metal atoms, with the increase of C-M bond length, the covalence of bonds decrease gradually. The metal bonds of Fe−Fe, Fe−Cr, Fe−Mn, and Cr−Mn have both ionic and covalent character, while Cr−Cr and Mn−Mn bonds are ionic, and bond spin of Mn−Mn is zero, indicating that the bond is paramagnetic. Compared with FC, the Poisson's ratio and bulk modulus of FCMC structure are significantly reduced, while the Young's modulus and shear modulus are increased, this means that FCMC has lower compressive deformation resistance and higher y-axis shear resistance capacity than FC, i.e., FCMC has a higher brittleness than FC. [ABSTRACT FROM AUTHOR]

The activation of CO on iron-based materials is a key elementary reaction for many chemical processes. We investigate CO adsorption and dissociation on a series of Fe, Fe3C, Fe5C2, and Fe2C catalysts through density functional theory calculations. We detect dramatically different performances for CO adsorption and activation on diverse surfaces and sites. The activation of CO is dependent on the local coordination of the molecule to the surface and on the bulk phase of the underlying catalyst. The bulk properties and the different local bonding environments lead to varying interactions between the adsorbed CO and the surface and thus yielding different activation levels of the C–O bond. We also examine the prediction of CO adsorption on different types of Fe-based catalysts by machine learning through linear regression models. We combine the features originating from surfaces and bulk phases to enhance the prediction of the activation energies and perform eight different linear regressions utilizing the feature engineering of polynomial representations. Among them, a ridge linear regression model with 2nd-degree polynomial feature generation predicted the best CO activation energy with a mean absolute error of 0.269 eV. [ABSTRACT FROM AUTHOR]

FeCx catalysts (Fe-CN-Py) were synthesized by co-pyrolyzing the mixture of iron nitrate and a CN source (melamine, bulk g-C3N4 (b-C3N4), or defective g-C3N4 (d-C3N4)). The physicochemical properties of Fe-CN-Py catalysts and their activities of CO2 hydrogenation to light hydrocarbon (C2-C6) were analyzed. The results indicated that Fe-d-C3N4-(0.3)-Py is the most promising with the highest CO2 conversion (47.2%), olefin yield (10.8%), and olefin space-time yield (STY = 4.5 µmol olefin/s/gFe). The promising activity of Fe-d-C3N4-(0.3)-Py was attributed to its high concentration of surface FeCx. The correlation between surface FeCx and the STY of hydrocarbons and olefins was established. [ABSTRACT FROM AUTHOR]

FeCx catalysts (Fe-CN-Py) were synthesized by co-pyrolyzing the mixture of iron nitrate and a CN source (melamine, bulk g-C3N4 (b-C3N4), or defective g-C3N4 (d-C3N4)). The physicochemical properties of Fe-CN-Py catalysts and their activities of CO2 hydrogenation to light hydrocarbon (C2-C6) were analyzed. The results indicated that Fe-d-C3N4-(0.3)-Py is the most promising with the highest CO2 conversion (47.2%), olefin yield (10.8%), and olefin space-time yield (STY = 4.5 µmol olefin/s/gFe). The promising activity of Fe-d-C3N4-(0.3)-Py was attributed to its high concentration of surface FeCx. The correlation between surface FeCx and the STY of hydrocarbons and olefins was established. [ABSTRACT FROM AUTHOR]

The right part of the Fe3C–100% C diagram has a certain incompleteness and fragmentation of the phase structure, which is associated with the impossibility of obtaining and studying alloys containing more than 5.0% C using known metallurgical smelting technologies under normal conditions. In this paper, three groups of problems are considered in the study of the right side of the Fe3C–100% C diagram. The first group of problems includes the absence of a sufficiently voluminous line of iron carbides on the diagram: FeC, Fe2C (except for Fe3C), Fe3C2, Fe4C, Fe5C2, Fe6C, Fe7C3, Fe8C, Fe20C9, Fe23C6. From this group of carbides, in addition to cementite θ-Fe3C, only three significant types of carbides are objectively distinguished: ε-carbide Fe2C, Hegg carbide χ-Fe5C2 and Ekström–Adcock carbide æ–Fe7C3, located to the right of the cementite line in chemical composition and not displayed on the Fe3C–100% C state diagram as full phases or diagram components, which are considered by default to be metastable, transitional phases. The absence of these carbides on the Fe3C–100% C diagram is explained by the fact that they cannot be obtained by metallurgical methods through the iron-graphite alloying and, therefore, their complete physicochemical and metallographic study can not be carried out with the subsequent introduction of the corresponding phase equilibrium fields into the Fe3C–100% C state diagram. The second group of problems is the fact that the "liquid–vapor" phase equilibrium area of the Fe–100% C diagram is currently not scientifically convincing, has not been studied, and no attempts have been made to study it. The third group of problems is that carbon as the main component of Fe–100% C alloys is currently considered as a pure graphite chemical in general with a "crystalline hexagonal layered lattice". It has been established that the structure of graphite inclusions in gray and high-strength cast iron does not correspond to an ideal hexagonal layered graphite lattice, but to a turbostratic, chaotic one. It is also shown that when assessing the graphite crystallization, the cast iron melt should be considered as a single-phase system in the form of a carbon-iron polymer, the structural basic elements of which are fullerenes and carbon nanoparticles based on them. The use of carbon allotropes to explain the problems of graphite structure formation is considered. It is proposed to designate the existing graphite line on the Fe3C–100% C diagram as a line of atomic carbon—Catom. [ABSTRACT FROM AUTHOR]

The pretreatment atmosphere has a significant impact on the performance of iron-based catalysts in carbon dioxide (CO2) hydrogenation. In this study, we investigated the effects of carbon monoxide (CO), syngas (H2/CO), and hydrogen (H2) on the performance of iron-based catalysts during the pretreatment process. To evaluate the structural changes in catalysts after activation and reaction, we analyzed their morphology and particle size, the surface and bulk phase composition, carbon deposition, the desorption of linear α-olefins and reaction intermediates using transmission electron microscope (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Mössbauer spectroscopy (MES), temperature-programmed desorption (TPD), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). Raman and XPS showed that the H2 pretreatment catalyst caused the absence of iron carbides due to the lack of carbon source, and the CO and syngas pretreatment catalysts promoted the formation of carbon deposits and iron carbides. While the bulk phase of the CO and syngas pretreatment catalyst mainly consists of iron carbide (FeCx), XRD and MES revealed that the bulk phase of the H2 pretreatment catalyst primarily consisted of metallic iron (Fe) and iron oxide (FeOx). The composition of the phase is closely associated with its performance at the initial stage of the reaction. The formation of olefins and C5+ products is more encouraged by CO pretreatment catalysts than by H2 and syngas pretreatment catalysts, according to in situ DRIFTS evidence. Ethylene (C2H4)/propylene (C3H6)-TPD indicates that the CO pretreatment catalyst is more favorable for the desorption of olefins which improves the olefins selectivity. Based on the analysis of the TEM images, H2 pretreatment stimulated particle agglomeration and sintering. In conclusion, the results show that the CO-pretreatment catalyst has higher activity due to the inclusion of more FeOX and Fe3C. In particular, the presence of Fe3C was found to be more favorable for the formation of olefins and C5+ hydrocarbons. Furthermore, carbon deposition was relatively mild and more conducive to maintaining the balance of FeOx/FeCx on the catalyst surface. [ABSTRACT FROM AUTHOR]

The effect of the action of temperature on the structure and phase composition of an Fe3C/Fe7C3/P-phase/Cam composite obtained by mechanosynthesis of Fe–75 at % C has been studied by the methods of X‑ray diffraction, Mössbauer spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. It is shown that the structural and phase changes at heating have a multistage character. In the temperature range 315–400°C crystallization of the paramagnetic P-phase with generation of Fe3C and/or Fe7C3 occur. In the course of heating to higher temperatures, complete decomposition of carbide Fe7C3 (in the range 450–550°C) and partial decomposition of Fe3C (at 600°C and above) are observed. After cooling from 800–1000°C the mechanocomposite consists of α-Fe, cementite Fe3C, and graphite. The phase transformations are accompanied by the processes of composite oxidation with the formation of Fe3O4 oxide and its subsequent reduction. The P-phase is a disorded amorphous carbide Fe1 – xCx, which is characterized by magnetic ordering at the temperature of liquid nitrogen. [ABSTRACT FROM AUTHOR]

K-doped FeC x catalysts derived from the carbothermal reduction of the Fe-chitosan complex were investigated and tested in the hydrogenation of CO 2 to light olefins (C 2-6 =). Catalyst characterization, including bulk composition, porosity, crystallinity, Fe coordination, morphology, surface property, and basicity, was conducted. The physicochemical properties of tested catalysts were correlated with their performances. The mechanistic study discovered that bidentate carbonates, monodentate carbonate, bicarbonate, and formate species were intermediates involved in C 2-6 = synthesis. The yield of C 2-6 = was found to be dependent on the composition of the carbide phases of K-doped FeC x catalysts. The most active catalyst, i.e., FeK@CS-(0.5)-py, showed the most promising space-time yield of C 2-6 = (13.7 μmol C2-6= /g Fe /s). [Display omitted] • K-doped FeC x catalysts were derived from the Fe-chitosan complexes. • A proper Fe-to-chitosan is important to increase the FeC x content. • A positive correlation between FeC x and the yield of C 2-6 = olefins was found. • Carbonates and formate were identified as the possible intermediates. [ABSTRACT FROM AUTHOR]

• The interaction between Fe and Zn was tuned by changing their proximity. • The closer proximity in ZnFe 2 O 4 phase enhances the Fe-Zn interaction. • The stronger interaction of Fe and Zn is conducive to the formation and stability of Fe 2(.2) C as well as carbon deposition. • More and stable Fe 2(.2) C facilitates CO conversion, but the severer carbon deposition lead to deactivation. In the Fischer–Tropsch synthesis (FTS) reaction, the intricate reaction conditions are prone to induce phase transitions in iron carbides, which markedly affect catalytic performance. Manipulating and stabilizing the active phase is paramount for ensuring the catalyst efficiency. In this study, we regulated the Fe-Zn interaction to investigate the influence of iron carbides and the characteristics of surface carbon species. The strong interaction significantly enhanced the adsorption and dissociation of CO on the catalyst surface, thereby facilitating the formation of carbon-rich Fe 2(.2) C. Consequently, the FeZn-s catalyst exhibited the highest iron time yield (FTY) of 959 μmol CO ·g Fe -1·s-1. But meanwhile, the stronger Fe-Zn interaction also accelerated the carbon deposition rate and elevated the graphitization degree of the surface carbon, expediting the catalyst deactivation. These results provide an insight into the modulation and stabilization of iron carbides by adjusting the interaction between the Fe and supports, which inspires the further development of Fe-based catalysts for FTS applications. [ABSTRACT FROM AUTHOR]

Nowadays, the scientific interest is focused more and more on the development of new strategies in recycling of waste products as well as on the development of clean technologies due to the increased environmental pollution. In this work we studied the valorization of an expired cheese-tomato flavor corn snack, which is polysaccharide food product, by producing advanced hybrid magnetic materials for environmental remediation purposes. The carbonization-chemical activation of this snack using potassium hydroxide leads to a microporous activated carbon with high surface area (SgBET ~800 m2/g). The magnetic hybrid material was synthesized via an in-situ technique using iron acetate complex as the precursor to produce iron based magnetic nanoparticles. The resulting material retains a fraction of the microporous structure with surface area SgBET ~500 m2/g. Such material consists, of homogenously dispersed magnetic isolated zero valent iron nanoparticles and of iron carbides (Fe3C), into the carbon matrix. The magnetic carbon exhibited high adsorption capacity in Cr(VI) removal applications following a pseudosecond order kinetic model. The maximum adsorption capacity was 88.382 mgCr(VI)/gAC at pH = 3. Finally, oxidation experiments, in combination with FT-IR, Mössbauer, and VSM measurements indicated that the possible Cr6+ removal mechanism involves oxidation of iron phases and reduction of Cr6+ to Cr3+.

Ceria-iron oxide mesoporous materials with Fe:Ce molar ratio of 5:5 and 9:1 were synthesized by hydrothermal method using CTAB as a template and subsequently modified with NiO (molar ratio Ni:Fe = 1:2) by incipient wetness impregnation technique. In order to increase the electro-capacitive properties and reduce the intrinsic impedance of the metal oxides, the samples were consecutively modified by reduction in hydrogen to obtain highly dispersed Ni–Fe alloys into ceria matrix. By exploiting the high permeability of carbon inside ferrous alloys, the metal phase has been further modified into ferrous carbides and metal alloys encapsulated within carbon nanofibers. For this purpose, a reaction, already widely studied for the production of hydrogen, was used, that is the decomposition of methanol vapors. In fact, this decomposition, in addition to producing syn-gas and methane, changes the catalysts in use through a chemical vapor deposition-carbon coating process. This fact, has been used by us to demonstrate how the newly obtained metal-carbon nanocomposites can be used for electro-catalytic purposes. The modified phases of the two molar ratios of the Fe–Ni–Ce catalysts were tested in the Oxygen Evolution Reaction (OER) in an alkaline environment (1 M KOH), showing a satisfactory and progressive increase in activity and a surprising decrease in the overpotential at 10 mA/cm2 of current density. The morphological, textural and physicochemical properties of the samples were characterized in details by XRD, N 2 -physisorption, TG-TPO, TEM, EDX, FTIR, XPS, Raman and Moessbauer spectroscopies. [Display omitted] • Reducing the Ce–Fe–Ni mixed oxides increases the intrinsic charge transfer properties. • CNFs grow with a tip-growth mechanism, encapsulating metallic Fe–Ni nanoparticles. • The different fragmentation mechanism of methanol influences the growth of CNFs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Selectivity towards C 2 /C 3 olefins are due to the formation of Fe x C y species; • The management of Fe and K introduction pathways optimized active site formation; • The management of Fe and K introduction pathways minimized carbide oxidation. Light olefins, traditionally sourced from petroleum, are currently being investigated through alternative routes, such as CO 2 hydrogenation, aligning with CCUS policies. Alkali-promoted Fe-based catalysts have shown promise in coupling reverse water–gas shift and Fischer-Tropsch processes to produce light olefins from CO 2 directly. The in-situ generated Fe 5 C 2 phase is commonly associated with the high activity of these catalysts. However, the preparation methods employed can lead to distinct catalyst properties, which considerably impact the performance during the reaction. Here, we investigate different strategies to introduce K and Fe in the KFeCeZr system and correlate catalyst properties with catalytic activity during CO 2 hydrogenation at ambient pressure. The catalyst prepared by one-pot precipitation of Fe, Ce, and Zr followed by K impregnation showed the most promising results in terms of conversion and productivity of light olefins. The effective incorporation of Fe in the CeZrO x structure during synthesis was essential for maintaining the high dispersion of metallic Fe particles after reduction, improving reactant chemisorption. Under reaction conditions, this catalyst showed the most remarkable propensity to form iron carbides (Fe x C y), mainly composed of Fe 5 C 2 phase. In general, this work highlights the impact that the construction steps of a multifunctional catalyst have on the physicochemical properties and, consequently, on the catalytic activity. [ABSTRACT FROM AUTHOR]

The effect of different active phases of Fe/SiO2 catalyst on the physio-chemical properties and the catalytic performance in CO2 hydrogenation under mild conditions (at 220 °C under an ambient pressure) was comprehensively studied in this work. The Fe/SiO2 catalyst was prepared by an incipient wetness impregnation method. Hematite (Fe2O3) in the calcined Fe/SiO2 catalyst was activated by hydrogen, carbon monoxide, and hydrogen followed by carbon monoxide, to form a metallic iron (Fe/SiO2-h), an iron carbide (Fe/SiO2-c), and a combination of a metallic iron and an iron carbide (Fe/SiO2-hc), respectively. All activated catalysts were characterized by XRD, Raman spectroscopy, N2 adsorption–desorption, H2-TPR, CO-TPR, H2-TPD, CO2-TPD, CO-TPD, NH3-TPD, and tested in a CO2 hydrogenation reaction. The different phases of the Fe/SiO2 catalyst are formed by different activation procedures and different reducing agents (H2 and CO). Among three different activated catalysts, the Fe/SiO2-c provides the highest CO2 hydrogenation performance in terms of maximum CO2 conversion, as well as the greatest selectivity toward long-chain hydrocarbon products, with the highest chain growth probability of 0.7. This is owing to a better CO2 and CO adsorption ability and a greater acidity on the carbide form of the Fe/SiO2-c surface, which are essential properties of catalysts for polymerization in FTs. [ABSTRACT FROM AUTHOR]

Nowadays, the scientific interest is focused more and more on the development of new strategies in recycling of waste products as well as on the development of clean technologies due to the increased environmental pollution. In this work we studied the valorization of an expired cheese-tomato flavor corn snack, which is polysaccharide food product, by producing advanced hybrid magnetic materials for environmental remediation purposes. The carbonization-chemical activation of this snack using potassium hydroxide leads to a microporous activated carbon with high surface area (SgBET ~800 m2/g). The magnetic hybrid material was synthesized via an in-situ technique using iron acetate complex as the precursor to produce iron based magnetic nanoparticles. The resulting material retains a fraction of the microporous structure with surface area SgBET ~500 m2/g. Such material consists, of homogenously dispersed magnetic isolated zero valent iron nanoparticles and of iron carbides (Fe3C), into the carbon matrix. The magnetic carbon exhibited high adsorption capacity in Cr(VI) removal applications following a pseudosecond order kinetic model. The maximum adsorption capacity was 88.382 mgCr(VI)/gAC at pH = 3. Finally, oxidation experiments, in combination with FT-IR, Mössbauer, and VSM measurements indicated that the possible Cr6+ removal mechanism involves oxidation of iron phases and reduction of Cr6+ to Cr3+. [ABSTRACT FROM AUTHOR]

As the only renewable organic carbon source, abundant biomass has long been established and developed to mass-produce functionalized carbon materials. Herein, an extremely facile and green strategy was executed for the first time to in situ construct K-Fe3C@C nanohybrids directly by one-pot carbonizing the pomelo peel impregnated with Fe(NO3)3 solutions. The pyrolytically self-assembled nanohybrids were successfully applied in Fischer–Tropsch synthesis (FTS) and demonstrated high catalytic performance. Accordingly, the optimized K-Fe3C@C catalysts revealed excellent FTS activity (92.6% CO conversion) with highlighted C5+ hydrocarbon selectivity of 61.3% and light olefin (C2-4=) selectivity of 26.0% (olefin/paraffin (O/P) ratio of 6.2). Characterization results further manifest that the high performance was correlated with the in situ formation of the core-shell nanostructure consisting of Fe3C nanoparticles enwrapped by graphitized carbon shells and the intrinsic potassium promoter originated in pomelo peel during high-temperature carbonization. This work provided a facile approach for the low-cost mass-fabrication of high-performance FTS catalysts directly utilizing waste biomass without any chemical pre-treatment or purification. [ABSTRACT FROM AUTHOR]

Comparative studies of the iron–75fullerite and iron–75graphite composites obtained by the mechanosynthesis method have been performed by X-ray diffraction and Mössbauer spectroscopy. It has been shown that a carbon polymorph and its deformation stability predetermine the kinetics of formation for the phase composition of composites. In the case of disordering in the crystal structure of fullerite with the further destruction of fullerene molecules, iron–75fullerite powders are characterized by the same phase composition as for the iron–75graphite composite: amorphous carbon, Fe3C, Fe7C3, and the P-phase. [ABSTRACT FROM AUTHOR]

Based on first-principle calculations in the framework of the density functional theory and structure prediction algorithms, we have determined iron carbide phases stable at the Earth's core pressures and temperatures. It is shown that Fe7C3 is unstable and decomposes into the mixture Fe2C + Fe3C over the entire range of pressures and temperatures specific to the Earth's inner core. Subsequent decomposition of Fe3C into the mixture Fe + Fe2C is unfavorable. We also predict a new low-temperature modification Fe3C-C2/m-II dynamically and thermodynamically stable over the pressure range 290-305 GPa. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a new line of magnetic hybrid nanostructured materials composed of spinel-type iron oxide to iron carbide nanoparticles grown on nanodiamond nanotemplates is reported in this study. The realization of these nanohybrid structures is achieved through thermal processing under vacuum at different annealing temperatures of a chemical precursor, in which very fine maghemite (γ-Fe2O3) nanoparticles seeds were developed on the surface of the nanodiamond nanotemplates. It is seen that low annealing temperatures induce the growth of the maghemite nanoparticle seeds to fine dispersed spinel-type non-stoichiometric ~5 nm magnetite (Fe3-xO4) nanoparticles, while intermediate annealing temperatures lead to the formation of single phase ~10 nm cementite (Fe3C) iron carbide nanoparticles. Higher annealing temperatures produce a mixture of larger Fe3C and Fe5C2 iron carbides, triggering simultaneously the growth of large-sized carbon nanotubes partially filled with these carbides. The magnetic features of the synthesized hybrid nanomaterials reveal the properties of their bearing magnetic phases, which span from superparamagnetic to soft and hard ferromagnetic and reflect the intrinsic magnetic properties of the containing phases, as well as their size and interconnection, dictated by the morphology and nature of the nanodiamond nanotemplates. These nanohybrids are proposed as potential candidates for important technological applications in nano-biomedicine and catalysis, while their synthetic route could be further tuned for development of new magnetic nanohybrid materials. [ABSTRACT FROM AUTHOR]

The main evolution stages of the phase diagram of Fe–C in the carbide region (on the right from the cementite line) is a recently discussed theoretical topic that has been considered. It has been determined that previously isolated ε-Fe2C, χ-Fe5C2, and g-Fe7C3 carbides (except for cementite Θ-Fe3C) are nonstoichiometric compounds. Thus, they are variable composition phases containing stoichiometric composition or second-type interstitial solid solutions based on daltonides and berthollides. It has been stated that the iron–cementite phase diagram can be identified as the iron–ε-Fe2C carbide phase diagram in the concentration range of 0–9.7% C. The following phase transformations are introduced to the diagram: reaction of nonvariant syntectic equilibrium of cementite Θ-Fe3C crystallization; reaction of peritectic equilibrium of carbide χ-Fe5C2 crystallization; low-temperature peritectoid transformation of carbide, at which solid solutions of ferrite and cementite form solid solution possessing broad homogeneity range based on ε-carbide Fe2C berthollide. It has been shown that the carbide phases represent a uniform isomorphous quasi-carbide solid solution, while the carbide phase crystallizes in the form of the carbide phase mixture as quasi-eutectics. [ABSTRACT FROM AUTHOR]

In Fe‐based Fischer‐Tropsch Synthesis (FTS), the Fe carbides form under the carburizing H2 : CO reaction atmosphere providing the active phases for hydrocarbon synthesis. H2 reduced Fe(−Na−S)/α‐Al2O3 catalyst materials, with and without Na−S promotion, were carburized under CO at 240–440 °C to form Fe carbides. X‐ray Powder Diffractometry (XRPD) with Rietveld Quantitative Phase Analysis (R‐QPA) and Mössbauer Absorption Spectroscopy (MAS) were used to identity and quantify the formed Fe carbide phases. The Fe carbides formed in order of increasing temperature are ϵ‐Fe3C, η‐Fe2C, χ‐Fe5C2 and θ‐Fe3C. θ‐Fe7C3 and a distorted χ‐Fe5C2 phase are formed at 25 bar CO (340 °C) from a Fe oxide precursor. Fe carbide formation was unaffected by Na−S addition, but it did increase Fe oxidation (≤290 °C) and preferred formation of χ‐Fe5C2 over θ‐Fe3C phase (≥390 °C). The results unify the often ambiguous Fe carbide identification and nomenclature and specify the role of Na−S in the carburization process. [ABSTRACT FROM AUTHOR]

Fisher‐Tropsch Synthesis (FTS) is industrially used for converting a carbon‐containing feedstock, such as coal, natural gas, biomass, and municipal waste, via the production of synthesis gas (a mixture of CO+H2) into hydrocarbons. This review article focuses on Fe‐based FTS catalysis, thereby focusing on the process conditions available for steering the various carbon pathways from input CO and their associated reactions. We will also discuss the effects of alkali‐sulphur chemical promotion and the identification of the FTS reaction active Fe carbides, which are assigned with precise crystal structures and nomenclature. Each observed Fe carbide crystal structure is further assigned with corresponding Mössbauer Absorption Spectroscopy (MAS) hyperfine fields. The expected formation temperatures and experimental conditions for the identified Fe carbides encountered in FTS research, namely ϵ‐Fe3C, η‐Fe2C, χ‐Fe5C2, θ‐Fe3C and θ‐Fe7C3, are reviewed. [ABSTRACT FROM AUTHOR]

Experimental results and geological observations suggesting the existence of a deep hydrocarbon cycle are presented. During immersion hydrocarbons accumulated in the Earth's crust in the slab generally maintain stability to a depth of 50 km. With deeper immersion, the hydrocarbon fluids contact the surrounding ferrous minerals forming a mixture of iron hydride and iron carbide on the depths of 100–290 km. Iron carbide transported into the asthenosphere by convective flows can act as carbon donors and react with hydrogen donors presented in the asthenosphere and form an aqueous-hydrocarbon fluid. This fluid can migrate through deep faults into the Earth's crust and form oil and gas deposits. [ABSTRACT FROM AUTHOR]

• CO 2 activation is more convenient on χ-Fe 5 C 2 and θ-Fe 3 C than Fe 3 O 4. • *CO is the dominating intermediate species on χ-Fe 5 C 2. • CO 2 activation undergoes both direct dissociation and *HCOO pathways on θ-Fe 3 C. In this work, periodic density functional theory (DFT) calculations were performed to investigate the CO 2 activation mechanism over thermodynamically stable χ-Fe 5 C 2 (510) and θ-Fe 3 C (031) facets. Four major pathways of CO 2 activation were examined, including the direct dissociation of CO 2 and the H-assisted intermediates of *COOH, *HCOO, and *CO + *OH. Both χ-Fe 5 C 2 and θ-Fe 3 C have proven to be active for CO 2 direct dissociation (E a =0.17 eV). As for H assisted CO 2 activation, the one-step formation of *CO + *OH is feasible on χ-Fe 5 C 2 (E a =0.24 eV). Furthermore, θ-Fe 3 C favors the *HCOO pathway (E a =0.20 eV) and *CO + *OH formation (E a = 0.11 eV), while neither phase favors the formation of *COOH. Both CO 2 direct activation and H-assisted CO 2 activation pathways are of vital importance under high ratio of H/C in CO 2 hydrogenation reaction. This work sheds light on CO 2 activation mechanism over iron carbides, improving the rational design of CO 2 hydrogenation catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electron-giving interaction of Ca, Sr, and Ba inhibited H species adsorption while enhancing CO dissociative adsorption, contributing to the formation of χ-Fe 5 C 2. • Mg decreased CO dissociative adsorption while enhancing H species adsorption. • This synergistic effect of Sr and Na resulted in the strongest CO dissociative adsorption capacity and the highest total amount of active iron carbide. • The light olefins yield of FeMnSrNa catalyst (427.0 g/(h·kg Cat)) was significantly higher than that of FeMnSr catalyst (341.4 g/(h·kg Cat)) and FeMnNa catalyst (80.6 g/(h·kg Cat)). For the high-temperature Fischer-Tropsch synthesis (HTFT) of light olefins (C 2 =-C 4 =), alkaline-earth metals (Mg, Ca, Sr, and Ba) modified FeMn catalysts were prepared using precipitation and impregnation methods. It was demonstrated that the electron-giving interaction of Ca, Sr, and Ba decreased the adsorption of H species while improving the dissociative adsorption of CO. This interaction resulted in the production of χ-Fe 5 C 2 , which increased CO conversion and C 2 =-C 4 = selectivity. Compared with Ca and Ba, the FeMnSr catalyst had the strongest CO dissociative adsorption capacity and the highest χ-Fe 5 C 2 content with the best FTS performance. When Sr and Na were introduced simultaneously, the synergistic effect of Sr and Na inhibited the formation of θ -Fe 3 C and the aggregation of catalyst particles, and also increased the χ-Fe 5 C 2 content. This synergistic effect resulted in the highest surface basicity of FeMnSrNa catalyst, the highest electron cloud density on the surface of Fe atoms, the weakest H species adsorption, the strongest CO dissociative adsorption, and the highest total amount of active iron carbide, which improved the FTS performance. The light olefins yield of FeMnSrNa catalyst (427.0 g/(h·kg Cat)) was significantly higher than that of FeMnSr catalyst (341.4 g/(h·kg Cat)) and FeMnNa catalyst (80.6 g/(h·kg Cat)). [ABSTRACT FROM AUTHOR]

Cementite occurs in steels, in meteorites, possibly at the core of the Earth and has uses in its pure form. It's composition can deviate from F e 3 C , but not by much because the Fe–C bond contributes to its cohesion. Its crystallographic unit cell is orthorhombic and primitive, with large lattice parameters, explaining its hardness. Many of its properties are anisotropic. Its single-crystal elastic properties have been investigated using first-principles calculations and by clever experiments. The iron atoms in the cell occupy two types of positions with different point symmetries; the four carbon atoms lodge within prismatic interstices. The structure can develop defects such as dislocations, faults and vacancies. Cementite is metallic and ferromagnetic with a Curie temperature of about 187 ∘ C. When alloyed, metallic solutes substitute on to the iron sites; smaller atoms such as boron replace carbon at interstitial sites. This review focuses on cementite as a single phase. [ABSTRACT FROM AUTHOR]

It is extremely important and challenging to develop exchange-coupled nanomagnets with soft/hard magnetic phase to apply for energy-related devices. Herein, we presented a simple strategy to synthesize the Fe 3 C/CoFe 2 O 4 nanocomposites by a chemical coprecipitation method, in which effective exchange coupling at hard and soft magnetic interfaces was achieved. The as-synthesized Fe 3 C/CoFe 2 O 4 nanocomposites show exceptional exchange-coupled effect and enhanced magnetic properties. Moreover, this work provides a new soft magnetic phase and an entirely new attempt for nanomagnets based on soft/hard magnetic exchange coupling. The Fe 3 C/CoFe 2 O 4 nanocomposites were prepared by chemical coprecipitation method, which show exceptional exchange-coupled effect and enhanced magnetic properties. This work provides a new soft magnetic phase and an entirely new attempt for nanomagnets based on soft/hard magnetic exchange coupling. Image 1 [ABSTRACT FROM AUTHOR]

Herein, the Fe3C nanoparticles encapsulated in carbon nanotubes (Fe3C/CNTs) were successfully prepared through a simple method using g‐C3N4 from thermal polymerization of melamine. The structure, morphology and magnetic properties of the prepared samples were investigated by various technologies. The results show that the Fe3C nanoparticles encapsulated in carbon nanotubes exhibit good magnetic properties with high saturated magnetization (83.7 emu g−1). Importantly, this simple approach provides a way to synthesize metal carbides encapsulated in carbon nanotubes. [ABSTRACT FROM AUTHOR]

N-doped iron carbide (N-doped Fe3C) nanomaterials have a wide range of applications. In this study, a hexadecyltrimethylammonium bromide (CTAB)-assisted two-step process was used to prepare N-doped Fe3C nanomaterials. The effect of CTAB addition on the phase, structure, and morphologies of the products has been investigated in detail based on the characterization results of X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectra (Raman), N2 adsorption–desorption, and elemental analysis. By tuning the amount of CTAB, N-doped Fe3C nanowires were prepared. The magnetic hysteresis loops at room temperature of the N-doped Fe3C nanomaterials show ferromagnetic characteristics. [ABSTRACT FROM AUTHOR]

Abstract—X-ray diffraction, Mössbauer and Raman spectroscopy have been used to study structural phase transformations during mechanosynthesis of Fe–С60/70 in toluene with 20 and 25 at % С60/70 fullerite. The quantitative carbon content determines the phase composition of mechanocomposites. (Fe3C)D forms at 20 at % С60/70, whereas Fe7C3 and a paramagnetic phase form at 25 at % С60/70 at the initial stage of mechanical alloying. Free carbon is in an amorphous state in the sample. The formation mechanism of carbides and a paramagnetic component is discussed. [ABSTRACT FROM AUTHOR]

Hägg carbide (χ) is, during the tempering of carbon-rich martensite, referred to as a transition carbide which eventually gives way to cementite. However, there are Fe–C binary phase diagrams estimated using thermodynamic data, that define a low-temperature phase field where a mixture of Hägg carbide and ferrite is more stable than that of cementite and ferrite. In this scenario, it may be cementite which is the transition carbide. Evidence is presented here that the predominance of Hägg carbide over cementite in the circumstances described is unlikely to be correct based on historical and new observations. Literature data are also interpreted to show that η and ϵ-carbides are best regarded as transition-phases relative to mixtures of cementite and ferrite. [ABSTRACT FROM AUTHOR]