A new synthesis of carbon encapsulated Fe5C2 nanoparticles for high-temperature Fischer-Tropsch synthesis

Graphene Nanoflake- and Carbon Nanotube-Supported Iron-Potassium 3D-Catalysts for Hydrocarbon Synthesis from Syngas

Transformation of carbon oxides into valuable feedstocks is an important challenge nowadays. Carbon oxide hydrogenation to hydrocarbons over iron-based catalysts is one of the possible ways for this transformation to occur. Carbon supports effectively increase the dispersion of such catalysts but possess a very low bulk density, and their powders can be toxic. In this study, spark plasma sintering was used to synthesize new bulk and dense potassium promoted iron-based catalysts, supported on N-doped carbon nanomaterials, for hydrocarbon synthesis from syngas. The sintered catalysts showed high activity of up to 223 μmol_CO/g_Fe/s at 300-340 °C and a selectivity to C₅₊ fraction of ~70% with a high portion of olefins. The promising catalyst performance was ascribed to the high dispersity of iron carbide particles, potassium promotion of iron carbide formation and stabilization of the active sites with nitrogen-based functionalities. As a result, a bulk N-doped carbon-supported iron catalyst with 3D structure was prepared, for the first time, by a fast method, and demonstrated high activity and selectivity in hydrocarbon synthesis. The proposed technique can be used to produce well-shaped carbon-supported catalysts for syngas conversion.

Effects of Silica Shell Encapsulated Nanocrystals on Active χ-Fe5C2 Phase and Fischer-Tropsch Synthesis

Among various iron carbide phases, χ-Fe₅C₂, a highly active phase in Fischer-Tropsch synthesis, was directly synthesized using a wet-chemical route, which makes a pre-activation step unnecessary. In addition, χ-Fe₅C₂ nanoparticles were encapsulated with mesoporous silica for protection from deactivation. Further structural analysis showed that the protective silica shell had a partially ordered mesoporous structure with a short range. According to the XRD result, the sintering of χ-Fe₅C₂ crystals did not seem to be significant, which was believed to be the beneficial effect of the protective shell providing restrictive geometrical space for nanoparticles. More interestingly, the protective silica shell was also found to be effective in maintaining the phase of χ-Fe₅C₂ against re-oxidation and transformation to other iron carbide phases. Fischer-Tropsch activity of χ-Fe₅C₂ in this study was comparable to or higher than those from previous reports. In addition, CO₂ selectivity was found to be very low after stabilization.

Preparation of Nitrogen Doped Biochar-Based Iron Catalyst for Enhancing Gasoline-Range Hydrocarbons Production

Developing catalysts to obtain high space time yield (STY) of gasoline-range hydrocarbons via Fischer-Tropsch synthesis (FTS) is a huge challenge due to the restriction of Anderson-Schulz-Flory distribution. Herein, a nitrogen doped biochar-based iron catalyst was synthesized by a one-step method using sugar cane bagasse as carbon precursor, which exhibited an excellent gasoline STY of 8.65 g_C5-12 g_Fe^-1 h^-1, exceeding most reported catalysts. A strong positive relationship between the amount of pyrrolic N and long-chain hydrocarbons selectivity was displayed. The characterization results indicated that pyrrolic N configuration on anchor sites tuned effectively the dispersion of iron species and metal-support interaction as well as CO adsorption, improving the FTS performance.

Single-Phase θ-Fe3C Derived from Prussian Blue and Its Catalytic Application in Fischer-Tropsch Synthesis

Elucidation of the intrinsic catalytic principle of iron carbides remains a substantial challenge in iron-catalyzed Fischer-Tropsch synthesis (FTS), due to possible interference from other Fe-containing species. Here, we propose a facile approach to synthesize single-phase θ-Fe3C via the pyrolysis of a molecularly defined Fe-C complex (Fe4[Fe(CN)6]3), thus affording close examination of its catalytic behavior during FTS. The crystal structure of prepared θ-Fe3C is unambiguously verified by combined XRD and MES measurement, demonstrating its single-phase nature. Strikingly, single-phase θ-Fe3C exhibited excellent selectivity to light olefins (77.8%) in the C2-C4 hydrocarbons with less than 10% CO2 formation in typical FTS conditions. This strategy further succeeds with promotion of Mn, evident for its wide-ranging compatibility for the promising industrial development of catalysts. This work offers a facile approach for oriented preparation of single-phase θ-Fe3C and provides an in-depth understanding of its intrinsic catalytic performance in FTS.

Hydrogenation of Carbon Dioxide to Value-Added Liquid Fuels and Aromatics over Fe-Based Catalysts Based on the Fischer–Tropsch Synthesis Route

Hydrogenation of CO2 to value-added chemicals and fuels not only effectively alleviates climate change but also reduces over-dependence on fossil fuels. Therefore, much attention has been paid to the chemical conversion of CO2 to value-added products, such as liquid fuels and aromatics. Recently, efficient catalysts have been developed to face the challenge of the chemical inertness of CO2 and the difficulty of C–C coupling. Considering the lack of a detailed summary on hydrogenation of CO2 to liquid fuels and aromatics via the Fischer–Tropsch synthesis (FTS) route, we conducted a comprehensive and systematic review of the research progress on the development of efficient catalysts for hydrogenation of CO2 to liquid fuels and aromatics. In this work, we summarized the factors influencing the catalytic activity and stability of various catalysts, the strategies for optimizing catalytic performance and product distribution, the effects of reaction conditions on catalytic performance, and possible reaction mechanisms for CO2 hydrogenation via the FTS route. Furthermore, we also provided an overview of the challenges and opportunities for future research associated with hydrogenation of CO2 to liquid fuels and aromatics.

A perspective on the controlled synthesis of iron-based nanoalloys for the oxygen reduction reaction

The worsening ecological environment is calling for clean energy alternatives, among which hydrogen fuel cells have been one of the hot topics. The commercialized Pt/C catalyst for the oxygen reduction reaction (ORR) in the cathode of fuel cells is suffering from its high cost, serious scarcity and so on. Hence, the exploration on alternative ORR catalysts has attracted much attention. Iron(Fe)-based nanoalloys have shown advantages of low cost, high abundance, and pleasant ORR activity. In this feature, we have summarized Fe-based nanoalloy structures and our recent progress on controllable synthesis as well as their ORR performance, including iron-platinum (Fe-Pt), iron carbide (Fe-C), and iron nitride (Fe-N). Finally, the perspective on this type of ORR electrocatalyst is also discussed.

Environmental STEM Study of the Oxidation Mechanism for Iron and Iron Carbide Nanoparticles

The oxidation of solution-synthesized iron (Fe) and iron carbide (Fe₂C) nanoparticles was studied in an environmental scanning transmission electron microscope (ESTEM) at elevated temperatures under oxygen gas. The nanoparticles studied had a native oxide shell present, that formed after synthesis, an ~3 nm iron oxide (Fe_xO_y) shell for the Fe nanoparticles and ~2 nm for the Fe₂C nanoparticles, with small void areas seen in several places between the core and shell for the Fe and an ~0.8 nm space between the core and shell for the Fe₂C. The iron nanoparticles oxidized asymmetrically, with voids on the borders between the Fe core and Fe_xO_y shell increasing in size until the void coalesced, and finally the Fe core disappeared. In comparison, the oxidation of the Fe₂C progressed symmetrically, with the core shrinking in the center and the outer oxide shell growing until the iron carbide had fully disappeared. Small bridges of iron oxide formed during oxidation, indicating that the Fe transitioned to the oxide shell surface across the channels, while leaving the carbon behind in the hollow core. The carbon in the carbide is hypothesized to suppress the formation of larger crystallites of iron oxide during oxidation, and alter the diffusion rates of the Fe and O during the reaction, which explains the lower sensitivity to oxidation of the Fe₂C nanoparticles.

Biosugarcane-based carbon support for high-performance iron-based Fischer-Tropsch synthesis

Exploiting new carbon supports with adjustable metal-support interaction and low price is of prime importance to realize the maximum active iron efficiency and industrial-scale application of Fe-based catalysts for Fischer-Tropsch synthesis (FTS). Herein, a simple, tunable, and scalable biochar support derived from the sugarcane bagasse was successfully prepared and was first used for FTS. The metal-support interaction was precisely controlled by functional groups of biosugarcane-based carbon material and different iron species sizes. All catalysts synthesized displayed high activities, and the iron-time-yield of Fe₄/C_bio even reached 1,198.9 μmol g_Fe⁻¹ s⁻¹. This performance was due to the unique structure and characteristics of the biosugarcane-based carbon support, which possessed abundant C−O, C=O (η¹(O) and η²(C, O)) functional groups, thus endowing the moderate metal-support interaction, high dispersion of active iron species, more active ε-Fe₂C phase, and, most importantly, a high proportion of Fe_xC/Fe_surf, facilitating the maximum iron efficiency and intrinsic activity of the catalyst.

•

A kind of carbon support, derived from the sugarcane bagasse, is prepared

•

This biochar catalyst reaches an excellent FTY value in Fischer-Tropsch synthesis

•

Functional groups and Fe species sizes regulate metal-support interactions

•

Superior performance is due to abundant functional groups and ε-Fe₂C

Duet Fe3C and FeNx Sites for H2O2 Generation and Activation toward Enhanced Electro-Fenton Performance in Wastewater Treatment

Heterogeneous electro-Fenton (HEF) reaction has been considered as a promising process for real effluent treatments. However, the design of effective catalysts for simultaneous H₂O₂ generation and activation to achieve bifunctional catalysis for O₂ toward •OH production remains a challenge. Herein, a core-shell structural Fe-based catalyst (FeNC@C), with Fe₃C and FeN nanoparticles encapsulated by porous graphitic layers, was synthesized and employed in a HEF system. The FeNC@C catalyst presented a significant performance in degradation of various chlorophenols at various conditions with an extremely low level of leached iron. Electron spin resonance and radical scavenging revealed that •OH was the key reactive species and Fe^IV would play a role at neutral conditions. Experimental and density function theory calculation revealed the dominated role of Fe₃C in H₂O₂ generation and the positive effect of FeN_x sites on H₂O₂ activation to form •OH. Meanwhile, FeNC@C was proved to be less pH dependence, high stability, and well-recycled materials for practical application in wastewater purification.

Stabilization of ε-iron carbide as high-temperature catalyst under realistic Fischer-Tropsch synthesis conditions

The development of efficient catalysts for Fischer–Tropsch (FT) synthesis, a core reaction in the utilization of non-petroleum carbon resources to supply energy and chemicals, has attracted much recent attention. ε-Iron carbide (ε-Fe₂C) was proposed as the most active iron phase for FT synthesis, but this phase is generally unstable under realistic FT reaction conditions (> 523 K). Here, we succeed in stabilizing pure-phase ε-Fe₂C nanocrystals by confining them into graphene layers and obtain an iron-time yield of 1258 μmol_CO g_Fe⁻¹s⁻¹ under realistic FT synthesis conditions, one order of magnitude higher than that of the conventional carbon-supported Fe catalyst. The ε-Fe₂C@graphene catalyst is stable at least for 400 h under high-temperature conditions. Density functional theory (DFT) calculations reveal the feasible formation of ε-Fe₂C by carburization of α-Fe precursor through interfacial interactions of ε-Fe₂C@graphene. This work provides a promising strategy to design highly active and stable Fe-based FT catalysts.

ε-Fe₂C has been identified as the highly active phase for Fischer-Tropsch synthesis (FTS), but is stable only at low-temperature. Here, the authors show that ε-Fe₂C phase can be stabilized even at ~ 573 K by being encapsulated inside graphene layers, and retains high activity in FTS.

Temperature-dependent c-axis lattice-spacing reduction and novel structural recrystallization in carbon nano-onions filled with Fe3C/α-Fe nanocrystals

Carbon nano-onions are approximately spherical nanoscale graphitic shells. When filled with ferromagnetic Fe3C/α-Fe nanocrystals, these structures have several important applications, such as point electron-sources, magnetic data recording, energy storage, and others, that exploit the interaction of either or both the shells and the magnetic moments in the filling. Despite these applications receiving much recent attention, little is known about the structural relationship between the carbon shells and the internal nanocrystal. In this work, the graphitic c-axis lattice-spacing in Fe3C/α-Fe-filled multi-shell structures was determined by XRD in the temperature range from 130 K to 298 K. A significant reduction in the c-axis lattice-spacing was observed in the multi-shell structures. A defect-induced magnetic transition was probed and ascribed to the formation of randomly oriented ferromagnetic clusters in the recrystallized disclination-rich regions of the CNOs-shells, in agreement with the percolative theory of ferromagnetism.

Preparation of Iron Carbides Formed by Iron Oxalate Carburization for Fischer–Tropsch Synthesis

Different iron carbides were synthesized from the iron oxalate precursor by varying the CO carburization temperature between 320 and 450 °C. These iron carbides were applied to the high-temperature Fischer–Tropsch synthesis (FTS) without in situ activation treatment directly. The iron oxalate as a precursor was prepared using a solid-state reaction treatment at room temperature. Pure Fe5C2 was formed at a carburization temperature of 320 C, whereas pure Fe3C was formed at 450 °C. Interestingly, at intermediate carburization temperatures (350–375 °C), these two phases coexisted at the same time although in different proportions, and 360 °C was the transition temperature at which the iron carbide phase transformed from the Fe5C2 phase to the Fe3C phase. The results showed that CO conversions and products selectivity were affected by both the iron carbide phases and the surface carbon layer. CO conversion was higher (75–96%) when Fe5C2 was the dominant iron carbide. The selectivity to C5+ products was higher when Fe3C was alone, while the light olefins selectivity was higher when the two components (Fe5C2 and Fe3C phases) co-existed, but the quantity of Fe3C was small.

Soft magnetic Fe5C2-Fe3C@C as an electrocatalyst for the hydrogen evolution reaction

Herein, cubic iron carbides encapsulated in an N-doped carbon shell (ICs@NC) were prepared by a simple two-step method. The two-step method included the preparation of iron oxalate dihydrate and the process of calcination with ethylenediamine. By changing the calcination temperature, we could control the type of iron carbide formed. Moreover, the prepared iron carbide@N-doped carbon core-shell particles exhibited regular cubic shapes and soft magnetic properties with high saturation magnetization. More importantly, we investigated the electrocatalytic activity of the iron carbide@N-doped carbon catalysts for the hydrogen evolution reaction (HER). The results show that the Fe5C2-Fe3C@NC catalyst has efficient HER catalytic activity with an overpotential of 209 mV@10 mA cm-2.

To heat or not to heat: a study of the performances of iron carbide nanoparticles in magnetic heating

Heating magnetic nanoparticles with high frequency magnetic fields is a topic of interest for biological applications (magnetic hyperthermia) as well as for heterogeneous catalysis. This study shows why FeC NPs of similar structures and static magnetic properties display radically different heating power (SAR from 0 to 2 kW g-1). By combining results from Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS) and static and time-dependent high-frequency magnetic measurements, we propose a model describing the heating mechanism in FeC nanoparticles. Using, for the first time, time-dependent high-frequency hysteresis loop measurements, it is shown that in the samples displaying the larger heating powers, the hysteresis is strongly time dependent. More precisely, the hysteresis area increases by a factor 10 on a timescale of a few tens of seconds. This effect is directly related to the ability of the nanoparticles to form chains under magnetic excitation, which depends on the presence or not of strong dipolar couplings. These differences are due to different ligand concentrations on the surface of the particles. As a result, this study allows the design of a scalable synthesis of nanomaterials displaying a controllable and reproducible SAR.

Substrate-induced hydrothermal synthesis of hematite superstructures and their Fischer–Tropsch synthesis performance

Fe foam substrates with different pore densities are used to fabricate versatile α-Fe₂O₃ nanostructures with different Fischer–Tropsch synthesis performance.

Sub-50 nm Iron-Nitrogen-Doped Hollow Carbon Sphere-Encapsulated Iron Carbide Nanoparticles as Efficient Oxygen Reduction Catalysts

Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe3C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m2 g-1), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe3C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe3C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H2O2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe3C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.

Fe2O3 hollow microspheres as highly selective catalysts for the production of α-olefins

Fe₂O₃ derived from Fe-glycerate with different interior structures and tunable pore sizes distinctly optimized the product selectivity.

Computational exploration of Fe55@C240-catalyzed Fischer–Tropsch synthesis

DFT calculations showed possible hydrocarbon chain growth on Fe55@C240 preferentially via a CO insertion mechanism.

Carbon-encapsulated highly dispersed FeMn nanoparticles for Fischer–Tropsch synthesis to light olefins

The peculiar structure of FeMn@C not only facilitates the formation of χ-Fe₅C₂, but it also promotes the product selectivity of light olefins.

Ferromagnetically filled carbon nano-onions: the key role of sulfur in dimensional, structural and electric control

A key challenge in the fabrication of ferromagnetically filled carbon nano-onions (CNOs) is the control of their thickness, dimensions and electric properties. Up to now literature works have mainly focused on the encapsulation of different types of ferromagnetic materials including α-Fe, Fe3C, Co, FeCo, FePd3 and others within CNOs. However, no report has yet shown a suitable method for controlling both the number of shells, diameter and electric properties of the produced CNOs. Here, we demonstrate an advanced chemical vapour deposition approach in which the use of small quantities of sulfur during the pyrolysis of ferrocene allows for the control of (i) the diameter of the CNOs, (ii) the number of shells and (iii) the electric properties. We demonstrate the morphological, structural, electric and magnetic properties of these new types of CNOs by using SEM, XRD, TEM, HRTEM, EIS and VSM techniques.

A Fe5C2 nanocatalyst for the preferential synthesis of ethanol via dimethyl oxalate hydrogenation

A Fe-based catalyst exhibits extremely high selectivity (89.6%) besides excellent catalytic activity in gas-phase dimethyl oxalate hydrogenation. The ethanol formation occurs via hydrogenation of methyl acetate instead of ethylene glycol over the active species Fe₅C₂.

Robust iron-carbide nanoparticles supported on alumina for sustainable production of gasoline-range hydrocarbons

A robust iron-carbide/alumina catalyst shows excellent catalytic performance for selective production of liquid fuels.

Effects of calcination and activation conditions on ordered mesoporous carbon supported iron catalysts for production of lower olefins from synthesis gas

Calcination and activation of CMK-3 supported iron catalysts for C₂–C₄ olefins production from syngas is investigated.