Atomically Defined Iron Carbide Surface for Fischer–Tropsch Synthesis Catalysis

Data-Driven Structure Recognition of Scanning Tunneling Microscopy Images in a Case of Iron Carbide

Scanning tunneling microscopy (STM) serves as a critical tool for high-resolution surface imaging, yet deciphering the atomic structures from STM images on multielement surfaces, such as oxides and carbides, remains a challenging task that heavily relies on the expertise and intuition of researchers. In this study, we introduce a data-driven method for rapid structural recognition from STM images. This method involves extracting structural features, filtering through a structural database, and matching with simulated STM images and surface energy analyses, thereby providing researchers with several of the most probable structures. We demonstrate the capabilities of this technique using our previously reported iron carbide grown on an Fe(110) crystal. By proposing a candidate structure set and establishing a comprehensive database linking STM images to corresponding structures and surface energies, we selected 6 out of more than 10 000 possible surfaces. On the basis of these 6 recommendations, researchers can conveniently determine the real surface structures. Our work provides an efficient tool for the structure recognition of STM images to construct surface structures, potentially serving as a universal auxiliary tool for STM structural analysis.

Adsorptive removal of phosphate from water with biochar from acacia tree modified with iron and magnesium oxides

A novel biochar (BC) from Acacia tortilis trees pruning waste was synthesized and tested for the removal of phosphate from aqueous solutions. The BC was prepared by calcination at 600 °C and doped with Fe3O4 and MgO by hydrothermal process. The presence of iron and magnesium ions in the modified BC was confirmed by EDS analysis and X-ray diffraction (XRD) methods. Both unmodified and doped BCs were tested for phosphate removal from synthetic 1-500 ppm aqueous solutions. While the unmodified BC did not show any significant removal of phosphate from aqueous solutions, the modified BC almost completely removed phosphate from water. The enhancement in removal efficiency is due to an increase in the overall surface charge and surface area of BC as a result of doping with Fe3O4 and MgO salts. The average porosity and BET surface area corresponding to the plain BC increased by more than 20% from 322 to 394 m2/g after modification by impregnation with iron oxide and magnesium oxide. The modificaiton of BC with Fe3O4 and MgO nanoparticles was observed to increase the point of zero electric charge (PZC) from pH 3.4 (corresponding to plain BC) to pH 5.3 (corresponding to modified BC). The adsorption process was very fast and a phosphate removal value of 82.5% was reached only after 30 min of adsorption, while the removal efficiency after 4 h of adsorption was 97.5%. The rapid removal efficiency in short contact time is attributed to the high surface area of BC and strong bonding between the modified BC surface and PO43- ions. The highest adsorption capacity was observed to correspond to 98.5 mg/g which was achieved at PO43- concentration of 500 ppm and pH 8.5. Moreover, after fitting the adsorption data onto four of the most widely used adsorption isotherm models, the adsorption of PO43- onto BC can be better described by the Langmuir isotherm model.

Tuning hydrogenation chemistry of Pd-based heterogeneous catalysts by introducing homogeneous-like ligands

Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H₂ activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (<50%) in hydrotreating of diphenyl ether (DPE, modelling the strongest C-O linkage in lignin) and enhanced ethene selectivity (>90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.

Selectively Tuning the Substrate Adhesion Strength of Aligned Carbon Nanotube Arrays via Thermal Postgrowth Processing

The excellent intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily formed into multifunctional 3D architectures motivate their use for a variety of commercial applications, such as batteries, chemical sensors for environmental monitoring, and energy harvesting devices. While controlling nanofiber adhesion to the growth substrate is essential for bulk-scale manufacturing and device performance, experimental approaches and models to date have not addressed tuning the CNT array-substrate adhesion strength with thermal processing conditions. In this work, facile "one-pot" thermal postgrowth processing (at temperatures T_p = 700-950 °C) is used to study CNT-substrate pull-off strength for millimeter-tall aligned CNT arrays. CNT array pull-off from the flat growth substrate (Fe/Al₂O₃/SiO₂/Si wafers) via tensile testing shows that the array fails progressively, similar to the response of brittle microfiber bundles in tension. The pull-off strength evolves nonmonotonically with T_p in three regimes, first increasing by 10 times through T_p = 800 °C due to graphitization of disordered carbon at the CNT-catalyst interface, and then decreasing back to a weak interface through T_p = 950 °C due to diffusion of the Fe catalyst into the substrate, Al₂O₃ crystallization, and substrate cracking. Failure is observed to occur at the CNT-catalyst interface below 750 °C, and the CNTs themselves break during pull-off after higher T_p processing, leaving residual CNTs on the substrate. Morphological and chemical analyses indicate that the Fe catalyst remains on the substrate after pull-off in all regimes. This work provides new insights into the interfacial interactions responsible for nanofiber-substrate adhesion and allows tuning to increase or decrease array strength for applications such as advanced sensors, energy devices, and nanoelectromechanical systems (NEMS).

Effects of Different Reductive Agents on Zn-Promoted Iron Oxide Phases in the CO2–Fischer–Tropsch to Linear α-Olefins

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.

Theoretically Predicted CO Adsorption and Activation on the Co-Doped hcp-Fe7C3 Catalyst

The Hcp-Fe7C3 phase has attracted more attention due to the high catalytic activity in Fischer–Tropsch synthesis (FTS) reactions. In this work, the adsorption and activation of CO on a Co-doped hcp-Fe7C3 catalyst were investigated by density functional theory (DFT) in order to understand the effect of Co doping on the initial step of FTS reactions on iron-based catalysts. Different Co-doped hcp-Fe7C3 001 and 11¯0 surfaces were constructed, and the CO adsorption configurations were studied. The calculated results show that the structure of the 001 surface remains basically unchanged after doping with Co atoms, while the replacement of Fe or C atoms on 11¯0 surfaces with Co atoms has a significant impact on the surface structure. The top sites on the doped Co atoms of hcp-Fe7C3 are disadvantages for the CO adsorption, whereas the T, 2F, or 3F sites around the doped Co atoms are beneficial for promoting the adsorption of CO. The CO direct dissociation pathways on the four types of Co-doped hcp-Fe7C3 001 surfaces are exothermic, while the H-assisted dissociation pathways of CO are endothermic. The H-assisted activation via HCO on the 3F1 site of the 2Co2-doped hcp-Fe7C3 001 surface shows the lowest energy barrier of 1.96 eV. For the Co-doped hcp-Fe7C3 11¯0 surfaces, the H-assisted activation via HCO is the preferred activation pathway for CO on the Co-doped surfaces with the energy barrier of approximately 1.30 eV.

In Situ Surface-Sensitive Investigation of Multiple Carbon Phases on Fe(110) in the Fischer–Tropsch Synthesis

Carbide formation on iron-based catalysts is an integral and, arguably, the most important part of the Fischer–Tropsch synthesis process, converting CO and H₂ into synthetic fuels and numerous valuable chemicals. Here, we report an in situ surface-sensitive study of the effect of pressure, temperature, time, and gas feed composition on the growth dynamics of two distinct iron–carbon phases with the octahedral and trigonal prismatic coordination of carbon sites on an Fe(110) single crystal acting as a model catalyst. Using a combination of state-of-the-art X-ray photoelectron spectroscopy at an unprecedentedly high pressure, high-energy surface X-ray diffraction, mass spectrometry, and theoretical calculations, we reveal the details of iron surface carburization and product formation under semirealistic conditions. We provide a detailed insight into the state of the catalyst’s surface in relation to the reaction.

In Situ Active Site for Fe-Catalyzed Fischer-Tropsch Synthesis: Recent Progress and Future Challenges

Fischer-Tropsch synthesis (FTS) that converts syngas into long-chain hydrocarbons is a key technology in the chemical industry. As one of the best catalysts for FTS, the Fe-based composite develops rich solid phases (metal, oxides, and carbides) in the catalytic reaction, which triggered the quest for the true active site in catalysis in the past century. Recent years have seen great advances in probing the active-site structure using modern experimental and theoretical tools. This Perspective serves to highlight these latest achievements, focusing on the geometrical structure and thermodynamic stability of Fe carbide bulk phases, the exposed surfaces, and their relationship to FTS activity. The current reaction mechanisms on CO activation and carbon chain growth are also discussed, in the context of theoretical models and experimental evidence. We also present the outlook regarding the current challenges in Fe-based FTS.

C2 weakens the turnover frequency during the melting of FexCy: insights from reactive MD simulations

The carbon accumulation in the form of C2 on the surface at high temperatures blocks the surface catalytic active sites, reducing the activity of melted FexCy nanoparticles.

Elucidating the Active Site and the Role of Alkali Metals in Selective Hydrodeoxygenation of Phenols over Iron-Carbide-based Catalyst

Iron-carbide-based catalysts have been explored in the selective hydrodeoxygenation (HDO) of phenol, aiming at elucidating the role of active site and alkali metal. Complementary characterization such as X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and scanning transmission electron microscopy coupled with electron energy loss spectroscopy, together with catalytic evaluations revealed a rapid structural reconstruction of iron carbide (Fe₃ C) catalysts, leading to a stable defective graphene-covered metallic Fe active phase (G@Fe) under reaction conditions. Further studies using different alkali metals (i. e., Na, K, and Cs) revealed that alkali metals showed negligible effect on the phase transformation of Fe₃ C. However, the reconstructed G@Fe doped with alkali metals inhibited the tautomerization, a facile reaction pathway to saturation of the aromatic ring, leading to enhanced selectivity to arene. The extent of inhibition of tautomerization or selectivity to arene was closely related to the degree of electron donation of alkali metal to Fe.

Subsurface-Carbon-Induced Local Charge of Copper for an On-Surface Displacement Reaction

Transition-metal carbides have sparked unprecedented enthusiasm as high-performance catalysts in recent years. Still, the catalytic properties of copper carbide remain unexplored. By introducing subsurface carbon to Cu(111), a displacement reaction of a proton in a carboxyl acid group with a single Cu atom is demonstrated at the atomic scale and room temperature. Its occurrence is attributed to the C-doping-induced local charge of surface Cu atoms (up to +0.30 e/atom), which accelerates the rate of on-surface deprotonation via reduction of the corresponding energy barrier, thus enabling the instant displacement of a proton with a Cu atom when the molecules adsorb on the surface. This well-defined and robust Cu^δ+ surface based on subsurface-carbon doping offers a novel catalytic platform for on-surface synthesis.

In Situ Active Site for CO Activation in Fe-Catalyzed Fischer-Tropsch Synthesis from Machine Learning

In situ-formed iron carbides (FeC_x) are the key components responsible for Fischer-Tropsch synthesis (FTS, CO + H₂ → long-chain hydrocarbons) on Fe-based catalysts in industry. The true active site is, however, highly controversial despite more than a century of study, which is largely due to the combined complexity in both FeC_x structures and mechanism of CO hydrogenation. Herein powered by machine learning simulation, millions of structure candidates for FeC_x bulk and surfaces are explored under FTS conditions, which leads to resolving the active site for CO activation. This is achieved without a priori input from experiment by first constructing the thermodynamics convex hull of bulk phases, followed by identifying the low surface energy surfaces and evaluating the adsorption ability of CO and H, and finally determining the lowest energy reaction pathway of CO activation. Rich information on FeC_x structures and CO hydrogenation pathways is gleaned: (i) Fe₅C₂, Fe₇C₃, and Fe₂C are the three stable bulk phases under FTS in producing olefins, where Fe₇C₃ and Fe₂C have multiple energetically nearly degenerate bulk crystal phases; (ii) only three low surface energy surfaces of these bulk phases, namely, χ-Fe₅C₂(510), χ-Fe₅C₂(111), and η-Fe₂C(111), expose the Fe sites that can adsorb H atoms exothermically, where the surface Fe:C ratio is 2, 1.75, and 2, respectively; (iii) CO activation via direct dissociation can occur at the surface C vacancies (e.g., with a barrier of 1.1 eV) that are created dynamically via hydrogenation. These atomic-level understandings facilitate the building of the structure-activity correlation and designing better FT catalysts.

Insight into the Fischer–Tropsch mechanism on hcp-Fe7C3 (211) by density functional theory: the roles of surface carbon and vacancies

Iron carbide phases discovered in the spent iron catalysts have proved to be active in the Fischer–Tropsch process. The surface carbon of the iron carbide played a key role in the Fischer–Tropsch mechanism. Since there are two surface carbons, C1 and C2, on the hcp-Fe₇C₃ (211), which are close to each other, their reaction mechanisms would be significant. Hence, the DFT calculations were performed to investigate the Fischer–Tropsch mechanism involving the surface carbon. It was found that the HC¹ + C² pathway was the major C–C coupling reaction pathway with an effective energy barrier of 0.97 eV. Ethane would be the major C₂ product from the HC¹C2 species through the stepwise hydrogenation pathway due to the high adsorption energy of ethylene (1.67 eV). After the desorption process of ethane, the carbon vacancy would form. The carbon vacancy was found to be the CO activation site through the CO direct dissociation pathway and the carbon vacancy would recover. It was concluded that the defect-hcp-Fe₇C₃ (211) is the high active facet of the Fischer–Tropsch synthesis, the carbon vacancy sites are the CO activation sites and the surface carbon sites are the C–C coupling sites. The surface carbons not only act as the chain initiation sites but also act as the chain growth sites in the Fischer–Tropsch mechanism on hcp-Fe₇C₃ (211).

The Fischer–Tropsch mechanism on hcp-Fe₇C₃ (211): the circulation of the surface carbon.

Facile fabrication of porous Fe@C nanohybrids from natural magnetite as excellent Fischer-Tropsch catalysts

Fe-Based catalysts play crucial roles in Fischer-Tropsch synthesis (FTS). Herein, we for the first time demonstrate a facile sol-gel approach using natural magnetite and citric acid to fabricate porous Fe@C nanohybrids as FTS catalysts. Excellent FTS activity and stability were revealed and attributed to the formation of an Fe3C active phase and a core-shell structure. This flexible synthesis strategy clearly highlights the promising application of such materials in FTS.

Magnetic N-doped 3D graphene-like framework carbon for extraction of cephalexin monohydrate and ceftiofur hydrochloride

Magnetic N-doped 3D graphene-like framework carbon (Fe₃O₄@N-3DFC) was prepared via direct pyrolysis of sodium citrate and further hydrothermal reaction, and employed in the extraction of two cephalosporin antibiotics including cephalexin monohydrate and ceftiofur hydrochloride. The scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy and X-ray diffraction were employed to confirm the successful synthesis of 3DFC-based adsorbents (3DFCs) including 3DFC, Fe₃O₄@3DFC and Fe₃O₄@N-3DFC. Fe₃O₄@N-3DFC sorbent with 3D graphene-like honeycomb architecture combined magnetic and N doping shows the attractive features including graphene carbon sheets, hierarchical porous structure, good wettability and higher surface affinity for cephalosporin antibiotics. Furthermore, the comparison of extraction efficiency with raw 3DFC and Fe₃O₄@3DFC sorbent also confirmed the superiority of Fe₃O₄@N-3DFC sorbent. Under optimized conditions, good linearity lines were obtained with the determination coefficients from 0.9953 to 0.9995. The limit of detections were in the range of 0.20-0.45 μg L^-1 and 0.03-0.10 μg L^-1 for cephalexin and ceftiofur, respectively. The spiked extraction recoveries were between 81.59% and 98.35% with the relative standard deviation values less than 6.98%. Combined with high performance liquid chromatography, Fe₃O₄@N-3DFC based magnetic solid-phase extraction was successfully applied in river water and zebrafish samples analysis.

Interstitial Boron Atoms in the Palladium Lattice of an Industrial Type of Nanocatalyst: Properties and Structural Modifications

It is well-established that the inclusion of small atomic species such as boron (B) in powder metal catalysts can subtly modify catalytic properties, and the associated changes in the metal lattice imply that the B atoms are located in the interstitial sites. However, there is no compelling evidence for the occurrence of interstitial B atoms, and there is a concomitant lack of detailed structural information describing the nature of this occupancy and its effects on the metal host. In this work, we use an innovative combination of high-resolution ¹¹B magic-angle-spinning (MAS) and ¹⁰⁵Pd static solid-state NMR nuclear magnetic resonance (NMR), synchrotron X-ray diffraction (SXRD), in situ X-ray pair distribution function (XPDF), scanning transmission electron microscopy-annular dark field imaging (STEM-ADF), electron ptychography, and electron energy loss spectroscopy (EELS) to investigate the B atom positions, properties, and structural modifications to the palladium lattice of an industrial type interstitial boron doped palladium nanoparticle catalyst system (Pd-^intB/C NPs). In this study, we report that upon B incorporation into the Pd lattice, the overall face centered cubic (FCC) lattice is maintained; however, short-range disorder is introduced. The ¹⁰⁵Pd static solid-state NMR illustrates how different types (and levels) of structural strain and disorder are introduced in the nanoparticle history. These structural distortions can lead to the appearance of small amounts of local hexagonal close packed (HCP) structured material in localized regions. The short-range lattice tailoring of the Pd framework to accommodate interstitial B dopants in the octahedral sites of the distorted FCC structure can be imaged by electron ptychography. This study describes new toolsets that enable the characterization of industrial metal nanocatalysts across length scales from macro- to microanalysis, which gives important guidance to the structure-activity relationship of the system.

Atomic-Scale View of the Oxidation and Reduction of Supported Ultrathin FeO Islands

By means of scanning tunneling microscopy (STM) measurements, we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O₂) and hydrogen (H₂), respectively. The FeO islands respond very dynamically toward O₂, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O₂ dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O₂ molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites. In contrast, we found that H₂ molecules only dissociate when vibrationally excited through the ion gauge and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H₂ dissociation. Our STM results reveal how excess O atoms are incorporated and released in O₂ and H₂ and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.