Relationship between Iron Carbide Phases (ε-Fe2C, Fe7C3, and χ-Fe5C2) and Catalytic Performances of Fe/SiO2 Fischer–Tropsch Catalysts

Efficient conversion of syngas to linear α-olefins by phase-pure χ-Fe5C2

Oil has long been the dominant feedstock for producing fuels and chemicals, but coal, natural gas and biomass are increasingly explored alternatives1-3. 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 present4-8. 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 efficiency9-14. 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 mixtures9-14. 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 (refs. 12-15), 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.

Effects of surface hydrophobization on the phase evolution behavior of iron-based catalyst during Fischer-Tropsch synthesis

Iron-based Fischer-Tropsch synthesis (FTS) catalyst is widely used for syngas conversion, but its iron carbide active phase is easily oxidized into Fe3O4 by the water produced during reaction, leading to the deterioration of catalytic performance. Here, we show an efficient strategy for protecting the iron carbide active phase of FTS catalyst by surface hydrophobization. The hydrophobic surface can reduce the water concentration in the core vicinity of catalyst during syngas conversion, and thus inhibit the oxidation of iron species by water, which enhances the C - C coupling ability of catalyst and promotes the formation of long-chain olefins. More significantly, it is unraveled that appropriate shell thickness plays a crucial role in stabilizing the iron carbide active phase without Fe3O4 formation and achieving good catalytic performance.

Machine learning predicts atomistic structures of multielement solid surfaces for heterogeneous catalysts in variable environments

Solid surfaces usually reach thermodynamic equilibrium through particle exchange with their environment under reactive conditions. A prerequisite for understanding their functionalities is detailed knowledge of the surface composition and atomistic geometry under working conditions. Owing to the large number of possible Miller indices and terminations involved in multielement solids, extensive sampling of the compositional and conformational space needed for reliable surface energy estimation is beyond the scope of ab initio calculations. Here, we demonstrate, using the case of iron carbides in environments with varied carbon chemical potentials, that the stable surface composition and geometry of multielement solids under reactive conditions, which involve large compositional and conformational spaces, can be predicted at ab initio accuracy using an approach that combines the bond valence model, Gaussian process regression, and ab initio thermodynamics. Determining the atomistic structure of surfaces under working conditions paves the way toward identifying the true active sites of multielement catalysts in heterogeneous catalysis.

An optimal Fe-C coordination ensemble for hydrocarbon chain growth: a full Fischer-Tropsch synthesis mechanism from machine learning

Fischer-Tropsch synthesis (FTS, CO + H2 → long-chain hydrocarbons) because of its great significance in industry has attracted huge attention since its discovery. For Fe-based catalysts, after decades of efforts, even the product distribution remains poorly understood due to the lack of information on the active site and the chain growth mechanism. Herein powered by a newly developed machine-learning-based transition state (ML-TS) exploration method to treat properly reaction-induced surface reconstruction, we are able to resolve where and how long-chain hydrocarbons grow on complex in situ-formed Fe-carbide (FeCx) surfaces from thousands of pathway candidates. Microkinetics simulations based on first-principles kinetics data further determine the rate-determining and the selectivity-controlling steps, and reveal the fine details of the product distribution in obeying and deviating from the Anderson-Schulz-Flory law. By showing that all FeCx phases can grow coherently upon each other, we demonstrate that the FTS active site, namely the A-P5 site present on reconstructed Fe3C(031), Fe5C2(510), Fe5C2(021), and Fe7C3(071) terrace surfaces, is not necessarily connected to any particular FeCx phase, rationalizing long-standing structure-activity puzzles. The optimal Fe-C coordination ensemble of the A-P5 site exhibits both Fe-carbide (Fe4C square) and metal Fe (Fe3 trimer) features.

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.

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.

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.

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.

Multiple CO2 reduction mediated by heteronuclear metal carbide cluster anions RhTaC2. Dalton Trans 2022;51:11491-11498. [PMID: 35833563 DOI: 10.1039/d2dt01612e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Noble metals dispersed on transition-metal carbides exhibit extraordinary activity in CO₂ catalytic conversion and bimetallic carbides generated at the interface were proposed to contribute to the observed activity. Heteronuclear metal carbide clusters (HMCCs) that compositionally resemble the bimetallic carbides are suitable models to get a fundamental understanding of the reactivity of the related condensed-phase catalysts, while the reaction of HMCCs with CO₂ has not been touched in the gas phase. Herein, benefiting from the newly designed double ion trap reactors, the reaction of laser-ablation generated and mass-selected RhTaC₂^- clusters with CO₂ was studied. The experimental results identified that RhTaC₂^- can reduce four CO₂ molecules consecutively and generate the product RhTaC₂O₄^-. The pivotal roles of Rh-Ta synergy and the C₂ ligand in driving CO₂ reduction were rationalized by theoretical calculations. The presence of an attached CO unit on the product RhTaC₂O₄^- was evidenced by the collision-induced dissociation experiment, providing a fundamental strategy to alleviate carbon deposition under a CO₂ atmosphere at elevated temperatures.

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.

Direct Construction of K-Fe3C@C Nanohybrids Utilizing Waste Biomass of Pomelo Peel as High-Performance Fischer–Tropsch Catalysts

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.

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.

Production of Light Olefins via Fischer-Tropsch Process Using Iron-Based Catalysts: A Review

The production of light olefins, as the critical components in chemical industries, is possible via different technologies. The Fischer–Tropsch to olefin (FTO) process aims to convert syngas to light olefins with high selectivity over a proper catalyst, reduce methane formation, and avoid the production of excess CO2. This review describes the production of light olefins through the FTO process using both unsupported and supported iron-based catalysts. The catalytic properties and performances of both the promoted and bimetallic unsupported catalysts are reviewed. The effect of support and its physico-chemical properties on the catalyst activity are also described. The proper catalyst should have high stability to provide long-term performance without reducing the activity and selectivity towards the desired product. The good dispersion of active metals on the surface, proper porosity, optimized metal-support interaction, a high degree of reducibility, and providing a sufficient active phase for the reaction are important parameters affecting the reaction. The selection of the suitable catalyst with enhanced activity and the optimum process conditions can increase the possibility of the FTO reaction for light-olefins production. The production of light olefins via the FTO process over iron-based catalysts is a promising method, as iron is cheap, shows higher resistance to sulfur, and has a higher WGS activity which can be helpful for the feed gas with a low H2/CO ratio, and also has higher selectivity towards light olefins.

Selective synthesis of core-shell structured catalyst χ-Fe5C2 surrounded by nanosized Fe3O4 for conversion of syngas to liquid fuels

Enhancing liquid hydrocarbons selectivity and simultaneously suppressing CO2 formation are highly desirable yet challenges in iron-based Fischer-Tropsch synthesis. Herein, we report an in-situ oxidation method for the fabrication of a...

PBA-derived high-efficiency iron-based catalysts for CO2 hydrogenation

The PBA-derived iron based catalyst effectively converts CO2 to hydrocarbons, especially C5+ hydrocarbons.

Selective olefin production on silica based iron catalysts in Fischer–Tropsch synthesis

Mixed phases of Fe3O4 and Fe5C2, interacting properly with SiO2, produce highly selective olefins from syngas.

Effect of the Zr promoter on precipitated iron-based catalysts for high-temperature Fischer–Tropsch synthesis of light olefins

FeMnxZr and FeMnxZr2Na catalysts prepared by coprecipitation and impregnation methods were applied to investigate the promoting effects of Zr on iron-based catalysts for high-temperature Fischer–Tropsch synthesis (HTFT).

Fischer–Tropsch Synthesis: Effect of the Promoter’s Ionic Charge and Valence Level Energy on Activity

In this contribution, we examine the effect of the promoter´s ionic charge and valence orbital energy on the catalytic activity of Fe-based catalysts, based on in situ synchrotron X-ray powder diffraction (SXRPD), temperature-programmed-based techniques (TPR, TPD, CO-TP carburization), and Fischer–Tropsch synthesis catalytic testing studies. We compared the promoting effects of K (a known promoter for longer-chained products) with Ba, which has a similar ionic radius but has double the ionic charge. Despite being partially “buried” in a crystalline BaCO3 phase, the carburization of the Ba-promoted catalyst was more effective than that of K; this was primarily due to its higher (2+) ionic charge. With Ba2+, higher selectivity to methane and lighter products were obtained compared to the K-promoted catalysts; this is likely due to Ba´s lesser capability of suppressing H adsorption on the catalyst surface. An explanation is provided in terms of a more limited mixing between electron-filled Ba2+ 5p and partially filled Fe 3d orbitals, which are expected to be important for the chemical promotion, as they are further apart in energy compared to the K+ 3p and Fe 3d orbitals.

Fischer–Tropsch Synthesis for Light Olefins from Syngas: A Review of Catalyst Development

Light olefins as one the most important building blocks in chemical industry can be produced via Fischer–Tropsch synthesis (FTS) from syngas. FT synthesis conducted at high temperature would lead to light paraffins, carbon dioxide, methane, and C5+ longer chain hydrocarbons. The present work focuses on providing a critical review on the light olefin production using Fischer–Tropsch synthesis. The effects of metals, promoters and supports as the most influential parameters on the catalytic performance of catalysts are discussed meticulously. Fe and Co as the main active metals in FT catalysts are investigated in terms of pore size, crystal size, and crystal phase for obtaining desirable light olefin selectivity. Larger pore size of Fe-based catalysts is suggested to increase olefin selectivity via suppressing 1-olefin readsorption and secondary reactions. Iron carbide as the most probable phase of Fe-based catalysts is proposed for light olefin generation via FTS. Smaller crystal size of Co active metal leads to higher olefin selectivity. Hexagonal close-packed (HCP) structure of Co has higher FTS activity than face-centered cubic (FCC) structure. Transition from Co to Co3C is mainly proposed for formation of light olefins over Co-based catalysts. Moreover, various catalysts’ deactivation routes are reviewed. Additionally, techno-economic assessment of FTS plants in terms of different costs including capital expenditure and minimum fuel selling price are presented based on the most recent literature. Finally, the potential for global environmental impacts associated with FTS plants including atmospheric and toxicological impacts is considered via lifecycle assessment (LCA).

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.

Cobalt Carbide Nanocatalysts for Efficient Syngas Conversion to Value-Added Chemicals with High Selectivity

Syngas conversion is a key platform for efficient utilization of various carbon-containing resources including coal, natural gas, biomass, organic wastes, and even CO₂. One of the most classic routes for syngas conversion is Fischer-Tropsch synthesis (FTS), which is already available for commercial application. However, it still remains a grand challenge to tune the product distribution from paraffins to value-added chemicals such as olefins and higher alcohols. Breaking the selectivity limitation of the Anderson-Schulz-Flory (ASF) distribution has been one of the hottest topics in syngas chemistry.Metallic Co⁰ is a well-known active phase for Co-catalyzed FTS, and the products are dominated by paraffins with a small amount of chemicals (i.e., olefins or alcohols). Specifically, a cobalt carbide (Co₂C) phase is typically viewed as an undesirable compound that could lead to deactivation with low activity and high methane selectivity. Although iron carbide (Fe_xC) can produce olefins with selectivity up to ∼60%, the fraction of methane is still rather high, and the required high reaction temperature (300-350 °C) typically causes coke deposition and fast deactivation. Recently, we discovered that Co₂C nanoprisms with preferentially exposed facets of (020) and (101) can effectively produce olefins from syngas conversion under mild reaction conditions with high selectivity. The methane fraction was limited within 5%, and the product distribution deviated greatly from ASF statistic law. The catalytic performances of Co₂C nanoprisms are completely different from that reported for the traditional FT process, exhibiting promising potential industrial application.This Account summarizes our progress in the development of Co₂C nanoprisms for Fischer-Tropsch synthesis to olefins (FTO) with remarkable efficiencies and stability. The underlying mechanism for the observed unique catalytic behaviors was extensively explored by combining DFT calculation, kinetic measurements, and various spectroscopic and microscopic investigation. We also emphasize the following issues: particle size effect of Co₂C, the promotional effect of alkali and Mn promoters, and the role of metal-support interaction (SMI) in fabricating supported Co₂C nanoprisms. Specially, we briefly review the synthetic methods for different Co₂C nanostructures. In addition, Co₂C can also be applied as a nondissociative adsorption center for higher alcohol synthesis (HAS) via syngas conversion. We also discuss the construction of a Co⁰/Co₂C interfacial catalyst for HAS and demonstrate how to tune the reaction network and strengthen CO nondissociative adsorption ability for efficient production of higher alcohols. We believe that the advances in the development of Co₂C nanocatalysts described here present a critic step to produce chemicals through the FTS process.

CO2 hydrogenation over functional nanoporous polymers and metal-organic frameworks

CO₂ is one of the major environmental pollutants and its mitigation is attracting huge attention over the years due to continuous increase in this greenhouse gas emission in the atmosphere. Being environmentally hazardous and plentiful presence in nature, CO₂ utilization as C1 resource into fuels and feedstock is very demanding from the green chemistry perspectives. To accomplish this CO₂ utilization issue, functional organic materials like porous organic polymers (POPs), covalent organic frameworks (COFs) as well as organic-inorganic hybrid materials like metal-organic frameworks (MOFs), having characteristics of large surface area, high thermal stability and tunability in the porous nanostructures play significant role in designing the suitable catalyst for the CO₂ hydrogenation reactions. Although CO₂ hydrogenation is a widely studied and emerging area of research, till date review exclusively focused on designing POPs, COFs and MOFs bearing reactive functional groups is very limited. A thorough literature review on this matter will enrich our knowledge over the CO₂ hydrogenation processes and the catalytic sites responsible for carrying out these chemical transformations. We emphasize recent state-of-the art developments in POPs/COFs/MOFs having unique functionalities and topologies in stabilizing metallic NPs and molecular complexes for the CO₂ reduction reactions. The major differences between MOFs and porous organics are critically summarized in the outlook section with the aim of the future benefit in mitigating CO₂ emission from ambient air.

Effects of Structure and Particle Size of Iron, Cobalt and Ruthenium Catalysts on Fischer–Tropsch Synthesis

This review emphasizes the importance of the catalytic conversion techniques in the production of clean liquid and hydrogen fuels (XTF) and chemicals (XTC) from the carbonaceous materials including coal, natural gas, biomass, organic wastes, biogas and CO2. Dependence of the performance of Fischer–Tropsch Synthesis (FTS), a key reaction of the XTF/XTC process, on catalyst structure (crystal and size) is comparatively examined and reviewed. The contribution illustrates the very complicated crystal structure effect, which indicates that not only the particle type, but also the particle shape, facets and orientation that have been evidenced recently, strongly influence the catalyst performance. In addition, the particle size effects over iron, cobalt and ruthenium catalysts were carefully compared and analyzed. For all Fe, Co and Ru catalysts, the metal turnover frequency (TOF) for CO hydrogenation increased with increasing metal particle size in the small size region i.e., less than the size threshold 7–8 nm, but was found to be independent of particle size for the catalysts with large particle sizes greater than the size threshold. There are some inconsistencies in the small particle size region for Fe and Ru catalysts, i.e., an opposite activity trend and an abnormal peak TOF value were observed on a Fe catalyst and a Ru catalyst (2 nm), respectively. Further study from the literature provides deeper insights into the catalyst behaviors. The intrinsic activity of Fe catalysts (10 nm) at 260–300 °C is estimated in the range of 0.046–0.20 s−1, while that of the Co and Ru catalysts (7–70 nm) at 220 °C are 0.1 s−1 and 0.4 s−1, respectively.