Graphene-Supported, Iron-Based Nanoparticles for Catalytic Production of Liquid Hydrocarbons from Synthesis Gas: The Role of the Graphene Support in Comparison with Carbon Nanotubes

Exploring Simultaneous Upgrading and Purification of Biomass−Gasified Gases Using Plasma Catalysis

Tar and substantial CH4 and CO2 are contained in gasified fuels, which pose an obstacle to direct chemical synthesis, and this is a predominant challenge for biomass gasification technology. Herein, a packed−bed dielectric barrier discharge (DBD) reactor was built for simultaneous CH4 dry reforming and tar removal with a La−Ni/γ−Al2O3 catalyst. The interaction between CH4 dry reforming and tar removal in plasma catalysis was investigated. The results indicated that plasma catalysis can achieve high−efficiency simultaneous tar removal and CH4 dry reforming, as indicated by the reactants’ conversion (14% increase for CCH4 and CCO2 at 450 °C in the presence of tar and a 37% increase for the tar removal rate at 360 °C when CH4 and CO2 were introduced), and the mechanism for mutual promotion of CH4 dry reforming and tar removal was elucidated through catalyst characterization results. In addition, a possible reaction mechanism for tar removal via plasma catalysis was proposed. These findings provide valuable insights for simultaneous upgrading and purification of gases generated by biomass gasification.

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...

Selective Conversion of Syngas to Olefins via Novel Cu-Promoted Fe/RGO and Fe-Mn/RGO Fischer-Tropsch Catalysts: Fixed-Bed Reactor vs Slurry-Bed Reactor

Fischer-Tropsch has become an indispensable choice in the gas-to-liquid conversion reactions to produce a wide range of petrochemicals using recently emerging biomass or other types of feedstock such as coal or natural gas. Herein we report the incorporation of novel Cu nanoparticles with two Fischer-Tropsch synthesis (FTS) catalytic systems, Fe/reduced graphene oxide (rGO) and Fe-Mn/rGO, to evaluate their FTS performance and olefin productivity in two types of reactors: slurry-bed reactor (SBR) and fixed-bed reactor (FBR). Four catalysts were compared and investigated, namely Fe, FeCu7, FeMn10Cu7, and FeMn16, which were highly dispersed over reduced graphene oxide nanosheets. The catalysts were first characterized by transmission electron microscopy (TEM), nitrogen physisorption, X-ray fluorescence (XRF), X-ray diffraction (XRD), and H-TPR techniques. In the SBR, Cu enhanced olefinity only when used alone in FeCu7 without Mn promotion. When used with Mn, the olefin yield was not changed, but light olefins decreased slightly at the expense of heavier olefins. In the FBR system, Cu as a reduction promoter improved the catalyst activity. It increased the olefin yield mainly due to increased activity, even if the CO2 decreased by the action of Cu promoters. The olefinity of the product was improved by Cu promotion but it did not exceed the landmark made by FeMn16 at 320 °C. The paraffinity was also enhanced by Cu promotion especially in the presence of Mn, indicating a strong synergistic effect. Cu was found to be better than Mn in enhancing the paraffin yield, while Mn is a better olefin yield enhancer. Finally, Cu promotion was found to enhance the selectivity towards light olefins C2-4. This study gives a deep insight into the effect of different highly dispersed FTS catalyst systems on the olefin hydrocarbon productivity and selectivity in two major types of FTS reactors.

In situ electrogeneration and activation of H2O2 by atomic Fe catalysts for the efficient removal of chloramphenicol

Heterogeneous electron-Fenton processes have been regarded as promising, environmentally friendly techniques for the removal of refractory organics. A new strategy has been brought forward for an electron-Fenton-like process with in situ H₂O₂ production, but regarding the catalysts, their geometric stability, H₂O₂ selectivity, and applicability under high pH values still need to be improved. Herein, bifunctional catalysts were proposed for a heterogeneous Fenton-like reaction by introducing Fe atoms into defect-enriched graphene sheets (Fe/N-DG). The structural and compositional results suggested that the excellent dispersing stability of Fe atoms is mainly attributed to the abundant pyridinic-N sites. Optimized Fe₁/N-DG exhibited superior mass activity (5.28 A mg_Fe^-1 at 0.6 V vs. RHE) and H₂O₂ selectivity (86%) under the synergistic effects of Fe‒N and Fe‒O sites. The Fe/N-DG catalysts maintained superior activities for chloramphenicol removal, even under extreme pH conditions (pH≤4 or pH≥10). Of these catalysts, Fe₁/N-DG with a predominant Fe-N structure exhibited the best catalytic performance, achieving the complete removal of chloramphenicol within 180 min under alkaline conditions. The possible mechanism for chloramphenicol removal under alkaline conditions was proposed, along with those for the production and activation of H₂O₂. This study gives new insights into atomic Fe-based catalysts exhibiting excellent selectivity and stability for antibiotic wastewater treatment.

Carbon-based catalysts for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (FTS) is an essential approach to convert coal, biomass, and shale gas into fuels and chemicals, such as lower olefins, gasoline, diesel, and so on. In recent years, there has been increasing motivation to deploy FTS at commercial scales which has been boosting the discovery of high performance catalysts. In particular, the importance of support in modulating the activity of metals has been recognized and carbonaceous materials have attracted attention as supports for FTS. In this review, we summarised the substantial progress in the preparation of carbon-based catalysts for FTS by applying activated carbon (AC), carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon spheres (CSs), and metal-organic frameworks (MOFs) derived carbonaceous materials as supports. A general assessment of carbon-based catalysts for FTS, concerning the support and metal properties, activity and products selectivity, and their interactions is systematically discussed. Finally, current challenges and future trends in the development of carbon-based catalysts for commercial utilization in FTS are proposed.

3D Graphene Materials: From Understanding to Design and Synthesis Control

Carbon materials, with their diverse allotropes, have played significant roles in our daily life and the development of material science. Following 0D C₆₀ and 1D carbon nanotube, 2D graphene materials, with their distinctively fascinating properties, have been receiving tremendous attention since 2004. To fulfill the efficient utilization of 2D graphene sheets in applications such as energy storage and conversion, electrochemical catalysis, and environmental remediation, 3D structures constructed by graphene sheets have been attempted over the past decade, giving birth to a new generation of graphene materials called 3D graphene materials. This review starts with the definition, classifications, brief history, and basic synthesis chemistries of 3D graphene materials. Then a critical discussion on the design considerations of 3D graphene materials for diverse applications is provided. Subsequently, after emphasizing the importance of normalized property characterization for the 3D structures, approaches for 3D graphene material synthesis from three major types of carbon sources (GO, hydrocarbons and inorganic carbon compounds) based on GO chemistry, hydrocarbon chemistry, and new alkali-metal chemistry, respectively, are comprehensively reviewed with a focus on their synthesis mechanisms, controllable aspects, and scalability. At last, current challenges and future perspectives for the development of 3D graphene materials are addressed.

Study of the Reduction of Fe on Reduced Graphene Oxide as a Catalyst for Carbon Monoxide Reduction

This work aims at optimizing the H2 reduction time of Fe/rGO as a preparatory step for the use of the reduced catalyst in Fisher-Tropsch synthesis (FTS). The catalytic system used was Iron Nanoparticles (NPs) loaded on reduced graphene oxide (rGO) support. The as prepared sample was analyzed by TEM, FTIR and XRD spectroscopy. Samples of the produced Fe/rGO catalyst were used to optimize the reduction conditions in the FBR reactor. The three samples were reduced under 1atm H2 gas flow of 50 sccm at 500°C for 8, 12 and 24 hrs. The samples were collected after reduction and analyzed by XRD, FTIR and TEM imaging. The best condition showing full reduction with minimal sintering was at 12hr.

Shedding Light Onto the Nature of Iron Decorated Graphene and Graphite Oxide Nanohybrids for CO2 Conversion at Atmospheric Pressure

We report on the design and testing of new graphite and graphene oxide-based extended π-conjugated synthetic scaffolds for applications in sustainable chemistry transformations. Nanoparticle-functionalised carbonaceous catalysts for new Fischer Tropsch and Reverse GasWater Shift (RGWS) transformations were prepared: functional graphene oxides emerged from graphite powders via an adapted Hummer's method and subsequently impregnated with uniform-sized nanoparticles. Then the resulting nanomaterials were imaged by TEM, SEM, EDX, AFM and characterised by IR, XPS and Raman spectroscopies prior to incorporation of Pd(II) promoters and further microscopic and spectroscopic analysis. Newly synthesised 2D and 3D layered nanostructures incorporating carbon-supported iron oxide nanoparticulate pre-catalysts were tested, upon hydrogen reduction in situ, for the conversion of CO2 to CO as well as for the selective formation of CH4 and longer chain hydrocarbons. The reduction reaction was also carried out and the catalytic species isolated and fully characterised. The catalytic activity of a graphene oxide-supported iron oxide pre-catalyst converted CO2 into hydrocarbons at different temperatures (305, 335, 370 and 405 °C), and its activity compared well with that of the analogues supported on graphite oxide, the 3-dimensional material precursor to the graphene oxide. Investigation into the use of graphene oxide as a framework for catalysis showed that it has promising activity with respect to reverse gas water shift (RWGS) reaction of CO2 to CO, even at the low levels of catalyst used and under the rather mild conditions employed at atmospheric pressure. Whilst the γ-Fe2O3 decorated graphene oxide-based pre-catalyst displays fairly constant activity up to 405 °C, it was found by GC-MS analysis to be unstable with respect to decomposition at higher temperatures. The addition of palladium as a promoter increased the activity of the iron functionalised graphite oxide in the RWGS. The activity of graphene oxide supported catalysts was found to be enhanced with respect to that of iron-functionalised graphite oxide with, or without palladium as a promoter, and comparable to that of Fe@carbon nanotube-based systems tested under analogous conditions. These results display a significant step forward for the catalytic activity estimations for the iron functionalised and rapidly processable and scalable graphene oxide. The hereby investigated phenomena are of particular relevance for the understanding of the intimate surface morphologies and the potential role of non-covalent interactions in the iron oxide-graphene oxide networks, which could inform the design of nano-materials with performance in future sustainable catalysis applications.

Highly dispersed cobalt Fischer–Tropsch synthesis catalysts supported on γ-Al2O3, CNTs, and graphene nanosheet using chemical vapor deposition

Highly dispersed 15.0 wt% cobalt catalysts were prepared on γ-Al2O3, carbon nanotubes (CNTs), and graphene nanosheet (GNS) using chemical vapor deposition (CVD) procedure. The physico-chemical properties of the catalysts were studied by inductively coupled plasma (ICP), Brunauer–Emmett–Teller (BET), X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and temperature-programmed reduction (TPR) techniques, and the Fischer–Tropsch synthesis (FTS) performance of the catalysts was assessed at 220 °C, 18 bar, H2/CO = 2 and feed flow rate of 45 ml/min g cat. Based on BET results, Co/GNS catalyst provided highest surface area in comparison to the other catalysts. XRD and FESEM results revealed that CVD method prepared smaller particles on GNS compared to the other supports and resulted in the most dispersed metal particles on GNS according to H2-chemisorption results. The performance of Co/Al2O3 catalyst prepared by CVD method was compared with conventional 15 wt% Co/Al2O3 catalyst prepared by impregnation method. The Co/Al2O3 catalyst prepared with CVD method showed 5.3% higher %CO conversion and 2.1% lower C5+ selectivity as compared with the Co/Al2O3 catalysts prepared by impregnation method. Among three catalysts prepared by CVD, Co/GNS showed higher %CO conversion of 78.4% and C5+ selectivity of 70.3%. Co/γ-Al2O3 catalyst showed higher stability.

Graphene/nickel oxide nanocomposites against isolated ESBL producing bacteria and A549 cancer cells

The synthesis of nickel oxide nanoparticles (NiO NPs) and graphene/nickel oxide nanocomposites (Gr/NiO NCs) was performed using a simple chemical reduction method. Powder X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used to examine the crystalline nature and thermal stability of the synthesized NiO NPs and Gr/NiO NCs, respectively. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to observe the morphology of NiO NPs and Gr/NiO NCs and estimate their size range. TEM suggested that the NiO NPs were speared onto the surface of Gr nanosheet. The efficiency of NiO NPs and Gr/NiO NCs against extended spectrum β-lacamase (ESBL) producing bacteria, which was confirmed by specific HEXA disc Hexa G-minus 24 (HX-096) and MIC strip methods (CLSI); namely Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa) was investigated using the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) methods. MIC results suggested that the NiO NPs and Gr/NiO NCs possess maximum growth inhibition of 86%, 82% and 94%, 92% at 50 and 30 μg/mL concentrations, respectively. Similarly, both nanomaterials were found to inhibit the β-lacamase enzyme at concentrations of 60 μg/mL and 40 μg/mL, respectively. The cytotoxicity of NiO NPs and Gr/NiO NCs was quantified against A549 human lung cancer cells. Cell death percentage values of 52% at 50 μg/mL against NiO NPs and 54% at 20 μg/mL against Gr/NiO NCs were obtained, respectively. The NCs were found to reduce cell viability, increase the level of reactive oxygen species (ROS) and modify both the mitochondrial membrane permeability and cell cycle arrest.

Effect of a potassium promoter on the Fischer–Tropsch synthesis of light olefins over iron carbide catalysts encapsulated in graphene-like carbon

Enhanced FTO catalyst performance and catalyst stability are achieved over a graphene-like carbon encapsulated iron carbide catalyst, which is prepared by a facile pyrolysis method.

Fe3O4 nanocubes assembled on RGO nanosheets: Ultrasound induced in-situ and eco-friendly synthesis, characterization and their excellent catalytic performance for the production of liquid fuel in Fischer-tropsch synthesis

In this study, Fe₃O₄ nanocubes (NCs) decorated on RGO nanosheets (NSs) structures were successfully synthesized through an innovative and environmentally-friendly rapid sonochemical method. More importantly, iron(II) sulfate heptahydrate and GO were employed as precursors and water as reaction medium, meanwhile, NaOH within the generated free radicals from the high intensity ultrasound were sufficient as reducing and base agent in our clean synthesis. Moreover, the hydrothermal method as a conventional approach was employed to synthesize the same catalysts for the comparison with the ultrasonocation technique. The as-synthesized Fe₃O₄ and RGO/Fe₃O₄ NSs catalysts were exposed to industrially relevant Fischer-tropsch synthesis (FTS) conditions at various reaction temperatures (250-290 °C), and they subjected to fully characterization before and after FTS reaction using XRD, TEM, HRTEM, EDS mapping, XPS, FTIR, BET, H₂-TPR, H₂-TPD and CO-TPD to understand the structure-performance relationships. Notably, the catalysts produced using the sonochemical method had a better CO conversion rate [Fe₃O₄ (80%), RGO/Fe₃O₄ (82%)] than the hydrothermally synthesized catalysts. However, compared to the naked-Fe₃O₄ catalysts, the sonochemically and hydrothermally synthesized RGO-supported Fe₃O₄ catalysts had higher long chain hydrocarbon (C5+) selectivity values (72% and 67%) and C₂-C₄ olefin/paraffin selectivity ratio (3.2 and 2) and low CH4 selectivity values (6% and 8.5%), respectively. This can be attributed to their high surface area, the degree of reducibility, and content of Hägg iron carbide (χ-Fe₅C₂) as the most active phase of the FTS reaction. Proposed reaction mechanisms for the sonochemical and hydrothermal reaction synthesis of Fe₃O₄ and RGO/Fe₃O₄ nanoparticles are discussed. In conclusion, our developed surfactantless-sonochemical method holds promise for the eco-friendly synthesis of highly efficient catalysts materials for FTS reaction.

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.

Utilizing FBR to produce olefins from CO reduction using Fe–Mn nanoparticles on reduced graphene oxide catalysts and comparing the performance with SBR

Mn was used as a promoter for Fe nanoparticles (NPs) loaded on reduced graphene oxide (rGO). The prepared catalysts were the unpromoted Fe/rGO catalysts along with two Mn promoted catalysts FeMn16 and FeMn29. These catalysts were used as Fischer–Tropsch catalysts in a Fixed Bed Reactor (FBR). The operating conditions of the reactor, namely temperature, pressure and space velocity, were varied to evaluate the catalyst performance and the olefin productivity. The olefins were produced in maximum yields of 34.5% and 31.3% with FeMn29 at 320 and 340 °C respectively. The ratio of light to heavy olefins was three times higher at 340 °C. The catalysts showed good stability up to 50 h of interrupted operation while varying the conditions at each interruption. The performance of the catalysts in the FBR was compared with a previous investigation carried out in an SBR under identical conditions with the same catalysts. The FBR was found to be more Mn tolerant than the SBR, giving very high conversion activity with high Mn concentrations (FeMn29). The FBR produced olefins in much higher yields than the SBR. The SBR was more selective to light olefins at low temperatures and high Mn loading levels, while the FBR produced light olefins at higher selectivities at high temperatures and high Mn concentrations.

Producing olefin rich products from the FTS reaction in both FBR and SBR reactors using Fe–Mn/rGO catalysts.

Enhanced Fischer–Tropsch performances of graphene oxide-supported iron catalysts via argon pretreatment

Argon pretreatment is used to modify the properties of graphene-supported iron catalysts with the purpose of enhancing FTS performances.

Precursor controlled synthesis of graphene oxide supported iron catalysts for Fischer–Tropsch synthesis

Iron precursors are used to tune the structure and FTS performance of graphene oxide supported iron catalysts.

Mn–Fe nanoparticles on a reduced graphene oxide catalyst for enhanced olefin production from syngas in a slurry reactor

Fe nanoparticles (NPs) supported on reduced graphene oxide (rGO) nano-sheets were promoted with Mn and used for the production of light olefins in Fischer–Tropsch reactions carried out in a slurry bed reactor (SBR). The prepared catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, N₂ physisorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopic (XPS) methods. Mn was shown to preferentially migrate to the Fe NP surface, forming a Mn-rich shell encapsulating a core rich in Fe. The Mn shell regulated the diffusion of molecules to and from the catalyst core, and preserved the metallic Fe phase by lowering magnetite formation and carburization, so decreasing water gas shift reaction (WGSR) activity and CO conversion, respectively. Furthermore, the Mn shell reduced H₂ adsorption and increased CO dissociative adsorption which enhanced olefin selectivity by limiting hydrogenation reactions. Modification of the Mn shell thickness regulated the catalytic activity and olefin selectivity. Simultaneously the weak metal–support interaction further increased the migration ability owing to the utilization of a graphene-based support. Space velocities, pressures and operating temperatures were also tested in the reactor to further enhance light olefin production. A balanced Mn shell thickness produced with a Mn concentration of 16 mol Mn/100 mol Fe was found to give a good olefin yield of 19% with an olefin/paraffin (O/P) ratio of 0.77. Higher Mn concentrations shielded the active sites and reduced the conversion dramatically, causing a fall in olefin production. The optimum operating conditions were found to be 300 °C, 2 MPa and 4.2 L g⁻¹ h⁻¹ of 1 : 1 H₂ : CO syngas flow; these gave the olefin yield of 19%.

Mn acted as a promoter by forming a Mn-rich layer around a core rich in Fe. The outer layer hindered the formation of magnetite, and impeded H₂ adsorption whilst encouraging CO dissociative adsorption, which gave the perfect conditions for olefin production.

Design and Fabrication of Highly Reducible PtCo Particles Supported on Graphene-Coated ZnO

Cobalt particles dispersed on an oxide support form the basis of many important heterogeneous catalysts. Strong interactions between cobalt and the support may lead to irreducible cobalt oxide formation, which is detrimental for the catalytic performance. Therefore, several strategies have been proposed to enhance cobalt reducibility, such as alloying with Pt or utilization of nonoxide supports. In this work, we fabricate bimetallic PtCo supported on graphene-coated ZnO with enhanced cobalt reducibility. By employing a model/planar catalyst formulation, we show that the surface reduction of cobalt oxide is substantially enhanced by the presence of the graphene support as compared to bare ZnO. Stimulated by these findings, we synthesized a realistic powder catalyst consisting of PtCo particles grafted on graphene-coated ZnO support. We found that the addition of graphene coating enhances the surface reducibility of cobalt, fully supporting the results obtained on the model system. Our study demonstrates that realistic catalysts with designed properties can be developed on the basis of insights gained from model catalytic formulation.

Facile Fabrication of BCN Nanosheet-Encapsulated Nano-Iron as Highly Stable Fischer-Tropsch Synthesis Catalyst

The few layered boron carbon nitride nanosheets (BCNNSs) have attracted widespread attention in the field of heterogeneous catalysis. Herein, we report an innovative one-pot route to prepare the catalyst of BCNNSs-encapsulated sub-10 nm highly dispersed nanoiron particles. Then the novel catalyst was used in Fischer-Tropsch synthesis for the first time and it exhibited high activity and superior stability. At a high temperature of 320 °C, CO conversion could reach 88.9%, corresponding catalytic activity per gram of iron (iron time yield, FTY) of 0.9 × 10^-4 mol_CO g_Fe^-1 s^-1, more than 200 times higher than that of pure iron. Notably, no obvious deactivation was observed after 1000 h running. The enhanced stability of the catalyst can be ascribed to the special encapsulated structure. Furthermore, the formation mechanism of highly dispersed iron nanoparticle also was elaborated. This approach opens the way to designing metal nanoparticles with both high stability and reactivity for nanocatalysts in hydrogenation application.

Pd–ZnO nanowire arrays as recyclable catalysts for 4-nitrophenol reduction and Suzuki coupling reactions

Hybrid Pd–ZnO nanowire arrays for catalysis: Pd–ZnO@Zn nanowire arrays have been found to be applicable as recyclable catalysts for 4-nitrophenol reduction and Suzuki coupling reactions.

Two-dimensional graphene-directed formation of cylindrical iron carbide nanocapsules for Fischer–Tropsch synthesis

Cylindrical Hägg carbide nanocapsules with a single (510) crystal facet were formed during the FTS reaction with the assistance of GO, which show excellent activity and selectivity for olefins.

Highly selective production of heavy hydrocarbons over cobalt–graphene–silica nanocomposite catalysts

Cobalt–graphene–silica nanocomposites catalysts were applied in FTS and showed highly selective production of heavy hydrocarbons.

Size and Promoter Effects on Stability of Carbon-Nanofiber-Supported Iron-Based Fischer-Tropsch Catalysts

The Fischer–Tropsch Synthesis converts synthesis gas from alternative carbon resources, including natural gas, coal, and biomass, to hydrocarbons used as fuels or chemicals. In particular, iron-based catalysts at elevated temperatures favor the selective production of C₂–C₄ olefins, which are important building blocks for the chemical industry. Bulk iron catalysts (with promoters) were conventionally used, but these deactivate due to either phase transformation or carbon deposition resulting in disintegration of the catalyst particles. For supported iron catalysts, iron particle growth may result in loss of catalytic activity over time. In this work, the effects of promoters and particle size on the stability of supported iron nanoparticles (initial sizes of 3–9 nm) were investigated at industrially relevant conditions (340 °C, 20 bar, H₂/CO = 1). Upon addition of sodium and sulfur promoters to iron nanoparticles supported on carbon nanofibers, initial catalytic activities were high, but substantial deactivation was observed over a period of 100 h. In situ Mössbauer spectroscopy revealed that after 20 h time-on-stream, promoted catalysts attained 100% carbidization, whereas for unpromoted catalysts, this was around 25%. In situ carbon deposition studies were carried out using a tapered element oscillating microbalance (TEOM). No carbon laydown was detected for the unpromoted catalysts, whereas for promoted catalysts, carbon deposition occurred mainly over the first 4 h and thus did not play a pivotal role in deactivation over 100 h. Instead, the loss of catalytic activity coincided with the increase in Fe particle size to 20–50 nm, thereby supporting the proposal that the loss of active Fe surface area was the main cause of deactivation.