Fischer-Tropsch Synthesis Performance Research Articles

High throughput screening of promoted bimetallic catalysts supported on alumina for production of light olefins via Fischer-Tropsch synthesis with artificial neural network and response surface methodology

The production of light olefins via Fischer-Tropsch Synthesis (FTS) was investigated by both experimental and modeling studies. Experimental studies were conducted by employing promoted FeCo/α-Al2O3 (FeCo) bimetallic catalysts. The Response Surface Methodology (RSM) and Artificial Neural Network (ANN) methods were used in modeling. Specifically, effects of the types and concentration of promoters on the catalytic performances of α-Al2O3 supported iron-cobalt bimetallic catalysts are investigated by using Ce, Ni, La, Ho, Ga Cu, Mn, and Zn used as promoters. A total of 49 catalysts were prepared by co-impregnation method and tested at 310 °C and 1 bar in a high-speed catalyst performance test system (HT-CPA). The FTS performances of catalysts showed that both the types and amounts of the promoters can affect light olefin production. The light olefin (C2=–C3=) production of unpromoted FeCo catalyst was about 3.87 × 10−3 (mol C/g active metal. h). The best performances were obtained from FeCo catalysts promoted with Ho, Cu, and Zn. Presence of these promoters increased olefin production up to ∼ 84 %, 76 % and 70 %, respectively.Performance parameters (MSE and R2) of RSM and ANN models revealed that both methods can be used to support experimental studies and provide a procedure for optimizing catalyst formulations yielding the desired product selectivity in FTS processes. The RSM outcomes aligned with experimental findings, suggest that that Ho, Cu, and Zn can serve as suitable promoters for the light olefin production in FTS. ANN model attained higher performance parameters (R2 = 0.95 and MSE = 2E−7) than RSM model (R2 = 0.85 and MSE = 3.4E−7). This indicates that ANN model, which designed as one hidden layer including 7 neurons, can provide better predictions and a fitting capacity. Therefore, it may be used in further optimization and sensitivity analysis approaches. Alignment degree between the experimental and modeling results demonstrates that the proposed methodology can provide a reliable and efficient way to guide experiments to optimize catalyst formulations through identification of potential promoters and their amounts to be used.

Highly effective MFe2O4 (M=Zn, Mg, Cu and Mn) spinel catalysts for Fischer-Tropsch synthesis

A series of spinel catalysts, including ZnFe2O4, MgFe2O4, CuFe2O4, and MnFe2O4, were prepared and applied to the Fischer-Tropsch synthesis (FTS). Zn, Mg, Cu and Mn easily form spinels with Fe. Among them, Zn and Mg can significantly maintain the spinel structure during the pretreatment and reaction, resulting in a low CO conversion. Cu and Mn are beneficial to the formation of iron carbide during the reaction, resulting in an apparent influence on FTS performance. ZnFe2O4 has little effect on the hydrocarbon distribution and the olefin/paraffin (O/P) ratio of C2–C4. MgFe2O4 exhibits low selectivity for C5+ hydrocarbons, and the selectivity of C= 2−C= 4 and the O/P ratio of C2–C4 in the product are increased due to the alkaline effect of Mg. Cu can promote the carbonization of the catalyst, so that CuFe2O4 has higher activity. Meanwhile, CuFe2O4 can significantly improve the selectivity of C5+ hydrocarbons. Moreover, Cu can promote the dissociation and activation of H2, which is beneficial to the secondary hydrogenation of olefins, thereby reducing the selectivity of C= 2−C= 4 and the O/P ratio of C2–C4. Mn promotes carbonization during the reaction, but MnFe2O4 has little effect on the chain growth of hydrocarbon. However, Mn can promote the generation of a certain amount of ɛ-Fe2C, which may explain the higher selectivity of C= 2−C= 4 and the O/P ratio of C2–C4 for MnFe2O4. All spinel catalysts exhibit low CO2 selectivity, which meets the current green environmental protection requirements.

Effects of Co3C formation on the catalytic performance for Fischer-Tropsch synthesis over Co/SiO2 catalysts

Cobalt carbides demonstrate higher selectivity towards light olefins and alcohols in the Fischer-Tropsch synthesis (FTS). In this study, we present an investigation of a series of supported cobalt catalysts that were carburized with CO for different time. The presence of Co3CCo, Co and Co2CCo phases in these catalysts was confirmed through Transmission Electron Microscopy (TEM). The Co/SiO2-RC-0.5 h catalyst with both Co3C and Co structure exhibits the highest CO conversion for the FTS reaction. By utilizing the chemisorption techniques, we studied the structure-performance relationships between the phase and activity, and confirmed that Co3C has the highest intrinsic activity, which is beneficial for CO activation. This study provides further insight into the positive effects of Co3C on the FTS reaction and guides us in the rational design of high active Co-based FTS catalysts.

Eco-friendly solid-state synthesis of Na-promoted Mn-Fe/ZrO2 catalyst for Fischer-Tropsch synthesis

The advancement of large-scale and environmentally-friendly solid-state synthesis methods is of paramount importance for the production of precipitated catalysts, notably Fe-based catalysts utilized in Fischer-Tropsch synthesis (FTS). In this study, we present an innovative solvent-independent, wastewater-free solid-state approach for the synthesis of highly efficient Fe/ZrO2 catalysts, elucidating the mechanisms of promotion within this specific methodology. The prepared catalysts underwent rigorous evaluation under industrially-relevant FTS conditions (230–270 °C, 20 bar, and WHSV = 5400 mL/(gcat⋅h)). A comprehensive suite of characterization techniques was employed to establish the intricate interplay between catalytic activity and structural attributes. We meticulously examined the impact of Na as an alkali promoter and the role of transition metals (Cu and Mn) in enhancing FTS catalytic performance. The catalytic outcomes obtained clearly demonstrate the pivotal role of the synergistic interaction between Mn and Na within the Na-Mn-Fe/ZrO2 catalyst system in driving the FTS reaction. This synergism significantly enhances FTS performance, yielding an impressive CO conversion rate (89.91 %) while maintaining low selectivity to CH4 (7.75 %). The incorporation of Mn and Na into the catalyst matrix enhances the dispersion of Fe2O3, thereby improving the reducibility of Mn/Fe oxides and elevating the electron density on the catalyst surface. These factors facilitate the formation of active Hägg iron carbide (χ-Fe5C2), stimulating chain propagation and ultimately leading to the generation of valuable C5+ hydrocarbon products, which underscores its potential as an optimal candidate for future industrial applications in Fe-based Fischer-Tropsch synthesis.

Enhanced Fischer-Tropsch synthesis performance using graphene Nanosheets-Supported Cobalt-Ruthenium Nanocatalysts: Comparative study with γ-Alumina and carbon nanotubes supports

In this research, graphene Nanosheets (GNS) as Cobalt-Ruthenium Nanocatalysts support have been used in Fischer-Tropsch synthesis. The characterizations of catalysts were evaluated by Fourier-transform infrared spectroscopy (FTIR), Inductively Coupled Plasma (ICP), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), Temperature Programmed Reduction (TPR), and Transmission Electron Microscope (TEM). Afterward, the catalysts' activity, selectivity, and performance were assessed in a fixed-bed reactor at 25 bar. The results were compared by changing the catalyst base to γ-alumina (γ-Al2O3) and carbon nanotubes (CNTs). Measurements were performed in the temperature range of 320––340 °C, the gas ratio (H2/CO) of 2, and the gas hourly space velocity of 3 Lh-1gCat.-1. Using GNS as cobalt catalyst support in Fischer-Tropsch synthesis leads to the 64.5 % and 25.9 % increase in the percentage of CO conversion compared to γ-Al2O3 and CNTs, respectively. Additionally, the utilization of GNS support has resulted in an 11.1 % increase in heavy hydrocarbons selectivity of C5+, in comparison to γ-Al2O3, and a 9.5 % increase compared to CNTs. GNS-based synthesized catalyst with 10 wt% of cobalt and 0.1 wt% of ruthenium (catalyst #6) indicates the highest activity of catalyst at 320 °C with 72.3 % selectivity than heavy hydrocarbons C5+.

Compositional Evolution of Individual CoNPs on Co/TiO2 during CO and Syngas Treatment Resolved through Soft XAS/X-PEEM.

The nanoparticle (NP) redox state is an important parameter in the performance of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Here, the compositional evolution of individual CoNPs (6-24 nm) in terms of the oxide vs metallic state was investigated in situ during CO/syngas treatment using spatially resolved X-ray absorption spectroscopy (XAS)/X-ray photoemission electron microscopy (X-PEEM). It was observed that in the presence of CO, smaller CoNPs (i.e., ≤12 nm in size) remained in the metallic state, whereas NPs ≥ 15 nm became partially oxidized, suggesting that the latter were more readily able to dissociate CO. In contrast, in the presence of syngas, the oxide content of NPs ≥ 15 nm reduced, while it increased in quantity in the smaller NPs; this reoxidation that occurs primarily at the surface proved to be temporary, reforming the reduced state during subsequent UHV annealing. O K-edge measurements revealed that a key parameter mitigating the redox behavior of the CoNPs were proximate oxygen vacancies (Ovac). These results demonstrate the differences in the reducibility and the reactivity of Co NP size on a Co/TiO2 catalyst and the effect Ovac have on these properties, therefore yielding a better understanding of the physicochemical properties of this popular choice of FTS catalysts.

Hydrophobic Fe‐Based Catalyst Derived from Prussian Blue for Enhanced Photothermal Conversion of Syngas to Light Olefins

AbstractAlthough there are many promising works in the field of Fischer–Tropsch synthesis (FTS), it is still a huge challenge to achieve the rational design of FTS catalysts for excellent selectivity toward light olefins with a high olefins/paraffins ratio (o/p ratio). Herein, a hydrophobic core–shell Fe‐based FTS catalyst is developed by calcination and hydrophobic modification of polyvinylpyrrolidone (PVP)‐decorated Prussian blue (PB, Fe4[Fe(CN)6]3) precursor. The hydrophobic modification is achieved by using trimethylchlorosilane (TMCS) as the hydrophobic agent. Under light irradiation and near ambient pressure (0.18 MPa), compared with the catalyst without the hydrophobic surface to mainly convert CO to valueless CO2, the optimal catalyst delivers excellent selectivity of 48.0% for value‐added light olefins (C2–4 = ) with a high o/p ratio of 10.1 but low selectivity for CO2 at a CO conversion of 22.6%. The excellent FTS performance can be attributed to that the hydrophobic surface of the catalyst regulates the chemical reaction pathway, which promotes the CO hydrogenation but suppresses the conversion of CO to CO2. The study demonstrates that a rationally designed functional surface of a catalyst can govern the reaction pathway along value‐added solar‐to‐chemicals conversion.

Formulation of KMoFe/CNTs catalyst for production of light olefins and liquid hydrocarbons via Fischer-Tropsch synthesis: Fixed-bed reactor simulation

Light olefins and C5+ are strategically important products of Fischer-Tropsch synthesis (FTS) over Fe catalysts. In the present work, FTS performance of 0.5K5Mo10Fe/CNTs (DOE 2) and 2 K0.5Mo30Fe/CNTs (DOE 5) catalysts was assessed in terms of CO conversion, C5+ selectivity, and C2-C4 selectivity under industrially relevant FTS conditions (H2/CO = 1.0; GHSV = 2000 h−1; T = 280 ℃; P = 4.13 MPa). The C2-C4 selectivity increased in DOE 2 catalyst to 51.2% with respect to DOE 5 catalyst (2.9%). Meanwhile, the selectivity toward C5+ hydrocarbons decreased from 93.1% in DOE 5 to 8.3% for DOE 2 catalyst. DOE 5 catalyst was more hydrophobic than DOE 2 as confirmed by the water contact angle measurement, lowering CO2 selectivity and water gas shift rate. H2-TPR and CO2-TPD indicated an increase in reducibility and basicity of DOE 5 compared with DOE 2 catalyst, which enhanced iron carbide formation and chain growth probability. The highest amount of metallic Fe in DOE 5 calcined and spent catalysts obtained by XPS and XAS spectra also suggested higher degree of carburization in DOE 5. Additionally, COMSOL Multiphysics software package was employed to provide a 2-dimensional simulation of the FTS fixed-bed reactor. Simulation results were validated using the experimental data for FTS over DOE 2 and DOE 5 catalysts. Effects of temperature on CO conversion as well as distribution of H2, CO, and ethylene were discussed. The simulation illustrated the effects of the fixed-bed reactor wall on both velocity and temperature profiles.

Influence of pretreatment conditions on the structure and catalytic performance of supported cobalt catalysts derived from metal-organic frameworks

Supported cobalt catalysts (Co@C-ZnZrO2 and Co/ZnZrO2) were prepared through a metal-organic frameworks (MOFs)-mediated synthesis strategy. The influence of MOFs pyrolysis on the structure and Fischer-Tropsch synthesis performance of supported cobalt catalysts was investigated. The crystalline phase and microstructure of supported cobalt catalysts were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), N2 adsorption-desorption and X-ray photoelectron spectroscopy (XPS). The Co/ZnZrO2 showed the CO conversion of 18.1% and the C5+ selectivity of 77.4%, whereas the Co@C-ZnZrO2 exhibited the CO conversion of 8.5% and the C5+ selectivity of 35.2%. The excellent CO conversion for Co/ZnZrO2 was attributed to the more exposure of active Co sites. Meanwhile, the activity of Co sites on Co@C-ZnZrO2 catalyst was restricted by the carbon layer, suppressing the adsorption and activation of syngas on Co sites.

Modulating Co reduction degree and Fischer-Tropsch synthesis performance by reducing cobalt silicate at different conditions

It is always hard to reduce Co2SiO4 to active Co0 on supported Co-based catalyst for Fischer-Tropsch synthesis (FTS). Here, amorphous Co2SiO4 material was prepared by in-situ doping TMOS during synthesis of ZIF-67, which was then reduced at different temperature and time to obtain ReX-2h (X = 400, 450, 500, 550, 600) and Re500-Yh (Y = 2, 4, 8) catalysts. The effects of reduction temperature and time on Co reduction degree and FTS performance were investigated. Results indicated that the ReX-2h and Re500-Yh catalysts exhibited low CH4 selectivity due to no obvious Co microcrystal found. The reduction degree of ReX-2h increased with reduction temperature, which led to enhanced FTS activity. However, both Re550-2h and Re600-2h exhibited poorer stability than Re500-2h, so the effects of reduction time on Re500-Yh were done. The reduction degree and FTS activity of Re500-Yh were also promoted with extending reduction time. The Re500-8h exhibited higher FTS activity with good stability.

Tailoring Carbon Deposits on a Single-Crystal Cobalt Catalyst for Fischer–Tropsch Synthesis without Further Reduction: The Role of Surface Carbon and Penetrating Carbon

The preparation of Fischer–Tropsch synthesis (FTS) catalysts that exhibit excellent catalytic performance and are suitable for direct use in a fixed-bed reactor without further reduction is crucial in industrial applications. However, only marginal progress has been made with respect to the preparation of cobalt-based FTS catalysts owing to tendency of metallic cobalt to oxidize easily; this tendency necessitates the activation of these catalysts via reduction despite having undergone reduction treatment during their preparation. Herein, this problem has been addressed by tuning the carbon deposits (surface C and penetrating C) on the surface of a single-crystal cobalt catalyst. Screen-like surface C on a catalyst pretreated with 5% CO (p-Co–CO) exhibits diffusion suppression and chemical inertness to oxidizing gas, thus preventing the oxidation of metallic cobalt and resulting simultaneously in high activity and low CH4 selectivity (7.2%) without requiring further reduction. Moreover, the exact role of surface C and penetrating C deposited on cobalt catalysts in the FTS performance is explored. Both surface C and penetrating C enhance the activity of the cobalt catalyst but with opposite effects on the FTS selectivity. Surface C improves the adsorption ability of bridged-type CO and the formation of long-chain hydrocarbons, whereas penetrating C is conducive to adsorbing linear CO and increases undesired CH4 selectivity. This study clarifies the effect of deposited carbon on the FTS reaction and provides insights into the design of high-performance nonconventional FTS catalysts that do not require further reduction.

Preparation of highly dispersed silicon spheres supported cobalt-based catalysts and their catalytic performance for Fischer-Tropsch synthesis

A series of silicon spheres supported cobalt catalysts were prepared by incipient wetness impregnation followed by decomposition under treatment of glow discharge plasma with different intensities. The catalysts were characterized by X-ray powder diffraction, N2 physical adsorption-desorption, H2 temperature-programmed reduction, transmission electron microscope and Fourier-Transform Infrared spectroscopy. The Fischer-Tropsch synthesis performance were tested on a fixed bed reactor. The influence of plasma treatment on cobalt dispersion, reducibility and cobalt-support interaction were analyzed and discussed. The results showed that the plasma-treated catalysts had better catalytic performance than the calcined sample. The Co/SP-P650W catalyst showed the highest reaction activity due to the proper cobalt dispersion and higher cobalt reducibility.

Interface-Induced Phase Evolution and Spatial Distribution of Fe-Based Catalysts for Fischer–Tropsch Synthesis

Fe-based catalysts undergo complex reduction, carburization, and phase interconversion as well as oxidation during Fischer–Tropsch synthesis (FTS). Unraveling the evolution of carbides and establishing the dominant factors are essential for rational design and modulation of Fe-based catalysts. In this work, Fe–oxide interfaces were constructed to investigate the effect on phase transformation and FTS performance. By correlating performance to the composition of iron species, the crucial role of Fe2.2C in high CO conversion and C5+ productivity has been confirmed. More importantly, a heterogeneous spatial distribution of iron carbides and oxides induced by Fe–oxide interfaces was observed. Fe3O4 preferentially forms at interfaces with the coexistence of carbon-rich Fe2.2C at subadjacent interfaces, while Fe5C2 locates at the region away from oxides or in the bare Fe catalyst. Further ex/in situ characterizations, including XPS, TPO, in situ XRD, and in situ IR, were conducted for revealing the mechanism. This study provides insights for iron carbides formation and stability by modulating interfaces, which guides the further development of Fe-based FTS catalysts.

Enhanced Fischer–Tropsch synthesis performance on MOFs‐derived Fe‐based catalysts constrained by SiO2

Fischer–Tropsch synthesis (FTS) is an essential technology to produce clean fuel, and catalyst will affect syngas conversion efficiency and product distribution. For traditional oxides‐supported catalysts, the mutual constraint between active metal loading and dispersion at high metal loading is hard to overcome, which will hinder the synthesis of high‐performance FTS catalysts. Herein, we report an efficient method for synthesizing iron‐based catalysts with not only high loading but also high dispersion. By using metal–organic frameworks Fe‐MIL‐88B as the precursor, we synthesized a series of Fe@C@SiO2 catalysts by tuning the amount of tetraethyl orthosilicate (TEOS) through a one‐step method; TEOS was in situ introduced and followed by high‐temperature carbonization. The prepared catalyst possesses a Fe@C core–shell structure with carbon layer encapsulation. Meanwhile, the SiO2 in the catalyst can effectively confine iron particles and avoid the agglomeration of iron species during high‐temperature carbonization. Among these catalysts, the Fe@C@SiO2‐0.005 showed the greatest FTS performance because high iron dispersion was achieved at high iron loading. Further increase SiO2 content enhanced the confinement effect of SiO2 but generated inert Fe2SiO4 species, which decreased the adsorption and activation of reactants and FTS activity.

Tuning Fischer-Tropsch synthesis product distribution toward light olefins over nitrided Fe-Mn bimetallic catalysts

A series of Fe-Mn bimetallic catalysts with various pretreatment atmosphere including H2, CO and alternatively NH3, were employed for the Fischer–Tropsch synthesis (FTS). The structure of Fe-Mn bimetallic catalysts was revealed by using multiple characterization techniques, where Mössbauer spectra was used to define the iron species. With direct pretreatment, the reduction efficiency is in the sequence of H2 > CO > NH3. Fe-Mn bimetallic catalysts pre-reduced by H2 that permit an easy nitrogenation. To correlate the phase compositions of spent catalysts with FTS performance, the individual contribution of Fe, iron carbides (χ-Fe5C2, ε-Fe2(.2)C and θ-Fe3C) and iron nitride (Fe2N) to hydrocarbon selectivity was deconvolved by linear regression. It was discovered that C2=-C4= selectivity was proportional to the content of epsilon-iron carbide, which was promoted by the doping with Mn and further nitriding treatment. Successfully nitrided Fe-Mn bimetallic catalysts exhibit a better stability of C2-C4 O/P along time on stream, while either direct CO treatment or NH3 treatment followed by CO treatment result in a rapid decent of C2-C4 O/P ratio against time. Furthermore, Mn enhances long-chain hydrocarbon selectivity and suppresses CH4 formation, whereas the nitrogenation inhibits the chain growth ability, concentrating the hydrocarbon selectivity into C2-C4 fraction. Hence, the nitrided Fe-Mn catalyst is able to maintain a high light olefin selectivity. This study highlights that nitrogenation could provide an opportunity to tune the C2-C4 olefins by optimizing the phase structure of Fe-Mn catalysts.

The Effect of Al2O3 Pore Diameter on the Fischer–Tropsch Synthesis Performance of Co/Al2O3 Catalyst

The Effect of Al2O3 Pore Diameter on the Fischer–Tropsch Synthesis Performance of Co/Al2O3 Catalyst

A hollow void catalyst of Co@C(Z-d)@void@CeO2 for enhancing the performance and stability of the Fischer-Tropsch synthesis.

To enhance the catalyst performance of Fischer-Tropsch synthesis (FTS), removing the mass-transfer restriction in the catalysis synthesis is essential. Although the core-shell nanostructures can improve the activity and stability of the catalyst, they can restrict the reactants' diffusion towards the active sites and the transfer of the products from these sites in FTS. Creating an adequate porosity between the core and the outer shell of the catalyst structure can tackle this issue. In this work, the synthesized cobalt-based nano-catalyst is encapsulated with two shells and a middle porous shell. The first shell is a carbon shell at the core of the catalyst derived from ZIF-67, the second one is the outer shell of ceria, and the middle porous shell is formed by removing the sacrificial silica shell through the etching technique. The characterization and performance tests represent significant evidence of the etching treatment's impact on the FTS catalyst performance. Besides, molecular dynamics simulation is also utilized to clarify its effect. The FTS catalytic performance is enhanced more than 2 times with the etched catalyst versus the catalyst without it at 17.5 bar and a (H2/CO) ratio of 1.2. In addition, not only does the etched catalyst with high porosity play the role of a nanoreactor and intensify its catalytic performance, but it also has higher stability.

Effects of Rare Earth Metal Promotion over Zeolite-Supported Fe-Cu-Based Catalysts on the Light Olefin Production Performance in Fischer-Tropsch Synthesis.

Fischer-Tropsch synthesis (FTS), a significant reaction for effective H2 utilization, is a promising approach for direct production of light olefins from syngas (H2 + CO). For the FT-Olefin process, an efficient catalyst restricting the product distribution of FTS to light olefins is required. Aligned with this goal, we synthesized 24 catalysts comprising Fe and Cu in combination with rare earth metals (La, Ce, Nd, Ho, Er) and zeolite supports (ultrastable Y and mordenite). FT-Olefin performances of these catalysts were screened using a high-throughput test system at atmospheric pressure, and then promising catalysts were tested under high pressure in a conventional test system. Results show that Nd increases selectivity to light olefins and Ho suppresses C5+ and coke formation. It is also demonstrated that zeolite-metal interaction, leading to a mixture of both acidic and basic sites, is significant in increasing light olefin production. The mordenite-supported 20 wt % Fe, 0.5 wt % Cu, and 0.5 wt % Ho catalyst provides the highest light olefin yield with the lowest coke and heavier hydrocarbon selectivity.

Activating nitrogen-doped carbon nanosheets by KOH treatment to promote the Fischer-Tropsch synthesis performance

Both appropriate support and promoter decoration are vital to improve the performance of Fe-based catalysts in Fischer-Tropsch Synthesis (FTS). In this work, KOH chemical activation was applied to modify N-doped carbon nanosheets supported Fe catalysts, which achieves support modification and promoter introduction simultaneously. The influence of KOH activation on pore structure, N configuration and residual K effect were discussed on Fe-based FTS catalyst. Upon KOH activation, the micro-porosity was enriched which improved the Fe dispersion. Meanwhile, the N configuration was regulated through KOH in-situ reacting with carbon matrix. Further investigation on residual K revealed a combined effect of K and N. The physical structure, electronic properties and phase compositions after activation were characterized by N2-physisorption, XRD, Raman, TEM, XPS and Mössbauer spectroscopy. Combing with the catalytic performance, we demonstrated that the pyrrolic N sites and suitable K residuals contribute to electron donation to Fe species, collectively promoting Fe carburization accompanied by higher C2–C4= productivity. This finding is expected to offer an activation method for preparation of N-doped carbon supported metal catalysts.

The promotion role of different crystal phase MnO2 in iron-based Fischer-Tropsch catalysts

Manganese oxide additive is an effective promoter for modulating the catalytic performance of iron-based Fischer-Tropsch synthesis (FTS) catalysts and studied extensively, however, the promotion effects of MnO2 crystal structures on iron-based FTS catalysts have never been reported. In this work, four types of MnO2 with crystal phases corresponding to α-, β-, γ- and δ-MnO2 were prepared, and their promotion functions on iron-based FTS catalysts were studied. These four types of MnO2 exhibit dramatically distinct promotion effects on the structure evolution and catalytic performances. The ferrite MnFe2O4 only forms in the α-MnO2 modified catalysts, while the β-, γ- and δ-MnO2 also present different existing forms in the catalysts. The presence of abundant oxygen vacancies on the surface of α- and δ-MnO2 notably facilitates the cleavage of Fe-O bonds and the dissociation of CO, leading to the enhancing formation of iron carbides in the activated and spent catalysts. Meanwhile, the strong Mn-Fe electronical interactions formed in α- and δ-MnO2 modified catalysts lead to high density of surface carbon species with a long residence time, dramatically improving the selectivity of linear high carbon α-olefins and heavy hydrocarbons. In contrast, the β- and γ-MnO2 modifying dramatically increases the catalytic activity, but suppresses the formation of olefins and long chain hydrocarbons, due to their relatively few amounts of surface oxygen vacancies and moderate Mn-Fe electronical interactions. This work provides new insights and in-depth understanding of the promotion roles of Mn additive, benefiting for the rational design of high performance iron-based FTS catalysts.