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As a prerequisite elementary step, carburization of iron oxides to form iron carbides is critically important to display activity for iron‐based Fischer–Tropsch synthesis catalysts. The positive role of the Cu promoter to advocate the reduction of the iron oxides has long been known. Nevertheless, its complex influence along the carburization of catalyst precursors remains controversial. Here, we resolve this decades‐long question by using a state‐of‐the‐art in situ X‐ray diffraction (XRD) method to elucidate the multi‐role of Cu promoter in modulation of carburization rate, composition of post‐carburization samples, and morphology evolution of prepared iron carbide particles under CO/He (v/v=2 %/98 %) and H2/CO/He (v/v/v=8 %/2 %/90 %) gaseous environment, by excluding the possible interference from promoter components other than Cu. We anticipate that this study enriches current tool box in the research of a specific promoter of the Fischer‐Tropsch synthesis catalysts. In situ seeing is believing: The complex influence of Cu promoter along the carburization of iron catalyst precursors has been elucidated by using a state‐of‐the‐art in situ X‐ray diffraction (XRD) method. Particularly, the multi‐role of Cu promoter in modulation of carburization rate, composition of post‐carburization samples, and morphology evolution of prepared iron carbide particles under CO/He (v/v=2 %/98 %) and H2/CO/He (v/v/v=8 %/2 %/90 %) gaseous environment, by excluding the possible interference from promoter components other than Cu. [ABSTRACT FROM AUTHOR]

The present study was undertaken to investigate the catalytic behavior of an industrial iron catalyst (Fe/Cu/K/SiO2) prepared from ferrous sulfate precursor for Fischer–Tropsch (FT) synthesis, in which different amount of Fe3O4 coexist with α-Fe2O3. The catalyst samples were characterized by BET, XRD, H2-TPR and Mössbauer effect spectroscopy (MES). The FT synthesis performance of the catalysts were carried out in a fixed bed reactor (FBR) under reaction conditions of 250 °C, 1.5 MPa, 2.0 nL/g-cat/h, and H2/CO=2/1 for 200 h. The results from XRD and MES for the catalyst samples of pre- and post-reduction indicate that more iron carbides form in the catalysts that have lower Fe3O4 contents. H2-TPR for the catalysts displays that Fe3O4 may facilitate the reduction of catalysts only when it was highly dispersed. FT reaction study in the FBR shows that the catalysts become more active with the decrease of Fe3O4 contents in the catalysts. However, the catalyst with certain amount of highly dispersed Fe3O4 exhibited high FT synthesis activity with CO conversion more than 75%. The catalyst also displayed much less olefins selectivity. A comparison of FTS performances of one of these catalysts with some known catalysts was also made in this paper. [ABSTRACT FROM AUTHOR]

[Display omitted] • The dynamic process of early-stage carburization of iron oxide has been systematically studied. • The underlying mechanism of morphology's impact on the carburization process has been elucidated. • The carburization process is determined by the competition between CO dissociation and reduction reactions. Transition metal carbide catalysts have garnered widespread attention due to their outstanding catalytic performance. The carbide catalysts, such as Fe x C and Mo x C are typically obtained from their oxide carburization. The reduction and carburization processes are key steps during the activation of oxide precursors, which determine the activity and stability. An understanding of the carburization process mechanism is very important for the optimization of carbide catalysts. Iron-based catalysts are widely used in the large-scale coal-to-liquid industry because of their low price and high activity. In this paper, the carburization process of iron oxide nanoparticles was simulated by the Reactive force field (ReaxFF) molecular dynamics method. The results illustrate that the competition between CO dissociation and reduction of oxides occurs at the 100 ps time scale, which can be tuned by particle size and oxygen vacancy. We have explained the counter-intuitive finding that small oxide particles are more difficult to achieve fully carburization than larger ones; this is because rapid carbon accumulation on the surface blocks the oxygen diffusion channels from bulk to surface. The atomic distribution heat map was carried out to demonstrate the carbon and oxygen penetration processes. Meanwhile, the volumetric strain analysis reveals the driving force comes from the strong tendency to form Fe-C bonds. The dependence on surface orientation was further systematically investigated. This work provides microscopic insights into the carburization process of iron-based materials and the fundamental knowledge for the optimization of catalyst synthesis procedures. [ABSTRACT FROM AUTHOR]

Abstract: Density functional theory calculations have been carried out on the reaction of ketene hydrogenation on Fe5C2(001) for the understanding of the Fischer–Tropsch synthesis mechanism. The main reaction pathway of ethene formation is CsH2CO→CCsH2 →CHCsH2 →CH2CsH2, and ethane formation follows CsH2CO→CCsH2 →CCsH3 →CCsH3 →CHCsH3 →CH2CsH3 →CH3CsH3, while that of ethanol is CsH2CO→[CsH3CO and/or CsH2CHO]→CsH3CHO→CsH3CH2O→CsH3CH2OH. Detailed comparison shows clearly that ketene dissociation with the formation of hydrocarbons is more favorable than the stepwise hydrogenation with the formation of ethanol. The expected product should be hydrocarbons rather than ethanol, in agreement with the experimental observation. [Copyright &y& Elsevier]

A comprehensive one-dimensional heterogeneous reactor model is developed to simulate the performance of fixed-bed Fischer–Tropsch reactors for hydrocarbon production. The detailed mechanistic kinetics is combined into the reactor model along with considering the fact that the catalyst pores are filled with liquid wax under realistic conditions. The equilibrium between the gases in the bulk and the wax in the catalyst pores is correlated by using a modified SRK equation of state (MSRK EOS). The model is solved by using Gear method to integrate the reactor model with the embedded pellet model discretized by orthogonal collocation on finite elements. The validity of the reactor model is tested against the measured data from different-scale demonstration processes. Satisfactory agreements between model predictions and experiment results are obtained. Detailed numerical simulations are performed to investigate the effect of major process parameters on the reaction behavior of fixed-bed FTS systems with recycle operation. [Copyright &y& Elsevier]

An industrial manganese-promoted ultrafine particle (UFP) iron catalyst for slurry phase Fischer–Tropsch synthesis was examined in a 1 dm3 stirred tank slurry reactor under various reaction conditions as a representative of industrial practice (533–573 K, 1.5–2.5 MPa, 1.0–2.5 l (STP)/g-cat h, H2/CO=0.65–2.0). The Fe–Mn UFP catalyst showed high CO conversion and gave high yields of C2–C4 alkenes, partially attributed to less secondary hydrogenation of alkenes during the slurry reaction. The catalyst displayed a slow deactivation rate with time on stream. The effects of reaction conditions on the catalytic activity, hydrocarbon product distribution and the ratio of alkene/alkane were discussed. [Copyright &y& Elsevier]

[Display omitted] Synthetic strategies in the preparation of embedded structure, impregnated, and precipitated catalysts have an important effect on the Fe-SiO 2 interaction, morphology, and catalytic performance. • Sintering of iron species is effectively inhibited by SiO 2 shell in Fe@Si catalyst. • Fe-SiO 2 interaction can be manipulated by preparation strategies. • The properties and catalytic performance of Fe-Si FTS catalysts are governed by the configuration and Fe-SiO 2 interaction. The physico-chemical properties of SiO 2 -promoted Fe-based Fischer-Tropsch synthesis catalysts were traditionally considered to be governed by Fe-SiO 2 interaction. Here we found that the configuration between SiO 2 and iron species also played a pivotal role in determining the structure and thus FTS performance of the catalysts. Fe@Si catalyst with embedded structure, impregnated Fe/Si catalyst as well as precipitated Fe-Si catalyst was fabricated by different methods respectively, and they were thoroughly characterized by multiple techniques. The results indicated that, despite the weaker Fe-SiO 2 interaction, the construction of SiO 2 shell outside the iron species core in Fe@Si catalyst strongly inhibited the reduction of iron oxides due to the confining effect of SiO 2 shell, which increased the diffusion resistance of H 2 O generated in the reduction process. However, the sintering of iron species was effectively hindered even during FTS reaction by the physical separation of SiO 2 shell. In contrast, iron species in impregnated Fe/Si and precipitated Fe-Si catalyst experienced aggregation with different degrees. The reduction behavior of these two catalysts were well correlated with the strength of Fe-SiO 2 interaction. The FTS performance showed that both Fe@Si and Fe/Si catalyst exhibited higher initial activity but deactivated gradually, while the activity of Fe-Si catalyst displayed an opposite trend. These phenomena were discussed in terms of the variation of iron carbides content at different stages, which was determined not only by Fe-SiO 2 interaction, but also by the manner that SiO 2 and iron species constructed. The present study contributed a new understanding of the structure-performance relationship for SiO 2 -promoted Fe-based FTS catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Real space evidence of p4g(2 × 2) Fe 2 C and Co 2 C surface carbides. • Tip-sample distance dependent STM imaging of Fe 2 C. • Fe 2 X (X = B, C, N, O) with increasing electrons the reconstruction becomes less favourable. Thin FCC (1 0 0) iron and cobalt carbide films were prepared on Cu(1 0 0) to study the connection between their structure, electronic properties and stability. We present the first detailed, real space experimental confirmation of the C-induced clock reconstruction on the FCC(1 0 0) surfaces of iron and cobalt. Both Fe and Co surface carbides show p4g (2 × 2) surface reconstruction with tetracoordinated square planar carbon and pure FCC (1 0 0) metal layers underneath. Combining tip-sample distance dependent STM imaging with theoretical calculations we present different imaging modes of Fe 2 C. Using a combination of angle-resolved x-ray photoelectron spectroscopy (AR-XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and theoretical calculations we provide detailed electronic and structural models for Fe 2 C and Co 2 C p4g (2 × 2) surface carbides and other 2D Fe 2 X interstitial compound systems. In various Fe 2 X (X = B, C, N, O) surface compounds moving to the right in the periodic table with increasing electrons the reconstruction becomes less favorable, while iron carbide shows the highest stability. [ABSTRACT FROM AUTHOR]

The structure and stability of seventeen facets of the Hägg carbide phase (χ-Fe 5 C 2 ) under the consideration of K 2 O promotion have been analyzed utilizing density functional theory approach and ab initio atomistic thermodynamics. On the basis of different interaction strengths of these facets with K 2 O promoter, the morphology of the χ-Fe 5 C 2 phase under the variation of K 2 O content has been predicated. At the surface Fe/K atomic ratio of 30, the morphology of the χ-Fe 5 C 2 phase can be modified apparently. Among the most exposed (1 1 1), (1 0 0), ( 1   1   1 ¯ ), (5 1 0) and ( 4 ¯   1   1 ) facets, the ( 1   1   1 ¯ ), (5 1 0) and ( 4 ¯   1   1 ) facets have more open surface structures as well as provide more surface Fe atoms and less carbon atoms. However, all these mostly exposed facets do not favor CO direct dissociation at low CO coverage. [ABSTRACT FROM AUTHOR]

Sn-containing Fe 5 C 2 interface not only can boost the rates of oxygen removal but also restrain the coke deposition by elevating the barrier of nucleation, which stabilizes the active iron carbide structure and enhanced the stability remarkablely. [Display omitted] • Oxygen removal by carbon oxide is facilitated for Fe-Sn interface； • Coking deposition was inhibited remarkably for Fe-Sn bimetallic catalysts; • Catalytic stability was significantly enhanced for Sn0.25 and Sn0.50 catalysts; • Provided a strategy to construct durable catalysts under the perspective of oxygen removal and coking resistance. Comprehending deactivation mechanisms and constructing durable Fe-based catalysts remain a substantial challenge for Fischer-Tropsch synthesis. In this contribution, the effects of Sn promoter on the structure and catalytic performance of Fe-based catalysts were investigated systematically. The catalytic stability was enhanced significantly for 100Fe/0.25Sn and 100Fe/0.50Sn catalysts, reducing the deactivation rate almost four-fold and seven-fold respectively. Combined with several characterizations and DFT calculation, it has been confirmed that Sn-containing Fe 5 C 2 interface not only can boost the rates of oxygen removal but also restrain the coke deposition by elevating the barrier of nucleation, which stabilizes the active iron carbide structure and inhibits the deactivation efficiently. This study may broaden the understanding of enhanced stability during Fe-based FTS under the perspective of oxygen removal and coking resistance and provide a strategy to construct durable catalysts. [ABSTRACT FROM AUTHOR]

Abstract: Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of χ-Fe5C2 under CO and synthesis gas (H2/CO) pretreatment conditions in catalyst activation for Fischer–Tropsch synthesis (FTS). On the basis of the computed surface free energy (γ) as a function of the carbon chemical potential (μ C) by considering temperature, pressure, and H2/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H2 added into CO leads to a large decrease in μ C and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H2/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets. [Copyright &y& Elsevier]

Abstract: Fe/SiO2 catalysts with different Fe/Si molar ratios were used to investigate the effects of silica on chemical/structural properties and Fischer–Tropsch synthesis (FTS) performance of iron-based catalysts. In the chemical aspect, silica interacts with Fe species by the formation of Feh name="sbnd" />Si structure, which further transforms into an Fe2SiO4 phase during FTS reaction. The interaction largely disturbs the electronic structure of Fe atoms in iron oxide phases and in turn resists the reduction and activation of catalysts. In the structural aspect, silica increases the dispersion of Fe species and inhibits the aggregation of active iron particles. Addition of silica largely changes the adsorption sites of catalysts, i.e., decreases the number of weak H adsorption sites but improves the adsorption strengths of H, C, and O on reduced or carburized catalysts. With increasing amounts of silica, the chemical and structural effects cause the firstly decrease and then the increase of the initial FTS activity and the selectivities of heavy hydrocarbons and olefins during the Fischer–Tropsch synthesis. In addition, an important finding is that a proper amount of silica apparently suppresses the methane selectivity and stabilizes the iron carbide in the FTS reaction. [Copyright &y& Elsevier]

Abstract: This paper designs four cases to investigate the performances of the polygeneration processes, which depend on the commercially ready technology to convert coal to liquid fuels (CTL) and electricity with CO2 sequestration. With Excel-Aspen Plus based models, mass and energy conversion are calculated in detail. The simulation shows that the thermal efficiency is down with the synfuels yield decrease though the electricity generation is increased. It also suggests that the largest low heat value (LHV) loss of coal occurs in the gasification unit. From the comparison of the four cases, prominent differences of coal energy transition appear in water–gas shift (WGS) units, Fischer–Tropsch (FT) synthesis and combined cycle processes. CO2 capture and vent are discussed and the results show that the vent amount of CO2 increases with the increase of percentage of the syngas going to produce electricity. The results also show that the ratio of carbon captured to total carbon increases from 58% to 93% which is an important contribution to cutting down the greenhouse gas vent. [Copyright &y& Elsevier]

Abstract: Kinetic Monte Carlo simulations were carried out for Fischer–Tropsch synthesis in a wide range of industrially relevant reaction conditions. The macroscopic performance of different reaction conditions obtained from simulations quite agrees with experimental trends. In all examined conditions, H and CO are dominant species on catalyst surface. The rest intermediates only exist in small amount, and are controlled by H and CO coverages. Activity and selectivity for Fischer–Tropsch synthesis and water gas shift reaction are determined by surface H/CO ratio. Reaction conditions can directly change surface coverage of H, CO, and vacant sites, and consequently exert an influence on macroscopic performance of iron catalyst. [Copyright &y& Elsevier]

Abstract: Spin-polarized density functional theory has been used to characterize hydrogen adsorption on the Fe3C(100), Fe3C(001) and Fe3C(010) surfaces. It is found that hydrogen adsorbs dissociatively on the three surfaces. On the Fe3C(100) surface, the most stable surface species is CH at 1/3 or 2/3 monolayer, and CH4 at 1 monolayer; in contrast on the Fe3C(001) surface the most stable surface intermediates are 3-fold H at 1/3 monolayer, and CH at 2/3 monolayer, and CH4 at 1 monolayer. On the metallic Fe3C(010) surface, hydrogen adsorbs at 2-fold or 3-fold sites. The computed energetic order of hydrogen adsorption is Fe3C(100)>Fe3C(010)>Fe3C(001), and this differs from that of CO adsorption (Fe3C(010)>Fe3C(100)>Fe3C(001)). [Copyright &y& Elsevier]

Abstract: The effect of co-feeding CO2 on the catalytic properties of an Fe–Mn catalyst during Fischer–Tropsch synthesis (FTS) was investigated in a spinning basket reactor by varying added CO2 partial pressure in the feed gas. It was found that co-feeding CO2 to syngas did not decrease the activity of the catalyst, on the contrary, a dramatic increase of the activity and an increase of methane selectivity were observed over the catalyst after removal of CO2 from the feed gas. The addition of CO2 led to an increase in olefin/paraffin ratios of low carbon hydrocarbons and a slight decrease in C19 + selectivity. It also slightly decreased CO2 formation rate on the catalyst by increasing the rate of reverse step of the water–gas shift (WGS) reaction and pushing the reaction towards equilibrium, and did not remarkably influence the hydrocarbon formation rate. However, the co-feeding CO2 can significantly increase the water formation rate and the overall oxygenate formation rate under these reaction conditions. [Copyright &y& Elsevier]

Abstract: FeCu/SiO2 catalysts, in which K or Na promoter is incorporated respectively, are prepared by a combination method of continuous co-precipitation and spray drying technology. The catalysts were characterized by temperature-programmed desorption and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts was studied in a continuously stirred tank slurry reactor. The basicity of the K-promoted catalyst is enhanced, as demonstrated by CO2-TPD results. MES results show that sodium can weaken the dispersion of α-Fe2O3 phase; either potassium or sodium can promote carburization of the catalyst, while the effect of sodium is weaker. FTS results indicate that the addition of K or Na can improve the catalyst activity, and shift the product distribution to heavy hydrocarbons to the different extent. [Copyright &y& Elsevier]

Abstract: Two model spherical iron catalysts (100Fe/0Al2O3 and 100Fe/15Al2O3) with free Cu and K promoters were prepared by the combination of co-precipitation and spray drying method for the application of slurry Fischer–Tropsch synthesis (FTS). The effect of Fe–Al2O3 interaction on the reduction/carburization behavior in H2/CO/syngas, surface basicity and the change of phase structure were comparatively studied by means of H2 or CO temperature-programmed reduction (TPR), CO2 temperature-programmed desorption (TPD) and Mössbauer effect spectroscopy (MES). The results showed that the catalyst incorporated with Al2O3 exhibits a strong Fe–Al2O3 interaction, which obviously weakens the surface basicity, stabilizes the FeO phase and inhibits the reduction of iron catalyst in H2 or syngas. Furthermore, Fe–Al2O3 interaction also restrains the carburization of iron catalyst in CO or syngas. In slurry FTS process, it was found that the strong Fe–Al2O3 interaction decreases the FTS activity and suppresses the water gas shift (WGS) reaction, but can stabilize the active sites of iron catalyst and improve its run stability. Due to the strong Fe–Al2O3 interaction, the weak surface basicity on the catalyst incorporated with Al2O3 greatly decreases the selectivity of heavy hydrocarbon products. [Copyright &y& Elsevier]

Abstract: The effects of reaction conditions on the performance of an Fe-Mn catalyst for Fischer-Tropsch synthesis (FTS) have been investigated in a continuous spinning basket reactor. Experiments were carried out under a wide range of industrially relevant reaction conditions including reaction temperature of 533–573K, pressure of 0.93–2.96MPa, inlet H2/CO molar ratio of 0.80–2.50 and space velocity of (0.46–1.85)×10−3 Nm3 kgcat −1 s−1. The selectivities to low molecular weight hydrocarbons basically increased with the increase of reaction temperature, total pressure and inlet H2/CO ratio, except for ethylene. The FTS reaction rate and the overall oxygenates formation rate increased with increasing reaction temperature, pressure, space velocity and H2/CO ratio on the whole. It was found that the catalyst has relatively high activity and selectivity in producing light alkenes. It also exhibited excellent stability during the 700h run at H2/CO ratio of 1.00, which indicates that the catalyst has good longevity and is suitable for syngas derived from coal with low H2/CO ratio to produce light olefins and high-quality liquid fuels. Meanwhile, it may have potential promising applications in advanced stirred tank slurry reactors. In addition, the detailed investigation of FTS reaction performance for the Fe-Mn catalyst can provide full and accurate basic information for industrial scale-up. [Copyright &y& Elsevier]

Abstract: Density functional theory (DFT) calculations have been carried out on the adsorption of CO on the (100), (001) and (010) surfaces of Fe3C. Both (100) and (001) have surface iron and carbon atoms, while (010) has only surface iron atoms. At 1/5ML on (100), the most stable adsorption configuration has adsorbed CO at a three-fold site (three Fe atoms), followed by adsorbed surface ketenylidene at a four-fold site (three iron atoms and one carbon atom). At 1/6ML on (001), the most stable adsorption configuration has adsorbed CO at a four-fold site (four iron atoms). With increased coverage, adsorption at different sites becomes possible and close in energy. On the metallic (010) surface, both two-fold and three-fold adsorptions are close in energy. The electronic states of the most stable adsorption structures have been analyzed accordingly. [Copyright &y& Elsevier]

Abstract: An industrial iron-based catalyst (100Fe/5Cu/6K/16SiO2, by weight) was characterized after reduction at different temperatures and after Fischer–Tropsch synthesis (FTS) in a stirred tank slurry reactor (STSR). The BET surface area and pore volume of the catalyst decreases with increasing reduction temperature, and the contrary trend was found for pore size. The iron phase compositions of catalysts reduced with syngas were strongly dependent on pretreatment conditions employed. Pretreatment with syngas at lower temperature prevents iron catalyst activation. Carburization was intensified with the increase in reduction temperature. The formation of iron carbides in reduced catalyst was necessary for obtaining stable high FTS activity. The relationship between the amount of CO2 in tail gas during activation and the Fe3+ (spm) content in the reduced catalyst was observed. The rapid carburization at high reduction temperature resulted in the formation of a superparamagnetic Fe3+ core and an iron carbide layer of the reduced catalyst. FTS activity decreased with the increase in the reduction temperature, but the stability distinctly improved. It was found that the working catalyst loss in the heavier waxy products resulted in higher deactivation rate of the catalyst reduced at lower temperature. With the increase in the reduction temperature, the product distribution shifted towards the lower molecular weight products. [Copyright &y& Elsevier]

Abstract: The effect of adding SiO2 to a precipitated iron-based Fischer–Tropsch synthesis (FTS) catalyst was investigated using N2 physical adsorption, H2 differential thermogravimetric analysis, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer spectroscopy. The FTS performances of the catalysts with or without SiO2 were compared in a fixed bed reactor. The characterization results indicated that SiO2 facilitates the high dispersion of Fe2O3 and significantly influences the Fe/Cu and Fe/K contacts, which play an important role in the surface basicity, reduction and carburization behaviors, as well as the FTS performances. The incorporation of SiO2 enhances the Fe/Cu contact, further enlarges the H2 adsorption and promotes the reduction of Fe2O3 →FeO x , while the transformation of FeO x →Fe is suppressed probably due to the strong Fe–SiO2 interaction. SiO2 indirectly weakens the surface basicity and severely suppresses the carburization and CO adsorption of the catalyst. In the FTS reaction, it was found that SiO2 decreases the FTS initial activity but improves the catalyst stability. Due to the lower surface basicity than the catalyst without SiO2, the catalyst incorporated with SiO2 has higher selectivity to light hydrocarbons and methane and decreased selectivity to the olefins and heavy hydrocarbons. [Copyright &y& Elsevier]

Abstract: The structural properties and iron–silica interaction of the precipitated iron catalysts incorporated with or without silica have been investigated by powder X-ray diffraction (XRD), Mössbauer spectroscopy, and temperature-programmed reduction (TPR) with H2 consumption quantitative analysis. The Fischer–Tropsch synthesis (FTS) performances of the catalysts were carried out in a slurry-phase continuously stirred tank reactor (CSTR). It was found that, for as-prepared samples, nano-particles of iron oxide mixtures (α- and/or γ-Fe2O3) formed on the silica-incorporated catalyst, whereas well-crystallized hematite (α-Fe2O3) is the only phase on the silica-free catalyst. After reduction in H2, the observed phase in the silica-free catalyst is α-Fe, whereas wüstite (Fe x O) and iron (II) silicate (Fe2SiO4) are the main phases in the silica-incorporated catalyst. In H2-TPR, the Fe3+ ions in the silica-incorporated catalyst are reduced to Fe (0) via Fe2+ ions as intermediate and the reduction extent of iron oxides obviously decreased comparing with the silica-free catalyst. The differences in structural properties and reduction behavior between the silica-free and the silica-incorporated catalysts may be due to the high dispersion effect of silica and the iron–silica interaction. The iron–silica interaction also decreases the FTS activity and improves the light hydrocarbon selectivity. [Copyright &y& Elsevier]

Abstract: A systematic study was undertaken to investigate the effects of Al2O3/SiO2 ratio on reduction, carburization and catalytic behavior of iron-based Fischer–Tropsch synthesis (FTS) catalysts promoted with potassium and copper. The catalysts were characterized by N2 physical adsorption, CO2 temperature-programmed desorption (TPD), H2 temperature-programmed reduction (TPR) and Mössbauer effect spectroscopy (MES). CO2-TPD indicated that Al2O3 binder has stronger acidity than SiO2 binder and weakens the surface basicity of the catalysts. H2-TPR profiles suggested that the lower Al2O3/SiO2 ratio promotes the reduction of Fe2O3→Fe3O4. With further increasing Al2O3/SiO2 ratio, the transformation of Fe2O3→Fe3O4 shifts to higher temperatures. The MES results showed that the increase of Al2O3/SiO2 ratio leads to the relatively large crystallite size of α-Fe2O3 and inhibits carburization of the catalyst. During reaction tests in a fixed bed reactor it was found that a maximum in catalyst activity is noted at the Al2O3/SiO2 ratio of 5/20 (weight basis). The selectivity to olefins shows a rapid decrease and the formations of methane and light hydrocarbons are promoted with increasing Al2O3/SiO2 ratio. The oxygenate selectivity in total products increases with increasing Al2O3/SiO2 ratio. [Copyright &y& Elsevier]

Abstract: A comprehensive Fischer–Tropsch synthesis (FTS) kinetics model including the hydrocarbon and oxygenate formation reactions as well as water gas shift (WGS) reaction over an Fe–Mn catalyst is developed in a continuous spinning basket reactor. On the basis of CH2 insertion alkyl mechanism, the kinetics expressions for paraffins, olefins, alcohols and acids are derived. Kinetics calculation results indicate that the formation of paraffins, olefins, alcohols and acids over the Fe–Mn catalyst are parallel competitive reactions. Reasonable and strict analysis in mathematics is given to interpret the results which the comprehensive kinetics model cannot predict the olefin/hydrocarbon ratio well though the olefin readsorption and secondary reactions are involved in the kinetics model. Oxygenates might readsorb over the catalyst surface and take part into the corresponding secondary reactions (chain growth and hydrogenation to the corresponding hydrocarbons), which leads to the exponentially decrease of the experimental alcohol/hydrocarbon and acid/hydrocarbon ratios with the increase of carbon number. [Copyright &y& Elsevier]

Abstract: The metal–silica interaction and catalytic behavior of Cu-promoted Fe–Mn–K/SiO2 catalysts were investigated by temperature-programmed reduction/desorption (TPR/TPD), differential thermogravimetric analysis, in situ diffuse reflectance infrared Fourier transform analysis, and Mössbauer spectroscopy. The Fischer–Tropsch synthesis (FTS) performance of the catalysts with or without copper was studied in a slurry-phase continuously stirred tank reactor. The characterization results indicate that several kinds of metal oxide–silica interactions are present on Fe–Mn–K/SiO2 catalysts with or without copper, which include iron–silica, copper–silica, and potassium–silica interactions. In addition to the well-known effect of Cu promoter on easing the reduction of iron-based FTS catalysts, it is found that Cu promoter can increase the rate of carburization, but does not vary the extent of carburization during the steady-state FTS reaction. The basicity of the Cu and K co-promoted catalyst is greatly enhanced, as demonstrated by CO2-TPD results. In the FTS reaction, Cu improves the rate of catalyst activation and shortens the induction period, whereas the addition of Cu has no apparent influence on the steady-state activity of the catalyst. Promotion of Cu strongly affects hydrocarbon selectivity. The product distribution shifts to heavy hydrocarbons, and the olefin/paraffin ratio is enhanced on the catalyst due to the indirect enhancement of surface basicity by the copper promotion effect. [Copyright &y& Elsevier]

The present study was undertaken to investigate the catalytic behavior of an industrial iron catalyst (Fe/Cu/K/SiO2) prepared from ferrous sulfate precursor for Fischer–Tropsch (FT) synthesis, in which different amount of Fe3O4 coexist with α-Fe2O3. The catalyst samples were characterized by BET, XRD, H2-TPR and Mössbauer effect spectroscopy (MES). The FT synthesis performance of the catalysts were carried out in a fixed bed reactor (FBR) under reaction conditions of 250 °C, 1.5 MPa, 2.0 nL/g-cat/h, and H2/CO=2/1 for 200 h. The results from XRD and MES for the catalyst samples of pre- and post-reduction indicate that more iron carbides form in the catalysts that have lower Fe3O4 contents. H2-TPR for the catalysts displays that Fe3O4 may facilitate the reduction of catalysts only when it was highly dispersed. FT reaction study in the FBR shows that the catalysts become more active with the decrease of Fe3O4 contents in the catalysts. However, the catalyst with certain amount of highly dispersed Fe3O4 exhibited high FT synthesis activity with CO conversion more than 75%. The catalyst also displayed much less olefins selectivity. A comparison of FTS performances of one of these catalysts with some known catalysts was also made in this paper. [ABSTRACT FROM AUTHOR]

Abstract: The kinetics of water gas shift (WGS) reaction over an Fe–Mn catalyst under Fischer–Tropsch synthesis (FTS) reaction conditions is studied in a spinning basket reactor. Experimental conditions are varied as follows: temperature of 533–573K, reactor pressure of 10.0–26.5bar, H2/CO feed ratio of 0.66–2.0 and space velocity of 0.66–2.65×10−3 Nm3 kgcat −1 s−1. By separately fitting WGS kinetics parameters with experimental data, which is possible in the spinning basket reactor with neglecting concentration and temperature gradients, different kinetics models of WGS are derived and discriminated on the basis of four sets of WGS elementary reactions. Kinetics experimental results show that the WGS reaction under FTS reaction conditions is far from equilibrium. Two types of WGS mechanisms are investigated. One is the formate mechanism, and the other is the direct oxidation mechanism. It is found that the formate mechanism is better in fitting experimental data than the direct oxidation mechanism over the Fe–Mn catalyst under the FTS reaction conditions. The optimized kinetics model with formate intermediate dissociation as the rate-determining step (RDS) can fit the WGS experimental results well. The simplified WGS kinetics model can easily be used for industrial modeling applications. [Copyright &y& Elsevier]

Abstract: The kinetics models of oxygenate formation in Fischer–Tropsch synthesis (FTS) over an industrial Fe–Mn catalyst are studied in a continuous spinning basket reactor. Detailed kinetics models on the basis of possible oxygenate formation mechanisms, namely adsorbed CO or CH2 insertion mechanisms, are derived. The calculated alcohol and acid distributions in FTS reaction fit the experimental data well with considering the esterification reactions between alcohols and acids. The alcohol formation via successive hydrogenation of intermediate [RCO-s] is more energetically favorable than the acid formation via the reaction between the [RCO-s] and [OH-s]. The alcohol formation is not via the successive hydrogenation of acid intermediates over the Fe–Mn catalyst under FTS reaction conditions. [Copyright &y& Elsevier]

Abstract: A systematic study has been carried out to investigate the impacts of SiO2 content, incorporation manner of SiO2 and drying process on the physico-chemical and catalytic performances of a precipitated iron–manganese catalyst for Fischer–Tropsch synthesis (FTS) in a fixed bed reactor (H2/CO=2.0, T=265°C, P=2.5MPa and GHSV=1000h-1). Characterization technologies of N2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES), temperature-programmed reduction (TPR) and scanning electron microscopy (SEM) were used to study the textural properties, bulk phase composition, reduction behavior and morphologies of the catalysts, and the SiO2 framework in the catalysts. The results of characterization showed that the incorporation of SiO2 leads to the increase in surface area. The catalyst with incorporated with precipitated SiO2 has the highest surface area and exhibits a stronger interaction between iron and SiO2 matrix than the interaction with binder SiO2. The strong Fe–SiO2 interaction restrains the reduction in H2 and the carburization in syngas of the catalysts. The addition of both binder and precipitated SiO2 greatly influences the activity and selectivity of FTS, and obviously improves its stability. For the FTS reaction tests, it was found that the catalyst activity is decreased with the increase of the amount of SiO2 incorporated. For the catalysts incorporated with the same levels of SiO2, the spray-dried catalyst has lower activity than the normal dried one. In addition, the catalyst incorporated with precipitated SiO2 has higher activity than that with binder SiO2. The FTS stability of the catalysts is improved with the incorporation of SiO2. The selectivity to gaseous hydrocarbons (C1–C4) and olefins decreases with the increase of SiO2 content. The spray-dried catalysts incorporated with either precipitated SiO2 or binder SiO2 produce more gaseous hydrocarbons and fewer olefins, while the normal-dried process and the addition of binder SiO2 cause a great increase in selectivity to light hydrocarbons. The selectivity to oxygenates in product decreases with the increase of SiO2 content in the catalyst. For the catalysts incorporated with binder SiO2, the spray-dried catalyst (SPUW) produces more oxygenates than the normal dried catalyst (FMSC), whereas the spray-dried catalyst incorporated with precipitated SiO2 (FSCP) produces the least oxygenates among all catalysts. [Copyright &y& Elsevier]

A systematic study has been carried out to investigate the impact of potassium promoter on the performance of a precipitated iron-manganese catalyst for Fischer–Tropsch synthesis (FTS). Characterization technologies of N2 physisorption, X-ray diffraction (XRD), Mössbauer effect spectroscopy (MES) and H2 thermal gravimetric analysis (H2-TGA) were used to study the effect of potassium on the textural properties, bulk phase composition and reduction behavior. FTS reaction test was performed in a fixed bed reactor. The results of characterization showed that the addition of potassium leads to the relatively large crystallite size of α-Fe2O3 and inhibits the reduction of catalyst. The carbonization of the catalyst is enhanced with the increase in both the potassium content and the reaction temperature. A maximum in FTS and water-gas shift (WGS) activity is noted upon increasing K content (0.7 wt.% K), followed by a sharp decline in activity at the potassium level in excess of the maximum. It is found that potassium is an effective promoter to suppress the hydrogenation function of the catalyst. The selectivity to olefins is promoted and the formation of methane and light hydrocarbons is restrained with the increasing potassium level. The selectivity to oxygenates shows a rapid and monotonic decrease with the increase of potassium loading and passes through a minimum at potassium loading of 0.7 wt.%. After the point, it increases slowly with further increasing in potassium content. At the same time, increasing reaction temperature results in a monotonic decrease in the weight percent of oxygenates over the un-promoted and potassium-promoted catalysts. [Copyright &y& Elsevier]

A precipitated iron catalyst containing sulfur for Fischer–Tropsch (F–T) synthesis was prepared by means of a novel method using a ferrous sulfate as precursor. Both fixed bed reactor (FBR) and continues stirred tank slurry reactor (STSR) were used to test long-term F–T reaction behaviors over the catalyst. A stability test (1600 h) in FBR showed that the catalyst was active even after 1500 h of time-on-stream with CO conversion of 78% and with C5+ hydrocarbon selectivity of 72 wt% at 250 °C, 2.0 MPa, 2.0 NL/g-cat/h, and H2/CO=2.0. The test (550 h) in STSR indicated that the catalyst exhibited relatively high activity with CO conversion of 70–76% and C5+ selectivity of 83–86 wt% in hydrocarbon products under the conditions of 260 °C, 2.0 MPa, 2.0 NL/g-cat/h, and H2/CO=0.67. The deactivation rate of the catalyst was low, accompanied by surprisingly low methane selectivity of 2.0–2.9 wt%. It is shown that a small amount of sulfur (existing as SO42−) may promote the catalyst by increasing activity and improving the heavier hydrocarbon selectivity. It is also comparable with other typical iron catalysts for F–T synthesis. [Copyright &y& Elsevier]

The kinetic experiments of Fischer–Tropsch synthesis (FTS) over an industrial Fe–Cu–K catalyst are carried out in a micro-fixed-bed reactor under the conditions as follows: temperature of 493–542 K, pressure of 10.9–30.9 bar, H2/CO feed ratio of 0.98–2.99, and space velocity of 4000–10 000 h−1. The effects of secondary reactions of olefins are investigated by co-feeding C2H4 and C3H6. A detailed kinetics model taking into account the increasingly proven evidence of the olefin re-adsorption mechanism is then proposed. In this model, different sites are assumed for FTS reactions and water gas shift (WGS) reaction, respectively. Rate expressions for FTS reactions are based on the carbide polymerisation mechanism, in which olefin re-adsorption is considered to be a reverse step of olefin desorption reaction. Rate expression for WGS reaction is based on the formate mechanism. An integral reactor model considering both FTS and WGS kinetics is used to describe the reaction system, and the simultaneous estimation of kinetic parameters is conducted with non-linear regression procedure. The optimal model shows that the rate determining steps in FTS reactions proceed via the desorption of hydrocarbon products and the adsorption of CO and the slowest step in WGS reaction is the desorption of gaseous carbon dioxide via formate intermediate species. The activation energies of FTS reactions and WGS reaction are in good agreement with literature values. [Copyright &y& Elsevier]

A theoretical chemistry approach combining Wulff construction, density functional theory, and microkinetics enables revealing the nature of facet-dependent key elementary reactions on single-phase iron carbide at single particle level, complementary to current limited understanding of this issue in experimental exploration of iron-catalyzed Fischer-Tropsch Synthesis. • Intrinsic catalysis of a single-phase iron carbide particle in FTS was studied. • Neither too strong nor too weak H adsorption is favorable for CH 4 formation. • Coupling of two H-deficient hydrocarbon intermediates is more facile. • The surface (111) and (10 1 -) are kinetically more viable for C 2 formation. • CH 4 formation is kinetically more facile on surface (010), (110) and (11 1 -). Elucidation of intrinsic working principle of single-phase iron carbide and its facet-dependent catalytic behavior remains a substantial challenge in iron-catalyzed Fischer-Tropsch synthesis. Here, we provided in-depth understanding of the iron carbide phase-dependent and facet-dependent properties on theoretically established Fe 5 C 2 particle model through an approach combining Wulff construction, density functional theory, and microkinetics. We studied two key probe reactions, C 2 formation via C 1 +C 1 coupling and CH 4 formation, by monitoring surface-dependent thermodynamics and microkinetics. Integration of results thereby allows us to assess the macroscopic catalytic properties at single particle level and evaluate contribution from individual exposed surface. The surface (111) and (10 1 -) are kinetically more viable for C 2 formation, whereas CH 4 formation is kinetically more facile on surface (010), (110) and (11 1 -). This study enriches the knowledge of the intrinsic working mechanism for single-phase iron carbide and provides fundamental insights into rational design of improved iron-based Fischer-Tropsch synthesis catalysts. [ABSTRACT FROM AUTHOR]

• The dispersion of iron species is greatly enhanced by B 2 O 3. • The interaction between iron and boron render the electron deficiency of iron species. • The promotion of B 2 O 3 result in higher tendency to H 2 rather than CO for doped catalysts. • B 2 O 3 greatly inhibit the coke deposition of catalysts and promotes the hydrogenation activity of catalyst. Experimental improvement on the suppression of coke deposition remains a substantial challenge in iron-catalyzed Fischer-Tropsch Synthesis. Here, we performed a thorough study to provide in-depth understanding of the coking resistant as well as intrinsic promotive effect of boron oxide on the iron-based catalysts, by excluding the possible interference from another components. It turns out that the dispersion of catalysts is improved by boron oxide doping, which simultaneously inhibit the reduction of iron species in the catalysts. Strikingly, both the coking resistant behavior and hydrogenation activity of single active site of catalysts is greatly enhanced with addition of boron oxide. Such dual pronounced influence has been tentatively reasoned as the electronic interaction between Fe and B that render the electron deficiency of iron species, which thereby result in weaker adsorption capability of CO while inversely promote the chemisorption of H 2 for B 2 O 3 -doped catalysts. As a consequence, the ratio of H/C on the surface of B 2 O 3 -doped catalysts is greatly enhanced with respect to undoped one, thus facilitate the formation of hydrocarbons via hydrogenation of surface carbon species rather than the competitive coke deposition. [ABSTRACT FROM AUTHOR]