DOI: 10.1002/cctc.201200241

Preparation and Catalysis of Carbon-Supported IronCatalysts for Fischer–Tropsch SynthesisBo Sun,[a] Ke Xu,[a] Luan Nguyen,[b] Minghua Qiao,*[a] and Franklin (Feng) Tao*[b]

1. Introduction

Fischer–Tropsch synthesis (FTS) is the core of the coal-to-liquid,gas-to-liquid, biomass-to-liquid,[1–3] and Fischer–Tropsch-to-ole-fins (FTO)[4] processes. Among these processes, FTS based onsyngas derived from biomass is believed to be a more promis-ing future technology because of the vast abundance and re-producibility of biomass and nearly zero net CO2 emissionwhen utilizing biomass.[5] Fe is especially suited for the produc-tion of liquid hydrocarbons from highly H2-deficient syngasfrom biomass because of its unique water–gas shift (WGS) ca-pability. Fe-based FTS catalysts have additional advantages ofwide availability, low price, and low sensitivity to poisons.[6–8] Inaddition, Fe-based catalysts offer more possible FTS products(paraffins, olefins, and alcohols) in comparison to typical Co-and Ru-based FTS catalysts. These merits make Fe a technologi-cally and economically competitive candidate in the develop-ment of new FTS catalysts with enhanced performance.[9]

It is well documented that in both academic research and in-dustrial application, the supports for Fe-based FTS catalysts aremostly limited to conventional oxides such as SiO2, Al2O3, TiO2,and mixed SiO2–Al2O3 oxides including zeolites.[10–14] Althoughcarbon materials have distinguished merits as supports, suchas high specific surface area, diverse pore structure, superiorchemical inertness, and good recycling characteristics,[15–18]

they were used only occasionally in the early years and mainlyrestricted to activated carbons (ACs), presumably owing to thelack of a fundamental understanding of many aspects ofcarbon materials in catalysis.[19] Since the beginning of this cen-tury, reports on FTS in the presence of catalysts using carbonsupports began to blossom along with a surge in interest incarbon materials.[20] To date, various types of carbon materials,such as ACs, carbon nanotubes (CNTs), carbon spheres (CSs),glassy carbon (GCs), and carbon nanofibers (CNFs), have beenemployed as supports for Fe-based FTS catalysts.

There are many advantages of using carbon materials ascatalyst supports. First, the texture of the support (specific sur-face area, pore size and distribution, and pore structure) re-markably influences the reduction and dispersion of activemetals.[21, 22] The higher surface area of carbon materials andtheir highly developed pores can directly lead to higher disper-sion and smaller particles of the active species, which usuallysignify a better catalytic performance.[19] Second, the rich sur-face chemistry of carbon materials makes the manipulation ofcatalytic behavior of carbon-supported catalysts possiblethrough proper activation and post-treatment methods of thesupport. The surface properties of carbon materials can be tail-ored by controlling the degree of graphitization, introducingnew surface sites (defects or functional groups), or changingthe relative population of the surface sites.[19, 23–25] For example,the degree of graphitization could influence the sintering re-sistance of metal crystallites.[26] The amounts and types ofoxygen-containing functionality on carbon materials (Figure 1)could change surface acid properties, thus influencing the in-teraction with the metal precursor and, consequently, the dis-persion of the metal. The beneficial effects of the surface func-tional groups on the catalytic selectivity have been nicely eluci-dated and interpreted by some groups.[27–29] Excellent reviewsare available on the detailed surface chemistry and electronic

Fischer–Tropsch synthesis (FTS) is essential for the transforma-tion of natural gas, coal, and biomass to clean transportationfuels and value-added chemicals. Traditionally, iron catalysts forFTS are predominantly fused iron catalysts and precipitatediron catalysts using silica as the support. Owing to an intensesurge in interest in carbon materials during recent years, alongwith the unique properties of these materials, such as high sur-face area, high porosity, and ample structures, carbon-support-ed iron-based FTS catalysts have attracted increasing attention.

In this detailed review of the progress of the Fe/C catalysts forFTS in the last three decades, particular emphasis is put ontheir preparation, characterization, and catalytic performancerelevant to the characteristics of carbon materials. This reviewis intended to be a valuable resource to researchers interestedin this exciting field of catalysis, as well as the foundation forthose investigating applications of novel carbon materials. Abrief discussion is also devoted to the challenges and opportu-nities regarding the future development of Fe/C FTS catalysts.

[a] B. Sun, K. Xu, Prof. Dr. M. QiaoDepartment of Chemistry and Shanghai Key Laboratory of Molecular Catal-ysis and Innovative MaterialsFudan University, Shanghai (P.R. China)E-mail : mhqiao@fudan.edu.cn

[b] L. Nguyen, Prof. Dr. F. (. TaoDepartment of Chemistry and BiochemistryUniversity of Notre DameNotre Dame, Indiana 46556 (USA)E-mail : ftao@nd.edu

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properties of carbon materials,[19, 25] which are essential for thepreparation of catalysts with improved performance. In addi-tion, an appropriate metal–support interaction would ensurea good catalytic performance, and in fundamental research,the use of an inert carbon material as the support would sub-stantially simplify the interpretation of the intrinsic behavior ofthe metal.[30] Third, the emergence of novel carbon materialswith intriguing structures could provide a new possibility forthe catalyst design based on different metal–carbon combina-tion modes.[31–34] In some carbon-supported Fe-based FTS cata-lysts, intimate contact between Fe nanoparticles and carbonsupports remarkably facilitates the formation of iron carbide,which is acknowledged as the real active phase in the Fe-cata-lyzed FTS reaction, thus giving very impressive FTS results.[35, 36]

In this Review, we comprehensively summarize research ef-forts concerning carbon-supported Fe-based FTS catalysts. Thephysical and chemical properties, along with the synthesisstrategies of the carbon materials involved in the preparationof the Fe/C FTS catalysts are concisely described in each sec-tion. The preparation methods, typical physical characteristics,and FTS results of diverse Fe/C catalysts are presented indetail, with a special emphasis on the fundamental interpreta-tion of the role of the carbon supports in the catalysts. Lastly,we briefly discuss some of the scientific challenges and oppor-tunities in this field.

2. Carbon-Supported Fe-Based Catalysts:Preparation, Characterization, and FTSPerformance

2.1. Activated carbon-supported Fe-based FTS catalysts

Activated carbon, also known as active carbon or activatedcharcoal, is a well-known porous carbon material with highlydeveloped micro–mesopores and surface area. The ACs aremanufactured in two ways: chemical activation and physicalactivation. The ACs are manufactured chemically by the simul-taneous carbonization and activation of the raw carbonaceousmaterials at 600–800 8C. The activating agent such as H3PO4 orZnCl2 is incorporated into the raw material before heatingstarts.[37] Physically activated carbon is manufactured froma precarbonized material, which is obtained by the thermal de-composition of a carbonaceous precursor at 600–800 8C in theabsence or under controlled flow of air.[38] The activation step

is usually performed in the presence of steam and/or CO2 at800–1100 8C.

Owing to the highly porous structure and large surface area,the ACs are extensively used for impurity or poison removaland decoloration for chemical refinement, as electrode materi-als for batteries, adsorbents for fuel gas storage, and catalyticsupports.[39] Table 1 summarizes the detailed FTS reaction con-ditions and typical FTS products over various AC-supported Fecatalysts. The ACs were initially used as the support for the Fe-based FTS catalyst by Vannice and co-workers.[40, 41] They pre-pared two kinds of Fe/C catalysts : Fe on V3G, a low-surfacearea graphitic carbon; Fe on Carbolac-1, a porous, high-surfacearea carbon black. The Fe particles on V3G were large, whereasthose on Carbolac-1 were highly dispersed, which revealed su-perparamagnetism and oxidation-sensitive characteristic ofvery small particles. The decreased reducibility of the Fe/Carbo-lac-1 catalyst was attributed to the presence of a support inter-action. The small Fe particles on the Fe/Carbolac-1 catalystshowed greatly decreased H2/CO chemisorption ratios at tem-peratures up to 200 8C. Accordingly, in the FTS reaction thiscatalyst demonstrated a high olefin/paraffin ratio. Venter et al.prepared highly dispersed bimetallic Fe–Mn catalysts ona high-surface area amorphous carbon black using Fe–Mn andK-Fe-Mn carbonyl clusters.[42] The Fe–Mn clusters, either pro-moted by K or not, showed a high selectivity to light olefins,which was presumably attributed to the formation ofa (Fe1�yMny)3O4 surface spinel. Jones et al. investigated the ef-fects of crystallite size on the FTS behavior of the Fe/AC cata-lysts.[43] Consistent with the previous work, the Fe/AC catalystswere highly selective for olefins as compared to unpromotedFe on Al2O3 or SiO2. The olefin/paraffin ratio increased with in-creasing crystallite size. The specific activity and the activationenergy increased significantly with increasing Fe crystallite size.The significantly lower activity of 1 and 3 wt % Fe/AC catalystswas partly attributed to electronic modifications in the smallFe clusters caused by the metal–support interaction.

Notably, in a recent in situ X-ray photoelectron spectroscopystudy, de Smit et al. evidenced that higher temperature andlonger time were required for the reduction of approximately5 nm iron oxide nanoparticles on the Si wafer than requiredfor the reduction of iron oxide nanoparticles of approximately100 nm in diameter.[21] This work provides an alternative ex-planation for the metal–support interaction claimedabove.[41, 43]

The effect of support treatment on the structure and stabili-ty of Fe particles on the Fe/AC catalysts was investigated byPhillips and co-workers.[44] A porous carbon black support wassubjected to different H2/air treatments, followed by impregna-tion with Fe3(CO)12. For AC that was not exposed to air after H2

treatment, a large fraction of the carbonyl-derived Fe particleswas highly dispersed and remained sinter-resistant. In contrast,Fe supported on H2-treated and air-exposed AC sintered morequickly, whereas AC without H2 treatment yielded large parti-cles similar to those on graphitic supports. The high stability ofthe Fe particles on AC treated solely by H2 was proposed to bea consequence of the formation of a Fe-O-C linkage betweenthe Fe particle and the surface carbon atom, in which the

Figure 1. Some types of oxygen surface groups in activated carbon. Repro-duced from Ref. [19] .

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bridging oxygen might come from the dissociation of the COligands during Fe3(CO)12 decomposition. Martin-Martinez andVannice examined the effects of porosity in the ACs and theimpregnation solvent on the dispersion and stability of ironparticles.[45] The 5 wt % Fe/AC catalysts were prepared fromFe3(CO)12 dissolved in dry degassed THF and benzene throughincipient wetness impregnation under anaerobic conditions. Byusing THF as the solvent and an AC with wide pore size distri-bution containing a large fraction of large micropores, opti-mum resistance to sintering under FTS reaction conditions orin H2 at higher temperatures was observed, with iron particlesize remaining at approximately 2 nm. However, there wasgenerally no significant effect of the differences in microporosi-ty and in pore size distribution on the activity or selectivity.Consistent with the finding by Jones et al. ,[43] the specific activ-ities increased with crystallite size, and the activation energiesfor both hydrocarbon formation and methanation were lowerfor small iron crystallites.

The early studies[40–45] mainly focused on the understandingof the interaction between the metal and the carbon surface,and usually C1–C6 light hydrocarbons at low conversion levelswere reported. Ma et al. investigated the relationship betweenthe origin of the AC precursors, surface property, and the FTSperformances of the Fe-Mo-Cu-K/AC catalysts at high conver-sion levels.[46] The FTS activity could be related directly to thetextural parameters and the relative fraction of metal crystalli-tes present in the meso- and macropores (wide pores) or inver-sely to the metal crystallite size. The surfaces of all four ACsare primarily covered by neutral and/or basic oxygen-contain-

ing groups, along with small amounts of acidic oxygen groups.The selectivity to C5 + products could be related to the numberof basic and neutral groups, whereas the high selectivity toCH4 correlated well to the high acidity of the external surface.This finding reveals the possibility of tailoring the distributionof FTS products by surface engineering of carbon materials.

In an attempt to synthesize hydrocarbons with longer chainlength, Ma et al. prepared unpromoted and Cu- and K-promot-ed Fe/almond AC through incipient wetness impregnation.[47]

The strong promoter–AC interaction enhanced the H2 desorp-tion ability of the promoted Fe/AC catalysts. The Fe-Cu-K/ACcatalyst presented moderate syngas conversion and high gas-eous selectivity in a slurry-phase reactor during 166 h of test-ing. Hydrocarbons up to C18 were formed. Cu facilitates the re-duction of Fe; however, high Cu loadings might suppress thecarbonization of iron, increase the deposition of carbon on thecatalyst surface, and/or form some Fe–Cu clusters during re-duction, which are adverse to the FTS activity. In some cases,Cu might enhance hydrogen adsorption and thus hydrogena-tion/isomerization and impede CO dissociation, which changesthe types of olefins and the selectivity and distribution of oxy-genates.[48]

To improve the catalytic performance of AC-supported Fe-based FTS catalysts, various promoters were added (seeabove). Ma et al. investigated the promotion effect of Mo onthe Fe-Cu-K/AC catalysts.[49] A strong interaction between Feand Mo oxides was identified, which lowered the reductiondegree of Fe. Hydrocarbons up to C34 were detected on theFe-Cu-K/AC catalyst with or without Mo. However, addition of

Table 1. FTS reaction conditions and catalytic results over Fe/AC catalysts.[a]

Catalyst Reaction conditions CO conv. [%] Selectivity [wt %] Olefin/paraffin ratio Ref.CH4 C5 + C=

Fe/Carbolac-1 250 8C, 0.1 MPa, H2/CO = 3,plug-flow reactor

3.2 (to hydrocarbons) 40 (mol %) 8 (mol %) 1.5 (C2–C3) [40, 41]

KMnFe/CSX-203 290 8C, 0.1 MPa, H2/CO = 3,plug-flow reactor

1.4 (to hydrocarbons) 25 (mol %) 0 75 (mol %)

[42]KMnFe2/CSX-203 250 8C, 0.1 MPa, H2/CO = 3,

plug-flow reactor1.7 (to hydrocarbons) 28 (mol %) 8 (mol %) 64 (mol %)

Fe–Mo–Cu–K/AC 320 8C, 2.2 MPa, H2/CO = 0.9,fixed-bed reactor

37.9–90.1 10.9–15.3 31.3–52.3 [43]

Fe/UU-1636 225 8C, 0.1 MPa, H2/CO = 2,fixed-bed reactor

4.1 [44]

Fe/CSX-203 225–265 8C, 0.1 MPa, H2/CO = 3,plug-flow reactor

2.3 (to hydrocarbons) 40–50 (mol %) �2.9 (C2 + C3) [45]

Fe/AC 275 8C, 0.1 MPa, H2/CO = 1,plug-flow reactor

3.7 (to hydrocarbons) 33 (mol %) 12 (mol %) 40 (mol %) 8.0 (C2 + C3) [46]

Fe-Cu-K/AC 304 8C, 3.0 MPa, H2/CO = 2,slurry-phase reactor

�45 (CO + H2) �14 �40 [47]

Fe-Cu-K/AC 260–270 8C, 2.2 MPa, H2/CO = 0.9,fixed-bed reactor

32.0–41.5 (CO + H2) 1.8–3.9 (C2)3.2–6.4 (C4)1.7–3.6 (C5)

[48]

Mo-Fe-Cu-K/AC 310–320 8C, 2.2 MPa, H2/CO = 0.9,fixed-bed reactor

7.2–15 39.4–51.6 2.0–6.0 (C2-C4)0.9–1.5 (C5-C11)

[49]

K-Fe-Cu-Mo 260–300 8C, 2.2 MPa, H2/CO = 0.9,fixed-bed reactor

29.4–96.9 5.7–34.4 16.5–61.1 0.10–6.86 (C2-C4)0.17–1.29 (C5-C11)

[50]

Fe/AFC 260–300 8C, 3 MPa, H2/CO = 2,fixed-bed reactor

51–95 11–27 (C1-C4) 46–82 [13]

[a] AC = activated carbon; AFC = activated fibrous carbon; FTS = Fischer–Tropsch synthesis.

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a suitable amount of Mo into the Fe-Cu-K/AC catalyst im-proved the catalytic stability without sacrificing the activity,which is attributed to the positive role of Mo in preventing Feparticles from agglomeration during reduction and reaction.Interestingly, Mo greatly enhanced secondary reactions of ole-fins, which leads to a large amount of internal olefins (int-ole-fins), an indication of the role of Mo as a solid acid. In a succes-sive work, the effect of K on the FTS performance and productdistribution (hydrocarbons and oxygenates) was studied overthe Fe-Cu-Mo/AC catalyst.[50] Both FTS and WGS activities ofthe catalyst were affected by the addition of K, which resultedin a significant suppression of the formation of CH4 and meth-anol, and shifted the selectivity to longer hydrocarbons (C5 +)and alcohols (C2–C5). The former was attributed to the forma-tion of more iron carbides facilitated by K. Meanwhile, Kchanged the distributions of paraffins and olefins. At least forC�25 hydrocarbons, increasing the amount of K to 0.9 wt %greatly reduced the amount of n-paraffins and int-olefins anddramatically increased iso-paraffins and 1-olefins. In addition,among the alcohol products, the amount of ethanol was thehighest, followed by propanol, methanol, butanol, andpentanol.

Fibrous activated carbon, also called activated carbon fiberor activated fibrous carbon (AFC), is a kind of AC in the formof fibers, filaments, yarns or rovings, and fabrics or felts accord-ing to the definition by IUPAC. AFC can be manufactured bycarbonization and activation of synthetic or natural fibrous car-bonaceous materials.[51–55] Similar to other ACs, AFC is charac-terized by high surface area and high porosity, which makes ita promising support for the preparation of catalysts with in-creased specific activity. Krylova et al. prepared Fe/AFC cata-lysts for FTS through incipient impregnation.[13] Their AFC wasmanufactured by the carbonization of a nonwoven fabric pre-pared from hydrated cellulose fiber, followed by steam activa-tion at a temperature up to 900 8C. The Fe/AFC catalystshowed a higher selectivity to liquid hydrocarbons than that ofconventional fused Fe catalyst, and this attractive result was at-tained at a lower temperature. The promotion of the Fe/AFCcatalysts with K and aluminum oxides or with Cu reduced theoptimum FTS temperature by 6–8 8C and enhanced the poly-merizing activity of the catalysts, which resulted in a chaingrowth probability (a) of 0.85–0.86; this is very impressive forFe catalysts, which usually have a chain propagation ability in-ferior to that of Co and Ru.

2.2. Carbon nanotube-supported Fe-based FTS catalysts

The CNTs can be envisioned as a tubular structure formed bya rolled-up sp2 hybridized-carbon (graphene) sheet(s).[56] Ac-cording to the number of the layer(s) constituting the wall,single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs)are defined. SWCNTs are similar to fullerenes in size and havesingle-layer cylinders extending from end to end with gooduniformity in diameter (1–2 nm).[57, 58] If produced in the vaporphase approaches, SWCNTs self-assemble into larger bundles(ropes) that consist of several tens to hundreds of nanotubes,owing to attractive dispersive forces.[59–61] MWCNTs were dis-

covered before SWCNTs.[62] The morphology of MWCNTs isclose to hollow graphite fibers, except for their much higherdegree of structural perfection. The structure of MWCNTs canbe regarded as a concentric assembly of two to several tens ofCNT units (cylinders). The high electrical and thermal conduc-tivity, high mechanical strength, and functionalizable surfacesof the CNTs have evoked wide interest for catalyticapplications.[63]

The synthetic methods for CNTs are mainly arc discharge,laser vaporization, catalytic combustion, and chemical vapordeposition (CVD).[64–67] The synthesis of CNTs by using the CVDmethod is facile, and the apparatus can be conveniently set upin a laboratory. As this method can be easily scaled up for mas-sive production, it has become the most popular technique forCNT synthesis.[68] In this method, hybrid metal/metal oxide, var-ious ceramics, and semiconductors can serve as catalysts forthe growth of CNTs.[69] Late 3d valence transition metals, suchas Fe, Co, and Ni, have been widely used in the CVD methodfor the catalytic growth of CNTs.[68] Several groups recentlyfound that the CNTs can also be grown on catalysts dependingon metals, such as Au, Ag, and Cu.[70]

The CNTs provide a well-defined structure at the atomiclevel and more uniform nanopore size distribution as com-pared to conventional AC supports. The inertness of the CNTsleads to relatively weak metal–support interactions, whichmakes it a unique support for the study of the catalytic perfor-mance of metals thereon.[71, 72] The special and robust structuralcharacteristics and abundant morphologies of the CNTs aresuitable for use as catalytic support materials in FTS.[73–75]

Only one year after the discovery of the MWCNTs, Moy pa-tented the preparation of a CNT-supported catalyst for the FTSby using the deposition precipitation method.[76] In a fluidized-bed reactor at 340 8C, 2.5 MPa, and H2/CO of 6, olefin selectivi-ties of 70 % in the C5–C10 fraction and 60 % in the C11–C18 frac-tion were reported. One decade later, van Steen and Prinsloostudied the effect of the preparation methods on the FTS per-formance of the Fe/CNT catalysts.[73] The catalysts were pre-pared through incipient wetness impregnation and depositionprecipitation with use of K2CO3 or urea of iron oxide on her-ringbone CNTs. These catalysts were promoted with Cu, andthe K2CO3-precipitated catalyst was also promoted by K. AfterH2 reduction, the Fe crystallite sizes of the catalysts preparedby incipient wetness impregnation and deposition precipita-tion with use of urea were 9.6 and 8.7 nm, respectively, where-as that prepared by deposition precipitation with use of K2CO3

was approximately 33 nm. Under reaction conditions of 220 8C,2.5 MPa, syngas containing 53.6 % H2, 32.3 % CO, 13.0 % CH4,0.4 % N2, 0.7 % CO2 (volume percentage), and 1860 cm3(STP)g�1 h�1, the catalyst prepared by incipient wetness impregna-tion was the most active. The possible difference in particlesize distribution was proposed to account for the activity dif-ference between the catalysts prepared by incipient wetnessimpregnation and deposition precipitation with use of urea. Incontrast, the selectivities to hydrocarbons seemed to be inde-pendent of the preparation method, and the a values of theC3–C6 fraction are 0.64–0.67 and the C8–C13 fraction is 0.83 onthese catalysts.

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Similarly, Bahome et al. prepared Fe/CNT catalysts by incipi-ent wetness and deposition precipitation with use of urea fol-lowed by K and/or Cu promotion.[77] Different from van Steenand Prinsloo,[73] they used tubular CNTs with both ends open.The Fe particles of approximately 15 nm homogeneously cov-ered the surface of CNTs. The preparation method did notaffect CO conversion and product selectivity, which is consis-tent with the previous report.[73] The K-promoted Fe/CNT cata-lyst prepared through deposition precipitation gave the high-est a value and a C2 olefin to total C2 hydrocarbon weightratio of 0.72. No deactivation was observed during 120 h onstream over all the catalysts used here, which is different fromthe finding of van Steen and Prinsloo.[73] This might be due tothe differences in the structure of their CNTs (herringboneversus tubular) and/or the reaction conditions between thesestudies. Guczi et al. compared the FTS performances of twoFe/MWCNT catalysts, prepared through simple impregnation ofiron acetate (I-Fe) and deposition of preformed iron oxidenanoparticles of approximately 6 nm in diameter (P-Fe) on theMWCNTs.[78] The I-Fe catalyst was highly reducible. Althoughthe Fe particles on the I-Fe catalyst were rather large and non-uniform in size after reduction, they became smaller and ratheruniform after the FTS reaction. The I-Fe catalyst had a highercatalytic activity and higher selectivity toward C2–C4 and C5 +

hydrocarbons than did the P-Fe catalyst. However, the effect ofparticle size differences on the FTS activity reported by Vanniceand co-workers[40] was not identified, which might be due tothe differences in the preparation methods for the I-Fe and P-Fe catalysts.

In the works reviewed above, the CNTs were used just likeconventional AC supports. The unique electronic propertiesand uniform channels of the CNTs on the FTS performancewere surprisingly not explored and used. For tubular CNTs, the-oretical studies have demonstrated that the deviation of thegraphene sheets from planarity shifts p-electron density fromthe concave inner surface to the convex outer surface(Figure 2),[79] which makes the inner surface electron deficientand the outer surface electron rich.[80, 81] Bao and co-workers el-

egantly presented that the autoreduction of Fe2O3 within theCNT channels was facilitated as compared to that of the parti-cles on the exterior.[82] Moreover, the reduction temperaturedropped monotonically with the channel size of the CNTs.[83]

These findings strongly hint that the location of the iron spe-cies on the CNTs might exert a significant effect on the FTSperformance. On the other hand, because the closeness of thesurrounding walls to the adsorbed molecule can maximize theattractive van der Waals interaction, the nanotube interior isexpected to have a high binding energy toward molecules.[84]

Moreover, at low concentrations of molecules, theoretical stud-ies pointed out that diffusion inside CNTs should be quick,mainly because of the smoothness of the potential along theCNT wall. At high adsorbate densities, there are strong indica-tions that the diffusion of some molecules inside SWCNTs issignificantly faster than in the bulk, as a result of ordering ofthe confined molecules in the nanotubes.[84] Thus, conductinga reaction on metal nanoparticles inside the CNTs does notnecessarily mean a sluggish kinetics.

Following their intriguing findings, Bao and co-workers[35]

successively studied the confinement effect of iron particles inthe CNT channels on the FTS performance. Close-capped CNTswere used for the deposition of iron particles selectively onthe exterior of the CNTs (Fe-out-CNT) by using the impregna-tion method. The catalyst with iron particles inside the CNTs(Fe-in-CNT) was prepared through the ultrasonication-aidedimpregnation of the iron salt into the 200–500 nm long CNTsthat were opened up and cut into segments by refluxing inconc. HNO3. The ratios of integral diffraction peaks of iron car-bide to those of iron oxide were approximately 4.7 for Fe-in-CNT and 2.4 for Fe-out-CNT under the same conditions, whichis most likely caused by the difference in electronic propertiesbetween the interior and exterior of the CNTs. This differencein phase composition readily correlated with the notably in-creased FTS activity and the twofold higher yield of C5+ hydro-carbons over Fe-in-CNT as compared with those over Fe-out-CNT. An additional advantage of channel encapsulation is thespatial restriction of the aggregation of the catalyst particles.The above results nicely exemplify that the confinement ofiron inside the CNTs remarkably modifies the catalytic perfor-mance, which leads to a more active, selective, and stable FTScatalyst. This intriguing work aroused great interest in thestudy of the preparation parameters of the Fe/CNT catalysts. Toexclude the possible interference between the differences inthe pretreatment method of the CNTs and the confinementeffect, Abbaslou et al. developed a simple method to controlthe position of the iron species on either the inner or theouter surface of the CNTs of the same origin.[85] More than 70–80 % of the iron oxide particles could be controlled to accom-modate at the inner or outer surface of the CNTs by their ap-proach. Although there was no apparent difference in the COconversion on these catalysts, easier reduction of iron oxideand higher selectivity for longer hydrocarbons were observedon the catalyst with iron inside the CNTs, which substantiatedthe confinement effect in such a catalyst system. In addition,the deposition of iron on the internal surface of the CNTs re-sulted in a more stable catalyst whereas its counterpart experi-

Figure 2. Contour for the charge density of a (6,0) CNT in a plane perpendic-ular to the tube axis. The circle represents a cross-section in which sixcarbon atoms are located. CNT = carbon nanotube. Reproduced fromRef. [79] .

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enced deactivation withina period of 125 h owing tosintering.

Acid treatment has a profoundeffect on the surface and textur-al properties of the CNTs,[86]

which closely relates to the dis-persion and position of theactive species. Dalai and co-workers systematically investi-gated the effects of acid treat-ment on the activity, selectivity,and stability of the Fe/CNT cata-lysts.[72] Two CNTs with low andhigh surface areas were treatedwith HNO3 at 25 and 110 8C. Ironwas introduced by using incipi-ent wetness impregnation. Acidtreatment led to larger Bruna-uer–Emmett–Teller (BET) surfaceareas and more defects, whichfavor the formation of smaller metal particles. The iron parti-cles were mainly homogeneously distributed in the channelsof the acid-treated CNTs. High-temperature acid treatment ledto stable Fe/CNT catalysts during 120 h on stream, whereasother catalysts experienced quick deactivation. The Fe catalystsupported on the CNTs with lower surface area and larger porediameter showed the highest activity, the lowest CH4 selectivi-ty, and the highest C5 + selectivity, which were attributed tothe high extent of carbonization of iron on this catalyst. Thesame group has also studied the effects of the channel size ofthe CNTs on the FTS performance.[87] Two CNTs with differentaverage channel dimensions of 12 and 63 nm but similar sur-face areas were used. Iron was again loaded through incipientwetness impregnation. The iron oxide nanoparticles weremainly encapsulated in both CNTs, with large pore size favor-ing large nanoparticles (Figure 3). The CO conversion on theFe/narrow-pore-CNT (Fe/np-CNT) catalyst was 2.5 times of thaton the Fe/wide-pore-CNT (Fe/wp-CNT) catalyst, and the CH4 se-lectivity of the former was much lower than that of the latter.The iron particles in the narrow-pore CNTs did not undergoa significant agglomeration, whereas those in the wide-poreCNTs increased from 14–25 nm for the fresh catalyst to 22–38 nm for the catalyst after 120 h on stream. The better FTSperformance of the Fe/np-CNT catalyst was correlated with thehigher dispersion and higher reduction degree of the ironnanoparticles, attributable to the difference in electronic prop-erties of the inner surface of the CNTs with different diameters.

Aside from depositing iron on the interior of the CNTs, thenitrogen functionalization of the CNTs is very effective in im-proving the stability of the Fe/CNT catalyst with the iron nano-particles situated even on the exterior of the CNTs.[88] The CNTswas treated by gas phase nitric acid vapor at 200 8C for oxygenfunctionalization (O-CNTs) and then ammonia at 400 8C for ni-trogen functionalization (N-CNTs). Under industrial relevantconditions for olefin production over iron-based FTS catalysts,both the Fe/N-CNT and Fe/O-CNT catalysts demonstrated high

selectivity to C3–C6 olefins, and the selectivity of the latter washigher than that of the former. This result is remarkable, be-cause these catalysts were not modified by conventional pro-moters for olefin production. Although the Fe/nonfunctional-ized CNT catalyst was nearly inactive under such a demandinghigh-temperature FTS condition, the Fe/N-CNT catalyst witha nominal iron loading of 40 wt % maintained a high andstable level of CO conversion during 80 h on stream (Figure 4),which showed promise as a selective and robust catalyst forthe production of short-chain olefins.

Studies on CNT-supported bimetallic Fe–M FTS catalystshave attracted increasing interest recently. The geometric and/or electronic effect of the second metal could influence the en-semble size and/or electronic structure of the active sites,which allows for the possibility of altering the adsorption andactivation behaviors of the reactants.[89, 90] The interaction be-tween the two metal components might also impose an effecton the particle size and reducibility of the active species,

Figure 3. TEM images of the iron catalysts: a) Fe/narrow-pore-CNT and b) Fe/wide-pore-CNT. Dark spots representthe iron oxide particles inside and outside the nanotubes. CNT = carbon nanotube. Reproduced from Ref. [87] .

Figure 4. CO conversion as a function of time on stream for the 40 wt % Fecatalysts. Reaction conditions: 340 8C, 2.5 MPa, H2/CO = 1, and833 mL g�1 min�1. Reproduced from Ref. [88].

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which are closely related to the catalytic performance. Bahomeet al. prepared K- and/or Cu-promoted bimetallic Fe–Ru/CNTcatalysts through the co-impregnation of Fe and Ru salts onCNTs.[91] The Fe loadings were 2.8–11.8 wt %, and the Ru load-ings were 0.06–0.5 wt %. The Fe–Ru particles of 2.1 nm onaverage were well dispersed on the surface of the CNTs withpredominantly closed ends. Increasing Fe loading led to de-creased CH4 selectivity and increased C5–C11 selectivity. Al-though the higher relative Ru content could improve the Fereducibility and CO conversion, Ru also unfavorably promotedCH4 formation. All the bimetallic catalysts were stable in ap-proximately 120 h FTS testing, which indicated that the smallFe–Ru particles possessed remarkable stability in the FTS reac-tion. The moderate metal–support interaction characteristic ofthe CNT supports was suggested to be the reason for superiorsintering resistance. Tavasoli et al. investigated the effect of theCo/Fe ratio on the activity and selectivity of the bimetallic Co–Fe/CNT catalysts in the FTS reaction.[75] Four bimetallic Co–Fe/CNT catalysts were prepared through co-impregnation. In thiscase, the content of Co was much higher than that of Fe. Thesmall amounts of Fe enhanced the reducibility and dispersionof the bimetallic catalysts and resulted in the formation of Co–Fe alloys. A majority of the metal particles were homogeneous-ly distributed inside the CNTs. The presence of Fe remarkablyincreased the FTS reaction rate and CO conversion. The highestCO conversion was obtained on the 10 Co–0.5 Fe/CNT catalyst.The selectivity to C5+ hydrocarbons and the olefin/paraffinratio over the bimetallic Co–Fe catalysts were in-betweenthose of the monometallic Co/CNT and Fe/CNT catalysts. Themost attractive trait of the bimetallic Co–Fe/CNT catalysts isthe enhanced alcohol formation. The alcohol selectivity over

the 10 Co–4 Fe/CNT catalyst was as high as 26.3 %, whereas itwas only 2.3 and 10.3 % over the Co/CNT and Fe/CNT catalysts,respectively. The Co–Fe alloy formation might be responsiblefor the remarkably high alcohol selectivity.

As oxidization of the active phase under FTS reaction condi-tions is one of the most important factors that lead to the de-activation of the Fe catalysts, Yang et al. designed a novel Fe-based catalyst with cubic FeN particles of 4–10 nm confined inthe CNT channels.[92] Because of the presence of nitrogenatoms, iron nitrides demonstrate high resistance to oxida-tion.[93] For the synthesis of this FexN-in catalyst, Fe2O3 nanopar-ticles were introduced into the CNT channels first, followed bynitridation under NH3. The FexN-in catalyst was 1.4 times moreactive than the FexN-out catalyst with nitride particles on theexterior of the CNTs. Both FexN-in and FexN-out catalysts were5–7 times more active than the Fe-in-CNT catalyst, which is anindication of the better stability of iron nitride against oxida-tion through water produced during the FTS reaction. Notably,instead of iron carbides as the FTS-active phase on convention-al Fe-based catalysts, iron carbonitrides (Fe2CxN1-x, FeCxN1-x, andg“-FeN) were formed on the FexN-in and FexN-out catalystsduring FTS. There was more FeCxN1-x on the FexN-in catalystthan on the FexN-out catalyst, which is attributable to thesmaller particle size and the stronger retention of nitrogenatoms in the confined FexN catalyst. To advance the applicationof this novel FexN-in catalyst, the C5+ selectivity over the FexNcatalysts needs substantial improvement and the long-termstability remains to be explored. Detailed FTS reaction condi-tions and typical FTS products over these CNT-supported Fecatalysts are listed in Table 2.

Table 2. FTS reaction conditions and catalytic results over Fe/CNT catalysts.[a]

Catalyst Reaction conditions CO conv. [%] Selectivity [wt. %] Olefin/paraffin ratio Ref.CH4 C5+

DPUK 270 8C, 0.8 MPa, H2/CO = 2,fixed-bed reactor

84.9 9.99 68.10 2.6 (C2) [77]

I-FeP-Fe

�251 8C, 1.0 MPa, H2/CO = 2,plug-flow reactor

�11�58

�42�5

7.313.3

[78]

Fe-in-CNTFe-out-CNT

270 8C, 2 MPa, H2/CO = 2,fixed-bed reactor

4029

1215

2919

[35]

in-Fe/CNTout-Fe/CNT

270 8C, 2 MPa, H2/CO = 2,fixed-bed reactor

�85�79

25.640.5

36.223.8

[85]

Fe/ha-lsa-CFe/ha-hsa-C

275 8C, 2 MPa, H2/CO = 2,fixed-bed micro reactor

8674

8.723.7

70.252.8

1.65(C2-C4)0.25(C2-C4)

[72]

Fe/np-CNTFe/wp-CNT

275 8C, 2 MPa, H2/CO = 2,fixed-bed reactor

3012

14.541.0

48.512.0

[87]

20Fe/N-CNT20Fe/O-CNT40Fe/N-CNT40Fe/O-CNT

340 8C, 2.5 MPa, H2/CO = 1,fixed-bed reactor

48.326.581.950.0

96118

2.3 (C2–C6)9.0 (C2–C6)2.6 (C2–C6)11.5 (C2–C6)

[88]

5 Fe/0.25 Ru10 Fe/0.25 Ru

275 8C, 0.8 MPa, H2/CO = 2,fixed-bed reactor

5538

29.421.3

26.537.0

0.07 (C2)0.15 (C2)

[91]

CoFe/CNT 220 8C, 2 MPa, H2/CO = 2,fixed-bed micro reactor

10.8–�54 9.4–16.9 46.7–85 0.96–1.95 [75]

FexN-inFexN-out

300 8C, 0.5 MPa, H2/CO = 1,fixed-bed reactor

26.6–27.229.9–31.8

22.5–22.918.2–20.9

2.6 (C2–C4)3.2–3.4 (C2–C4)

[92]

[a] CNT = carbon nanotube; Fe-in-CNT = iron particles inside the CNT; Fe/np-CNT = Fe/narrow-pore-CNT; Fe-out-CNT = iron particles outside the CNT; Fe/wp-CNT = Fe/wide-pore-CNT; FTS = Fischer–Tropsch synthesis; hsa = high surface area; lsa = low surface area; N-CNT = nitrogen-functionalized CNT; O-CNT = oxygen-functionalized CNT.

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Carbon-Supported Iron Catalysts for Fischer–Tropsch Synthesis

2.3. Carbon sphere-supported Fe-based FTS catalysts

Here, the term CS is used to represent all carbon materials thathave a spherical or near-spherical contour, irrespective of theirpossible differences in size, texture, and structure.[94] The CSs—when functionalized—show promising applications in fieldssuch as drug delivery, lithium ion secondary battery, lubricatingmaterials, high-strength composites, and catalyst supports.[95]

The advantage of the CSs as catalyst supports associates withthe ease of synthesis without the use of catalysts, ability tocontrol the physical properties (size, purity, porosity), and highyields of pure materials.[94]

According to Deshmukh et al. ,[94] the CSs can be categorizedin terms of 1) texture (solid, core–shell, or hollow),[96] 2) stack-ing mode of the carbonaceous layers (concentric, radial, orrandom),[97] 3) dimension,[98] and 4) synthetic strategies. Thesynthetic methods can be either physical (e.g. , arc discharge,laser ablation, ultrasonic) or chemical (e.g. , CVD, pyrolysis, hy-drothermal treatment) synthesis. Several reviews on thesemethods have been published recently.[94, 99, 100] Among thesesynthetic strategies, the hydrothermal treatment of biomasshas gained increasing interest because this strategy fabricatesCSs in high quantity under relatively mild conditions (usually<250 8C and under autogenous pressure),[101, 102] and the appa-ratus for synthesis is technically much less demanding andthus less expensive than that for the physical approaches.[100]

Moreover, biomass is a renewable resource that is in abun-dance and readily available.[99] Through this hydrothermal strat-egy, it is facile to tailor the structure of the CSs (e.g. , size, sur-face functionality), which leads to versatile properties for mis-cellaneous applications.[102–106]

As to the Fe-CS materials, Ma et al. reported the synthesis ofcarbon-coated iron nanoparticles by means of a laser-inductionheating evaporation technique.[107] An iron cake was placed ina graphite crucible under 2 kPa of Ar. A high-frequency induc-tion power was then applied to melt the iron cake. Simultane-ously, a CO2 laser was used to irradiate and vaporize themolten iron. CH4 was introduced into the chamber and decom-posed therein to give the CSs. The diameter of the resultingcore–shell Fe-CSs was in the range of 5–50 nm.

a-Fe2O3- or Fe3O4 nanoparticle-loaded CSs from 4-ferrocenylbutyric acid through a facile one-step solvothermal methodwere prepared with use of the medium ethylene glycol.[95]

Smooth CSs with a diameter of approximately 900 nm wereobtained in large quantity, with round and uniform iron oxideof 16 nm in size mainly located on the surface. A mechanisminvolving dehydration–nucleation–carbonization and hydroly-sis-controlled oxidation of Fe was proposed for the formationof iron oxide-CSs. The size and morphology of iron oxide-CSscould be tuned by adjusting the preparation parameters, andthe surface of iron oxide-CSs had large numbers of functionalgroups, which make such material adaptable to demands ofvarious application fields.

A series of Fe/CS catalysts were prepared through incipientwetness impregnation and homogeneous deposition precipita-tion of iron oxide on functionalized CSs.[108] The CSs were pre-pared by using the CVD method at 900 8C, with acetylene as

the carbon source. Because the as-synthesized CSs had a lowBET surface area of approximately 1 m2 g�1, the spheres weretreated by HNO3 or KMnO4 to generate functional groups to fa-cilitate the anchorage of the iron species. The FTS performanceof the Fe/CS catalysts was studied under reaction conditions of275 8C, 0.8 MPa, H2/CO of 2, and 2760 h�1 in a fixed-bed micro-reactor. The weak Fe–C interaction is beneficial for metal re-duction. The promoting effects of K and Cu on the Fe/CS cata-lysts were generally in line with other literature reports. TheFe/CSs-K-IM catalyst (K refers to treatment with KMnO4 ; IMrefers to impregnation) demonstrated the lowest CH4 selectivi-ty of 3.2 % and the highest C5 + selectivity of 91.6 %. The excel-lent selectivity to long-chain hydrocarbons was attributed tothe effect of residual manganese oxide on the catalyst duringsupport functionalization, which enhanced the olefin formationand promoted the chain propagation. The Fe/CSs-C-DP catalyst(C refers to treatment with HNO3 at 90 8C; DP refers to deposi-tion precipitation) demonstrated the highest metal time yieldamong these Fe/CSs catalysts. For HNO3-treated Fe/CS cata-lysts, the higher treatment temperature led to the higher irondispersion and, consequently, the higher catalytic activity.These results offer a new means to adjust the FTS performanceby tailoring the surface chemistry of the CSs. However, the sta-bility of these low-surface area catalysts was not presented.

A two-step hydrothermal approach was used for the prepa-ration of Fe3O4–CS composite with Fe3O4 inside.[109] In this ap-proach, Fe3O4 particles of approximately 150 nm were presyn-thesized by using a hydrothermal method and then added toan aqueous solution containing glucose and Na2SO4. Hydro-thermal treatment at 160 8C for 24 h generated the Fe3O4–CScomposite with a dimension of approximately 8 mm. The ag-glomeration of the magnetic Fe3O4 particles might be responsi-ble for the formation of large Fe3O4–CS composite. Althoughthe two-step approach is less convenient than the one-stepapproach, it can take advantage of the abundant preparationprotocols of iron oxides.[110–115] In addition, it is a predictablemethod that can encapsulate iron oxide nanoparticles insidethe CSs.

One-pot synthesis of the iron oxide–CS hybrid material withencapsulation and high dispersion of iron oxide nanoparticlesin CS, which can effectively restrict the aggregation of thenanoparticles and thus maximize their utility, has long beena great challenge. Yu et al. demonstrated a novel but facileone-pot hydrothermal cohydrolysis–carbonization processusing glucose and Fe(NO3)3 as starting materials for the fabrica-tion of CSs embedded with iron oxide nanoparticles.[36] FexOy

nanoparticles as small as approximately 1 nm in size werehighly dispersed in the CSs of approximately 6 mm in diameter,which leads to a unique “plum-pudding” structure. The forma-tion mechanism of this FexOy@CS material was proposed(Figure 5). If (NH4)2Fe(SO4)2 is used as the iron precursor undersimilar hydrothermal conditions, the iron oxide nanoparticleswere predominantly located near the hydrophilic shell of theCSs,[116] which signify the important role of the iron source informing FexOy@CS with the plum-pudding structure.

Following this one-pot approach, Jia et al. prepared the g-Fe2O3/activated CS through CO2 activation of the FexOy@CS

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material at 700 8C.[117] The spherical morphology was well re-tained even after high-temperature carbonization. The g-Fe2O3/activated CS had a large BET surface area of 315 m2 g�1 and themost probable pore size of approximately 2 nm, which rendersit a good adsorbent for the removal of methyl orange asa model pollutant and a potential material in separation pro-cesses.

The reduced FexOy@CS showed excellent performance undertypical FTS reaction conditions of 270 8C, 2.0 MPa, and H2/COof 2 and in a tubular fixed-bed reactor.[36] Its activity and stabili-ty were superior to those of the Fe/CNT catalyst.[73] The normal-ized activity of the reduced FexOy@CS with respect to theweight of iron was comparable to those iron catalysts reportedin the literature, which is a direct evidence of the high accessi-bility of the iron species embedded in the catalyst to the reac-tants. Moreover, the C5–C12 fraction was up to 40 % in the hy-drocarbon products, which excelled the values reported so faron promoter-free Fe catalysts.[78] Mçssbauer absorption spec-troscopy (MAS) of 57Fe revealed that in the reduced FexOy@CS,more than 88 % of the iron species was iron carbides, whichmight explain its high activity and high C5 + selectivity.[118] After108 h on stream, iron carbide nanoparticles enlarged onlyslightly from approximately 7 to 9 nm. It was proposed thatthe surrounding carbonaceous materials not only facilitatedthe formation of more iron carbides during catalyst activation,which is essential for the formation of C5 + hydrocarbons, butalso restricted the aggregation of the iron carbide nanoparti-cles during the activation and reaction processes.

To obtain porous Fe-CS material with even larger surfacearea, a single-step continuous process using ultrasonic spraypyrolysis (USP) was developed by Atkinson et al.[119] In this ap-proach, solution containing sucrose and iron salt was ultrasoni-cally aerosolized and pyrolyzed, during which sucrose was de-hydrated and the iron salt was converted to iron oxide. PorousUSP Fe-CSs of 0.5–3 mm in diameter were harvested. The ironoxide nanoparticles of 4–90 nm were well dispersed in USP Fe-CSs with iron loadings between 1 and 35 wt %. A high disper-

sion of iron oxide was remarkably retained even at the highloading of 35 wt %. Surface areas up to 800 m2 g�1 were ob-tained. As demonstrated by the authors, good accessibility ofthe internal iron species along with high dispersion of highlyloaded iron makes the USP Fe-CS material a potentially high-performance catalyst for the FTS reaction.

2.4. Fe-based FTS catalysts supported on miscellaneouscarbon

Beside carbon materials mentioned in above sections, thereare scattered reports on GC- and CNF-supported Fe catalystsfor FTS. GCs with glassy appearance are nongraphitizingcarbon materials that combine glassy and ceramic propertieswith those of graphite. They are usually prepared by pyrolyz-ing cross-linked polymers such as phenolic resins,[120] thermo-setting resins,[121] and lignins and lignin condensates[122] undercontrolled conditions.[123] A recent research disclosed that GCscontain a fullerene-related structure (Figure 6).[121] The uniquestructure renders the GCs distinguished properties of very highthermal stability and extreme resistance to oxygen attack andacid attack.

Moreno-Castilla et al. explored different ways of adding ironto the GC-forming mixture, which showed pronounced effectson the nature of the porosity in the resulting GCs.[124] The GCscould be further modified by adding K or B to the mixture. Theauthors suggested the potential of these Fe-containing GCs ascatalysts; however, the catalytic application of Fe/GCs in FTS

Figure 5. Illustration of the formation process of the FexOy@C spheres:a) aqueous solution containing glucose and iron nitrate, b) carbonaceouscolloids and iron oxide nanoparticles generated at the initial stage of the hy-drothermal treatment, c) carbonaceous nanorods embedded with iron oxidenanoparticles, d) large aggregate from further polymerization of the nano-rods, and e) the FexOy@C sphere with iron oxide nanoparticles highly dis-persed inside. Reproduced from Ref. [36].

Figure 6. Models for the structure of a) low-temperature and b) high-tem-perature glassy carbon. Reproduced from Ref. [121].

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Carbon-Supported Iron Catalysts for Fischer–Tropsch Synthesis

appeared only recently.[30] It is very difficult to reduce Fe com-pletely on traditional supports, such as SiO2 and Al2O3, becauseof the strong metal–support interaction. The presence of inter-mediate iron species (i.e. , silicates or aluminates) in combina-tion with metallic Fe complicates the interpretation of the roleof individual iron phase in FTS. The chemical inertness of theGC makes it an especially suitable support for the differentia-tion of the behavior of the iron phase at different reductionstages on the FTS performance owing to the expected weakmetal–support interaction. Bengoa et al. prepared two GCswith low (22 m2 g�1) and high (292 m2 g�1) surface areas, denot-ed as C(l.s) and C(h.s), respectively, to support Fe through in-cipient wetness impregnation.[30] The Fe loadings on both cata-lysts were approximately 5.4 wt %. Mçssbauer absorption spec-troscopy of 57Fe revealed that the only iron species in Fe/C(l.s)and Fe/C(h.s) before reduction was Fe3O4. On Fe/C(l.s), therewas only one type of ferrimagnetic Fe3O4 larger than 28 nm indiameter. On Fe/C(h.s), there were two types of Fe3O4: ferri-magnetic one of approximately13.5 nm in diameter and an ex-tremely small one with superpar-amagnetic (sp) behavior. Afterreduction, all the Fe3O4 on Fe/C(l.s) was reduced to a-Fewhereas the two types of Fe3O4

with smaller sizes on Fe/C(h.s)showed lower reduction degree.After the FTS reaction, all a-Feswere carburized to c-Fe5C2 andsp carbides on Fe/C(l.s) whereasthe presence of the intermediatereduction species of iron on Fe/C(h.s) led to the formation ofa nonstoichiometric carbide (“O”carbide). The better FTS perfor-mance of p-Fe/C(h.s) was attrib-uted to the presence of this “O”carbide that was more activeand selective to olefins than c-Fe5C2. Illustrated in Figure 7 arethe phase evolutions of theseFe/GCs catalysts at differentstages.

The main difference betweenCNFs and CNTs lies in the lack ofa hollow cavity for the former.The CNFs are usually fabricatedby the catalytic decompositionof small hydrocarbon moleculeson Fe- or Ni-based catalysts athigh or moderately high temper-atures in a flow reactor.[15] Newstrategies based on water-solu-ble alkali salt catalyst[125] and cat-alytic thermal decomposition ofpoly(ethylene glycol)[126] havebeen reported recently. The

CNFs are unique high-surface area materials (�200 m2 g�1)that can expose exclusively either basal graphite planes oredge planes.[63] Some investigations indicated that the applica-tion of the CNFs as the catalyst support often resulted in im-proved catalytic activity and selectivity as compared to tradi-tional supports.[127–130] The application of CNFs in FTS has alsodrawn increasing attention, with the aim to improve the dis-persion and reducibility of Co nanoparticles, which are conduc-tive to high activity and selectivity.[131–134]

Very recently, de Jong and co-workers prepared a Fe/CNFcatalyst and applied used it in a so-called FTO process.[4] Lightolefins (C2–C4) are fundamental feedstocks in petrochemicaland fine chemical industries for the synthesis of a wide rangeof products such as polymers, solvents, drugs, cosmetics, anddetergents.[135] The FTO process is born out of FTS but is con-sidered as a direct route to transform syngas into light olefinswithout intermediate steps. The Fe/CNF catalyst, along withother oxide-supported Fe catalysts, was synthesized through

Figure 7. Schematic representation of the different steps followed by each solid to reach the structure of the“working” catalyst at the steady state. Reproduced from Ref. [30] .

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incipient wetness impregnation to achieve a nominal Fe load-ing of 10 wt % with use of ammonium iron citrate containinglow amounts of Na and S. The Fe2O3 crystallite size on CNFwas 7 nm. The FTO reaction was conducted under conditionsof 350 8C, 0.1 MPa, and H2/CO of 1 at a CO conversion of 0.5–1 % to restrict secondary reactions of olefins. The Fe/CNF cata-lyst demonstrated the highest selectivity to light olefins (61 %)and the second highest activity expressed as iron time yieldamong the supported and unsupported Fe catalysts investigat-ed. The high activity was attributed to the weak Fe–CNF inter-action, which is advantageous to the formation of catalyticallyactive iron carbides. Under industrially relevant conditions of340 8C, 2.0 MPa, and H2/CO of 1, the Fe/CNF catalyst still dem-onstrated an impressive selectivity to light olefins as high as52 % and an activity 20 times than at 0.1 MPa. The suppressioneffect of the promoters on the methanation reaction was ob-served only if “inert” supports such as CNF and a-Al2O3 wereused, which was interpreted as the close interaction betweenFe and promoters (Na plus S) on these inert supports.

3. Summary and Outlook

The Fischer–Tropsch (FTS) process is currently a topical issue inthe fields of fuel production and biomass utilization. The re-newed interest in Fe-based FTS catalysis and the intense inter-est in carbon materials have stimulated great research effortson carbon-supported Fe-based catalysts that led to many intri-guing results. In this Review, we have reviewed examples ofFe/C FTS catalysts since the early 1980s with use of supportssuch as ACs, CNTs, CSs, GCs, and CNF. In general, their high sta-bility in reducing atmospheres and robustness to water attackunder typical FTS conditions ensure that they are stable sup-ports for this reaction. Their high specific surface area andwell-developed pore structure make them very suitable forpreparing catalysts of high dispersion. Their relatively inert sur-face is conductive to catalysts of high degree of reduction.Comparing with other supports such as SiO2 and Al2O3, theiron species on carbon materials are generally more reducibleand can be transformed to iron carbides more facilely in theFTS reaction.[40–44] The formation of iron carbides was observedeven after H2 activation.[35, 36] The strong interaction with oxidesusually prevents the complete reduction of all the iron speciesand thus disallows low iron loadings on many oxide surfa-ces.[45] For example, SiO2 in Fe/SiO2 catalysts interacts with theiron species by forming the Fe–O–Si linkage, which disturbsthe electronic structure of Fe atoms in iron oxide phases andin turn retards the reduction and carburization of the cata-lyst.[14] Similar results have also been reported on precipitatedFe catalysts such as Fe/Cu/K/SiO2

[136] and Fe/Cu/K/Al2O3.[137] Theunique pore structure and the electronic property of the CNTs,as well as the spatial restriction of the CSs, endow Fe/C FTScatalysts with improved catalytic performance. Specifically, itseems that the carbon materials, either in the form of AC,[40–43]

CNT,[77, 78] or CNF,[4] are quite powerful in the preparation ofsupported Fe FTS catalysts, which are highly selective towardlight olefins. The Fe-based FTS catalyst with an outstandingpolymerizing ability could be prepared with use of supports

AFC[13] and CS.[108] . The modification of the Fe/C catalysts bysome elements could direct FTS to products other than paraf-fins and olefins.[75] In spite of the inertness of carbon materials,superior sintering resistance was observed on Fe/CNT cata-lysts[35, 72, 85, 87, 91] and the Fe/CS catalyst[36] owing to spatial re-striction and/or enhanced metal–support interaction associat-ed with the reduced dimension of the iron nanoparticles. Incontrast, the inertness of carbon materials offered a goodchance for the elucidation of some fundamental aspects of Fe-based FTS catalysis, such as the particle size effect[43, 45] and therelationship between the reduction degree of iron and the FTSperformance.[30]

In spite of the remarkable advances in Fe/C catalysts for FTS,some clear limitations of the carbon supports have hamperedtheir further application. For example, the ACs have a micropo-rous structure, which causes transport limitations and cokingto occur during the reaction. The complicated synthesis andpurification procedures and the high cost of the CNTs bringproblems to their industrial application. Hence, more efforts incatalyst design built on further expansion of the library ofcarbon materials, synthesis strategies, and a combinationmode between iron and carbon support would be beneficialfor the preparation of cost-effective, high-performance Fe/CFTS catalysts. For example, the disadvantage of the ACs’ micro-porous structure might be circumvented by use of orderedmesoporous carbon materials as the support.[138] Natural mate-rials and some byproducts of industrial production processeshave been explored as carbon sources for the synthesis ofCNTs, which provides an attractive way to lower the costs ofthis carbon material.[139] More attention should be paid to thefollowing issues in future research and development of the Fe/C FTS catalysts:

1) Although there were indications that the surface acid/basic property or surface functionalities of carbon materialscould exert an effect on the activity and distribution of the FTSproducts,[28, 46, 108] the rich surface chemistry of carbon materialshas not yet been fully made use of. Thermal treatment, surfaceoxidation, reduction, grafting, and incorporation of heteroa-toms in the carbon matrix are useful means to tailor the sur-face properties of carbon materials.

2) The promoters for the Fe/C FTS catalysts have beenmainly limited to K and Cu and, to a lesser extent, Mn and Mo.Other promoters proven effective for Fe-based FTS catalystsshould also deserve more attention. For example, rare-earthoxides, such as La2O3 and CeO2, showed positive promoting ef-fects on the Fe catalysts.[140–142] MgO could suppress catalystdeactivation and raise the ratio of olefins to paraffins.[143, 144] Be-cause the surface reactivity of carbon materials is anticipatedto deviate from that of conventional oxides, this would resultin different interaction modes of the promoter with the sup-port and generate new promoting effect. Moreover, alloying ofFe with other metals would be promising for the manipulationof the kinds and distribution of the FTS products.

3) Although the confinement effect of the channels of theMWCNTs has been extensively studied, the confinement effectof the SWCNTs remains open. Depending on the wall thickness,the electronic structure of the external surface of the MWCNTs

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might be more similar to that of graphite or in-between thoseof SWCNTs and graphite. Therefore, the effect of the differencein the internal and external electronic properties of the CNTson the physicochemical properties and FTS performance of Femight be different for MWCNTs and SWCNTs. Moreover, de-pending on the chiral angle between hexagons and the tubeaxis, the SWCNTs could be either metallic or semiconducting.And the electronic structure of the semiconducting SWCNTscould be finely tuned by changing the diameter.[64] Thus, therich electronic structures of the SWCNTs offer many possibili-ties to tailor the FTS performance of Fe supported on or insidethe SWCNTs.

4) Because metallic Fe is not the active phase for FTS, the ac-tivation atmosphere (H2, CO, syngas) remarkably influences theFTS activity, selectivity, and stability of the Fe catalysts by gen-erating different iron phases.[145] Systematic investigation ofthe activation atmosphere for the Fe/C catalysts would behelpful for the further optimization of the FTS performance.Much work also remains to be performed in developing relia-ble characterization methods that would aid in unambiguousstructural identification of the Fe/C catalysts during the activa-tion, reaction, as well as the preparation stages.

We hope that the overview on the status of the Fe/C FTScatalysts in the last three decades and our viewpoint on thechallenges and opportunities in this field could provide an in-structive background for researchers interested in this excitingfield of heterogeneous catalysis as well as for those investigat-ing applications of novel carbon materials. It is anticipated thatthe great surge in versatile and powerful methods for materialssynthesis[146] and the quick advance in in situ or operando char-acterization techniques[147–149] would lead to rationally designedFe/C FTS catalysts with the desired performance.

Acknowledgements

This work was supported by the Science & Technology Commis-sion of Shanghai Municipality (10JC1401800, 08DZ2270500), theNSF of China (21073043), the Program of New Century ExcellentTalents (NCET-08-0126), the National Basic Research Program ofChina (2012CB224804), the Key Laboratory of Resource Chemistryof Ministry of Education, Shanghai Normal University, and by theChemical Sciences, Geosciences and Biosciences Division, Officeof Basic Energy Sciences, Office of Science, U.S. Department ofEnergy under the grant DE-FG02-12ER16353, ACS petroleum re-search fund, and seed funds from Center for Sustainable Energyat Notre Dame.

Keywords: iron · carbon materials · Fischer–Tropsch ·heterogeneous catalysis · water–gas shift

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Received: April 16, 2012Published online on && &&, 0000

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MINIREVIEWS

B. Sun, K. Xu, L. Nguyen, M. Qiao,*F. (. Tao*

&& –&&

Preparation and Catalysis of Carbon-Supported Iron Catalysts for Fischer–Tropsch Synthesis

Carbon-supported Fe catalysts have at-tracted increasing interest in Fischer–Tropsch synthesis because of theunique properties of carbon materials,such as high surface area, high porosity,and ample electronic and topologicalstructures. This Review focuses on theprogress made in the synthesis and ap-plications of these catalysts.

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