DOI 10.1515/ntrev-2013-0025 Nanotechnol Rev 2013; 2(5): 547–576

Review

Peng Zhai, Geng Sun, Qingjun Zhu and Ding Ma* Fischer-Tropsch synthesis nanostructured catalysts: understanding structural characteristics and catalytic reaction

Abstract: One key goal of heterogeneous catalysis study is energywise and carbon-resourcewise industry to replace to understand the correlation between the catalyst struc- the current petroleum-based economy. ture and its corresponding catalytic activity. In this review, Catal. → we focus on recent strategies to synthesize well-defined CO+H2n2003− 50°C CH2n+2 +Cn2H+n22n HO Fischer-Tropsch synthesis (FTS) nanostructured catalysts The search for suitable catalysts for FTS reactions can be and their catalytic performance in FTS. The development dated back to the 1920s when the FTS process was invented of those promising catalysts highlights the potentials of [1]. Varieties of transition metals have been employed for nanostructured materials to unravel the complex and the syngas conversion, and it is confirmed that Fe-, Co-, dynamic reaction mechanism, particularly under the in and Ru-containing catalysts are most efficient to catalyze situ reaction conditions. The crucial factors associated syngas transformation to the desired liquid hydrocarbons. with the catalyst compositions and structures and their Usually, saturated hydrocarbons are preferably formed on effects on the FTS activities are discussed with an empha- Ru and Co catalysts, while more olefins are produced on sis on the role of theoretical modeling and experimental Fe-based catalyst. However, because of the relatively high results. price of Ru, the much cheaper Fe and Co have attracted most of the attentions from a practical point of view. Keywords: catalysis; Fischer-Tropsch synthesis; Indeed, the catalysts used in the commercial plants are all nanostructure. based on Co and/or Fe. Most of the researches have been well reviewed in some recent publications from different *Corresponding author: Ding Ma, Beijing National Laboratory perspectives [2–10]. for Molecular Sciences, College of Chemistry and Molecular In the FTS reaction, the reactants (CO and H2) undergo Engineering, Peking University, Beijing 100871, P. R. China, elementary chemical steps such as adsorption of reac- e-mail: [email protected] tants from the syngas mixture, surface diffusion, reaction Peng Zhai and Geng Sun: Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking of adsorbed species, and desorption of products to form University, Beijing 100871, P. R. China hydrocarbons over a metal surface. It is well known that Qingjun Zhu: National Institute of Clean-And-Low-Carbon Energy, traditional methods for the preparation of heterogene- Future Science and Technology City, Changping District, Beijing ous catalysts normally lead to a mixture of different metal 102209, P. R. China species, thus, offer little control over the product selec- tivity. Recent advancements on the design/fabrication of nanostructured metal catalysts with altered product selectivity or increased reaction activity for the syngas 1 Introduction conversion is one of the most important topics in hetero- geneous catalysis research. Apparently, those nanostruc- Fischer-Tropsch synthesis (FTS) is an exothermic catalytic tured materials not only provide platform for the product process that transforms mixed gas of CO and H2 (syngas) selectivity engineering but also offer new insights on the into a wide spectrum of hydrocarbons. It has been one FTS reaction mechanism. In this review, we summarize of the most important chemical processes in the chemi- some recent progresses on the FTS nanostructured cata- cal industry. As syngas can be obtained from coal, nature lyst design and their applications in the FTS reaction. We gas, and biomass, the establishment of designated syngas do not intend to cover all the nanostructured metal cata- conversion routes might help us to develop an alternative lysts in the syngas conversions. The focuses are the key 548 P. Zhai et al.: Understanding structural characteristics and catalytic reaction factors that contribute to the unique characteristics of liquid phase clearly demonstrate that solvent play impor- those nanostructured catalysts and their catalytic effects tant roles, and it is demanding to discuss the solvent in FTS reaction. Our objective is to identify the active phase functions as a category. Moreover, it is desired to gain and correlate it with reaction performance. We mainly use the knowledge between the catalytic properties and the examples of nanostructured catalysts containing Fe and/ catalyst structure, especially that under working reaction or Co because they are in line with current industrial FTS condition. Therefore, we stress the importance of using researches. In addition, we extend our discussion slightly in situ characterization techniques to study the catalytic to several case studies concerning Cu or Ru nanoparticles performance of those nanostructured catalysts in FTS to (NPs) because their applications in syngas conversion can gain deep understanding between the catalyst structure provide supplementary evidence for our understanding. and catalytic performance. Moreover, the theoretical The structure of this review is illustrated in Scheme 1. modeling is an inseparable approach to overcome experi- First, we summarize recent advancements of synthetic mental limitation to study the catalyst nature and the FTS strategies on the nanostructured catalysts, particularly for reaction process, which will be discussed in the last part the well-designed metal NPs that are widely used in the of the review. In the end, we summarize the knowledge we FTS. In the following parts, the sections (core metal, aux- have gained on the nanostructural characteristics of cata- iliary metal, and environment) are divided with respect to lysts and FTS catalytic performance and offer our view for the key issues that have strong impact on the FTS perfor- further research. mance. Under them, there are six key factors (size effect, surface crystallography, promotional effect, bimetallic effect, support effect, and solvent effect) that play impor- tant roles in determining the catalyst properties, leading 2 Preparation of FTS to different catalytic activities. The impact of each factor in nanostructured catalysts FTS will be discussed in detail. One needs to mention that, among those key factors, solvent effect is rarely reported The design/fabrication of catalysts, the intrinsic struc- in literature. This is because overwhelming majority of ture of prepared catalysts, and the resulting catalytic catalysts studied in FTS are heterogeneous catalysts in gas activities are inseparable in the catalysis studies. The phase reactors, such as a fixed bed reactor, and solvent synthesis method determines the catalyst structure and, effect is not present in such reaction systems. However, thus, plays crucial roles in its catalytic application. It is recent progresses of using nanostructured catalysts in the noted that many researchers demonstrate that subtle changes in the synthesis method affect the catalyst structures, leading to remarkable difference in cata- lytic performance due to different structures of active phases [11]. Theoretical modeling One of the most important industrial catalyst synthe- Surface crystallo- sis methods is the impregnation, that is, the core metal graphy precursor is loaded on supports like alumina, silica, or Promotionaleffect Size ef carbon and followed by the treatments such as drying or Auxiliary fect Core calcination to get the catalysts prepared by impregnation metal metal Bimetallic effect method. Oxides support is commonly used for such cata- FTS Nanostructured lysts. As oxide support typically have high surface area of catalyst more than 200 m2/g, the supports provide a large surface to stabilize small metal crystallites, adsorb reactant gas, W ell-designed and enhance the mechanical strength of the catalysts. structure Thus, if a precursor salt of metal can be effectively dis- Environment In-situ tributed over this surface by impregnation or any other

characterization

Suppor Solvent ef

ef methods, small nanoscale metallic particles are formed fect fect after decomposition, resulting in the formation of nano- t structured catalysts. The intrinsic properties of those metal NPs can be tuned by the surface properties of the Scheme 1 The key factors affecting nanostructured metal catalysts supports and a variety of treatment methods. For instance, and the experimental approaches for the catalyst study. in a typical process to prepare Co catalysts for FTS, a P. Zhai et al.: Understanding structural characteristics and catalytic reaction 549 precursor of cobalt nitrate is dissolved in water first; then, diameter of 3–5 nm was synthesized by the sol-gel method it is impregnated onto the supports like silica or alumina and was found to have a stable FTS activity [20]. by using incipient wetness method. The obtained catalyst In recent years, there have been tremendous devel- is dried after the impregnation and calcined at certain opments in the preparation of nanostructured catalysts temperature and atmosphere. Cobalt exists as Co3O4 after with well-defined size, shape, and composition, thus, the calcination, and the Co3O4 should be reduced in situ allowing tailoring of their catalytic properties. One or ex situ before the reaction. The reduction is a two-step example is that Kou et al. developed a kind of innova- process on silica. First, Co3O4 is reduced to CoO at 623– tive catalyst system with metal NPs dispersed in solvent 673 K under inert gas atmosphere. The second reduction such as ionic liquids (ILs), water, or glycol [21–24]. In a step varies depending on the specific treatment situation typical experiment for a cobalt-based NP catalyst prepa-

[12]. It is well known that for some systems, the reduction ration, Co (acac)2 and K2PtCl4 are dissolved in water con- degree of Co species determines their FTS activity, and the taining polyvinyl pyrrolidone (PVP) [22]. The mixture nature of the Co NPs might be changed during the reaction is then hydrogenated by H2 [22]. The illustration of the process [10]. The impregnation method is very facile for nanostructure Pt-Co catalyst preparation is shown in operation that the additional/auxiliary metal component Scheme 2. The catalyst exhibits promising FTS catalytic + can be added in the process. The auxiliary metals can be activity and C5 selectivity. This kind of catalyst prepa- impregnated with the core metal simultaneously, or they ration approach can be used for other metals such as can be impregnated with successive impregnation proce- ruthenium, platinum, and iron. Besides hydrogen, alco- dures [13–15]. Apparently, different preparation methods hols and sodium borohydride can also be used as reduct- lead to different interactions between the core metal and ants [23, 24]. the auxiliary metals, affecting the nature of resulting Another interesting development in the catalyst prep- catalytic materials. The impregnation method can also be aration is to synthesize hybrid materials with different extended to a wide range of metals and supports such as compositions, which have different catalytic functions. carbon nanotubes (CNTs) [15, 16] or molecular sieves For instance, Tsubaki et al. developed a core-shell struc- [17, 18]. The crucial factors that affect the catalytic activi- ture catalyst by the hydrothermal method [25, 26]. The ties of those nanostructured catalysts are the function core is alumina-supported cobalt active catalyst for FTS, of the core metal, the interactions between the auxiliary and the shell is acidic zeolite membrane used for crack- metal and core metal, and the impact of support on the ing and isomerization of heavier FTS products. This kind nature of active phases. of core-shell structure catalyst is very selective for C5–C12 Another important preparation method is the pre- products in FTS [27]. The concept for the catalyst structure cipitation method that deposits the active metals on the is exhibited in Figure 1. Another example is highly dis- support by adding suitable precipitator into the metal persed iron oxide/graphene oxide hybrid catalyst. It was precursor solution, followed by drying and calcination. also prepared by a facile one-pot hydrothermal hydrolysis-

The control of appropriate pH value and temperature reduction process [28]. Moreover, γ-Fe2O3/SiO2 core-shell during the precipitation process is important to get well- NPs were observed using iron nitrate and TEOS as precur- defined nanostructured catalyst. Moreover, coprecipita- sors at 200°C [29]. The iron oxide core has an average size tion is also the most common and effective method for of 20 nm, and it was coated by a 3-nm SiO2 shell. Those introducing auxiliary metals on the iron FTS catalyst. studies demonstrate that hydrothermal method is also Generally, iron nitrate is used as precursor salt, while very useful in synthesizing well-shaped crystallite and ammonium iron (III) citrate, iron acetate, and iron (III) homogeneous metal-support nanocomposite. acetylacetonate are suggested to decompose slowly and, thus, are able to help the grafting of metal complexes onto support oxides. Apart from common drying, spray 2- PtCl4 Pt seeds drying is also used in the posttreatment. For instance, nanosized iron particles were synthesized in a water-in- O O H 333 K oil microemulsion system [19]. 2 Co2+ Pt-Co NPs In addition, sol-gel method is an effective pathway O O for synthesizing homogenous metal-support compos- ites. Tetraethylorthosilicate (TEOS) is commonly used to Scheme 2 The illustration of one-step hydrogen reduction of Pt-Co produce sol, then, act as a support matrix in catalysts. bimetallic NPs (Reprinted with permission from ref [22]. Copyright

Uniformly dispersed Co-SiO2 catalysts with a Co average (2013) American Chemical Society). 550 P. Zhai et al.: Understanding structural characteristics and catalytic reaction

Zeolite membrane reaction. It is widely accepted that reactions containing (cracking and isomerization) deposition or formation of C-C bonds are structure-sen- sitive reactions, while the reactions only containing C-H

CO+H2 bonding deposition or formation are structured-insensi-

Isoparaffin tive. Therefore, FTS reaction is recognized as a structure- sensitive reaction [31], which means that the catalytic performance is strongly related with the particle size of those core metals. However, size effect for heterogeneous catalysts is a very complex phenomenon, and changing Co/Al2O3 catalyst (FT Synthesis) the core metal NP size often affects the FTS catalytic per- formance significantly [32]. The correlations between the particle size and the activity should be treated with great caution, particularly for the comparison among different

Figure 1 Acidic zeolite coated core-shell structure (Reprinted from supports. This is because the oxide supports, which are ref [27]. Copyright (2008), with permission from John Wiley and widely used in the FTS catalysts, can change the chemical Sons). properties of metal catalysts. It is believed that the strong metal support interaction (SMSI) exist between the inter- face of support oxide and core metal [33]. 3 Key factors of FTS In order to elucidate the metal size effect on the FTS, it is necessary to exclude the strong metal-support inter- nanostructured catalysts action and use relatively inert support such as carbon materials. For example, carbon nanofiber (CNF) was used 3.1 Core metal as an inert support for different diameters of cobalt NPs from 2.6 to 27 nm. Such carbon support does not affect the A variety of transition metals have been applied in the Co particle electronic structure as those traditional oxide syngas conversion to chemicals. In general, FTS catalysts supports. de Jong et al. demonstrate that when the parti- include unsupported Fe, Co, Ru metals catalysts, or those cle size is smaller than 6–8 nm, the FTS activity, as well supported on oxides or other supports. We label those as the selectivity to methane, increase with the increase metals as “core metal” because they are indispensable in particle size, but the turnover frequency (TOF) is inde- to produce desired aliphatic hydrocarbons from syngas. pendent of the particle size if the Co size is larger than The “core” stresses their crucial functions in the CO disso- 6–8 nm [34]. This size effect of the cobalt catalyst was also ciation and subsequent important reaction steps such as investigated by the Steady-state isotopic transient kinetic C-C coupling to form a dominant portion of paraffins and analysis (SSITKA) [35]. The SSIKA reveals the differences olefins in the overall products. In other words, it is their in coverages and residence times when the particle sizes intrinsic electronic structures, although can be modified are smaller than 6–8 nm. The results demonstrated that slightly, that determine the activity toward CO activation for smaller cobalt particles, CHx has longer residence time and the trend of product distribution [30]. and lower coverages, which are not advantageous for the C-C coupling reactions, so the activities of small particles are low. Nevertheless, hydrogen coverage is higher on 3.1.1 Size effect the small particles compared with the large particles that prefer the methanation reaction, the small particles still Usually, the core metal particles, which play the catalytic show high methane selectivity. functions, are nanosized crystallites on the support mate- The size effects of Fe NP catalysts are also studied on rials. The difference in crystallite size is normally a result the inert carbon supports. It was observed that the FTS of different contents of metal precursors during synthesis, activation energies and CO turnover frequencies increase different type of supports, or different synthetic proce- when the average Fe crystallite size increase from 0.6 nm dure. The overall particle surface area increases with the to 54 nm on Fe/C, and similar results are concluded on decrease in crystallite size, leading to an increased expo- another series of Fe/C in which Fe crystallite size increase sure of metal to the syngas reactants. Traditionally, if the from 0.6 nm to 7.9 nm [36]. However, the resulting olefin/ catalytic performance of a reaction is related with parti- paraffin ratios show different trends on those two series of cle size of the catalyst, it is dubbed as structure-sensitive Fe/C. Moreover, iron NPs were incorporated into ordered P. Zhai et al.: Understanding structural characteristics and catalytic reaction 551

mesoporous carbon supports, and an obvious relation- catalyst prepared from those well-defined Fe2O3 NPs was ship between iron particle size and FTS performances investigated in the FTS [41]. The highest catalytic activity have been displayed [37]. That is, the size of iron NPs can be attained with Fe NP diameter of 6.1 nm. Increase or decreases from 22.1 nm to 8.3 nm, leading to the increase decrease in the Fe NP size results in the decrease in FTS in FTS activity as well as the suppression in CH4 formation activity. CH4 and C2-C4 selectivity all decrease when Fe + + and increase in C5 selectivity. NP size increases, giving rise to C5 selectivity. TOF also de Jong and coworkers have developed an excellent increases with the increase in Fe NP size from 2.4 to 6.2 Fe-based catalyst for Fischer-Tropsch to olefins (FTO) nm, and then, it remains almost constant up to the Fe NP process [38] and investigated the Fe NP size effect in this size of 11.5 nm. Therefore, it can be concluded that the cat- system [39]. The apparent TOF corresponding to the initial alytic activity and selectivity of those Fe/Al2O3 are strongly activity of unpromoted Fe/CNF catalysts decreased six to affected by the particle size of iron oxide in the size range eight times when the average iron size increased from 2 from 2.0 to 12.0 nm. to 7 nm; however, the selectivity for methane and lower The size effect of ruthenium was also studied over Ru/ olefins were almost constant. In contrast to the behavior Al2O3 catalyst. It was found that the FTS activity is depend- observed for unpromoted catalysts, the product selectivity ent on the particle size, i.e., FTS activity decreased with of Na, S-promoted Fe/CNF samples exhibited a clear effect the decrease in Ru particle size when the Ru particle size of iron particle size. Lower olefin selectivity increased, was smaller than 10 nm, whereas FTS activity was nearly while CH4 selectivity showed a trend of decrease when constant when the Ru particle size was larger than 10 nm. iron size increased from 2 to 7 nm. The TOF as a function of SSITKA characterization results indicate that CO is more iron size is shown in Figure 2. This trend may be attributed ready for dissociation on smaller Ru particles, possibly by to size sintering or carbon deposition. It is suggested that blocking the active sites and decrease the FTS activity [42]. the sites at corners and edges play diverse roles from sites Advanced characterizations were employed for size at terraces in activity and selectivity. The former is associ- effect research in FTS. For instance, the dissociation of ated to size, while the latter is independent of size. These CO on cobalt NPs with size ranging from 4 to 15 nm was results give a rigorous and novel discussion in size effect studied by employing the in situ soft X-ray absorption and make some important suggestions for the design of spectroscopy (XAS) [43]. The oxide peak, which refers to FTO catalyst. surface oxygen species that was produced by CO disso- Another interesting method to investigate the size ciation, showed a size-dependent trend as derived from effect is to impregnate well-synthesized iron oxide NPs O K-edge and Co L-edge XAS spectra. Figure 3 shows the on the support. For example, monodispersed Fe3O4 NPs relationship between the particle size and the ability for can be synthesized in the presence of oleic acid and CO dissociation, which is measured from the XAS spectra. oleylamine, and particle diameter can be tuned from 3 to 20 nm by seed-mediated growth [40]. The Fe/δ-Al2O3

0.5 1.0 250°C TOF C2+ 150°C TOF n 0.4 0.8 CH4 RT ) -1 0.6 0.3 × 10 (s 0.4

OF 0.2

T 15 nm 4 nm 10 nm 0.2 Relative CO dissociatio 0.1 XAS (oxide)/XAS (CO adsorbed)

0 02468 0 Fe carbide particle size (nm) Nanoparticle size

Figure 2 Apparent turnover frequencies (TOF) as a function of iron Figure 3 Relative concentration of dissociated CO species on 4, 10, carbide size (TOS = 1 h). TOF + corresponds to the CO conversion to and 15 nm NPs after exposure to CO/He at different temperatures. C 2 + hydrocarbons and conversion to C2 hydrocarbons, respectively. The relative concentration was calculated as the ratio of the areas

The reaction was performed at 340°C, 20 bar, and a H2/CO ratio of of the oxide XAS peak and the π* peak from intact adsorbed CO 1 (v/v) on promoted catalysts (Reprinted with permission from ref (Reprinted with permission from ref [43]. Copyright (2013) American [39]. Copyright (2012) American Chemical Society). Chemical Society). 552 P. Zhai et al.: Understanding structural characteristics and catalytic reaction

Oxide formation was clearly observed on the 15-nm and presence of iron may induce the bcc structure of cobalt 10-nm NPs, but hardly detected on the NPs with particle (instead of the fcc structure of pure Co NPs), change the size of 4 nm. Further research offered strong evidence exposed facets, therefore, leading to higher activity. It is for hydrogen-assisted CO-dissociation mechanism. This interesting to note that the surface crystallography might result confirmed the conclusion that the dissociation of H2 be affected by many factors. For instance, it is proposed is responsible for the activity decrease with decrease in Co that Co shows hcp crystal structures at room tempera- NP size [44]. ture, but they will convert to fcc crystal structures at high temperature. However, some types of Co nanostructured particles exhibit fcc structure even at room temperature 3.1.2 Surface crystallography [46, 47] resulting in different catalytic activities [46]. It is acknowledged that NPs with different morphologies have The progress of surface science in the last decades shows different facets exposed to the reactants. that heterogeneous catalysis, i.e., acceleration of a chemi- Recent progress in shape-controlled synthesis of mag- cal reaction on the metal surfaces, is caused by the high netic NPs has led to several well-defined shapes. Spheri- reactivity of the metal surface atoms that facilitate bond cal and rod, nanocubes, or irregular-shaped iron NPs breaking and bond rearrangement of adsorbed molecules. have been reported in literature [48–50]. For instance,

The core metal of nanostructured catalyst is composed of MnFe2O4 superlattice array of 12 nm cubic-like or polyhe- many crystal facets. For the structure-sensitive reactions, dron-shaped NPs are synthesized [51]. Spherical and cubic it is believed that the catalytic activities on those crystal CoFe2O4 are also obtained [52]. Some of them have been surfaces are different because of the different coordina- employed for FTS reaction, such as the as-prepared Fe3O4 tion status on those surface atoms. For instance, signifi- NPs supported on Al2O3 by the wet incipient method [53]. cant change in the binding strength of a given molecule is It would be a very interesting research topic to study iron observed when it is located at surface sites of lower coor- NPs with different facets in FTS under the same reaction dination, such as steps or a kink, indicating the increase condition to obtain the relationship between the activity in adsorption heat on the transition metal surfaces and and surface crystallography. higher dissociation of an adsorbed molecule. For a high- To overcome the experimental obstacles, DFT cal- index facet such as (310) or (211), it contains more steps culations are widely employed to establish the correla- and kinks for the catalytic reactions and might show tions between FTS catalytic activities and the core metal higher catalytic activity in FTS. Clearly, different elemen- facets. One of the interesting examples is the studies of tary reaction steps have different activation energies on adsorption and reaction property on Fe5C2 facets as Fe5C2 different facets, resulting in different product distribu- is widely considered as the active phase in iron FTS cata- tions and reaction profile. It can be easily imaged that the lyst. The theoretical studies reveal that (010) Miller index surface crystallography is closely related with the NP mor- plane is the most stable surface, while (101) surface is the phology of the core metals. For nanostructured catalysts least stable surface [54]. Numerous stable surface species with the same loading of core metal, the density of step or coexist under H2 and CO on Fe5C2 (001), (110), and (100) kink increases with the decrease in the core metal particle facets [55]. Ketene is the important intermediates on size if their morphologies are the same, leading to a poten- Fe5C2 (001) and (100), and hydrocarbons are produced tially higher catalytic activity. more favorably than ethanol because the energy barrier A great deal of the evidence have been accumulated of ketene dissociation is lower than its hydrogenation, for the relationship between the catalytic activity and the confirming the unique FTS catalytic properties on Fe5C2 surface crystallography, but such studies on the more [56]. Moreover, CO adsorption on Fe5C2 (010)0.25, Fe5C2 complex reactions such as FTS are rare, particularly under (110)0.00, and Fe5C2 (110)0.80 are also investigated, suggest- the reaction conditions. Calderone et al. designed a core- ing that adsorption and activation of CO on Fe5C2 surface shell Fe-Co catalyst, and such Fe@Co is synthesized by the are strongly influenced by the vacancy sites on stepped precipitation method on the parent magnetite particles and corrugated surfaces [57]. Not only the pure model with a mean size of 7 nm [45]. In contrast to the uncoated compound surface facets but also the facets in the K2O-Fe iron particle, Fe@Co supported on Al2O3 exhibit excel- system, in which K2O presence promotes Fe activities, + lent stability for 90 h and higher C5 selectivity, whereas are also studied using computational methods [58]. The the iron-based catalyst lose half activity after 10 h. adsorption energies of K2O and surface energies on this It is suggested that the improved catalytic performance is system indicate that the high-index facets become more caused by the interaction between iron and cobalt. The thermodynamically favored as the K/Fe ratio increases. P. Zhai et al.: Understanding structural characteristics and catalytic reaction 553

Among all Fe facets, (211) and (321) show the strongest A 0.6 0.4 Fe C (100) 2 increase in stability, whereas (100) is found to exhibit the 4 R =0.97 slightest increase. Based on these results, the equilibrium 0.2 0 Fe C (001) shapes of bcc Fe crystallites are studied at K/Fe = 0, 1/48, Fe C (011) 3 -0.2 2 Fe C (010) and 1/12. In the clean Fe system, the (110) facet has the -0.4 5 2 E (eV) biggest contribution (39%) to the exposed surface area. r -0.6 ∆ Fe5C2 (001) However, when the K content increases to 1/48 and 1/12, -0.8 Fe3C (100) -1.0 the percentage of (211) facet reach, 35% and 70%, respec- Fe2C (001) -1.2 tively. This stabilizing effect in theoretical modeling is Fe4C (111) confirmed by transmission electron microscope (TEM) -1.4 -1.6 -0.76 -0.72 -0.68 -0.64 -0.60 -0.56 and X-ray diffraction (XRD) results of the synthesized H2- q (C )/e reduced Fe/K catalysts to some extent. Because high facets surf have higher density of atomic steps, more active sites for B 2.4 activating chemical bonds may lead to the improved per- R2=0.96 2.2 formance in FTS. This work sheds light on promoter effect Fe C (001) and highlights the design of the outstanding catalyst with 2.0 3 Fe4C (100) a unique surface structure. 1.8 (eV) f

An interesting topic in FTS is the transformation of ef

E 1.6 carbon atoms on metal surface after CO dissociation. It Fe5C2 (010) appears that surface carbon deposition might be found 1.4 Fe C (011) on Fe (100), while formation of iron carbide is preferred 1.2 2 on Fe (110) [59]. The activity toward CO dissociation has 1.0 the order of Fe (100) > Fe (111) > Fe (110) [60]. Therefore, -1.45 -1.40 -1.35 -1.30 -1.25 -1.20 the carbon atom strongly binds with Fe(100) due to the εd (eV) shorter distance between carbon atom and the subsurface Figure 4 Relationships between reaction energy (Δ E) of CH iron atom [61]. In contrast, diffusion of carbon atom into r 4 formation and Mulliken charge (q) of the surface C atom as well as Fe (110) has a lower barrier energy, and graphite formation effective barrier (Eeff) of CH4 formation and d-band center (εd) of the will be more favorable. Compared with the surfaces that surface (Reprinted with permission from ref. [63]. Copyright (2009) both iron and carbon atoms are present, the surfaces with American Chemical Society). iron termination have the strongest CO adsorption ener- gies; these surfaces include Fe (111), Fe5C2 (110), and Fe3C

(010) [62]. Fe5C2 (010) and Fe2C (011) prefer CH4 formation, great challenge for the direct characterization although whereas CH4 formation is inactive on Fe4C (100) and Fe3C remarkable differences of electronic structure of the core (001) [63] (Figure 4). Those theoretical modelings clearly metal can be observed. In contrast, if the radius of the confirm that different facets of the core Fe metal play dif- core metal and auxiliary metal are similar, it is likely ferent catalytic roles. that auxiliary metal can form certain compounds or even alloys. The appearance of such newly formed compound phase, in some cases, can be identified by XRD or more 3.2 Auxiliary metal sophisticated characterization techniques such as X-ray absorption fine structure (XAFS). We denote the former The employment of auxiliary metals on the core metal effect (solid solution/chickenpox structures) as the pro- can alter the catalytic performance by acting as elec- motional effect of auxiliary metals upon core metals and tronic promoter and/or structural promoter of the core the latter effect (new compound) as the bimetallic effect metal. Normally, the auxiliary metals can function in for the purpose of discussion. Usually, the promotional two ways to change the physicochemical properties of effect exhibits a volcano curve, indicating that there is an the core metal. One is to form a kind of solid solution optimization of the auxiliary metal effect. For example, or chickenpox structure, in which the auxiliary metal is it was found that Fe-Mn catalyst reached a maximum dissolved in the core metal lattice or stay on the surface FTS activity when potassium content was 0.7%, which so as to affect the electronic properties of the core metal. corresponded to the sample with the highest Fe5C2 con- However, the crystal structure of the core metal remains. centrate in iron species [64]. It is noted that not all the Such solid solutions/chickenpox structures impose formed compounds, e.g., amorphous materials, can be 554 P. Zhai et al.: Understanding structural characteristics and catalytic reaction identified. In other words, there is no clear distinction Apparently, the addition of Mo hindered the carbon depo- between those two effects. sition on the iron surface, hence, stabilized the active site for long-chain hydrocarbon growing [74]. Interestingly, Wang et al. used several characterization techniques to 3.2.1 Promotional effect investigate the influence of Zn, Mn, and Cr on the iron

catalyst [75]. The study indicates that Zn-Fe form ZnFe2O4 The influences of auxiliary metals on iron-based catalysts compound, while Mn-Fe and Cr-Fe form solid solutions, have been investigated extensively, and various auxiliary which are consistent with their atom radius. Zn-Fe and α- metals have been used to enhance the catalytic perfor- Fe2O3 catalysts showed higher activity, but CO conversion mance of Fe catalysts in the FTS reaction. The auxiliary declined remarkably after a long time on stream in FTS metals can change the crystallite size of iron particles reaction. In the case of Mn-Fe and Cr-Fe, the catalysts dis- and increase surface area and then alter the activity and played a good FTS stability after a 200-h reaction; lower + product distribution through the influence on reduction CH4 selectivity and an increase in C5 hydrocarbons were and carburization process. For instance, manganese is observed despite the relatively low CO conversion. Smit widely recognized to be an excellent promoter for light et al. confirmed that the addition of auxiliary Cu cannot olefin production on iron-based FTS catalysts. Its addition only improve the reduction of iron oxide but also change to the core metal of iron decreased the crystallite size of iron the surface status of the reduced catalyst, as indicated by oxides, led to more heavy hydrocarbons, and suppressed in situ X-ray photoelectron spectroscopy (XPS) and XAS the formation of CH4 [65]. Similarly, auxiliary metal like [76]. A spillover of H2 or CO absorbed on Cu (0) may exist

Mg also led to the small crystallite size of iron oxide and that facilitates the reduction from Fe3O4 to Fe (0), and facilitated the reduction and carburization process [66]. the surface coverage of oxygen and carbon species were In contrast to Mn and Mg, potassium addition to Fe-Mn strongly increased by the presence of Cu (0). Moreover, catalyst resulted in larger crystallite size and lower surface copper facilitated the reduction of Fe2O3 to Fe3O4 or metal- area of the Fe catalysts and inhibited the reduction and lic Fe [77, 78], leading to heavy hydrocarbons and olefins carburization of iron [64]. It was found that alkali metals by the enhancement of surface basicity [79]. It is inter- change the FTS activity of iron catalyst through the asso- esting to note that although indium, similar to copper, ciation with water gas shift (WGS) reaction activity [67], decreased the reduction temperature of iron catalyst. Its as evidenced by the study of auxiliary metal of potassium addition caused low Fe reducibility and weak CO adsorp- upon Fe and Fe-Mn catalysts using SSITKA [68]. K and tion, resulting in lower FTS activity [80]. Pd was found to

Na are effective promoters for enhancing the FTS acti­vity, decrease the crystallite size of Fe3O4 NPs, which was syn- whereas Li, Cs, and Rb exhibit negative effects upon CO thesized by flame spray pyrolysis method [81]. However, conversions, but all the alkali metals can increase the its promotional function might be more attributed to the ethylene selectivity among the hydrocarbon products. good hydrogenation ability of Pd. Its presence improved This is particularly true for potassium that it is found to the reducibility of Fe as well as the carburization of the Fe be able to improve the olefin selectivity and long-chain NPs. As a result, the increase in activity and the selectiv- hydrocarbon yield by increasing the CO adsorption on ity shift to long-chain hydrocarbons with higher paraffin metal surface and reducing the CO dissociation activation content were found on the promoted catalyst. energy ­[69–71]. The promotional effect of auxiliary alkali The presence of more than one auxiliary metal can metals are also attributed to the enhancement of surface modify the core metal from different aspects, which might polarizability on the core metal substrate [72]. be more beneficial to improve the desired product selec- The promotional effects are not limited to crystallite tivity. The introduction of a third metal (Cr, Mn, Mo, Ta, size and surface area of the core metal on the nanostruc- V, or Zr, except W) in Fe-Cu-based catalyst was found to tured catalyst. Indeed, the auxiliary metal can change enhance the degree of Fe dispersion in a different scale the electronic properties of the core metal, thus, affecting [82]. Both CO conversion and WGS reaction were increased, their catalytic performance. For example, strong interac- while no obvious change was observed for the selectivity tions were observed between auxiliary Mo promoter and and chain growth probability. In the case of Mn and Zr, the core Fe [73]. The addition of Mo obviously increased the highest intrinsic site activities, which were derived from acidity of catalyst surface and inhibited the reduction and the reaction rate/amount CO chemisorbed (TOFchem) may carburization of iron oxide by covering the iron surface contribute to the highest activity among all the promoted sites. Consequently, the loss of almost half of the activity catalysts. Table 1 lists the representative catalytic perfor- + was detected, whereas more C12 diesels were produced. mance of catalytic materials discussed in this section. P. Zhai et al.: Understanding structural characteristics and catalytic reaction 555

3.2.2 Bimetallic effect [65] [68] [81] [78] [73] [64] [66] [69] [79]

As discussed in the section above, the promotional effect References caused by the auxiliary metal on the core metal enhances the catalytic performance to the desired catalytic proper- ties to some extent, but the “promotion” does not affect the fundamental properties, as exhibited without the presence of auxiliary metal, of the core metal. However, if new compound phase is formed among the core metal and auxiliary metals, the catalytic performance, particu- larly the main products, on such alloys may be much dif- ferent from that of the compositional metals. For instance, although metallic cobalt is believed to be the active phase

and olefins, shifts sel. olefins, to gasoline and in FTS to produce hydrocarbons, it is interesting to note 4 HC selectivity HC , shifts sel. to long-chain HC and olefins and HC , shifts sel. to long-chain 4 12 + that cobalt carbide is suggested as the active phase for C is not included. Some values are recalculated from the from are recalculated Some values included. is not 2 alcohol synthesis in Cu-Co alloy [83], indicating that the bimetallic effect could change the catalytic performance of the compositional metals dramatically. The intrinsic , shifts sel. to heavy HC and olefins and HC , shifts sel. to heavy

4 catalytic performance of this alloy might be attributed to the synergistic effects of the compositional metals. It is well recognized that copper, by assisting the nondis- sociative activation of CO, can catalyze the C-O insertion process. On the other hand, metallic cobalt is active in the dissociative adsorption of CO and C-C chain growth, thus, Promotional effect in catalysis performance in catalysis effect Promotional K: enhances activity, suppresses CH activity, K: enhances WGS and activity enhances Cu: K: enhances activity, shift sel. to heavy HC and olefins and HC sel. shift to heavy activity, K: enhances Mn: suppresses CH product shifts sel. to gasoline activity, Mg: enhances HC shifts heavy activity, K: enhances enhanced activity, Mo: decreases olefins and HC shortens sel. to heavy induction period, enhances Cu: Pd: enhances activity, suppresses CH activity, Pd: enhances

a produces a spectrum of hydrocarbons. The combination NA 3.6 5.8 0.6 6.1 1.9 1.4 O/P 10.1 1.95 of those two metals to a bimetallic catalyst may perform ratio a unique catalytic property in alcohol synthesis, aside sel.

2 from the FTS products as Co usually function to catalyze. 67 15.8 (%) 40.4 NA 40.6 38 33.3 NA CO NA Not only two metals but also several metals can form the 12 +

C alloys together. From this point of view, the bimetallic 35.1 43.7 41.2 38.7 31.5 24.1 effect is a simplification of the multimetallic effect. c c 11 7 14

-C Fe-Co or Fe-Cu, Fe-Ni bimetallic catalysts are syn- 5 26.6 28.5 27.1 31.3 38.1 41.8 89.1 C thesized and investigated as they have unique catalytic 4 77 19 -C 8.9 2 properties in the FTS reaction [84–86]. Fe-Co bimetal- 27.8 23.4 23.7 28.6 21.7 23.7 C

4 lic catalyst can be synthesized by a number of methods 23 67 4.4 8.1 1.4 8.8 2.0 8.2 CH

10.6 such as coprecipitation [87], coimpregnation [88], plasma Sel. in hydrocarbons/wt% sprayed [89], sol-gel method [90], microwave hydro- 38 50 8.9 6.8 (%) 87.8 86.8 2.12 27.8 62.1 thermal synthesis [91], or by just physical mixing [92]. Conv.

Moreover, well-defined Fe-Co/SiO2 catalyst with Fe-Co 2.0 1.2 2.0 2.0 1.2 2.0 2.0 /CO 2 0.67 0.67 NPs smaller than 10 nm can be prepared by high-temper- . H 4

-C ature supercritical drying of alcogels [93, 94]. Fe usually 2 2.5 1.5 0.2 1.2 1.5 2.0 1.5 0.5

0.25 appears to be a bcc crystal phase, while the active phase of Press (MPa) Co is generally considered to be an hcp/fcc structure. The ° C) ( 270 250 250 280 230 280 200 270 300 bimetallic Fe-Co NPs prefer to form bcc alloys when the Temp concentration of Fe is high [95], whereas the hcp/fcc struc- b 2 hydrocarbons selectivity. hydrocarbons 5

+ ture retain when the catalyst have a lower Fe concentra- C 2 2 2 tion [96]. Alloy [97] and spinel structure [87, 98] are proven 2 2 2 b to be the compound phases when Fe and Co coexist in the -Cu 4 composite. In the sample with high Fe concentration, iron immigrate to the surface and undergo carburization to Other products including oxygenates. Other products including Olefin to paraffin weight ratios for C ratios weight to paraffin Olefin Data represent represent Data NA, data not available; carbon selectivity is defined as the carbon atoms in the total number of C atoms in hydrocarbon products, CO atoms in hydrocarbon C of number atoms in the total as the carbon is defined selectivity carbon available; not NA, data Fe-Cu-0.07K-Mg/SiO Fe-5% K/SiO Catalytic performance on promoted iron F-T catalysts. iron F-T performance on promoted 1 Catalytic Table Catalyst a b c original data reported in literature. data original Fe-Mn-0.7K Fe-Mn-Cu-K/SiO Fe-Zn-K Fe-Mn–K/SiO Fe-Cu-1.5K/SiO Fe-K-Mo5-1/SiO Fe-0.1% Pd/MgO form iron carbides, while the cobalt carbides are formed 556 P. Zhai et al.: Understanding structural characteristics and catalytic reaction in the center [99]. However, the enrichment effect seems to example to illustrate the bimetallic effects. A possible be dependent on the catalyst support. Fe showed surface reaction mechanism is proposed in Scheme 3. Co@Cu enrichment in the Fe-Co/TiO2 system [95], whereas cobalt core-shell and Cu-Co mixed NPs with different Cu:Co ratios was enriched at the NP surface in the Fe-Co/Al2O3 system have been synthesized by the wet chemical methods [106]. [100]. Among all the four catalysts, the coreduced Cu-Co sample Generally, the addition of Co can greatly decrease the (24:1) exhibited the highest selectivity toward ethanol, and + reduction temperature of the Fe-Co catalyst and increase Cu-Co (3:1) showed the highest selectivity for C2 oxygen- the degree of iron reduction [101, 102]. In terms of the FTS ates at 270°C, indicating that varying the compositions of reaction, the Fe-based catalyst is known to have a good the alloys can effectively change the product distribution.

WGS activity and is suitable for H2-deficient syngas, while Moreover, such alloys can be loaded on different supports the Co-based catalysts show a higher activity with more to further modify their catalytic properties. For example, heavy hydrocarbon formation under lower reaction tem- the CNT supported Co3Cu1 catalyst achieved a highly perature. In the bimetallic catalyst, strong interaction selective formation of oxygenates, especially for BuOH between iron and cobalt exists as metal-metal bonds, and DME [107]. The addition of CNTs led to an increase which affect CO adsorption and metal reducibility. As a in the concentration of active Co species, CoO(OH), at the consequence, Fe-Co bimetallic catalysts prefer to produce surface of the catalyst was suspected be responsible for fewer CH4 and wax, but with more gasoline and olefin por- the selective formation of higher alcohols [108]. In addi- tions in the product distribution compared to the mono- tion, Cu-Co supported on Al2O3 displayed a higher selec- metallic catalyst. Fe-Co/SiO2 bimetallic catalyst showed tivity toward alcohols, and more ethanol was formed than higher activity and stability than pure Fe/SiO2 catalyst Cu-Co supported on SiO2 or CNT [109]. Introduction of Fe

[103]. An equal amount of Fe and Co is suggested to be the in Cu-Co/Al2O3 as promoter further enhanced the activ- optimization in the Fe-Co/TiO2 system [92]. Moreover, the ity with threefold increase in alcohol productivity, due to order of depositing Fe and Co on the Al2O3 surface seems higher metal dispersion and degree of reduction. to affect the catalytic performance in CO2 hydrogena- Lanthanum seems to be an efficient promoter for such tion [104]. Highest activity was obtained on the double- bimetallic catalyst as well. The LaCo1-xCuxO3-δ perovskite- impregnation catalyst, which first deposited Fe and then type catalysts were prepared for a high alcohol synthesis deposited Co on the Al2O3 surface. It is suggested that the process [110]. TPR data suggested that a strong cobalt-cop- presence of Fe prevents Co from forming a strong interac- per interaction exists in perovskites, which enhanced the tion with the support, resulting in a well-dispersed Fe-Co metallic dispersion of cobalt and prevented copper from alloy on the aluminum surface. sintering. The perovskite catalyst prepared by mecha- Because Co-Cu alloys are widely studied in the lit- nosynthesis showed a higher surface area and smaller erature for the alcohol synthesis, we take this as another diameter than that synthesized by the citrate complex

Hydrocarbons

CH3 Co CH CHx 4 Hx C CH2 [H] O CH H O 2 Co C O C [H] CH [H] [H] [H] O x CH CH OH CO H2 3 2 CO Cu Insertion [H] + C2 Oxygenates Cu CH3 CO O [H] CH3OH

Methanol

+ Scheme 3 A possible mechanism of C2 oxygenate formation via a CO insertion route over Cu-Co bimetallic catalyst (Adapted with permis- sion from ref [105]. Copyright (2008) American Chemical Society). P. Zhai et al.: Understanding structural characteristics and catalytic reaction 557 method [111]. Higher alcohol productivity was detected on nanostructured catalysts on the support, particularly the former catalyst than the latter, and the selectivity of oxides and carbon, have attracted remarkable attention alcohols was about 40% [112]. In contrast, Co3O4 was also due to their unique properties. It is believed that the SMSI loaded on the surface of LaFe0.7Cu0.3O3 by the wet impreg- exist between the oxide and iron interface [33]. Obviously, nation method [113]. A profile of the evolution of the the interaction may change the property of the iron metal catalyst structure was proposed based on XRD and XPS. surface, thus, alter the adsorption or dissociation energy

Interestingly, Cu in the perovskite was dislodged when the of H2 and CO and regulate the activity and selectivity in catalyst was pretreated in H2, forming the Cu@Co or Cu-Co FTS. Recent studies also reveal that oxide supports not alloy on LaFeO3. During the reaction, Co undergone car- only affect the metal morphology but also change the burization to nanosized Co2C in the presence of La2O3. The phase of catalyst before the FTS reaction. dissolved Cu and carbonized Co2C were believed to play important roles in alcohols synthesis. 3.3.1.1 Oxide support effect

SiO2 is one of the oxides that are most studied in iron- 3.3 Environment based catalysts. Generally, silica-iron composites are syn- thesized by impregnation or coprecipitation method, and 3.3.1 Support effect core-shell structure can be formed by the hydrothermal method [29]. It is found that the degree of Fe reduction The employment of support on the active metal compo- decreased with the increase in silica surface area [114]. Iron nent is a most used approach in the heterogeneous cata- phase was transformed from hematite to α-Fe after reduc- lyst preparation for the apparent advantage to disperse tion on silica-free catalyst, whereas it was converted to the metal active phase to enhance the efficiency. It is wüstite and Fe (II) silicate on the silica-supported catalyst noted that the supports can also change the physicochem- [115]. The introduction of silica decreased the Fe activity ical properties of metal catalysts, thus, influence their and yield toward light hydrocarbons and increased heavy catalytic performance. The existence of strong bonding hydrocarbon production in the FTS reaction. However, between the support and the metal component can affect the catalyst became more stable due to the lower carbon the size and shape of the metal particles. Meanwhile, good deposition rate [114]. We can observe an apparent support dispersion of metal on support can effectively inhibit influence on Fe-Si catalysts from Table 2. A variety of char- the agglomeration of the active phase and improve the acterization methods such as IR, TEM, and H2-TPR were mechanical properties of the catalyst. In some cases, high used to study the interaction between the Fe active phase surface area of supports and the SMSI effect can decrease and the silica support [116]. The result revealed that silica the reducibility of the metal component, resulting in a low had apparent influence on the chemisorption property degree of reduction. of H2 and CO over the iron catalyst. XRD and Mössbauer Although unsupported or bulk Fe catalysts are spectroscopy confirmed that a strong interaction existed used in industrial FTS process, the preparation of iron in silica-supported magnetite catalysts [117]. In situ XRD

Table 2 Reduction extents, dispersions, and average iron crystallite diameters for Fe-Si catalysts.

Catalyst Fe0Si Fe1Si Fe5Si Fe10Si Fe15Si Fe25Si

Reduction extent (%)a 100.0 100.0 95.0 75.0 67.2 33.1 Dispersion (%) 1.4 1.6 5.4 9.4 10.6 13.4 Average iron crystallite diameter(nm)b 85.7 76.7 22.9 13.0 11.6 9.2 CO conversion (%)c 76.6 65.9 34.7 26.7 51.4 46.7 CO conversion (%)d 40.8 45.4 41.9 34.0 55.0 53.4 c CO2 selectivity (mol%) 42.7 32.7 13.2 10.1 15.1 13.2 c CH4 selectivity (wt%) 22.3 22.9 25.3 24.4 16.3 15.6 c C2-C4 selectivity (wt%) 49.6 50.1 46.8 46.5 39.6 39.2 + c C5 selectivity (wt%) 28.1 27.0 27.9 29.1 44.1 45.2 a Catalysts were reduced in pure H2 at 350°C for 10 h. The reduction extents of catalysts were measured by MES. b Determined by H2 uptake on metallic iron. cTime of stream of 24 h. dTime of stream of 192 h (Data is adapted from ref [116]. Copyright (2012), with permission from Elsevier). 558 P. Zhai et al.: Understanding structural characteristics and catalytic reaction

confirmed the conjecture that additional silica in iron cat- activity and more olefin productions, but a lower CO2 alyst tend to stabilize the wüstite or Fe (II) phase as shown selectivity. The weak interaction between the iron oxide in Figure 5. NPs and the alumina NPs may be responsible for these

In addition to SiO2, other oxides, including ZrO2, Al2O3 unusual properties. Addition of ZrO2 in Fe-O-Si composite

[118], MgO [119], CeO2 [120], or Nb2O3 [121] were used as could weaken the Si-Fe bonds by forming Zr-O-Si linkages support. Wan et al. studied the Fe-Al2O3 interaction over [123]. The stability of iron carbides was improved by the the precipitated iron catalyst in a slurry reactor and con- synergistic effects, and the potassium doping can acceler- cluded that alumina inhibited the weak H2 adsorption on ate the reduction rate of Fe/ZrO2 catalyst [124, 125]. Promo- iron surface but favored strong H2 adsorption [122]. More­ tion of Fe-based FTS catalysts with cerium is only effective over, CO adsorption exhibited a similar trend, as evidenced when Fe-O-Ce bridges are formed in the precursor prepa- by CO-TPD. CO2-TPD demonstrated that alumina support ration. The CO dissociation increased because the C-O reduced the surface basicity; therefore, the carburization bond is weakened in the CO tilted configuration, resulting and reduction process of Fe2O3 were suppressed, and the in a higher olefin content and selectivity to heavier hydro- FeO phase was stable after pretreatment. The catalytic carbons [126]. activity decreased with the increased selectivity toward The support effects are not only dependent on the

CH4 and C5–C12 hydrocarbons in the FTS reaction [122]. oxide precursor, but are also related with the methods of Another interesting example is that the Fe-Al nanocata- preparation of catalyst compositions. It was found that lyst can be synthesized by impregnation using Fe3O4 NPs potassium impregnation on sol-gel synthesized Fe-Cu-Al as the precursor [53]. The nanostructured active phase led to the highest CO activity, and Fe-Cu-K impregnation + remained spherical and almost the same particle size on alumina showed 97.5% C5 selectivity compared to during the preparation. This Fe-Al catalyst showed higher other impregnated or coprecipitated K-Fe-Cu-Al catalysts [11]. Another example is the influence of different silica

sources in the synthesis when SiO2 is used as the support Si/Fe=0 mmol/mol α-Fe [127] Compared with the acidic silica sol (AcSS), TEOS

derived SiO2 support showed more uniform pore size dis- tribution and higher dispersion of iron oxides. Moreover, γ-Fe2O3

Fe3O4 it has more adsorption sites for H2 and CO on the surface.

The higher iron carbide content in Fe/SiO2-TEOS catalyst, 450 compared with Fe/SiO -AcSS catalyst, is responsible for its 350 2 250 high activity in FTS reaction. Additionally, the methane

150 formation is inhibited on the Fe/SiO2-TEOS catalyst prob-

50

mperature (°C) mperature Te ably due to the higher dispersion of iron carbides and less 20 40 60 80 iron oxides. Θ Diffraction angle (Co-Kα), 2 , ° The effects of oxide supports on the FTS activity or product selectivity are inconsistent in the literature. This Si/Fe=98 mmol/mol FeO (wüstite) is mainly because the studies were conducted under dif- α-Fe ferent conditions and over catalysts with multiple com-

γ-Fe2O3 ponents; a pervasive tendency of oxide effect in FTS can hardly be given. Generally, it is accepted that a balanced Fe3O4 Fe3O4 interaction between the oxide support and active phase is 450 most suitable for FTS. 350 250 150

mperature (°C) mperature 50

Te 3.3.1.2 Carbon support effect 20 40 60 80 In addition to the oxide support, carbon material is also Θ Diffraction angle (Co-Kα), 2 , ° a kind of important catalyst support for loading the metal component. The less hydroxyl groups on carbon materials, In situ XRD of the hydrogen reduction of the unmodi- Figure 5 compared with oxide support, lead to weaker interactions fied sample (top) and the sample containing 98 mmol Si/mol Fe (bottom). Reduction: pure hydrogen, 400 ml (NTP)/min/g; heating between metal and support. Moreover, new carbon materi- rate: 0.5°C/min – only main diffraction peaks indicated (Reprinted als with various morphologies such as CNT, mesoporous­ from ref [19]. Copyright (2012), with permission from Elsevier). carbon, and graphene have attracted remarkable attention. P. Zhai et al.: Understanding structural characteristics and catalytic reaction 559

Studies of the carbon support effect on the Fe-based cata- A lyst were available in the early 1980s. For example, it was reported that well-dispersed Fe/C catalysts exhibited high 15 Fe-in-CNT reaction activity and stability for CO hydrogenation at a before reaction temperature of 235°C [128, 129]. It was also found that Fe/ 10 carbon catalysts exhibited high selectivity for olefins com- pared with Fe/Al2O3 and Fe/SiO2 as well as it offered high 5 activity [36]. As carbon materials have many unique prop- erties, they may become a very important field for improv- 15 0 Fe-out-CNT ing the FTS performance over iron catalysts. For example, B ) before reaction the CNT can restrict the size of metal NPs within their 10 diameter scale, as well as to prevent the metal sintering during reactions [130]. This effect is named as confinement 5 effect. Moreover, it is proposed that the π electron density Percentage (% of the CNT may shift from the concave inner to the convex 0 246810 12 14 outer surface so the interior surfaces are electron deficient, Particle size (nm) whereas the exterior ones are electron rich. This proposed electric potential difference could be used to explain the change of physicochemical properties that the metal NPs Figure 6 TEM images and particle size distribution of the activated in contact with either surface, leading to different catalytic catalysts (A) Fe-in-CNT and (B) Fe-out-CNT before reaction (Reprinted performances between the particles deposited inside and with permission from ref [133]. Copyright (2008) American Chemical outside of the nanotube. For the metal NPs staying on the Society). external surface of CNTs, an interesting example is that a

Ru/CNT catalyst can achieve diesel oil fraction (C10–C20) expected [135]. A striking enhancement of the catalytic selectivity as high as 60%, which is the highest in litera- activity of Rh-Mn particles confined inside CNTs for the ture [131]. It is believed that CNTs or other carbon materials conversion of syngas to C2 oxygenates was also reported with arranged channels not only provide the confinement [136]. As the functional groups on carbon surface were effect to improve the core metal performance in FTS but demonstrated to be able to affect the FTS performance of also play roles in mild hydrocracking of heavier hydrocar- the iron catalysts, Dalai et al. studied acid treatment effect bons like the molecular sieves. in iron/CNT catalyst [137]. They found that HNO3 treat- With the above-mentioned concept of confinement ment increased the surface area as well as the number of effect by CNTs, Bao and coworkers developed an interest- defects. The average of crystallite size decreased about ing area in syngas conversion [132]. They used a delicate 20% as a result of the opening of CNT caps, leading to method to synthesize Fe-in/CNT catalyst in which over enhanced FTS stability and activity. The same acid treat- 70% of iron particles were located inside CNTs channels ment effect was also reported on Co/CNT catalyst [138]. (as shown in Figure 6) [133]. The catalytic performance results proved that Fe-in/CNT catalyst favored the forma- Fe-in 400 + Fe-out tion of long-chain products, and the yield of C5 hydro- carbons was remarkably higher at 270°C compared with 45 300 Fe-out/CNT (outside the CNTs channel; as shown in h) ca t Figure 7). Temperature-programmed desorption experi- 30 ments and Raman spectroscopy proved that the under- 200 eld (g/k g Yi lying reason is attributed to the interaction between the 5+ C CNT surface and the encapsulated iron oxides, which is in CO Conversion and 15 product selectivity (C%) 100 agreements with the combined first principles and Monte Carlo study [134]. In addition, the channels of CNT may 0 0 provide an environment to prolong the contact time of the CH4 C2-C4 C5+ CO2 CO C5+ reactants with metal component inside the tubes, leading Conv. yield C+ to higher selectivity to 5 hydrocarbons. The catalytic Figure 7 The effect of confinement in CNTs on the activity of FTS performance of cubic iron nitride NPs inside and outside iron catalyst (Reprinted with permission from ref [132]. Copyright CNT was also in accordance to the confinement effect as (2011) American Chemical Society). 560 P. Zhai et al.: Understanding structural characteristics and catalytic reaction

Comparisons were performed on three CNT-supported stability and good selectivity for long-chain hydrocar- iron catalysts prepared by different methods, the activ- bons were observed on Fe-RGO when they were applied ity of catalysts prepared by incipient wetness was higher in FTS reaction. The defective nature might hinder the than those by precipitation and deposition of as-prepared coalescence of iron NPs, and the acidic oxygen-containing iron NPs on CNT [139, 140]. The confinement effect is groups on RGO may help to crack heavier hydrocarbons not only limited to CNTs. Ordered mesoporous carbon to the gasoline fraction like molecular sieves. Zhao et al. was synthesized with iron-containing NPs by a chelate- also studied the effect of oxygenated groups on graphene assisted multicomponent co-assembly method [37]. As the oxide, on which the presynthesized iron oxide NPs were iron NPs were partially embedded into the mesoporous loaded, in order to avoid the influence of support on Fe carbon channels, the confinement effect was also effec- NP growth [142]. They conducted thermal treatment by tive for this catalyst, showing significantly improved FTS H2 at different temperatures on pyrolytic graphene oxide + performances with C5 selectivity as high as 68%. Another (PGO) to recover the original graphitic surface. C-O and interesting syngas conversion on Fe-loaded carbon mate- C = O species on the carbon surface decreased, while gra- rials was performed on FexOy@C spheres, which was pre- phitization degree increased with the increase in thermal pared by the hydrothermal method with iron nitrate and treatment temperature. The FTS activity improved glucose as carbon source at 80°C [141]. The microscale and produced more long-chain hydrocarbons with the spheres were constructed of nanorods with iron oxide NPs decreased oxygen species. Fe K-edge X-ray absorption highly dispersed inside. This material showed high activ- near edge structure (XANES), EXAFS data, and XPS exper- ity and a noteworthy stability in FTS reaction at 270°C. iments revealed that the valence states of this series of Fe/ + Moreover, the selectivity toward C5 long-chain hydrocar- PGO were almost identical. Moreover, the used catalysts bons reached up to 60%, with 40% C5–C12 petrol fraction. had fine structures more close to Fe5C2. The evidence sug- It is suggested that the confinement effect also provides gested that oxygenated groups may change the surface a suitable environment for the thermodynamic stability property of iron, thus, influence their performance in FTS. of iron carbides, which is beneficial for the long-chain Table 3 lists the representative catalytic performance of hydrocarbon formation. catalytic materials discussed in this section. de Jong et al. have made a breakthrough on iron- based catalyst, by using CNFs as catalyst support. They greatly enhanced the yield of C2–C4 olefins products, while 3.3.2 Solvent effect limiting methane selectivity in the FTS reaction at 340°C, in comparison with the iron supported on high surface In a gas-liquid-solid FTS reaction system, the solvent play area γ-Al2O3 or SiO2 [38]. This result breaks the product dis- a very important role. In addition to the most common tribution predicted by the Anderson-Schulz-Flory model. “green” solvent of water, polyethylene glycol (PEG) can

The maximum selectivity achievable for the C2–C4 frac- be used as solvent for both Fe and Co NPs in FTS at mild tion according to the ASF model, including olefins and conditions [23]. The Fe NPs dispersed in PEG showed the paraffins, is approximately 50% with about 30% methane highest activity among the solvents including ethanol, selectivity. It is believed that Na and S can act as promot- cyclohexanol, and ILs. If water is used as solvent for Co ers to suppress the methanation reaction, corresponding or Fe catalysts, oxidation of the catalyst is unavoidable, to the “surface carbide” or “alkyl” mechanism. The results which was considered to be one of the main reasons for shed light on the future study to use carbon support to the catalyst deactivation. In the PEG system, the isolation tune product distribution in the FTS. of water produced during the FTS from the iron NPs in the Graphene is one of the most attractive topics in the PEG environment might be the cause for the good activ- realm of chemistry after it was discovered. It is a two- ity and stability of the catalyst. Interestingly, outstand- dimensional single-layer sheet of carbon material with p ing selectivity toward oxygenates are also obtained in CO electrons fully delocalized on the plane. Because of the hydrogenation in the liquid environments. A dissolved weak interaction between graphene and NPs, it is dif- base/oxygenated solvent was found to be able to facili- ficult for metal oxide deposition on graphene. Reduced tate the selective production of oxygenate products [144]. graphene oxide (RGO) has oxygen-containing groups and The syngas conversion over Fe2O3 supported on Al2O3 in more atomic defects on its surface, which make it suitable the oxygenated solvent PEG-400 can reach 95% selectivity + for anchoring metal oxide NPs. Three-nanometer Fe2O3 toward mixed alcohols with more than 40% C2 alcohols

NPs were highly dispersed on RGO by a one-pot hydro- [145]. Results of H2-TPR and XPS confirmed that FeO phase thermal hydrolysis-reduction strategy [28]. Remarkable was responsible for such high selectivity, and PEG-400 P. Zhai et al.: Understanding structural characteristics and catalytic reaction 561

Table 3 Catalytic performance on carbon-supported iron FTS catalysts.

-5 Catalyst Temp Press H2/CO FTY (10 Selectivity in hydrocarbons/wt% CO2 Olefins/ References

(°C) (MPa) molco/gFe.s) + selectivity paraffins ratio CH4 C2-C4 C5-C11 C12 10% Fe/C 200 0.1 2.0 NA 12 39 42 7 45 4.1a [36] Fe/MWNTs 252 1.0 2.0 5.0 12 44 42c NA 7.3 [140] Fe-in-CNT 270 5.1 2.0 ∼33.1 15 50 35c 18 NA [133] Fe-out-CNT 270 5.1 2.0 ∼24.0 15 54 19c 12 NA [133] Fe/ha-lsa-C 275 2.0 2.0 3.87 8.7 21.1 70.2c 33.3 1.65 [137]

FexOy@C 270 2.0 NA ∼3.1 13 27 40 20 23 ∼0.6 [141] Fe-MesoC-8 270 2.0 1.0 ∼5.4 8.2 23.8 48.8 19.2 25.1 1.7b [37] FeA/CSs-B-IM 275 0.8 2.0 4.5 20.2 22.2 57.6c 21.2 ∼0.7b [143] Fe-rGO 270 2.0 2.0 ∼5.0 8.1 25.9 48 18 NA ∼0.8 [28] Fe/PGO-800d 270 3.0 2.0 ∼21.8 12.6 36.8 47.4c 10.1 1.44 [142] Fe/CNFd 340 2.0 1.0 2.98 13 64 18c 42 4.3b [38] d c b Fe/α-Al2O3 340 2.0 1.0 1.35 11 59 21 40 8.8 [38] a Olefin to paraffin weight ratios for C3-C7. b Olefin to paraffin weight ratios for C2-C4. c + Data represent C5 hydrocarbons selectivity. dOther products including oxygenates.

NA, data not available. Carbon selectivity is defined as the carbon atoms in the total number of C atoms in hydrocarbon products, CO2 is not included. Iron time yield (FTY) represents moles of CO converted to hydrocarbons per gram of Fe per second; some values are recalculated from the original data reported in literature.

was also indispensable for the reaction results. It is noted in general. The catalysis system combines both the advan- that supercritical hydrocarbons are used as the reaction tages of homogeneous catalysis and heterogeneous catal- media for FTS reaction in literatures [146]. The influence ysis, showing unique mass and heat transfer properties of pressure and composition of the supercritical phase and resulting in high reaction activity. This is particularly on the catalytic property was reported [147, 148]. It was true for FTS in which the solvent environment, as the reac- found that supercritical media could inhibit the deactiva- tion media, facilitates the interaction between the core tion of Co/Al2O3 catalyst [149]. Higher CO conversion and metal NPs and syngas reactants as well as to control the more long-chain hydrocarbon products with decreased reaction temperature effectively by good mass transfer

CH4 and CO2 selectivity were obtained in supercritical and heat transfer. reaction media [150]. Different solvents (hydrophilic or Kou et al. set a precedent of low-temperature aqueous hydrophobic medium) affect both the catalyst activity and FTS reaction and developed several excellent catalyst selectivity toward products by changing the stability of systems [21]. Ruthenium nanoclusters with an average intermediate species, and they may also influence diffu- diameter of 2.0 nm were prepared by high-pressure H2 sion and desorption of products in FTS reaction. reduction. Those nanostructured catalysts were employed The application of quasi-homogeneous catalysts in in FTS in a stainless steel autoclave. The researchers inves- FTS represents one of the most important progresses in tigated the solvent effect by changing different solvent the syngas conversion researches from a scientific point media. Ru NPs that are stabilized by ethanol, dioxane, or of view in the last decades, although the employments of cyclohexane, even the IL, [BMIM] [BF4], show remarkable such catalysts in the commercial scale plant face the diffi- FTS activity at 150°C. More importantly, the reaction in culty of the product separation and catalyst regeneration. water exhibit an unprecedented high activity (6.9 molCO/

Those so-called quasi-homogeneous catalysts are metal molRu/h) compared with other solvent systems. The activ- NPs that are dispersed in the solvent such as aqueous ity is even higher than traditional heterogeneous Ru cata- phase or ILs. The solvent (liquid) provides a three-dimen- lysts working at higher reaction temperature. The 80.9 sional freedom environment for those nanostructured wt% of the products are C5–C20 hydrocarbons, and only metal particles to rotate; thus, the reactant (gas) can 3.3% CH4 is detected. Size effect was also observed over contact the active sites of metal particles more easily. For this catalyst. Detailed product distribution with different the small nanostructured metal particles, the catalytic diameters are shown in Figure 8. Further study found that activity is higher than their heterogeneous counterparts, this catalyst showed good stability in a continuous flow 562 P. Zhai et al.: Understanding structural characteristics and catalytic reaction

1.0 A 60 B 1.4 nm 0.5 1.9 nm 50 ) 0 2.5 nm 40 -0.5 3.0 nm ) -1.0 3.5 nm 30 -1.5 20 -2.0 lg (yield (mmol) -2.5 10 -3.0 Selectivity (wt% 0 -3.5 C1 C2-C4 C5-C12 C13-C20 C21+ 0510 15 20 25 30 Products Carbon number (N)

Figure 8 (A) Hydrocarbon selectivities for 2.0-nm-diameter nanoclusters; (B) ASF distribution of products for Ru nanocluster catalysts with -4 different diameters. Reaction conditions: 150°C, 2.0 MPa H2, 1.0 MPa CO, 2.79 × 10 mol Ru, 20 ml water (Adapted from ref [21]. Copyright (2008), with permission from John Wiley and Sons).

reactor [151]. Moreover, high selectivity toward long-chain produced as iron is facile for reactions under the syngas products was observed over 5–8 nm Co NPs, which were atmosphere at elevated temperatures. For instance, synthesized and dispersed in imidazolium ILs [152]. Pt-Co metallic iron is observed in hydrogen, cementite, and

NPs synthesized by one-step H2 reduction method was Hägg carbide are the main phases in CO and in syngas, found to be an excellent catalyst for aqueous-phase FTS at respectively. And iron carbide is also observed to have 160°C [22]. In addition, Hensen et al. achieved a very high ε-χ-θ transition behavior during the FTS reaction at dif- aldehyde selectivity as high as 70% for an aqueous phase ferent conditions [154] confirming the structure evolu- 2.2-nm Ru NP catalyst at 145°C [153]. tion of iron oxide catalyst from hematite to magnetite and Apparently, quasi-homogeneous catalyst is a new finally to iron carbide in the FTS [155]. It is well recognized research field, and more studies are needed to explore that the activation conditions before the FTS reaction their potential applications, to investigate the related can influence the final iron structure in the steady reac- reaction mechanisms and to reveal in detail the solvent tion conditions [156]. Noting that the iron structure could functions in the liquid FTS reactions. change during FTS, it is difficult to correlate the relation- ship between the initial catalyst status prior to use and the catalyst performance in FTS steady state; hence, the moni- toring of surface chemical processes under reaction condi- 4 Characterization of FTS tions is extremely crucial to identify the active Fe phase. nanostructured catalysts It is a great challenge to study the detailed process and identify the “real” active phase because of the rela- Generally, there are several approaches for studying the tively harsh conditions used in FTS. Table 4 lists some active phase of nanostructured catalysts for the syngas commonly used methods for the in situ characterization conversion: (1) observing the catalyst structure by in situ techniques. characterization; (2) synthesizing pure active phase and Great effort was devoted to the in situ investigations. comparing its activity with the real catalyst; (3) induc- Weckhuysen et al. investigated the iron catalyst by com- ing promoters on core metal in catalyst or modifying bined in situ characterization techniques and theoreti- active phase (e.g., treating the metal particle by H2, CO, cal methods at relative high pressure [157]. As shown in or syngas) to establish relation between structural change Figure 9, in situ EXAFS, XANES, XRD, Raman spectro­ and catalytic activity; and (4) theoretical modeling from scopy, and online mass spectrometry were combined computational methods. Because enormous literature to investigate two samples of iron oxide. One was pre- have been published to investigate the Fe active phase in treated under CO, and another one was pretreated under the FTS, we take the studies of Fe active phase as exam- 1% CO in H2 at elevated temperatures. The results give a ples to illustrate how those approaches are used together detailed change trend of iron restructures and conclude to provide complementary information on the nature of that χ-Fe5C2, θ-Fe3C, and FexC are formed under differ- the catalyst. ent conditions, exhibiting different catalytic activities in Typically, FTS use iron oxide NPs as the catalyst core FTS, which is consistent with the theoretical prediction metal component, and a variety of iron phases can be [63]. In contrast, Bao et al. found that iron encapsulated P. Zhai et al.: Understanding structural characteristics and catalytic reaction 563

Table 4 Comparison of the in situ characterization used in iron catalyst.

Characterization methods Advantage Disadvantage

XRD In situ at high pressure Bulk phase information, poor for noncrystallite sample MES Suitable for phase analysis Only bulk information of metal is given XAFS In situ at high pressure Bulk information, low fitting accuracy of crystalline phase XPS Surface structure analysis Lower pressure than real reaction condition TEM Direct observation of the surface Lower pressure than real reaction condition Computational chemistry Overcome the extreme experiment conditions As supplementary evidence in CNTs showed higher activity as well as higher selectiv- respectively; the addition of water obviously inhibits the + ity toward C5 products than the iron NPs supported on carburization of Fe species, implicating the importance of the CNT outside surface [133]. In situ XRD revealed that water during the FTS process.

FexCy/FeO ratio was higher in Fe-in-CNT than that in Fe- In addition to the in situ characterization techniques out-CNT. Fe5C2 and Fe2C were the most likely carbide above that can be operated under more realistic reactions phases under the reaction conditions. This obvious differ- in practical use, some surface techniques such as TEM ence in iron structure may be caused by the confinement and XPS, which can only be used at relatively low pres- effect in the system. Moreover, the carburization process sure, are also employed for the FTS study. Fe-based cata- of Mn-promoted iron-based catalyst was also studied by lyst was studied by in situ TEM-EELS during carburization in situ EXAFS/XANES [158]. It was found that the pre- in low pressure CO to elucidate the activation mechanism treatment atmosphere affect the performance of catalysts using specially designed equipment [161]. Severe sintering dramatically; the sample pretreated by H2 showed lower occurred following hematite reduction, and no evidence of

CH4 formation and higher selectivity toward olefins than carbon deposition was observed in contrast to literature. the one pretreated by CO. The additive Mn can hinder the Nevertheless, the metal surface was coated with carbon carburization rate, thus, leading to χ-Fe5C2/Fe3O4/MnO(1-x) layers when it was exposed to air or after passivation [162]. mixed structure. The method was also applied to study Another example is that a delicate in situ equipment made the influence of alkali metal promoters on iron, reveal- it possible for imaging the iron nanostructured catalyst ing that alkali metals promote the carburization of Fe under scanning transmission X-ray microscopy (STXM), [159]. Interestingly, the effect of water on iron oxide and which is capable of operating up to 500°C at 1.2 bars. The iron carbide under syngas and hydrogen was also investi- Fe-Cu-K/SiO2 catalyst was investigated for in situ reduction gated by in situ XAS [160]. Fe2O3 is transformed to FeCx and in H2 and followed by the FTS reaction in syngas. Reduc- α-Fe as expected under syngas and hydrogen at 350°C, tion, carburization, and spillover of carbon from metal to

XRD Scintillator

detectors

Raman objective Raman XRD Circle diffractometer spectrometer

Storage ring Sample capillary Ionization chamber (I ) Ionization chamber (It) o (XAFS) (XAFS+XRD) X-rays Reference foil

ionization chamber (Ir) (XAFS) Gas manifold N 2 (mass flow controllers, Heater gun valves, pressure controller) Mass spectrometer

Figure 9 Schematic representation of the combined XAFS/XRD/Raman experimental setup (Reprinted with permission from ref [157]. Copy- right (2010) American Chemical Society). 564 P. Zhai et al.: Understanding structural characteristics and catalytic reaction support were visibly demonstrated from STXM informa- A 40 tion [163, 164]. Furthermore, in situ XPS was conducted Fe5C2 35 to study the structure change during low pressure treat- Reduced Fe2O3 ment on the surface of iron oxide NP and bulk iron oxide 30 [165]. The two samples showed different performance in ) 25 the same treatment atmosphere. The bulk iron oxide was 20 easily reduced to metallic iron, whereas Fe2O3 NP may be

partly converted to FeO phase. The results provide a direct Conversion (% 15 experimental evidence for the size dependency of iron 10 oxide in FTS reaction. The in situ investigation results above all indicate that 5 iron carbide phase plays crucial roles in FTS. Indeed, iron 0 carbide was considered to be necessary in CO hydrogena- 0 10 20 30 40 50 60 70 80 90 100 tion, as hematite and magnetite did not show any activity Time on stream (h) in the early studies of FTS. α-Fe, Fe3O4, bulk and surface B Reduced Oxygenate iron carbides have all been suggested as active phases for Fe O Olefin 40 2 3 Paraffin FTS. Nevertheless, iron carbides attracted more attention Fe C Fe C 5 2 as more data have been accumulated as the characteriza- 5 2 Reduced tion methods, including XRD and Mössbauer spectrum, 30 Reduced Fe2O3 always display a mixture of various iron carbide phases Fe2O3 after the FTS. Fe2C was prepared and investigated in the 20

Selectivity (% ) Fe C 1950s, it was suggested that Fe2C was not the active phase 5 2 in FTS due to the poor CO adsorption property [166]. Although the exact iron carbide phase for the FTS activity 10 Reduced Fe5C2 and the relationship between the structure and activity/ Fe2O3 selectivity remain unclear, they are believed to play very 0 important roles in FTS reaction. It is possible that some, CH4/CH3OH C2-C4 C5-C12 C12+ if not all, iron carbide phases are active for converting the Products syngas into hydrocarbons. Figure 10 Overall catalytic performance of Fe C NPs/SiO and Nanosized iron carbides are mainly observed as 5 2 2 reduced Fe2O3/SiO2 catalysts: (A) CO conversion and (B) product product in carburization of iron, and the synthesis of selectivity (Reprinted with permission from ref [168]. Copyright pure phase of iron carbide impose a great challenge. Fe3C (2012) American Chemical Society). NPs were synthesized through a urea-glass route, which have a size range from 5 to 10 nm [167]. Recently, Yang et al. have synthesized Fe5C2 NPs, for the first time, via TPSR reaction over Fe5C2 catalyst. The results clearly indi- an elegant route [168]. Fe2C5 particles with a well-defined cate that Fe5C2 is an active phase for FTS. Furthermore, it particle size of 20 nm were obtained after the reactions of offers a new possibility in investigating the active phase of iron carbonyl, Fe(CO)5, with octadecylamine in the pres- iron FTS catalysts, based on the progress in materials syn- ence of bromide under 350°C. In contrast to the traditional thesis. However, it is still desired to synthesize other pure reduced-hematite catalyst, the Fe5C2 NPs supported on phase iron carbides to see which type of iron carbides is

SiO2 showed higher activity and selectivity in FTS, result- the most active phase for FTS. + ing in 39% selectivity toward C5 hydrocarbons at 270°C Modification of the core metal such as iron by the pro- without the pretreatment by H2 or CO. The activity and moters such as alkali metals is a very traditional method selectivity are shown in Figure 10. Moreover, the induc- to enhance the catalytic activity and study the structure- tion period observed on conventional Fe2O3 with respect activity relationships. Iglesia et al. combined the kinetic to pure phase Fe5C2 catalyst was coincided with the con- analysis of the initial stages of FTS with XAS to study the clusion of temperature programming surface reaction structure evolution of K and Cu-promoted iron oxide cata-

(TPSR) experiment, which demonstrate that Fe2O3 catalyst lyst during the initial contact with syngas [169]. Accord- needs to be converted to iron carbide before the formation ing to XAS results, Fe2O3 was converted to Fe3O4 and, then, of hydrocarbons could be observed, whereas the hydro- rapidly transformed to FeCx after contact with syngas for carbon products can be detected at a very early stage of 120 s. The K and Cu in Fe2O3 facilitate the formation of P. Zhai et al.: Understanding structural characteristics and catalytic reaction 565

more nucleation sites for Fe3O4 and FeCx as well as smaller chemistry system with abundant atoms can be simulated; crystallites on the surface of the catalyst. These smaller thus, complex reactions can be studied by the theoretical particles can be regarded as active sites for FTS, which modeling. The most popular method in the heterogeneous shorten the structure evolution paths of Fe2O3. It was catalysis is based on the density functional theory (DFT) also observed that the Mg-promoted Fe/Cu/K/SiO2 cata- that expresses the total energy of the chemical system + lyst could enhance the catalytic activity and C5 selectiv- as the function of electron density. Compared with the ity; the highest Fe5C2 percentage of the iron phase in this Hartree-Fock method, the DFT method is more efficient at outstanding catalyst after activation was observed in the the expense of slight inaccuracy. Energy errors in normal Mössbauer emission spectroscopy (MES) [66]. The FTS DFT calculations are about 0.2 eV, which is acceptable activity was also decreased with a decrease in the Fe5C2 to describe most chemical reactions semiquantitatively. concentration, which was caused by the addition of silica Generally, the DFT calculation can illustrate the reac- [170]. In addition to the alkali metals, Mn was applied to tants’ stable structure or the preferable pathway of chemi- decrease the iron carbide concentration and stabilize the cal reactions. Practically, different models such as slab

Fe3O4 phase as evidenced by XANES, but the promoter models or cluster models are used to simulate catalysts. manganese could effectively improve the FTS activity [171]. The DFT calculations offer us the adsorption energy and

The authors suggest that the mixed oxide (Fe1-yMny)3O4 the reaction barriers in catalysis, which are very useful to may contribute to produce smaller iron carbide particle, the analysis experimental data in real catalysis. Based on which is more active in the FTS reaction. Mo is also sug- those data, advanced methods like molecular dynamics or gested to play a similar role in the restructure process of Monte Carlo simulation can be performed to obtain more iron catalyst [172]. Because iron particles are ready for oxi- information about catalysis. Moreover, adsorption ener- dation in air, the obtained structural information of used gies and reaction barriers calculated from DFT are often catalysts are difficult to be correlated with their activities used to deduce the reaction rate equations on surfaces. in the reaction. Syngas conversion is one of the most complicated DFT studies also reveal that iron carbide might play a reaction systems in heterogeneous catalysis research positive role in FTS, which is different from cobalt carbide. with many parallel reactions and consecutive reactions. Although CO dissociation is difficult on iron carbide, CO Gas, liquid, and solid are involved, and the intermediates hydrogenation on the surface more readily proceeds than and products consist of hundreds of chemicals. Remark- metallic iron. It is proposed that the CH4 selectivity on iron able researches have been performed by the theoretical carbide will be similar to that on metallic iron; hence, studies on the FTS reaction. Abundant knowledge can be iron carbide can be considered to be the active phase in obtained from the computational chemistry [174, 175], and FTS. Pretreatment effect was investigated to understand those computational results rationalize our understand- the surface structure of χ-Fe5C2 using ab initio atomis- ing of complex syngas conversion, particularly the reac- tic thermodynamics [173]. It is found that exposed facets tion mechanism at atomic level such as CO activation, C-C strongly depend on the gas pressure and H2/CO ratio. CO coupling, olefin selectivity, etc.; thus, guiding us on the pretreatment is beneficial for carbon-rich facets, whereas synthesis of more efficient and stable catalysts. the addition of H2 into CO favors the stable carbon-poor Theoretical studies of the FTS could be divided into facets. As a consequence, the higher activity of surface two fields: the first one is based on the iron carbide cata- carbon toward hydrogenation may be the cause for the lyst, which represents the most important industrial iron- higher initial activity on iron catalyst pretreated by CO based catalyst. The iron carbides can be characterized by than that pretreated in syngas. This case study clearly Mössbauer spectroscopy, and the existence of such iron exhibits that the computational technique is a powerful carbides have been well related to the catalytic activity. method to assist us to understand the active phase in the Nevertheless, Mössbauer spectroscopy cannot identify reaction condition. the structure of catalyst surface, which is very essential for their catalytic roles. Compared with spectroscopy tech- niques, theoretical study is more straightforward to reveal the structure-activity relationship. Adsorption of syngas 5 Theoretical study of FTS and stability of different iron carbide structures have been nanostructured catalysts studied systemically [55, 176–181]. It is noted that the metal surface, which plays the catalytic role under the reaction With the improving power of the computer science and conditions, might be very complex, and the presence of the rapid development of computational chemistry, the active iron carbide mixture cannot be excluded. Moreover, 566 P. Zhai et al.: Understanding structural characteristics and catalytic reaction the involvement of gas atmosphere is also important for a form graphite species. In contrast, the C atoms prefer to reliable theoretical study. Some reports stressed the issue be hydrogenated first to form the CHx species with a high and calculated the preferable surface structure in the reac- hydrogen pressure, then, the CHx coupling pathway may tion atmosphere [173]. The second category of catalysts is be preferable [187]. This is consistent with the experimen- based on Co or Ru metals. Cobalt and ruthenium are often tal observation that hydrogen pressure influences the FTS believed to act as zero-valance metal in the FTS reaction. significantly. They are located on the near top of the volcano-like profile Methane is an undesired by-product in Fischer- [182], so they are often studied comparatively with other Tropsch synthesis, so tuning the catalyst to lower the metals such as Rh, Ni, and Cu. The activity trend of these methane selectivity is a very interesting topic. In the exper- metals can be described very well by the d-band center imental work on iron-based catalyst, very little knowledge model. The d-band model may act as a very promising is gained on the CH4 formation upon catalysts, partially descriptor for screening of the bimetallic FTS catalyst. because of the complex nature of the catalysts. DFT cal- The CO dissociation is very important for FTS, usually culation offers the atomic-level understanding of adsorp- being regarded as the rate-determining step, so it is neces- tion, reaction, and desorption in the reaction process, sary to understand the pathways and factors that influence thus, one can specify the methane formation pathways on the CO dissociation. There are two suggested pathways for different surface sites. Four different surface of Fe2C(011),

CO dissociation: H-assisted CO dissociation and direct CO Fe5C2(010), Fe3C(001), and Fe4C(100) were employed to dissociation. The two pathways are both studied on the study their activities on methanation, indicating that the iron carbide surface. Jiao et al. studied the CO dissocia- Cs vacancy site is very active for CO dissociation on all four tion on Fe3C (001) surface. The results show that if CO is surfaces, but the methanation activities are different. Fe5C2 adsorbed on the ideal surface site, the H-assisted CO dis- (010) and Fe2C (010) are active for methanation because of sociation pathway has lower barriers than the direct CO the suitable CHx hydrogenation barriers and stability. In dissociation. However, if the CO is adsorbed on a vacancy contrast, Fe3C (001) and Fe4C (100) are not very active for site from the surface Cs hydrogenation, CO dissociation methane formation [63]. barrier is reduced, indicating that direct CO dissociation Potassium is the most important auxiliary metal, is also very favorable [183]. The result that the vacancy site usually dubbed as promoter, for the industrial Fe (core of surface carbon species is very active for CO dissociation metal) catalyst in FTS and ammonia synthesis. Potassium is confirmed by a similar study on four other different iron is added as potassium carbonate, which forms K2O after carbide surfaces, including Fe2C (011), Fe5C2 (010), Fe3C calcination at high temperature, in the FTS. The function

(001), and Fe4C (100) [63]. of K2O is concluded, by the experimental results, as elec-

The formation of initial C2 compounds is also essential tron donor, i.e., the addition of potassium can reduce the for FTS. In the classical FTS mechanism, CO dissociates work function of the surface so the CO dissociation barrier first, then the C atom is hydrogenated to form CHx(x = 1,2), is reduced at the promoted Fe surface. The alkali potas- and CHx coupling occurs successively to form long-chain sium promotional effects on CO and H2S adsorptions were hydrocarbons. However, based on the study of CO adsorp- studied on Fe(100) surface by the theoretical modeling, tion on iron carbide of Fe3C(001), a different picture is indicating that the presence of potassium promotes the concluded: C2 compounds may be formed by the CO CO adsorption but hinders the adsorption of the H2S [188]. coupling with surface Cs atom, then, the CsCO species is Furthermore, this result implied that the alkali metal pro- hydrogenated followed by the dissociation of the oxygen moter could reduce the deactivation of the Fe core metal atom [183]. The initial C2 compound formation pathways from the impurity of H2S, thus, extending the catalyst life. on Fe5C2 (001), (100), (110) surfaces are very similar with Moreover, another function of the potassium promoter that on Fe3C (001). It is concluded that the adsorbed CO was proposed by combining the DFT calculation and favors coupling with surface Cs into C2 compounds, then C2 experimental results. The results suggest that the potas- compounds are hydrogenated, and, subsequently, the FTS sium (or precisely potassium oxide) can stabilize the high are initiated [57, 62, 184, 185]. Similar to the C2 compound index surfaces of Fe, such as Fe (211) and Fe (310), which formation, C3 compounds are formed by the CCO coupling are more active for CO dissociation. The percentage of high with C atom, which is competitive with pathways of CCO index surface in all exposed surfaces can be evaluated by hydrogenation to CCHx on Fe5C2(001) [186]. It is noted that the Wulff model, which shows the probability of a certain the C-C coupling pathways are drastically affected by surface appearing in the catalyst in statistics. Hence, it the hydrogen pressure. When the hydrogen pressure is was found that the (211) and (310) surface of Fe gradually low, surface C atoms prefer to couple with each other to dominate the Wulff model, as shown in Figure 11, with P. Zhai et al.: Understanding structural characteristics and catalytic reaction 567

A B

(211) (310)

(100) (110)

(111)

K/Fe=0 K/Fe=1/48

100% 10 C 22 31 80%

32 60% 70 35 6 40%

39 20% 30 20 0% K/Fe=0 1/48 1/12 The contribution of each facet K/Fe=1/12 to the total surface area

Figure 11 The effect of potassium contents on the Wulff shape of nanostructure Fe. The figures above show that the high index surface (211) are gradual domains of exposed surfaces of this Wulff shape (Reprinted from ref. [58]. Copyright (2011), with permission from John Wiley and Sons). the increasing content of potassium promoter. This result factors, including space velocity of reactants, tempera- shows that the potassium can stabilize the high active ture, and the aforementioned gas pressure and promoters. surface as well as reduce the work function, implying the The knowledge shows the complexity of the active sites importance of auxiliary metals for the core metal on those under working conditions and offers us valuable informa- nanostructured catalysts [58]. tion to prepare well-defined nanostructured catalysts. The surface structure of the catalyst is not only affected Cobalt and ruthenium share some interesting prop- by the promoters but also by the atmospheres of the gas erties in FTS such as the high selectivity for long-chain reactants. However, the surface structure of the working hydrocarbons and the zero-valence active center, suggest- catalyst under high pressure and high temperature is dif- ing that the FTS mechanism on those two metals might be ficult to be studied by some important characterization similar. The reaction mechanisms of CO dissociation on techniques, e.g., the atomic resolution characterization cobalt and ruthenium have been studied extensively [176]. methods like TEM and XPS are impracticable at high pres- The theoretical study concluded that the CO dissociation sures. Instead, one can evaluate the Gibbs free energy and prefers the step sites on the surface, and the results are chemical potential of a system at different temperatures consistent on many metals such as Ru [189, 190], Rh, Co and pressures, with the knowledge from statistic thermo- [191], and even zero-valence Fe [192, 193]. Some experi- dynamics, by DFT calculation that simulates an isolated mental results at ultrahigh vacuum also underpin this system at zero Kelvin. The stable species at a real catalysis conclusion. For example, it is found that CO dissociation atmosphere of Fe5C2 are studied, and it is concluded that occurs at step sites by high-resolution electron micro­ the surface of Fe5C2 is changing at the reaction conditions: scopy [189], and the H-assisted CO dissociation have been the presence of hydrogen in gas or the high reaction tem- elaborated in literature [192, 194]. Iglesia et al. combined perature reduces the chemical potential of carbon dra- the kinetics analysis and DFT calculation proposing that matically and influences the chemical state of the carbon direct CO dissociation on the Fe surface may be a domi- species on the Fe5C2 surface remarkably [173]. The cata- nating pathway, but CO prefers the H-assisted dissociation lyst surface at real atmosphere is influenced by complex mechanism on the Co surface [195]. The higher activity of 568 P. Zhai et al.: Understanding structural characteristics and catalytic reaction step sites on CO dissociation might also arise from the CO insertion and CHO insertion barriers on Rh (0001) and shallow d-band center and geometric effects. The conclu- Co (0001) were compared, and the results show that the sion seems solid from a theoretical point-of-view, but it is CHO insertion is more energetically favorable [200]. This often challenged by a fact that the active step sites may be is a new viewpoint for CO insertion, but the proportion poisoned by carbon atoms or other impurity atoms. More of the CHO insertion in reaction need be reconsidered detailed studies from both theory and experiments are due to the poor stability of CHO. Interestingly, theoretical demanded to elucidate the dispute. study and microkinetic analysis are combined to study The growth mechanism of hydrocarbons is indis- the Fischer-Tropsch reaction on the Ru (0001) surface, pensable for the understanding of FTS, and two popular suggesting that CO insertion is more favorable thermody- mechanisms are often discussed on the cobalt surface and namically compared with the carbene mechanism [201], ruthenium surface. The first one is dubbed as carbene and the hydrogenation of CO is the initiation reaction of mechanism, which suggests that the CO molecule first the hydrocarbon polymerization process. In conclusion, dissociates to the C atom and O atom, then, the C atom is the mechanism of the chain growth is still on debate. It hydrogenated to form the CHx species, and subsequently, seems that the chain growth mechanism is dependent on the CHx species couple consecutively to form long-chain the reaction conditions and the catalyst used. The reac- hydrocarbons. The carbene mechanism well explains the tion results on well-designed nanostructured catalyst ASF distribution of FTS. The coupling reactions of the dif- could provide more detailed picture of the chain-growth ferent CHx on the Ru (0001) surface are studied, and the mechanism. results suggest that CH is the most stable intermediate among the CHx, the coupling reactions prefer the step sites, not the terrace sites [196]. Extensive effort were devoted to the study of CHx-CHx coupling on different metals (Co, 6 Summary and perspective Ru), and it is concluded that coupling reaction barriers are dependent on the core metals and the surfaces, but We have seen a substantial shift of FTS catalyst prepara- leading to the similar FTS product distributions except tion from the empirical approach, which lacks the know- for small differences in activities and chain-growth prob- how knowledge, to the well-defined nanostructured abilities [197]. Another well-studied mechanism in FTS is catalyst materials in the past decades. Those nanostruc- denoted as the CO-insertion mechanism. The mechanism tured catalysts offer superior model compounds for the is inspired by the homogeneous syngas reaction route, FTS scientific studies, although their thermal and chemi- such as hydroformylation, for which the mechanism is cal stabilities are major concerns in FTS reactions. Various well established. In the CO-insertion mechanism, CO preparation strategies for their synthesis as applicable inserts to the alkyl on the surface before CO dissociation; catalysts are being established in the interest of supple- this is strongly supported by the experimental evidence menting, and ultimately replacing, the current commer- that oxygenates like aldehyde and alcohol are present in cial FTS catalysts. The employments of such model the products. A reaction pathway of CO-insertion mech- catalysts have exhibited great advantages in the academic anism on the Co (0001) surface was proposed, which research, although those nanomaterials are usually more suggests that the CO inserting to an RCH- group needs susceptible to environmental changes than the bulk much lower barriers compared with inserting to the RCH2- material. In this review, we have shown key factors that group. The CO-insertion mechanism in some situations determine their catalyst properties and the underlying seems to be more reasonable compared with the carbene physiochemical phenomena associated with the improve- mechanism because of the high barriers for direct CO dis- ments of the catalytic performance. Clearly, the different sociation and high barrier of CO hydrogenation in the electronic properties, due to the different core metal with H-assisted CO dissociation [198]. The high CO coverage on different surface crystallography, play crucial roles for the Co (0001) surface was studied by DFT calculation, and the activation of the reactant (CO and H2) and the subse- the results show that a stable surface adsorption structure quent elementary reaction steps to form different prod- could be maintained at a large range of CO pressure. It is ucts. However, the core metal electronic properties that suggested that the high CO coverage benefits the CO-inser- are related to the size, shape, defects, shear planes, etc. tion mechanism under the real catalytic condition [199]. in the crystalline structure, can be affected by the auxil- As the CO-insertion mechanism is often challenged with iary metal(s) and the environment, typically the support, the high barrier of CO insertion, another possible mecha- resulting in a variety of product distribution when the core nism of CO insertion was suggested to address the issue. metal is chemically modified. We note here that although P. Zhai et al.: Understanding structural characteristics and catalytic reaction 569 different key factors are categorized in this review, they the reaction mechanism at the atomic scale under the are, indeed, intertwined in the reality and should not be real FTS working conditions (elevated temperature and discussed separately. high pressure) is sparse in the literature. Those insights Several new important issues need to be addressed in into the atomic-scale processes can guide the nanostruc- the development and study of the nanostructured FTS cat- tured catalyst design for the FTS reaction to develop more alyst area and summarized as follows. (1) It is necessary energy-efficient and environmentally benign catalysis to design new synthetic approaches that are simpler and process. (5) We stress the importance of using the com- are conducted under relatively mild conditions to gain bined approaches such as theoretical modeling and in the production cost advantage compared with the tradi- situ techniques to disclose the FTS reaction mechanism tional FTS catalyst. It is also highly desirable to gain the and the nature of the catalyst active phase. The theoreti- knowledge about the fundamental formation mechanism cal descriptions of the FTS reaction provides information for the further catalyst improvement. (2) Another impor- that are hardly obtained in the practical experiments, but tant aspect is to explore novel FTS nanostructured cata- the gap between the modeling and experiments need to be lysts such as the bimetallic catalysts that are discussed bridged to shed light on the FTS process under the working extensively in this review. Apparently, each core metal has condition. Maybe, a higher accuracy than the widely used its own characteristics in determining the single reaction DFT method is required to gain the predicative power. events such as the CO dissociation and C-C coupling. The The complexity of the real FTS reaction with many paral- combination of those different characteristics could lead lel subreactions should also be taken into consideration to enhanced catalytic activities and versatile product dis- in the computational methodology. (6) The well-defined tribution compared with each component on the bimetal- nanostructured catalysts, especially for those materials lic catalysts or multimetallic catalysts. (3) The FTS research that are promising in the FTS reactions, are highly desired under the liquid phase is also an important field. Some to be studied by the new surface characterization tech- of the intrinsic obstacles in the FTS commercial applica- niques with higher spatial, temporal, and energy resolu- tion on the heterogeneous catalyst are the control of heat tion to get a complete picture on the catalyst composition transfer, the product separation from the catalysts. The and surface structure. This is because FTS reaction results development of a suitable reactor such as the slurry-bed are critically dependent on the catalyst structure. The lack reactor cannot solve those problems completely. However, of knowledge by the limited characterization methods the FTS in the liquid phase might pave a new way for the might hinder further improvement of the catalyst materi- future FTS commercialization because of its low reaction als. On the basis of these considerations, we could expect temperature, facile separation of the reaction products that nanostructured catalyst study in FTS reaction would from the catalysts, and good control on the heat transfer. make a major contribution to the heterogeneous catalysis (4) We emphasize the need to synthesize well-defined and chemical industry. nanostructured catalysts and gain the understanding of their activities in the syngas conversion with their unique Acknowledgments: This work received financial support structural characteristics in the atomic scale. It is clear from the Natural Science Foundation of China (21173009, that we need the observation of the reaction events in situ 21222306), 973 Projects (2011CB201402, 2013CB933100). because, as discussed in this review, the metal active sites are highly likely to be different from their as-synthesized state prior to the FTS reaction. The direct observation about how exactly a specific crystalline structure affects Received May 8, 2013; accepted May 28, 2013

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Qingjun Zhu joined the National Institute of Clean-and-Low-Carbon Peng Zhai obtained his bachelor’s degree in 2010 from the Univer- Energy as a researcher in 2011. He received his PhD from the sity of Science and Technology of China. Now, he is working for his Eindhoven University of Technology, Netherlands, in 2003. He did PhD degree under the supervision of Professor D. Ma at the Beijing his postdoc research in Tokyo Institute of Technology, Japan, and National Laboratory for Molecular Sciences (BNLMS, China) in Northwestern University, USA. His research interests include Peking University. He was addressed in research on high efficient heterogeneous catalysis and design of novel catalytic materials. fixed bed reactors, and his research interests are in the area of experimental studies in the syngas conversion process that cover nanocatalysts synthesis and operando characterization of catalytic reactions.

Ding Ma is a professor in the College of Chemistry and Molecular Engineering, Peking University. He took up Chemistry in Sichuan University (1996) and obtained his PhD from the State Key Labora- tory of Catalysis, Dalian Institute of Chemical Physics (2001). After Geng Sun obtained his bachelor’s degree in 2011 from the Peking his postdoctoral stay in Oxford University (Prof. M.L.H. Green) and University. Currently, he is pursuing his PhD degree under the University of Bristol (Prof. S. Mann), he started his research career supervision of Professor H. Jiang at the Theoretical Materials Group in Dalian Institute of Chemistry as associate professor (2005). He in the Peking University. His research interests are theoretic and was promoted as a full professor in 2007 and moved to Peking Uni- experimental study of catalysis reactions on transition metals, versity in 2009. His research interests are heterogeneous catalysis, especially on syngas conversion. C1 chemistry, and development of in situ spectroscopic method that can be operated at working reaction conditions to study reaction mechanisms.