Abstract
One key goal of heterogeneous catalysis study is to understand the correlation between the catalyst structure and its corresponding catalytic activity. In this review, we focus on recent strategies to synthesize well-defined Fischer-Tropsch synthesis (FTS) nanostructured catalysts and their catalytic performance in FTS. The development of those promising catalysts highlights the potentials of nanostructured materials to unravel the complex and dynamic reaction mechanism, particularly under the in situ reaction conditions. The crucial factors associated with the catalyst compositions and structures and their effects on the FTS activities are discussed with an emphasis on the role of theoretical modeling and experimental results.
1 Introduction
Fischer-Tropsch synthesis (FTS) is an exothermic catalytic process that transforms mixed gas of CO and H2 (syngas) into a wide spectrum of hydrocarbons. It has been one of the most important chemical processes in the chemical industry. As syngas can be obtained from coal, nature gas, and biomass, the establishment of designated syngas conversion routes might help us to develop an alternative energywise and carbon-resourcewise industry to replace the current petroleum-based economy.
The search for suitable catalysts for FTS reactions can be dated back to the 1920s when the FTS process was invented [1]. Varieties of transition metals have been employed for the syngas conversion, and it is confirmed that Fe-, Co-, and Ru-containing catalysts are most efficient to catalyze syngas transformation to the desired liquid hydrocarbons. Usually, saturated hydrocarbons are preferably formed on Ru and Co catalysts, while more olefins are produced on Fe-based catalyst. However, because of the relatively high price of Ru, the much cheaper Fe and Co have attracted most of the attentions from a practical point of view. Indeed, the catalysts used in the commercial plants are all based on Co and/or Fe. Most of the researches have been well reviewed in some recent publications from different perspectives [2–10].
In the FTS reaction, the reactants (CO and H2) undergo elementary chemical steps such as adsorption of reactants from the syngas mixture, surface diffusion, reaction of adsorbed species, and desorption of products to form hydrocarbons over a metal surface. It is well known that traditional methods for the preparation of heterogeneous catalysts normally lead to a mixture of different metal species, thus, offer little control over the product selectivity. Recent advancements on the design/fabrication of nanostructured metal catalysts with altered product selectivity or increased reaction activity for the syngas conversion is one of the most important topics in heterogeneous catalysis research. Apparently, those nanostructured materials not only provide platform for the product selectivity engineering but also offer new insights on the FTS reaction mechanism. In this review, we summarize some recent progresses on the FTS nanostructured catalyst design and their applications in the FTS reaction. We do not intend to cover all the nanostructured metal catalysts in the syngas conversions. The focuses are the key factors that contribute to the unique characteristics of those nanostructured catalysts and their catalytic effects in FTS reaction. Our objective is to identify the active phase and correlate it with reaction performance. We mainly use examples of nanostructured catalysts containing Fe and/or Co because they are in line with current industrial FTS researches. In addition, we extend our discussion slightly to several case studies concerning Cu or Ru nanoparticles (NPs) because their applications in syngas conversion can provide supplementary evidence for our understanding.
The structure of this review is illustrated in Scheme 1. First, we summarize recent advancements of synthetic strategies on the nanostructured catalysts, particularly for the well-designed metal NPs that are widely used in the FTS. In the following parts, the sections (core metal, auxiliary metal, and environment) are divided with respect to the key issues that have strong impact on the FTS performance. Under them, there are six key factors (size effect, surface crystallography, promotional effect, bimetallic effect, support effect, and solvent effect) that play important roles in determining the catalyst properties, leading to different catalytic activities. The impact of each factor in FTS will be discussed in detail. One needs to mention that, among those key factors, solvent effect is rarely reported in literature. This is because overwhelming majority of catalysts studied in FTS are heterogeneous catalysts in gas phase reactors, such as a fixed bed reactor, and solvent effect is not present in such reaction systems. However, recent progresses of using nanostructured catalysts in the liquid phase clearly demonstrate that solvent play important roles, and it is demanding to discuss the solvent functions as a category. Moreover, it is desired to gain the knowledge between the catalytic properties and the catalyst structure, especially that under working reaction condition. Therefore, we stress the importance of using in situ characterization techniques to study the catalytic performance of those nanostructured catalysts in FTS to gain deep understanding between the catalyst structure and catalytic performance. Moreover, the theoretical modeling is an inseparable approach to overcome experimental limitation to study the catalyst nature and the FTS reaction process, which will be discussed in the last part of the review. In the end, we summarize the knowledge we have gained on the nanostructural characteristics of catalysts and FTS catalytic performance and offer our view for further research.
The key factors affecting nanostructured metal catalysts and the experimental approaches for the catalyst study.
2 Preparation of FTS nanostructured catalysts
The design/fabrication of catalysts, the intrinsic structure of prepared catalysts, and the resulting catalytic activities are inseparable in the catalysis studies. The synthesis method determines the catalyst structure and, thus, plays crucial roles in its catalytic application. It is noted that many researchers demonstrate that subtle changes in the synthesis method affect the catalyst structures, leading to remarkable difference in catalytic performance due to different structures of active phases [11].
One of the most important industrial catalyst synthesis methods is the impregnation, that is, the core metal precursor is loaded on supports like alumina, silica, or carbon and followed by the treatments such as drying or calcination to get the catalysts prepared by impregnation method. Oxides support is commonly used for such catalysts. As oxide support typically have high surface area of more than 200 m2/g, the supports provide a large surface to stabilize small metal crystallites, adsorb reactant gas, and enhance the mechanical strength of the catalysts. Thus, if a precursor salt of metal can be effectively distributed over this surface by impregnation or any other methods, small nanoscale metallic particles are formed after decomposition, resulting in the formation of nanostructured catalysts. The intrinsic properties of those metal NPs can be tuned by the surface properties of the supports and a variety of treatment methods. For instance, in a typical process to prepare Co catalysts for FTS, a precursor of cobalt nitrate is dissolved in water first; then, it is impregnated onto the supports like silica or alumina by using incipient wetness method. The obtained catalyst is dried after the impregnation and calcined at certain temperature and atmosphere. Cobalt exists as Co3O4 after the calcination, and the Co3O4 should be reduced in situ or ex situ before the reaction. The reduction is a two-step process on silica. First, Co3O4 is reduced to CoO at 623–673 K under inert gas atmosphere. The second reduction step varies depending on the specific treatment situation [12]. It is well known that for some systems, the reduction degree of Co species determines their FTS activity, and the nature of the Co NPs might be changed during the reaction process [10]. The impregnation method is very facile for operation that the additional/auxiliary metal component can be added in the process. The auxiliary metals can be impregnated with the core metal simultaneously, or they can be impregnated with successive impregnation procedures [13–15]. Apparently, different preparation methods lead to different interactions between the core metal and the auxiliary metals, affecting the nature of resulting catalytic materials. The impregnation method can also be extended to a wide range of metals and supports such as carbon nanotubes (CNTs) [15, 16] or molecular sieves [17, 18]. The crucial factors that affect the catalytic activities of those nanostructured catalysts are the function of the core metal, the interactions between the auxiliary metal and core metal, and the impact of support on the nature of active phases.
Another important preparation method is the precipitation method that deposits the active metals on the support by adding suitable precipitator into the metal precursor solution, followed by drying and calcination. The control of appropriate pH value and temperature during the precipitation process is important to get well-defined nanostructured catalyst. Moreover, coprecipitation is also the most common and effective method for introducing auxiliary metals on the iron FTS catalyst. Generally, iron nitrate is used as precursor salt, while ammonium iron (III) citrate, iron acetate, and iron (III) 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 drying is also used in the posttreatment. For instance, nanosized iron particles were synthesized in a water-in-oil microemulsion system [19].
In addition, sol-gel method is an effective pathway for synthesizing homogenous metal-support composites. Tetraethylorthosilicate (TEOS) is commonly used to produce sol, then, act as a support matrix in catalysts. Uniformly dispersed Co-SiO2 catalysts with a Co average diameter of 3–5 nm was synthesized by the sol-gel method and was found to have a stable FTS activity [20].
In recent years, there have been tremendous developments in the preparation of nanostructured catalysts with well-defined size, shape, and composition, thus, allowing tailoring of their catalytic properties. One example is that Kou et al. developed a kind of innovative catalyst system with metal NPs dispersed in solvent such as ionic liquids (ILs), water, or glycol [21–24]. In a typical experiment for a cobalt-based NP catalyst preparation, Co (acac)2 and K2PtCl4 are dissolved in water containing polyvinyl pyrrolidone (PVP) [22]. The mixture is then hydrogenated by H2 [22]. The illustration of the nanostructure Pt-Co catalyst preparation is shown in Scheme 2. The catalyst exhibits promising FTS catalytic activity and C5+ selectivity. This kind of catalyst preparation approach can be used for other metals such as ruthenium, platinum, and iron. Besides hydrogen, alcohols and sodium borohydride can also be used as reductants [23, 24].
![Scheme 2 The illustration of one-step hydrogen reduction of Pt-Co bimetallic NPs (Reprinted with permission from ref [22]. Copyright (2013) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_scheme2.jpg)
The illustration of one-step hydrogen reduction of Pt-Co bimetallic NPs (Reprinted with permission from ref [22]. Copyright (2013) American Chemical Society).
Another interesting development in the catalyst preparation is to synthesize hybrid materials with different compositions, which have different catalytic functions. For instance, Tsubaki et al. developed a core-shell structure catalyst by the hydrothermal method [25, 26]. The core is alumina-supported cobalt active catalyst for FTS, and the shell is acidic zeolite membrane used for cracking and isomerization of heavier FTS products. This kind of core-shell structure catalyst is very selective for C5–C12 products in FTS [27]. The concept for the catalyst structure is exhibited in Figure 1. Another example is highly dispersed iron oxide/graphene oxide hybrid catalyst. It was also prepared by a facile one-pot hydrothermal hydrolysis-reduction process [28]. Moreover, γ-Fe2O3/SiO2 core-shell NPs were observed using iron nitrate and TEOS as precursors at 200°C [29]. The iron oxide core has an average size of 20 nm, and it was coated by a 3-nm SiO2 shell. Those studies demonstrate that hydrothermal method is also very useful in synthesizing well-shaped crystallite and homogeneous metal-support nanocomposite.
![Figure 1 Acidic zeolite coated core-shell structure (Reprinted from ref [27]. Copyright (2008), with permission from John Wiley and Sons).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig1.jpg)
Acidic zeolite coated core-shell structure (Reprinted from ref [27]. Copyright (2008), with permission from John Wiley and Sons).
3 Key factors of FTS nanostructured catalysts
3.1 Core metal
A variety of transition metals have been applied in the syngas conversion to chemicals. In general, FTS catalysts include unsupported Fe, Co, Ru metals catalysts, or those supported on oxides or other supports. We label those metals as “core metal” because they are indispensable to produce desired aliphatic hydrocarbons from syngas. The “core” stresses their crucial functions in the CO dissociation and subsequent important reaction steps such as C-C coupling to form a dominant portion of paraffins and olefins in the overall products. In other words, it is their intrinsic electronic structures, although can be modified slightly, that determine the activity toward CO activation and the trend of product distribution [30].
3.1.1 Size effect
Usually, the core metal particles, which play the catalytic functions, are nanosized crystallites on the support materials. The difference in crystallite size is normally a result of different contents of metal precursors during synthesis, different type of supports, or different synthetic procedure. The overall particle surface area increases with the decrease in crystallite size, leading to an increased exposure of metal to the syngas reactants. Traditionally, if the catalytic performance of a reaction is related with particle size of the catalyst, it is dubbed as structure-sensitive reaction. It is widely accepted that reactions containing deposition or formation of C-C bonds are structure-sensitive reactions, while the reactions only containing C-H bonding deposition or formation are structured-insensitive. 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 the core metal NP size often affects the FTS catalytic performance significantly [32]. The correlations between the particle size and the activity should be treated with great caution, particularly for the comparison among different supports. This is because the oxide supports, which are widely used in the FTS catalysts, can change the chemical properties of metal catalysts. It is believed that the strong metal support interaction (SMSI) exist between the interface of support oxide and core metal [33].
In order to elucidate the metal size effect on the FTS, it is necessary to exclude the strong metal-support interaction and use relatively inert support such as carbon materials. For example, carbon nanofiber (CNF) was used as an inert support for different diameters of cobalt NPs from 2.6 to 27 nm. Such carbon support does not affect the Co particle electronic structure as those traditional oxide supports. de Jong et al. demonstrate that when the particle size is smaller than 6–8 nm, the FTS activity, as well as the selectivity to methane, increase with the increase in particle size, but the turnover frequency (TOF) is independent of the particle size if the Co size is larger than 6–8 nm [34]. This size effect of the cobalt catalyst was also investigated by the Steady-state isotopic transient kinetic analysis (SSITKA) [35]. The SSIKA reveals the differences in coverages and residence times when the particle sizes are smaller than 6–8 nm. The results demonstrated that for smaller cobalt particles, CHx has longer residence time 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 the small particles compared with the large particles that prefer the methanation reaction, the small particles still show high methane selectivity.
The size effects of Fe NP catalysts are also studied on the inert carbon supports. It was observed that the FTS activation energies and CO turnover frequencies increase when the average Fe crystallite size increase from 0.6 nm to 54 nm on Fe/C, and similar results are concluded on another series of Fe/C in which Fe crystallite size increase from 0.6 nm to 7.9 nm [36]. However, the resulting olefin/paraffin ratios show different trends on those two series of Fe/C. Moreover, iron NPs were incorporated into ordered mesoporous carbon supports, and an obvious relationship between iron particle size and FTS performances have been displayed [37]. That is, the size of iron NPs decreases from 22.1 nm to 8.3 nm, leading to the increase in FTS activity as well as the suppression in CH4 formation and increase in
de Jong and coworkers have developed an excellent Fe-based catalyst for Fischer-Tropsch to olefins (FTO) process [38] and investigated the Fe NP size effect in this system [39]. The apparent TOF corresponding to the initial activity of unpromoted Fe/CNF catalysts decreased six to eight times when the average iron size increased from 2 to 7 nm; however, the selectivity for methane and lower olefins were almost constant. In contrast to the behavior observed for unpromoted catalysts, the product selectivity of Na, S-promoted Fe/CNF samples exhibited a clear effect of iron particle size. Lower olefin selectivity increased, while CH4 selectivity showed a trend of decrease when iron size increased from 2 to 7 nm. The TOF as a function of iron size is shown in Figure 2. This trend may be attributed to size sintering or carbon deposition. It is suggested that the sites at corners and edges play diverse roles from sites at terraces in activity and selectivity. The former is associated to size, while the latter is independent of size. These results give a rigorous and novel discussion in size effect and make some important suggestions for the design of FTO catalyst.
![Figure 2 Apparent turnover frequencies (TOF) as a function of iron carbide size (TOS=1 h). corresponds to the CO conversion to hydrocarbons and conversion to hydrocarbons, respectively. The reaction was performed at 340°C, 20 bar, and a H2/CO ratio of 1 (v/v) on promoted catalysts (Reprinted with permission from ref [39]. Copyright (2012) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig2.jpg)
Apparent turnover frequencies (TOF) as a function of iron carbide size (TOS=1 h).
Another interesting method to investigate the size effect is to impregnate well-synthesized iron oxide NPs on the support. For example, monodispersed Fe3O4 NPs can be synthesized in the presence of oleic acid and oleylamine, and particle diameter can be tuned from 3 to 20 nm by seed-mediated growth [40]. The Fe/δ-Al2O3 catalyst prepared from those well-defined Fe2O3 NPs was investigated in the FTS [41]. The highest catalytic activity can be attained with Fe NP diameter of 6.1 nm. Increase or decrease in the Fe NP size results in the decrease in FTS activity. CH4 and C2-C4 selectivity all decrease when Fe NP size increases, giving rise to
The size effect of ruthenium was also studied over Ru/Al2O3 catalyst. It was found that the FTS activity is dependent on the particle size, i.e., FTS activity decreased with the decrease in Ru particle size when the Ru particle size was smaller than 10 nm, whereas FTS activity was nearly constant when the Ru particle size was larger than 10 nm. SSITKA characterization results indicate that CO is more ready for dissociation on smaller Ru particles, possibly by blocking the active sites and decrease the FTS activity [42].
Advanced characterizations were employed for size effect research in FTS. For instance, the dissociation of CO on cobalt NPs with size ranging from 4 to 15 nm was studied by employing the in situ soft X-ray absorption spectroscopy (XAS) [43]. The oxide peak, which refers to surface oxygen species that was produced by CO dissociation, showed a size-dependent trend as derived from O K-edge and Co L-edge XAS spectra. Figure 3 shows the relationship between the particle size and the ability for CO dissociation, which is measured from the XAS spectra. Oxide formation was clearly observed on the 15-nm and 10-nm NPs, but hardly detected on the NPs with particle size of 4 nm. Further research offered strong evidence for hydrogen-assisted CO-dissociation mechanism. This result confirmed the conclusion that the dissociation of H2 is responsible for the activity decrease with decrease in Co NP size [44].
![Figure 3 Relative concentration of dissociated CO species on 4, 10, and 15 nm NPs after exposure to CO/He at different temperatures. The relative concentration was calculated as the ratio of the areas of the oxide XAS peak and the π* peak from intact adsorbed CO (Reprinted with permission from ref [43]. Copyright (2013) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig3.jpg)
Relative concentration of dissociated CO species on 4, 10, and 15 nm NPs after exposure to CO/He at different temperatures. The relative concentration was calculated as the ratio of the areas of the oxide XAS peak and the π* peak from intact adsorbed CO (Reprinted with permission from ref [43]. Copyright (2013) American Chemical Society).
3.1.2 Surface crystallography
The progress of surface science in the last decades shows that heterogeneous catalysis, i.e., acceleration of a chemical reaction on the metal surfaces, is caused by the high reactivity of the metal surface atoms that facilitate bond breaking and bond rearrangement of adsorbed molecules. The core metal of nanostructured catalyst is composed of many crystal facets. For the structure-sensitive reactions, it is believed that the catalytic activities on those crystal surfaces are different because of the different coordination status on those surface atoms. For instance, significant change in the binding strength of a given molecule is observed when it is located at surface sites of lower coordination, such as steps or a kink, indicating the increase in adsorption heat on the transition metal surfaces and higher dissociation of an adsorbed molecule. For a high-index facet such as (310) or (211), it contains more steps and kinks for the catalytic reactions and might show higher catalytic activity in FTS. Clearly, different elementary reaction steps have different activation energies on different facets, resulting in different product distributions and reaction profile. It can be easily imaged that the surface crystallography is closely related with the NP morphology of the core metals. For nanostructured catalysts with the same loading of core metal, the density of step or kink increases with the decrease in the core metal particle size if their morphologies are the same, leading to a potentially higher catalytic activity.
A great deal of the evidence have been accumulated for the relationship between the catalytic activity and the surface crystallography, but such studies on the more complex reactions such as FTS are rare, particularly under the reaction conditions. Calderone et al. designed a core-shell Fe-Co catalyst, and such Fe@Co is synthesized by the precipitation method on the parent magnetite particles with a mean size of 7 nm [45]. In contrast to the uncoated iron particle, Fe@Co supported on Al2O3 exhibit excellent stability for 90 h and higher
Recent progress in shape-controlled synthesis of magnetic NPs has led to several well-defined shapes. Spherical and rod, nanocubes, or irregular-shaped iron NPs have been reported in literature [48–50]. For instance, MnFe2O4 superlattice array of 12 nm cubic-like or polyhedron-shaped NPs are synthesized [51]. Spherical and cubic CoFe2O4 are also obtained [52]. Some of them have been employed for FTS reaction, such as the as-prepared Fe3O4 NPs supported on Al2O3 by the wet incipient method [53]. It would be a very interesting research topic to study iron NPs with different facets in FTS under the same reaction condition to obtain the relationship between the activity and surface crystallography.
To overcome the experimental obstacles, DFT calculations are widely employed to establish the correlations between FTS catalytic activities and the core metal facets. One of the interesting examples is the studies of adsorption and reaction property on Fe5C2 facets as Fe5C2 is widely considered as the active phase in iron FTS catalyst. The theoretical studies reveal that (010) Miller index plane is the most stable surface, while (101) surface is the least stable surface [54]. Numerous stable surface species coexist under H2 and CO on Fe5C2 (001), (110), and (100) facets [55]. Ketene is the important intermediates on Fe5C2 (001) and (100), and hydrocarbons are produced more favorably than ethanol because the energy barrier of ketene dissociation is lower than its hydrogenation, confirming the unique FTS catalytic properties on Fe5C2 [56]. Moreover, CO adsorption on Fe5C2 (010)0.25, Fe5C2 (110)0.00, and Fe5C2 (110)0.80 are also investigated, suggesting that adsorption and activation of CO on Fe5C2 surface are strongly influenced by the vacancy sites on stepped and corrugated surfaces [57]. Not only the pure model compound surface facets but also the facets in the K2O-Fe system, in which K2O presence promotes Fe activities, are also studied using computational methods [58]. The adsorption energies of K2O and surface energies on this system indicate that the high-index facets become more thermodynamically favored as the K/Fe ratio increases. Among all Fe facets, (211) and (321) show the strongest increase in stability, whereas (100) is found to exhibit the slightest increase. Based on these results, the equilibrium shapes of bcc Fe crystallites are studied at K/Fe=0, 1/48, and 1/12. In the clean Fe system, the (110) facet has the biggest contribution (39%) to the exposed surface area. However, when the K content increases to 1/48 and 1/12, the percentage of (211) facet reach, 35% and 70%, respectively. This stabilizing effect in theoretical modeling is confirmed by transmission electron microscope (TEM) and X-ray diffraction (XRD) results of the synthesized H2-reduced Fe/K catalysts to some extent. Because high facets have higher density of atomic steps, more active sites for activating chemical bonds may lead to the improved performance in FTS. This work sheds light on promoter effect and highlights the design of the outstanding catalyst with a unique surface structure.
An interesting topic in FTS is the transformation of carbon atoms on metal surface after CO dissociation. It appears that surface carbon deposition might be found on Fe (100), while formation of iron carbide is preferred on Fe (110) [59]. The activity toward CO dissociation has the order of Fe (100)>Fe (111)>Fe (110) [60]. Therefore, the carbon atom strongly binds with Fe(100) due to the shorter distance between carbon atom and the subsurface iron atom [61]. In contrast, diffusion of carbon atom into Fe (110) has a lower barrier energy, and graphite formation will be more favorable. Compared with the surfaces that both iron and carbon atoms are present, the surfaces with iron termination have the strongest CO adsorption energies; these surfaces include Fe (111), Fe5C2 (110), and Fe3C (010) [62]. Fe5C2 (010) and Fe2C (011) prefer CH4 formation, whereas CH4 formation is inactive on Fe4C (100) and Fe3C (001) [63] (Figure 4). Those theoretical modelings clearly confirm that different facets of the core Fe metal play different catalytic roles.
![Figure 4 Relationships between reaction energy (ΔrE) of CH4 formation and Mulliken charge (q) of the surface C atom as well as effective barrier (Eeff) of CH4 formation and d-band center (εd) of the surface (Reprinted with permission from ref. [63]. Copyright (2009) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig4.jpg)
Relationships between reaction energy (ΔrE) of CH4 formation and Mulliken charge (q) of the surface C atom as well as effective barrier (Eeff) of CH4 formation and d-band center (εd) of the surface (Reprinted with permission from ref. [63]. Copyright (2009) American Chemical Society).
3.2 Auxiliary metal
The employment of auxiliary metals on the core metal can alter the catalytic performance by acting as electronic promoter and/or structural promoter of the core metal. Normally, the auxiliary metals can function in two ways to change the physicochemical properties of the core metal. One is to form a kind of solid solution or chickenpox structure, in which the auxiliary metal is dissolved in the core metal lattice or stay on the surface so as to affect the electronic properties of the core metal. However, the crystal structure of the core metal remains. Such solid solutions/chickenpox structures impose great challenge for the direct characterization although remarkable differences of electronic structure of the core metal can be observed. In contrast, if the radius of the core metal and auxiliary metal are similar, it is likely 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 sophisticated characterization techniques such as X-ray absorption fine structure (XAFS). We denote the former effect (solid solution/chickenpox structures) as the promotional effect of auxiliary metals upon core metals and the latter effect (new compound) as the bimetallic effect for the purpose of discussion. Usually, the promotional effect exhibits a volcano curve, indicating that there is an optimization of the auxiliary metal effect. For example, it was found that Fe-Mn catalyst reached a maximum FTS activity when potassium content was 0.7%, which corresponded to the sample with the highest Fe5C2 concentrate in iron species [64]. It is noted that not all the formed compounds, e.g., amorphous materials, can be identified. In other words, there is no clear distinction between those two effects.
3.2.1 Promotional effect
The influences of auxiliary metals on iron-based catalysts have been investigated extensively, and various auxiliary metals have been used to enhance the catalytic performance of Fe catalysts in the FTS reaction. The auxiliary metals can change the crystallite size of iron particles and increase surface area and then alter the activity and product distribution through the influence on reduction and carburization process. For instance, manganese is widely recognized to be an excellent promoter for light olefin production on iron-based FTS catalysts. Its addition to the core metal of iron decreased the crystallite size of iron oxides, led to more heavy hydrocarbons, and suppressed the formation of CH4 [65]. Similarly, auxiliary metal like Mg also led to the small crystallite size of iron oxide and facilitated the reduction and carburization process [66]. In contrast to Mn and Mg, potassium addition to Fe-Mn catalyst resulted in larger crystallite size and lower surface area of the Fe catalysts and inhibited the reduction and carburization of iron [64]. It was found that alkali metals change the FTS activity of iron catalyst through the association with water gas shift (WGS) reaction activity [67], as evidenced by the study of auxiliary metal of potassium upon Fe and Fe-Mn catalysts using SSITKA [68]. K and Na are effective promoters for enhancing the FTS activity, whereas Li, Cs, and Rb exhibit negative effects upon CO conversions, but all the alkali metals can increase the ethylene selectivity among the hydrocarbon products. This is particularly true for potassium that it is found to be able to improve the olefin selectivity and long-chain hydrocarbon yield by increasing the CO adsorption on metal surface and reducing the CO dissociation activation energy [69–71]. The promotional effect of auxiliary alkali metals are also attributed to the enhancement of surface polarizability on the core metal substrate [72].
The promotional effects are not limited to crystallite size and surface area of the core metal on the nanostructured catalyst. Indeed, the auxiliary metal can change the electronic properties of the core metal, thus, affecting their catalytic performance. For example, strong interactions were observed between auxiliary Mo promoter and core Fe [73]. The addition of Mo obviously increased the acidity of catalyst surface and inhibited the reduction and carburization of iron oxide by covering the iron surface sites. Consequently, the loss of almost half of the activity was detected, whereas more
The presence of more than one auxiliary metal can modify the core metal from different aspects, which might be more beneficial to improve the desired product selectivity. The introduction of a third metal (Cr, Mn, Mo, Ta, V, or Zr, except W) in Fe-Cu-based catalyst was found to enhance the degree of Fe dispersion in a different scale [82]. Both CO conversion and WGS reaction were increased, while no obvious change was observed for the selectivity and chain growth probability. In the case of Mn and Zr, the highest intrinsic site activities, which were derived from the reaction rate/amount CO chemisorbed (TOFchem) may contribute to the highest activity among all the promoted catalysts. Table 1 lists the representative catalytic performance of catalytic materials discussed in this section.
Catalytic performance on promoted iron F-T catalysts.
| Catalyst | Temp (°C) | Press (MPa) | H2/CO | Conv. (%) | Sel. in hydrocarbons/wt% | CO2 sel. (%) | O/P ratioa | Promotional effect in catalysis performance | References | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| CH4 | C2-C4 | C5-C11 | | |||||||||
| Fe-Mn-0.7Kb | 270 | 2.5 | 2.0 | 87.8 | 10.6 | 27.8 | 26.6 | 35.1 | 40.4 | 3.6 | K: enhances activity, shift sel. to heavy HC and olefins | [64] |
| Fe-Mn–K/SiO2 | 250 | 1.5 | 1.2 | 8.9 | 4.4 | 23.4 | 28.5 | 43.7 | NA | 5.8 | Mn: suppresses CH4, shifts sel. to heavy HC and olefins | [65] |
| Fe-Cu-0.07K-Mg/SiO2b | 250 | 0.25 | 2.0 | 86.8 | 8.1 | 23.7 | 27.1 | 41.2 | 40.6 | 0.6 | Mg: enhances activity, shifts sel. to gasoline product | [66] |
| Fe-Cu-1.5K/SiO2 | 280 | 0.2 | 2.0 | 2.12 | 23 | 77 | 7c | 67 | 10.1 | K: enhances activity, suppresses CH4, shifts sel. to long-chain HC and olefins | [68] | |
| Fe-5% K/SiO2 | 230 | 1.2 | 0.67 | 38 | 1.4 | 28.6 | 31.3 | 38.7 | 38 | 6.1 | K: enhances activity, shifts heavy HC | [69] |
| Fe-K-Mo5-1/SiO2 | 280 | 1.5 | 1.2 | 27.8 | 8.8 | 21.7 | 38.1 | 31.5 | 33.3 | 1.95 | Mo: decreases activity, enhanced  | [73] |
| Fe-Zn-K4-Cu2 | 200 | 2.0 | 2.0 | 50 | 2.0 | 8.9 | 89.1c | 15.8 | NA | Cu: enhances activity and WGS | [78] | |
| Fe-Mn-Cu-K/SiO2 | 270 | 1.5 | 0.67 | 62.1 | 8.2 | 23.7 | 41.8 | 24.1 | NA | 1.9 | Cu: shortens induction period, enhances sel. to heavy HC and olefins | [79] |
| Fe-0.1% Pd/MgO | 300 | 0.5 | 2.0 | 6.8 | 67 | 19 | 14 | NA | 1.4 | Pd: enhances activity, suppresses CH4 and olefins, shifts sel. to gasoline | [81] | |
aOlefin to paraffin weight ratios for C2-C4.
bOther products including oxygenates.
cData represent
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. Some values are recalculated from the original data reported in literature.
3.2.2 Bimetallic effect
As discussed in the section above, the promotional effect caused by the auxiliary metal on the core metal enhances the catalytic performance to the desired catalytic properties 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, particularly the main products, on such alloys may be much different from that of the compositional metals. For instance, although metallic cobalt is believed to be the active phase in FTS to produce hydrocarbons, it is interesting to note that cobalt carbide is suggested as the active phase for alcohol synthesis in Cu-Co alloy [83], indicating that the bimetallic effect could change the catalytic performance of the compositional metals dramatically. The intrinsic 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 nondissociative 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, produces a spectrum of hydrocarbons. The combination of those two metals to a bimetallic catalyst may perform a unique catalytic property in alcohol synthesis, aside from the FTS products as Co usually function to catalyze. Not only two metals but also several metals can form the alloys together. From this point of view, the bimetallic effect is a simplification of the multimetallic effect.
Fe-Co or Fe-Cu, Fe-Ni bimetallic catalysts are synthesized and investigated as they have unique catalytic properties in the FTS reaction [84–86]. Fe-Co bimetallic catalyst can be synthesized by a number of methods such as coprecipitation [87], coimpregnation [88], plasma sprayed [89], sol-gel method [90], microwave hydrothermal synthesis [91], or by just physical mixing [92]. Moreover, well-defined Fe-Co/SiO2 catalyst with Fe-Co NPs smaller than 10 nm can be prepared by high-temperature supercritical drying of alcogels [93, 94]. Fe usually appears to be a bcc crystal phase, while the active phase of Co is generally considered to be an hcp/fcc structure. The bimetallic Fe-Co NPs prefer to form bcc alloys when the concentration of Fe is high [95], whereas the hcp/fcc structure retain when the catalyst have a lower Fe concentration [96]. Alloy [97] and spinel structure [87, 98] are proven to be the compound phases when Fe and Co coexist in the composite. In the sample with high Fe concentration, iron immigrate to the surface and undergo carburization to form iron carbides, while the cobalt carbides are formed in the center [99]. However, the enrichment effect seems to be dependent on the catalyst support. Fe showed surface enrichment in the Fe-Co/TiO2 system [95], whereas cobalt was enriched at the NP surface in the Fe-Co/Al2O3 system [100].
Generally, the addition of Co can greatly decrease the reduction temperature of the Fe-Co catalyst and increase the degree of iron reduction [101, 102]. In terms of the FTS reaction, the Fe-based catalyst is known to have a good WGS activity and is suitable for H2-deficient syngas, while the Co-based catalysts show a higher activity with more heavy hydrocarbon formation under lower reaction temperature. In the bimetallic catalyst, strong interaction between iron and cobalt exists as metal-metal bonds, which affect CO adsorption and metal reducibility. As a consequence, Fe-Co bimetallic catalysts prefer to produce fewer CH4 and wax, but with more gasoline and olefin portions in the product distribution compared to the monometallic catalyst. Fe-Co/SiO2 bimetallic catalyst showed higher activity and stability than pure Fe/SiO2 catalyst [103]. An equal amount of Fe and Co is suggested to be the optimization in the Fe-Co/TiO2 system [92]. Moreover, the order of depositing Fe and Co on the Al2O3 surface seems to affect the catalytic performance in CO2 hydrogenation [104]. Highest activity was obtained on the double-impregnation catalyst, which first deposited Fe and then deposited Co on the Al2O3 surface. It is suggested that the presence of Fe prevents Co from forming a strong interaction with the support, resulting in a well-dispersed Fe-Co alloy on the aluminum surface.
Because Co-Cu alloys are widely studied in the literature for the alcohol synthesis, we take this as another example to illustrate the bimetallic effects. A possible reaction mechanism is proposed in Scheme 3. Co@Cu core-shell and Cu-Co mixed NPs with different Cu:Co ratios have been synthesized by the wet chemical methods [106]. Among all the four catalysts, the coreduced Cu-Co sample (24:1) exhibited the highest selectivity toward ethanol, and Cu-Co (3:1) showed the highest selectivity for
![Scheme 3 A possible mechanism of oxygenate formation via a CO insertion route over Cu-Co bimetallic catalyst (Adapted with permission from ref [105]. Copyright (2008) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_scheme3.jpg)
A possible mechanism of
Lanthanum seems to be an efficient promoter for such bimetallic catalyst as well. The LaCo1-xCuxO3-δ perovskite-type catalysts were prepared for a high alcohol synthesis process [110]. TPR data suggested that a strong cobalt-copper interaction exists in perovskites, which enhanced the metallic dispersion of cobalt and prevented copper from sintering. The perovskite catalyst prepared by mechanosynthesis showed a higher surface area and smaller diameter than that synthesized by the citrate complex method [111]. Higher alcohol productivity was detected on the former catalyst than the latter, and the selectivity of alcohols was about 40% [112]. In contrast, Co3O4 was also loaded on the surface of LaFe0.7Cu0.3O3 by the wet impregnation method [113]. A profile of the evolution of the catalyst structure was proposed based on XRD and XPS. Interestingly, Cu in the perovskite was dislodged when the catalyst was pretreated in H2, forming the Cu@Co or Cu-Co alloy on LaFeO3. During the reaction, Co undergone carburization to nanosized Co2C in the presence of La2O3. The dissolved Cu and carbonized Co2C were believed to play important roles in alcohols synthesis.
3.3 Environment
3.3.1 Support effect
The employment of support on the active metal component is a most used approach in the heterogeneous catalyst preparation for the apparent advantage to disperse the metal active phase to enhance the efficiency. It is noted that the supports can also change the physicochemical properties of metal catalysts, thus, influence their catalytic performance. The existence of strong bonding between the support and the metal component can affect the size and shape of the metal particles. Meanwhile, good dispersion of metal on support can effectively inhibit the agglomeration of the active phase and improve the mechanical properties of the catalyst. In some cases, high surface area of supports and the SMSI effect can decrease the reducibility of the metal component, resulting in a low degree of reduction.
Although unsupported or bulk Fe catalysts are used in industrial FTS process, the preparation of iron nanostructured catalysts on the support, particularly oxides and carbon, have attracted remarkable attention due to their unique properties. It is believed that the SMSI exist between the oxide and iron interface [33]. Obviously, the interaction may change the property of the iron metal surface, thus, alter the adsorption or dissociation energy of H2 and CO and regulate the activity and selectivity in FTS. Recent studies also reveal that oxide supports not only affect the metal morphology but also change the phase of catalyst before the FTS reaction.
3.2.1.1 Oxide support effect
SiO2 is one of the oxides that are most studied in iron-based catalysts. Generally, silica-iron composites are synthesized by impregnation or coprecipitation method, and core-shell structure can be formed by the hydrothermal method [29]. It is found that the degree of Fe reduction decreased with the increase in silica surface area [114]. Iron phase was transformed from hematite to α-Fe after reduction on silica-free catalyst, whereas it was converted to wüstite and Fe (II) silicate on the silica-supported catalyst [115]. The introduction of silica decreased the Fe activity and yield toward light hydrocarbons and increased heavy hydrocarbon production in the FTS reaction. However, the catalyst became more stable due to the lower carbon deposition rate [114]. We can observe an apparent support influence on Fe-Si catalysts from Table 2. A variety of characterization methods such as IR, TEM, and H2-TPR were used to study the interaction between the Fe active phase and the silica support [116]. The result revealed that silica had apparent influence on the chemisorption property of H2 and CO over the iron catalyst. XRD and Mössbauer spectroscopy confirmed that a strong interaction existed in silica-supported magnetite catalysts [117]. In situ XRD confirmed the conjecture that additional silica in iron catalyst tend to stabilize the wüstite or Fe (II) phase as shown in Figure 5.
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 |
| CO2 selectivity (mol%)c | 42.7 | 32.7 | 13.2 | 10.1 | 15.1 | 13.2 |
| CH4 selectivity (wt%)c | 22.3 | 22.9 | 25.3 | 24.4 | 16.3 | 15.6 |
| C2-C4 selectivity (wt%)c | 49.6 | 50.1 | 46.8 | 46.5 | 39.6 | 39.2 |
 | 28.1 | 27.0 | 27.9 | 29.1 | 44.1 | 45.2 |
aCatalysts were reduced in pure H2 at 350°C for 10 h. The reduction extents of catalysts were measured by MES.
bDetermined 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).
![Figure 5 In situ XRD of the hydrogen reduction of the unmodified sample (top) and the sample containing 98 mmol Si/mol Fe (bottom). Reduction: pure hydrogen, 400 ml (NTP)/min/g; heating rate: 0.5°C/min – only main diffraction peaks indicated (Reprinted from ref [19]. Copyright (2012), with permission from Elsevier).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig5.jpg)
In situ XRD of the hydrogen reduction of the unmodified sample (top) and the sample containing 98 mmol Si/mol Fe (bottom). Reduction: pure hydrogen, 400 ml (NTP)/min/g; heating rate: 0.5°C/min – only main diffraction peaks indicated (Reprinted from ref [19]. Copyright (2012), with permission from Elsevier).
In addition to SiO2, other oxides, including ZrO2, Al2O3 [118], MgO [119], CeO2 [120], or Nb2O3 [121] were used as support. Wan et al. studied the Fe-Al2O3 interaction over the precipitated iron catalyst in a slurry reactor and concluded that alumina inhibited the weak H2 adsorption on iron surface but favored strong H2 adsorption [122]. Moreover, CO adsorption exhibited a similar trend, as evidenced by CO-TPD. CO2-TPD demonstrated that alumina support reduced the surface basicity; therefore, the carburization and reduction process of Fe2O3 were suppressed, and the FeO phase was stable after pretreatment. The catalytic activity decreased with the increased selectivity toward CH4 and C5–C12 hydrocarbons in the FTS reaction [122]. Another interesting example is that the Fe-Al nanocatalyst can be synthesized by impregnation using Fe3O4 NPs as the precursor [53]. The nanostructured active phase remained spherical and almost the same particle size during the preparation. This Fe-Al catalyst showed higher activity and more olefin productions, but a lower CO2 selectivity. The weak interaction between the iron oxide NPs and the alumina NPs may be responsible for these unusual properties. Addition of ZrO2 in Fe-O-Si composite could weaken the Si-Fe bonds by forming Zr-O-Si linkages [123]. The stability of iron carbides was improved by the synergistic effects, and the potassium doping can accelerate the reduction rate of Fe/ZrO2 catalyst [124, 125]. Promotion of Fe-based FTS catalysts with cerium is only effective when Fe-O-Ce bridges are formed in the precursor preparation. The CO dissociation increased because the C-O bond is weakened in the CO tilted configuration, resulting in a higher olefin content and selectivity to heavier hydrocarbons [126].
The support effects are not only dependent on the oxide precursor, but are also related with the methods of preparation of catalyst compositions. It was found that potassium impregnation on sol-gel synthesized Fe-Cu-Al led to the highest CO activity, and Fe-Cu-K impregnation on alumina showed 97.5%
The effects of oxide supports on the FTS activity or product selectivity are inconsistent in the literature. This is mainly because the studies were conducted under different conditions and over catalysts with multiple components; a pervasive tendency of oxide effect in FTS can hardly be given. Generally, it is accepted that a balanced interaction between the oxide support and active phase is most suitable for FTS.
3.3.1.2 Carbon support effect
In addition to the oxide support, carbon material is also a kind of important catalyst support for loading the metal component. The less hydroxyl groups on carbon materials, compared with oxide support, lead to weaker interactions between metal and support. Moreover, new carbon materials with various morphologies such as CNT, mesoporous carbon, and graphene have attracted remarkable attention. Studies of the carbon support effect on the Fe-based catalyst were available in the early 1980s. For example, it was reported that well-dispersed Fe/C catalysts exhibited high reaction activity and stability for CO hydrogenation at a temperature of 235°C [128, 129]. It was also found that Fe/carbon catalysts exhibited high selectivity for olefins compared with Fe/Al2O3 and Fe/SiO2 as well as it offered high activity [36]. As carbon materials have many unique properties, they may become a very important field for improving the FTS performance over iron catalysts. For example, the CNT can restrict the size of metal NPs within their diameter scale, as well as to prevent the metal sintering during reactions [130]. This effect is named as confinement effect. Moreover, it is proposed that the π electron density of the CNT may shift from the concave inner to the convex outer surface so the interior surfaces are electron deficient, 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 in contact with either surface, leading to different catalytic performances between the particles deposited inside and outside of the nanotube. For the metal NPs staying on the external surface of CNTs, an interesting example is that a Ru/CNT catalyst can achieve diesel oil fraction (C10–C20) selectivity as high as 60%, which is the highest in literature [131]. It is believed that CNTs or other carbon materials with arranged channels not only provide the confinement effect to improve the core metal performance in FTS but also play roles in mild hydrocracking of heavier hydrocarbons like the molecular sieves.
With the above-mentioned concept of confinement effect by CNTs, Bao and coworkers developed an interesting area in syngas conversion [132]. They used a delicate method to synthesize Fe-in/CNT catalyst in which over 70% of iron particles were located inside CNTs channels (as shown in Figure 6) [133]. The catalytic performance results proved that Fe-in/CNT catalyst favored the formation of long-chain products, and the yield of
![Figure 6 TEM images and particle size distribution of the activated catalysts (A) Fe-in-CNT and (B) Fe-out-CNT before reaction (Reprinted with permission from ref [133]. Copyright (2008) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig6.jpg)
TEM images and particle size distribution of the activated catalysts (A) Fe-in-CNT and (B) Fe-out-CNT before reaction (Reprinted with permission from ref [133]. Copyright (2008) American Chemical Society).
![Figure 7 The effect of confinement in CNTs on the activity of FTS iron catalyst (Reprinted with permission from ref [132]. Copyright (2011) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig7.jpg)
The effect of confinement in CNTs on the activity of FTS iron catalyst (Reprinted with permission from ref [132]. Copyright (2011) American Chemical Society).
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 limiting methane selectivity in the FTS reaction at 340°C, in comparison with the iron supported on high surface area γ-Al2O3 or SiO2 [38]. This result breaks the product distribution predicted by the Anderson-Schulz-Flory model. The maximum selectivity achievable for the C2–C4 fraction according to the ASF model, including olefins and paraffins, is approximately 50% with about 30% methane selectivity. It is believed that Na and S can act as promoters to suppress the methanation reaction, corresponding to the “surface carbide” or “alkyl” mechanism. The results shed light on the future study to use carbon support to tune product distribution in the FTS.
Graphene is one of the most attractive topics in the realm of chemistry after it was discovered. It is a two-dimensional single-layer sheet of carbon material with p electrons fully delocalized on the plane. Because of the weak interaction between graphene and NPs, it is difficult for metal oxide deposition on graphene. Reduced graphene oxide (RGO) has oxygen-containing groups and more atomic defects on its surface, which make it suitable for anchoring metal oxide NPs. Three-nanometer Fe2O3 NPs were highly dispersed on RGO by a one-pot hydrothermal hydrolysis-reduction strategy [28]. Remarkable stability and good selectivity for long-chain hydrocarbons were observed on Fe-RGO when they were applied in FTS reaction. The defective nature might hinder the coalescence of iron NPs, and the acidic oxygen-containing groups on RGO may help to crack heavier hydrocarbons to the gasoline fraction like molecular sieves. Zhao et al. also studied the effect of oxygenated groups on graphene oxide, on which the presynthesized iron oxide NPs were loaded, in order to avoid the influence of support on Fe NP growth [142]. They conducted thermal treatment by H2 at different temperatures on pyrolytic graphene oxide (PGO) to recover the original graphitic surface. C-O and C=O species on the carbon surface decreased, while graphitization degree increased with the increase in thermal treatment temperature. The FTS activity improved and produced more long-chain hydrocarbons with the decreased oxygen species. Fe K-edge X-ray absorption near edge structure (XANES), EXAFS data, and XPS experiments revealed that the valence states of this series of Fe/PGO were almost identical. Moreover, the used catalysts had fine structures more close to Fe5C2. The evidence suggested that oxygenated groups may change the surface property of iron, thus, influence their performance in FTS. Table 3 lists the representative catalytic performance of catalytic materials discussed in this section.
Catalytic performance on carbon-supported iron FTS catalysts.
| Catalyst | Temp (°C) | Press (MPa) | H2/CO | FTY (10-5 molco/gFe.s) | Selectivity in hydrocarbons/wt% | CO2 selectivity | Olefins/paraffins ratio | References | |||
|---|---|---|---|---|---|---|---|---|---|---|---|
| CH4 | C2-C4 | C5-C11 |  | ||||||||
| 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] | |
| Fe/α-Al2O3d | 340 | 2.0 | 1.0 | 1.35 | 11 | 59 | 21c | 40 | 8.8b | [38] | |
aOlefin to paraffin weight ratios for C3-C7.
bOlefin to paraffin weight ratios for C2-C4.
cData represent
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.
3.3.2 Solvent effect
In a gas-liquid-solid FTS reaction system, the solvent play a very important role. In addition to the most common “green” solvent of water, polyethylene glycol (PEG) can be used as solvent for both Fe and Co NPs in FTS at mild conditions [23]. The Fe NPs dispersed in PEG showed the highest activity among the solvents including ethanol, cyclohexanol, and ILs. If water is used as solvent for Co or Fe catalysts, oxidation of the catalyst is unavoidable, which was considered to be one of the main reasons for the catalyst deactivation. In the PEG system, the isolation of water produced during the FTS from the iron NPs in the PEG environment might be the cause for the good activity and stability of the catalyst. Interestingly, outstanding selectivity toward oxygenates are also obtained in CO hydrogenation in the liquid environments. A dissolved base/oxygenated solvent was found to be able to facilitate the selective production of oxygenate products [144]. The syngas conversion over Fe2O3 supported on Al2O3 in the oxygenated solvent PEG-400 can reach 95% selectivity toward mixed alcohols with more than 40%
The application of quasi-homogeneous catalysts in FTS represents one of the most important progresses in the syngas conversion researches from a scientific point of view in the last decades, although the employments of such catalysts in the commercial scale plant face the difficulty of the product separation and catalyst regeneration. Those so-called quasi-homogeneous catalysts are metal NPs that are dispersed in the solvent such as aqueous phase or ILs. The solvent (liquid) provides a three-dimensional freedom environment for those nanostructured metal particles to rotate; thus, the reactant (gas) can contact the active sites of metal particles more easily. For the small nanostructured metal particles, the catalytic activity is higher than their heterogeneous counterparts, in general. The catalysis system combines both the advantages of homogeneous catalysis and heterogeneous catalysis, showing unique mass and heat transfer properties and resulting in high reaction activity. This is particularly true for FTS in which the solvent environment, as the reaction media, facilitates the interaction between the core metal NPs and syngas reactants as well as to control the reaction temperature effectively by good mass transfer and heat transfer.
Kou et al. set a precedent of low-temperature aqueous FTS reaction and developed several excellent catalyst systems [21]. Ruthenium nanoclusters with an average diameter of 2.0 nm were prepared by high-pressure H2 reduction. Those nanostructured catalysts were employed in FTS in a stainless steel autoclave. The researchers investigated the solvent effect by changing different solvent media. Ru NPs that are stabilized by ethanol, dioxane, or cyclohexane, even the IL, [BMIM] [BF4], show remarkable FTS activity at 150°C. More importantly, the reaction in water exhibit an unprecedented high activity (6.9 molCO/molRu/h) compared with other solvent systems. The activity is even higher than traditional heterogeneous Ru catalysts working at higher reaction temperature. The 80.9 wt% of the products are C5–C20 hydrocarbons, and only 3.3% CH4 is detected. Size effect was also observed over this catalyst. Detailed product distribution with different diameters are shown in Figure 8. Further study found that this catalyst showed good stability in a continuous flow reactor [151]. Moreover, high selectivity toward long-chain products was observed over 5–8 nm Co NPs, which were synthesized and dispersed in imidazolium ILs [152]. Pt-Co NPs synthesized by one-step H2 reduction method was found to be an excellent catalyst for aqueous-phase FTS at 160°C [22]. In addition, Hensen et al. achieved a very high aldehyde selectivity as high as 70% for an aqueous phase 2.2-nm Ru NP catalyst at 145°C [153].
![Figure 8 (A) Hydrocarbon selectivities for 2.0-nm-diameter nanoclusters; (B) ASF distribution of products for Ru nanocluster catalysts with different diameters. Reaction conditions: 150°C, 2.0 MPa H2, 1.0 MPa CO, 2.79×10-4 mol Ru, 20 ml water (Adapted from ref [21]. Copyright (2008), with permission from John Wiley and Sons).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig8.jpg)
(A) Hydrocarbon selectivities for 2.0-nm-diameter nanoclusters; (B) ASF distribution of products for Ru nanocluster catalysts with different diameters. Reaction conditions: 150°C, 2.0 MPa H2, 1.0 MPa CO, 2.79×10-4 mol Ru, 20 ml water (Adapted from ref [21]. Copyright (2008), with permission from John Wiley and Sons).
Apparently, quasi-homogeneous catalyst is a new research field, and more studies are needed to explore their potential applications, to investigate the related reaction mechanisms and to reveal in detail the solvent functions in the liquid FTS reactions.
4 Characterization of FTS nanostructured catalysts
Generally, there are several approaches for studying the active phase of nanostructured catalysts for the syngas conversion: (1) observing the catalyst structure by in situ characterization; (2) synthesizing pure active phase and comparing its activity with the real catalyst; (3) inducing promoters on core metal in catalyst or modifying active phase (e.g., treating the metal particle by H2, CO, or syngas) to establish relation between structural change and catalytic activity; and (4) theoretical modeling from computational methods. Because enormous literature have been published to investigate the Fe active phase in the FTS, we take the studies of Fe active phase as examples to illustrate how those approaches are used together to provide complementary information on the nature of the catalyst.
Typically, FTS use iron oxide NPs as the catalyst core metal component, and a variety of iron phases can be produced as iron is facile for reactions under the syngas atmosphere at elevated temperatures. For instance, metallic iron is observed in hydrogen, cementite, and Hägg carbide are the main phases in CO and in syngas, respectively. And iron carbide is also observed to have ε-χ-θ transition behavior during the FTS reaction at different conditions [154] confirming the structure evolution of iron oxide catalyst from hematite to magnetite and finally to iron carbide in the FTS [155]. It is well recognized that the activation conditions before the FTS reaction can influence the final iron structure in the steady reaction conditions [156]. Noting that the iron structure could change during FTS, it is difficult to correlate the relationship between the initial catalyst status prior to use and the catalyst performance in FTS steady state; hence, the monitoring of surface chemical processes under reaction conditions is extremely crucial to identify the active Fe phase.
It is a great challenge to study the detailed process and identify the “real” active phase because of the relatively harsh conditions used in FTS. Table 4 lists some commonly used methods for the in situ characterization techniques.
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 |
Great effort was devoted to the in situ investigations. Weckhuysen et al. investigated the iron catalyst by combined in situ characterization techniques and theoretical methods at relative high pressure [157]. As shown in Figure 9, in situ EXAFS, XANES, XRD, Raman spectroscopy, and online mass spectrometry were combined to investigate two samples of iron oxide. One was pretreated under CO, and another one was pretreated under 1% CO in H2 at elevated temperatures. The results give a detailed change trend of iron restructures and conclude that χ-Fe5C2, θ-Fe3C, and FexC are formed under different conditions, exhibiting different catalytic activities in FTS, which is consistent with the theoretical prediction [63]. In contrast, Bao et al. found that iron encapsulated in CNTs showed higher activity as well as higher selectivity toward
![Figure 9 Schematic representation of the combined XAFS/XRD/Raman experimental setup (Reprinted with permission from ref [157]. Copyright (2010) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig9.jpg)
Schematic representation of the combined XAFS/XRD/Raman experimental setup (Reprinted with permission from ref [157]. Copyright (2010) American Chemical Society).
In addition to the in situ characterization techniques above that can be operated under more realistic reactions in practical use, some surface techniques such as TEM and XPS, which can only be used at relatively low pressure, are also employed for the FTS study. Fe-based catalyst was studied by in situ TEM-EELS during carburization in low pressure CO to elucidate the activation mechanism using specially designed equipment [161]. Severe sintering occurred following hematite reduction, and no evidence of carbon deposition was observed in contrast to literature. Nevertheless, the metal surface was coated with carbon layers when it was exposed to air or after passivation [162]. Another example is that a delicate in situ equipment made it possible for imaging the iron nanostructured catalyst under scanning transmission X-ray microscopy (STXM), which is capable of operating up to 500°C at 1.2 bars. The Fe-Cu-K/SiO2 catalyst was investigated for in situ reduction in H2 and followed by the FTS reaction in syngas. Reduction, carburization, and spillover of carbon from metal to support were visibly demonstrated from STXM information [163, 164]. Furthermore, in situ XPS was conducted to study the structure change during low pressure treatment on the surface of iron oxide NP and bulk iron oxide [165]. The two samples showed different performance in the same treatment atmosphere. The bulk iron oxide was easily reduced to metallic iron, whereas Fe2O3 NP may be partly converted to FeO phase. The results provide a direct experimental evidence for the size dependency of iron oxide in FTS reaction.
The in situ investigation results above all indicate that iron carbide phase plays crucial roles in FTS. Indeed, iron carbide was considered to be necessary in CO hydrogenation, as hematite and magnetite did not show any activity in the early studies of FTS. α-Fe, Fe3O4, bulk and surface iron carbides have all been suggested as active phases for FTS. Nevertheless, iron carbides attracted more attention as more data have been accumulated as the characterization methods, including XRD and Mössbauer spectrum, always display a mixture of various iron carbide phases after the FTS. Fe2C was prepared and investigated in the 1950s, it was suggested that Fe2C was not the active phase in FTS due to the poor CO adsorption property [166]. Although the exact iron carbide phase for the FTS activity and the relationship between the structure and activity/selectivity remain unclear, they are believed to play very important roles in FTS reaction. It is possible that some, if not all, iron carbide phases are active for converting the syngas into hydrocarbons.
Nanosized iron carbides are mainly observed as product in carburization of iron, and the synthesis of pure phase of iron carbide impose a great challenge. Fe3C 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 an elegant route [168]. Fe2C5 particles with a well-defined particle size of 20 nm were obtained after the reactions of iron carbonyl, Fe(CO)5, with octadecylamine in the presence of bromide under 350°C. In contrast to the traditional reduced-hematite catalyst, the Fe5C2 NPs supported on SiO2 showed higher activity and selectivity in FTS, resulting in 39% selectivity toward
![Figure 10 Overall catalytic performance of Fe5C2 NPs/SiO2 and reduced Fe2O3/SiO2 catalysts: (A) CO conversion and (B) product selectivity (Reprinted with permission from ref [168]. Copyright (2012) American Chemical Society).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig10.jpg)
Overall catalytic performance of Fe5C2 NPs/SiO2 and reduced Fe2O3/SiO2 catalysts: (A) CO conversion and (B) product selectivity (Reprinted with permission from ref [168]. Copyright (2012) American Chemical Society).
Modification of the core metal such as iron by the promoters such as alkali metals is a very traditional method to enhance the catalytic activity and study the structure-activity relationships. Iglesia et al. combined the kinetic analysis of the initial stages of FTS with XAS to study the structure evolution of K and Cu-promoted iron oxide catalyst during the initial contact with syngas [169]. According to XAS results, Fe2O3 was converted to Fe3O4 and, then, rapidly transformed to FeCx after contact with syngas for 120 s. The K and Cu in Fe2O3 facilitate the formation of more nucleation sites for Fe3O4 and FeCx as well as smaller crystallites on the surface of the catalyst. These smaller particles can be regarded as active sites for FTS, which shorten the structure evolution paths of Fe2O3. It was also observed that the Mg-promoted Fe/Cu/K/SiO2 catalyst could enhance the catalytic activity and
DFT studies also reveal that iron carbide might play a positive role in FTS, which is different from cobalt carbide. Although CO dissociation is difficult on iron carbide, CO hydrogenation on the surface more readily proceeds than metallic iron. It is proposed that the CH4 selectivity on iron carbide will be similar to that on metallic iron; hence, iron carbide can be considered to be the active phase in FTS. Pretreatment effect was investigated to understand the surface structure of χ-Fe5C2 using ab initio atomistic thermodynamics [173]. It is found that exposed facets strongly depend on the gas pressure and H2/CO ratio. CO pretreatment is beneficial for carbon-rich facets, whereas the addition of H2 into CO favors the stable carbon-poor facets. As a consequence, the higher activity of surface carbon toward hydrogenation may be the cause for the higher initial activity on iron catalyst pretreated by CO than that pretreated in syngas. This case study clearly exhibits that the computational technique is a powerful method to assist us to understand the active phase in the reaction condition.
5 Theoretical study of FTS nanostructured catalysts
With the improving power of the computer science and the rapid development of computational chemistry, the chemistry system with abundant atoms can be simulated; thus, complex reactions can be studied by the theoretical modeling. The most popular method in the heterogeneous catalysis is based on the density functional theory (DFT) that expresses the total energy of the chemical system as the function of electron density. Compared with the Hartree-Fock method, the DFT method is more efficient at the expense of slight inaccuracy. Energy errors in normal DFT calculations are about 0.2 eV, which is acceptable to describe most chemical reactions semiquantitatively. Generally, the DFT calculation can illustrate the reactants’ stable structure or the preferable pathway of chemical reactions. Practically, different models such as slab models or cluster models are used to simulate catalysts. The DFT calculations offer us the adsorption energy and the reaction barriers in catalysis, which are very useful to the analysis experimental data in real catalysis. Based on those data, advanced methods like molecular dynamics or Monte Carlo simulation can be performed to obtain more information about catalysis. Moreover, adsorption energies and reaction barriers calculated from DFT are often used to deduce the reaction rate equations on surfaces.
Syngas conversion is one of the most complicated reaction systems in heterogeneous catalysis research with many parallel reactions and consecutive reactions. Gas, liquid, and solid are involved, and the intermediates and products consist of hundreds of chemicals. Remarkable researches have been performed by the theoretical studies on the FTS reaction. Abundant knowledge can be obtained from the computational chemistry [174, 175], and those computational results rationalize our understanding of complex syngas conversion, particularly the reaction mechanism at atomic level such as CO activation, C-C coupling, olefin selectivity, etc.; thus, guiding us on the synthesis of more efficient and stable catalysts.
Theoretical studies of the FTS could be divided into two fields: the first one is based on the iron carbide catalyst, which represents the most important industrial iron-based catalyst. The iron carbides can be characterized by Mössbauer spectroscopy, and the existence of such iron carbides have been well related to the catalytic activity. Nevertheless, Mössbauer spectroscopy cannot identify the structure of catalyst surface, which is very essential for their catalytic roles. Compared with spectroscopy techniques, theoretical study is more straightforward to reveal the structure-activity relationship. Adsorption of syngas and stability of different iron carbide structures have been studied systemically [55, 176–181]. It is noted that the metal surface, which plays the catalytic role under the reaction conditions, might be very complex, and the presence of active iron carbide mixture cannot be excluded. Moreover, the involvement of gas atmosphere is also important for a reliable theoretical study. Some reports stressed the issue and calculated the preferable surface structure in the reaction atmosphere [173]. The second category of catalysts is based on Co or Ru metals. Cobalt and ruthenium are often believed to act as zero-valance metal in the FTS reaction. They are located on the near top of the volcano-like profile [182], so they are often studied comparatively with other metals such as Rh, Ni, and Cu. The activity trend of these metals can be described very well by the d-band center model. The d-band model may act as a very promising descriptor for screening of the bimetallic FTS catalyst.
The CO dissociation is very important for FTS, usually being regarded as the rate-determining step, so it is necessary to understand the pathways and factors that influence the CO dissociation. There are two suggested pathways for CO dissociation: H-assisted CO dissociation and direct CO dissociation. The two pathways are both studied on the iron carbide surface. Jiao et al. studied the CO dissociation on Fe3C (001) surface. The results show that if CO is adsorbed on the ideal surface site, the H-assisted CO dissociation pathway has lower barriers than the direct CO dissociation. However, if the CO is adsorbed on a vacancy site from the surface Cs hydrogenation, CO dissociation barrier is reduced, indicating that direct CO dissociation is also very favorable [183]. The result that the vacancy site of surface carbon species is very active for CO dissociation is confirmed by a similar study on four other different iron carbide surfaces, including Fe2C (011), Fe5C2 (010), Fe3C (001), and Fe4C (100) [63].
The formation of initial C2 compounds is also essential for FTS. In the classical FTS mechanism, CO dissociates first, then the C atom is hydrogenated to form CHx(x=1,2), and CHx coupling occurs successively to form long-chain hydrocarbons. However, based on the study of CO adsorption on iron carbide of Fe3C(001), a different picture is concluded: C2 compounds may be formed by the CO coupling with surface Cs atom, then, the CsCO species is hydrogenated followed by the dissociation of the oxygen atom [183]. The initial C2 compound formation pathways on Fe5C2 (001), (100), (110) surfaces are very similar with that on Fe3C (001). It is concluded that the adsorbed CO favors coupling with surface Cs into C2 compounds, then C2 compounds are hydrogenated, and, subsequently, the FTS are initiated [57, 62, 184, 185]. Similar to the C2 compound formation, C3 compounds are formed by the CCO coupling with C atom, which is competitive with pathways of CCO hydrogenation to CCHx on Fe5C2(001) [186]. It is noted that the C-C coupling pathways are drastically affected by the hydrogen pressure. When the hydrogen pressure is low, surface C atoms prefer to couple with each other to form graphite species. In contrast, the C atoms prefer to be hydrogenated first to form the CHx species with a high hydrogen pressure, then, the CHx coupling pathway may be preferable [187]. This is consistent with the experimental observation that hydrogen pressure influences the FTS significantly.
Methane is an undesired by-product in Fischer-Tropsch synthesis, so tuning the catalyst to lower the methane selectivity is a very interesting topic. In the experimental work on iron-based catalyst, very little knowledge is gained on the CH4 formation upon catalysts, partially because of the complex nature of the catalysts. DFT calculation offers the atomic-level understanding of adsorption, reaction, and desorption in the reaction process, thus, one can specify the methane formation pathways on different surface sites. Four different surface of Fe2C(011), Fe5C2(010), Fe3C(001), and Fe4C(100) were employed to study their activities on methanation, indicating that the Cs vacancy site is very active for CO dissociation on all four surfaces, but the methanation activities are different. Fe5C2 (010) and Fe2C (010) are active for methanation because of the suitable CHx hydrogenation barriers and stability. In contrast, Fe3C (001) and Fe4C (100) are not very active for methane formation [63].
Potassium is the most important auxiliary metal, usually dubbed as promoter, for the industrial Fe (core metal) catalyst in FTS and ammonia synthesis. Potassium is added as potassium carbonate, which forms K2O after calcination at high temperature, in the FTS. The function of K2O is concluded, by the experimental results, as electron donor, i.e., the addition of potassium can reduce the work function of the surface so the CO dissociation barrier is reduced at the promoted Fe surface. The alkali potassium promotional effects on CO and H2S adsorptions were studied on Fe(100) surface by the theoretical modeling, indicating that the presence of potassium promotes the CO adsorption but hinders the adsorption of the H2S [188]. Furthermore, this result implied that the alkali metal promoter could reduce the deactivation of the Fe core metal from the impurity of H2S, thus, extending the catalyst life. Moreover, another function of the potassium promoter was proposed by combining the DFT calculation and experimental results. The results suggest that the potassium (or precisely potassium oxide) can stabilize the high index surfaces of Fe, such as Fe (211) and Fe (310), which are more active for CO dissociation. The percentage of high index surface in all exposed surfaces can be evaluated by the Wulff model, which shows the probability of a certain surface appearing in the catalyst in statistics. Hence, it was found that the (211) and (310) surface of Fe gradually dominate the Wulff model, as shown in Figure 11, with the increasing content of potassium promoter. This result shows that the potassium can stabilize the high active surface as well as reduce the work function, implying the importance of auxiliary metals for the core metal on those nanostructured catalysts [58].
![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).](/document/doi/10.1515/ntrev-2013-0025/asset/graphic/ntrev-2013-0025_fig11.jpg)
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 surface structure of the catalyst is not only affected by the promoters but also by the atmospheres of the gas reactants. However, the surface structure of the working catalyst under high pressure and high temperature is difficult to be studied by some important characterization techniques, e.g., the atomic resolution characterization methods like TEM and XPS are impracticable at high pressures. Instead, one can evaluate the Gibbs free energy and chemical potential of a system at different temperatures and pressures, with the knowledge from statistic thermodynamics, by DFT calculation that simulates an isolated system at zero Kelvin. The stable species at a real catalysis atmosphere of Fe5C2 are studied, and it is concluded that the surface of Fe5C2 is changing at the reaction conditions: the presence of hydrogen in gas or the high reaction temperature reduces the chemical potential of carbon dramatically and influences the chemical state of the carbon species on the Fe5C2 surface remarkably [173]. The catalyst surface at real atmosphere is influenced by complex factors, including space velocity of reactants, temperature, and the aforementioned gas pressure and promoters. The knowledge shows the complexity of the active sites under working conditions and offers us valuable information to prepare well-defined nanostructured catalysts.
Cobalt and ruthenium share some interesting properties in FTS such as the high selectivity for long-chain hydrocarbons and the zero-valence active center, suggesting that the FTS mechanism on those two metals might be similar. The reaction mechanisms of CO dissociation on cobalt and ruthenium have been studied extensively [176]. The theoretical study concluded that the CO dissociation prefers the step sites on the surface, and the results are consistent on many metals such as Ru [189, 190], Rh, Co [191], and even zero-valence Fe [192, 193]. Some experimental results at ultrahigh vacuum also underpin this conclusion. For example, it is found that CO dissociation occurs at step sites by high-resolution electron microscopy [189], and the H-assisted CO dissociation have been elaborated in literature [192, 194]. Iglesia et al. combined the kinetics analysis and DFT calculation proposing that direct CO dissociation on the Fe surface may be a dominating pathway, but CO prefers the H-assisted dissociation mechanism on the Co surface [195]. The higher activity of step sites on CO dissociation might also arise from the shallow d-band center and geometric effects. The conclusion seems solid from a theoretical point-of-view, but it is often challenged by a fact that the active step sites may be poisoned by carbon atoms or other impurity atoms. More detailed studies from both theory and experiments are demanded to elucidate the dispute.
The growth mechanism of hydrocarbons is indispensable for the understanding of FTS, and two popular mechanisms are often discussed on the cobalt surface and ruthenium surface. The first one is dubbed as carbene mechanism, which suggests that the CO molecule first dissociates to the C atom and O atom, then, the C atom is hydrogenated to form the CHx species, and subsequently, the CHx species couple consecutively to form long-chain hydrocarbons. The carbene mechanism well explains the ASF distribution of FTS. The coupling reactions of the different CHx on the Ru (0001) surface are studied, and the 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, Ru), and it is concluded that coupling reaction barriers are dependent on the core metals and the surfaces, but leading to the similar FTS product distributions except for small differences in activities and chain-growth probabilities [197]. Another well-studied mechanism in FTS is denoted as the CO-insertion mechanism. The mechanism is inspired by the homogeneous syngas reaction route, such as hydroformylation, for which the mechanism is well established. In the CO-insertion mechanism, CO inserts to the alkyl on the surface before CO dissociation; this is strongly supported by the experimental evidence that oxygenates like aldehyde and alcohol are present in the products. A reaction pathway of CO-insertion mechanism on the Co (0001) surface was proposed, which suggests that the CO inserting to an RCH- group needs much lower barriers compared with inserting to the RCH2- group. The CO-insertion mechanism in some situations seems to be more reasonable compared with the carbene mechanism because of the high barriers for direct CO dissociation and high barrier of CO hydrogenation in the H-assisted CO dissociation [198]. The high CO coverage on the Co (0001) surface was studied by DFT calculation, and the results show that a stable surface adsorption structure could be maintained at a large range of CO pressure. It is suggested that the high CO coverage benefits the CO-insertion mechanism under the real catalytic condition [199]. As the CO-insertion mechanism is often challenged with the high barrier of CO insertion, another possible mechanism of CO insertion was suggested to address the issue. CO insertion and CHO insertion barriers on Rh (0001) and Co (0001) were compared, and the results show that the CHO insertion is more energetically favorable [200]. This is a new viewpoint for CO insertion, but the proportion of the CHO insertion in reaction need be reconsidered due to the poor stability of CHO. Interestingly, theoretical study and microkinetic analysis are combined to study the Fischer-Tropsch reaction on the Ru (0001) surface, suggesting that CO insertion is more favorable thermodynamically compared with the carbene mechanism [201], and the hydrogenation of CO is the initiation reaction of the hydrocarbon polymerization process. In conclusion, the mechanism of the chain growth is still on debate. It seems that the chain growth mechanism is dependent on the reaction conditions and the catalyst used. The reaction results on well-designed nanostructured catalyst could provide more detailed picture of the chain-growth mechanism.
6 Summary and perspective
We have seen a substantial shift of FTS catalyst preparation from the empirical approach, which lacks the know-how knowledge, to the well-defined nanostructured catalyst materials in the past decades. Those nanostructured catalysts offer superior model compounds for the FTS scientific studies, although their thermal and chemical stabilities are major concerns in FTS reactions. Various preparation strategies for their synthesis as applicable catalysts are being established in the interest of supplementing, and ultimately replacing, the current commercial FTS catalysts. The employments of such model catalysts have exhibited great advantages in the academic research, although those nanomaterials are usually more susceptible to environmental changes than the bulk material. In this review, we have shown key factors that determine their catalyst properties and the underlying physiochemical phenomena associated with the improvements of the catalytic performance. Clearly, the different electronic properties, due to the different core metal with different surface crystallography, play crucial roles for the activation of the reactant (CO and H2) and the subsequent elementary reaction steps to form different products. However, the core metal electronic properties that are related to the size, shape, defects, shear planes, etc. in the crystalline structure, can be affected by the auxiliary metal(s) and the environment, typically the support, resulting in a variety of product distribution when the core metal is chemically modified. We note here that although different key factors are categorized in this review, they are, indeed, intertwined in the reality and should not be discussed separately.
Several new important issues need to be addressed in the development and study of the nanostructured FTS catalyst area and summarized as follows. (1) It is necessary to design new synthetic approaches that are simpler and are conducted under relatively mild conditions to gain the production cost advantage compared with the traditional FTS catalyst. It is also highly desirable to gain the knowledge about the fundamental formation mechanism for the further catalyst improvement. (2) Another important aspect is to explore novel FTS nanostructured catalysts such as the bimetallic catalysts that are discussed extensively in this review. Apparently, each core metal has its own characteristics in determining the single reaction events such as the CO dissociation and C-C coupling. The combination of those different characteristics could lead to enhanced catalytic activities and versatile product distribution compared with each component on the bimetallic catalysts or multimetallic catalysts. (3) The FTS research under the liquid phase is also an important field. Some of the intrinsic obstacles in the FTS commercial application on the heterogeneous catalyst are the control of heat transfer, the product separation from the catalysts. The development of a suitable reactor such as the slurry-bed reactor cannot solve those problems completely. However, the FTS in the liquid phase might pave a new way for the future FTS commercialization because of its low reaction temperature, facile separation of the reaction products from the catalysts, and good control on the heat transfer. (4) We emphasize the need to synthesize well-defined nanostructured catalysts and gain the understanding of their activities in the syngas conversion with their unique structural characteristics in the atomic scale. It is clear that we need the observation of the reaction events in situ 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 the reaction mechanism at the atomic scale under the real FTS working conditions (elevated temperature and high pressure) is sparse in the literature. Those insights into the atomic-scale processes can guide the nanostructured catalyst design for the FTS reaction to develop more energy-efficient and environmentally benign catalysis process. (5) We stress the importance of using the combined approaches such as theoretical modeling and in situ techniques to disclose the FTS reaction mechanism and the nature of the catalyst active phase. The theoretical descriptions of the FTS reaction provides information that are hardly obtained in the practical experiments, but the gap between the modeling and experiments need to be bridged to shed light on the FTS process under the working condition. Maybe, a higher accuracy than the widely used DFT method is required to gain the predicative power. The complexity of the real FTS reaction with many parallel subreactions should also be taken into consideration in the computational methodology. (6) The well-defined nanostructured catalysts, especially for those materials that are promising in the FTS reactions, are highly desired to be studied by the new surface characterization techniques with higher spatial, temporal, and energy resolution to get a complete picture on the catalyst composition and surface structure. This is because FTS reaction results are critically dependent on the catalyst structure. The lack of knowledge by the limited characterization methods might hinder further improvement of the catalyst materials. On the basis of these considerations, we could expect that nanostructured catalyst study in FTS reaction would make a major contribution to the heterogeneous catalysis and chemical industry.
About the authors
Peng Zhai obtained his bachelor’s degree in 2010 from the University of Science and Technology of China. Now, he is working for his PhD degree under the supervision of Professor D. Ma at the Beijing National Laboratory for Molecular Sciences (BNLMS, China) in Peking University. He was addressed in research on high efficient 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.
Geng Sun obtained his bachelor’s degree in 2011 from the Peking University. Currently, he is pursuing his PhD degree under the supervision of Professor H. Jiang at the Theoretical Materials Group in the Peking University. His research interests are theoretic and experimental study of catalysis reactions on transition metals, especially on syngas conversion.
Qingjun Zhu joined the National Institute of Clean-and-Low-Carbon Energy as a researcher in 2011. He received his PhD from the Eindhoven University of Technology, Netherlands, in 2003. He did his postdoc research in Tokyo Institute of Technology, Japan, and Northwestern University, USA. His research interests include heterogeneous catalysis and design of novel catalytic materials.
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 Laboratory of Catalysis, Dalian Institute of Chemical Physics (2001). After his postdoctoral stay in Oxford University (Prof. M.L.H. Green) and University of Bristol (Prof. S. Mann), he started his research career in Dalian Institute of Chemistry as associate professor (2005). He was promoted as a full professor in 2007 and moved to Peking University in 2009. His research interests are heterogeneous catalysis, C1 chemistry, and development of in situ spectroscopic method that can be operated at working reaction conditions to study reaction mechanisms.
This work received financial support from the Natural Science Foundation of China (21173009, 21222306), 973 Projects (2011CB201402, 2013CB933100).
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