Abstract
Fischer–Tropsch Synthesis (FTS) is an important process in the production of liquid fuels in the energy sector, due to its flexibility for use with other technologies that can produce carbon monoxide (CO) and hydrogen. Catalysts have found substantial use in FTS to improve the process efficiency. However, the use of conventional FTS catalyst reduction techniques using (hydrogen (H2), CO and syngas) to activate the metal precursor has been accompanied by strong metal-support interactions. Such limitations have driven the quest for better technologies to ensure FTS catalysis reaches its full capacity. In this article, we review the activation techniques used, with emphasis on the contemporary auto-reduction technique, which has revealed energy-saving merits. Auto-reduction has the advantage of reducing the number of steps involved in catalyst preparation prior to FTS as well as eliminating costly reducing agents such as H2, CO and syngas. Auto-reduction in this article refers to the reduction of the metal precursor using a carbon support. We firstly provide a comprehensive review of the traditional reducing agents, followed by a review of the contemporary auto-reduction technique. A comparison of the conventional FTS catalyst reduction and auto-reduction techniques is provided to allow for a fundamental understanding of the merits and demerits of both techniques. The different types of nanostructured carbon materials used in aiding auto-reduction for the FTS process are reviewed.
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1 Introduction
Fischer–Tropsch Synthesis (FTS) is a promising route for establishing energy independence globally. However, several shortfalls in the process need to be resolved. The growing demand for fuel globally prompts the need to re-think new technologies that can ensure energy demands are met. X-to-liquid (XTL) (X-means any carbon rich feed) technologies are increasingly becoming important for the production synthetic hydrocarbon fuels. Within the X-to-liquid (XTL) field are the gas-to-liquid (GTL) and coal-to-liquid (CTL) technologies that are employed in producing fuels. Over the years, these two technologies have been the largest pools of feedstock for the production of fuels. However, while the feedstock obtained from these two technologies has contributed immensely to global warming and other environmental issues, these shortcomings have not affected the use of the FTS process, as it is not entirely limited to the two technologies.
Another challenge facing the energy sector today is developing more economical processes to produce fuel. The FTS process has problems in terms of efficiency, selectivity to higher hydrocarbons and the stability of the catalysts. Traditionally, catalysis has been the key to circumventing these problems; however, it has not yet reached its full potential. Catalysts are the driving force behind most industrial systems, as they ease the operating temperatures and pressures vital to the modern industry. Most industrial practices are dependent on catalysis to lower the temperature and ensure efficient results in FTS [1,2,3]. Catalysis is an integral part of the development of economic processes in order to reduce costs, for example improving selectivity towards desired products. Higher hydrocarbons are fabricated following further treatments, including hydro-isomerisation, hydrocracking and hydrogenation.
The catalysts must be activated before the FTS process is carried out, hence the pre-treatment stage. Pre-treatment of catalysts is an inevitable stage in FTS that follows catalyst preparation. It entails the calcination and reduction of catalysts. The reduction step is one of the many important steps in FTS as it facilitates the activation of the catalyst, which is usually inactive, and this plays a role in determining the characteristics of the catalysts [4,5,6,7,8]. Previous studies have revealed a strong correlation between catalyst pre-treatment conditions and the activity of the catalyst in some situations [9,10,11,12,13]. Therefore, catalyst activity is mainly dependent on the pre-treatment conditions applied to the catalyst in question.
Knowledge of how the composition of the catalysts change is thus crucial; therefore, in situ (XRD) is usually employed during reduction to show such catalyst phases. Conventional oxide support materials exhibit strong metal support interactions and thus require a very high temperature in order for activation to take place [14]. Initially, metallurgical coke was employed in the reduction of iron ore in industry, however, this was costly and led to the introduction of relatively low-cost reducing agents [15].
Later, various articles were published on the reduction of catalysts using H2, CO and H2/CO (syngas) to improve the performance of catalysts [16,17,18]. However, the need to eliminate energy-intensive processes prompted researchers in the field of catalysis to explore new avenues that involve milder temperatures. A large volume of detailed review papers has been dedicated to improving the performance of FTS catalysts, and in the wake of recent technologies, a critical understanding of the mechanism of activating catalysts is necessary.
In this review, we look at the potential of an area under development in the field of catalysis as a possible solution to some of the problems that have been encountered when using conventional reducing techniques, i.e. auto-reduction. This manuscript briefly revisits the conventional techniques used, to clarify the complementarity between the auto-reduction technique and conventional techniques when activating catalysts. The progress seen with auto-reduction in recent years will then be detailed. The role of auto-reduction and its applications in FTS catalysis is then discussed, including the opportunities and challenges that this technique presents. Finally, future projections and guidelines are presented regarding possible measures to improve the reduction of catalysts to ensure efficient catalysis processes.
1.1 Metal Oxide Supports for FTS
Alumina, silica and titania are some of the supports that have been used with conventional iron and cobalt catalysts [19, 20]. However, oxide and hydroxide supports lead to strong metal support interactions (SMSIs) [21]. For example, cobalt forms cobalt aluminates (CoAl2O4), which are difficult to reduce with an alumina support [22]. SMSIs are triggered by H2 reduction of the metal oxide support interaction via different routes, as illustrated in Fig. 1 [23]. Figure 1 (A1) shows that activated hydrogen species spill-over from the metal surface; (A2) illustrates the direct injection of electrons from hydrogen adsorbed on the metal surface into the metal oxide support; (A3) illustrates direct molecular H2 reduction [23]. With the (B1) and (B2) routes, reduced metal cation formation alters the Fermi levels of metal oxides, which initiates electron transfer between metals and metal oxides, and leads to the formation of mixed oxides [23]. Finally, with route C, the reduced species move onto the surface of the metal to form “decoration” layers of reduced support oxides. Subsequently, the layer formation impedes the reception of reducing gases such as H2 and CO, which leads to the suppression of chemisorption [23].
Illustration of strong metal-support interactions in routes (A), (B) and (C) [23]
2 Auto-reduction
Auto-reduction, or ‘self-reduction’ as Kuroda and Iwamoto [24] term it, is a research frontier that is under development for FTS. It is a technique for reducing catalysts in situ in the presence of a carbon support without the use of external reagents such as H2 [25]. Auto-reduction is believed to take place during the calcination stage in catalyst preparation [26]. During this stage, the carbon layer is pyrolysed, which results in weaker interactions between the conventional support (silica, titania or alumina) and the cobalt species, and the cobalt species are activated [27]. The carbon layer can be removed by simple pyrolysis, which leaves unoccupied active sites, and this facilitates further reduction of cobalt oxides in the absence of externally-sourced H2 or syngas [28]. The main routes that have been explored for the reduction of catalysts include using H2, CO, syngas and, more recently, auto-reduction [29,30,31,32]. Auto-reduction has emerged as a self-sustaining process that possibly decreases metal-support interactions and the cost of using externally sourced reducing gases such as hydrogen through the use of carbon as a support and reducing agent. It can be said to be self-sustaining, as the reducing gas is produced during the process, and it simultaneously activates the catalyst. It should be pointed out that this review specifically looks at the carbon support as a reducing agent used in the FTS process. Therefore, the carbon support can be a polymer, carbon nanotube, carbon fiber, activated carbon et cetera and any other gases evolving from the carbon support other than CO could also reduce the catalyst. However, these gases are not part of the auto-reduction definition described in this review as the focus is on the carbon support acting as a reducing agent.
Before looking at the effect that carbon nanostructures have on auto-reduction of FTS-based catalysts, how auto-reduction is a self-sustaining process will be considered, by comparing it to a conventionally-reduced catalyst using H2. In terms of the auto-reduction technique, one study compared Co-based catalysts reduced under an inert medium with one reduced under H2 [33]. With the former, auto-reduction was employed to transform cobalt (II) formate dihydrate under a high-purity nitrogen atmosphere at 380 °C for 4 h, in order to produce metallic Co nanoparticles. The formate used in this case has the formula Co(OOCH)2⋅2H2O; thus, it was used as a source of CO and H2. In the process, reducing gases (CO and H2) were released, which simultaneously auto-reduces the sample, since formate is consumed through oxidation to CO2 and H2 [34]. In a similar experiment carried out to compare an auto-reduced catalyst with an H2-reduced catalyst [34], it was realized that the auto-reduced Co/γ-Al2O3 catalyst had a CO conversion almost twice that of the H2-reduced catalyst, owing to a higher number of active sites as a result of smaller Co nanoparticles being formed during auto-reduction. (See Fig. 2).
CO conversion as a function of time on stream (TOS) for decomposed cobalt (II) formate dihydrate in a fixed bed reactor at T = 230 °C, P = 8 bar, H2/CO = 2 and GHSV = 3600 h−1 [33]
The following mechanism has been proposed for the decomposition reaction of cobalt formate in an inert environment from ambient temperature to 380 °C:
The higher surface area of auto-reduced catalysts (Co/γ–Al2O3) shown in Table 1 corresponds to the higher CO conversion exhibited by these catalysts.
As shown in Table 1, the auto-reduced catalyst shows higher light gas selectivity compared to the H2-reduced catalyst. The authors attributed this to large cobalt nanoparticles (51 nm) for the H2-reduced catalyst, which favour higher carbon chain growth than do smaller auto-reduced cobalt nanoparticles (22 nm). The different carbon nanostructures that have been employed as support materials during auto-reduction of FTS catalysts are explained below.
Carbon nanostructures have been used as support materials to aid in reducing metal catalysts under an inert atmosphere. Figure 3 is an illustration of carbon nanostructures that have been employed for auto-reduction.
Illustration of carbon-based supports used for auto-reduction
2.1 Auto-reduction of Cobalt Supported on Nanostructured Carbon Species
Auto-reduction has proven to be successful in cobalt catalysts, as reported by several studies [30, 35, 36]. This section provides a discussion of the impact of the carbon support structure on FT performance in cobalt catalysts.
2.1.1 Auto-reduction of Cobalt Supported on Carbon Spheres
In a recent study, auto-reduced cobalt oxide showed superior performance compared to cobalt oxide reduced using hydrogen, because of the well-dispersed particles of cobalt entrapped in the carbon spheres (CS) vacancies [30].
Some studies have shown that auto-reduction yields higher activity than compared hydrogen-reduced Co catalysts. For example, an investigation carried out by Xiong et al. [30] showed higher activity for a 2.3 wt% Co/N-CSs catalyst pre-treated using auto-reduction compared to one pre-treated in H2. The activity of the catalysts pre-treated under inert conditions was double that of the catalyst pre-treated under hydrogen—see Fig. 4.
Activity of 2.3 wt% Co/N-CSs over time pretreated under H2 and Ar for FTS [30]
In another study, Xiong et al. [30] reported complete reduction of cobalt oxides using a nitrogen-doped CS support under reduction in an inert atmosphere. Auto-reduced catalysts showed better performance than the catalyst reduced in H2. This was ascribed to improved dispersion of cobalt particles created when carbon was removed by oxidation. These findings were supported by the findings reported by Yang et al. [35], i.e. that the resultant oxygen atoms that escaped from the annealing treatment of the cobalt oxide lattice reacted with carbon and produced a completely reduced Co catalyst. (See Eq. 1.)
2.1.2 Auto-reduction of Cobalt Supported on Ordered Mesoporous Carbons
Yang et al. [29] explored the auto-reduction of cobalt catalysts supported on ordered mesoporous carbon (OMC) for FTS. Interestingly, a comparison between the H2-reduced and auto-reduced catalysts showed favourable high activity with the auto-reduced catalyst, as well as a change in the shape of the auto-reduced cobalt catalysts. (See Fig. 5) The auto-reduction mechanism postulated is that oxygen atoms liberated from the cobalt oxide combine with carbon to form CO. During hydrogen reduction, the oxygen atoms furthest from the carbon substrate react with hydrogen to produce water, after which a deficit in oxygen atoms on the outer layer triggers the inner O2 atoms bound within the inner lattice to rummage for H2.
Mechanism of CoO/OMCs during the auto-reduction and hydrogen-reduction processes [29]
In a recent study, Yang et al. [35] evaluated the effect of nitrogen-doped ordered mesoporous carbon (NMC) supports on auto-reduction. The cobalt particles tended to cluster on the availed sp2-hybridised nitrogen sites, which induced both physical and chemical irregularities in the carbon matrix. The carbon support then acted as a convenient recipient for the oxygen atoms, which weakened the point of contact between the cobalt and the parent oxide. Contrary to the H2 reduction route, oxygen atoms favour a reaction with the carbon substrate under an inert atmosphere. An interesting finding by Yang et al. [32] is the ellipsoidal shape formed when exciting the oxygen atoms at an elevated temperature leads to a large number of cobalt particles settling on the carbon surface. The pressure exerted on the layer of cobalt atoms on the carbon surface compacts these particles into an ellipsoidal shape.
2.1.3 Auto-reduction of Cobalt Supported on Carbon Nanotubes
Apart from auto-reduction on graphitic carbon (GC), some authors [37] have studied the auto-reduction of cobalt supported on carbon nanotubes. Auto-reduction of cobalt oxides to metallic cobalt was found to occur in two successive stages, but it was inconclusive which of the two routes the activation process followed. (See Route 1 and Route 2 below). However, it was clear that auto-reduction yields better performance than the analogous (H2) process.
The third peak in the (TPR) profiles (Fig. 6) corresponds to (CNT) methanation. This decreases with an increase in heating temperature to 800 °C, because auto-reduction consumes a portion of the CNTs.
a TPR profiles of 20Co/CNTs heated at different temperatures in different atmospheres [37]
However, the presence of reduction peaks from CoO to Co for samples heated at 500 and 800 °C in the XRD scheme (Fig. 7) demonstrates that auto-reduction does not proceed to completion, even at an elevated temperature (800 °C).
XRD patterns of 20Co/CNTs calcined at different temperatures in different atmospheres [37]
Though similar findings were obtained in Xiong et al.’s [30] study involving temperatures below 500 °C, Li et al. [26] noted contradictory findings at temperatures above 500 °C, as the former did not observe any Co2C species in their work. In conclusion, the results of the experiment done by Li et al. [26] found clear support for auto-reduction of Co by carbon nanotubes at temperatures below 480 °C. Due to methanation of CNTs in hydrogen, auto-reduction at temperatures above 500 °C resulted in a decrease in FTS activity.
2.1.4 Auto-reduction of Cobalt Supported on GC Nitride
The upside of auto-reduction is that it also extends to GC. Park et al. [36] were able to demonstrate this and indicated that GC promotes auto-reduction in oxide-supported catalysts because it weakens the bond between the catalyst and the oxygen when it is sandwiched between the two. It could be that a build-up of carbon on the surface of the catalyst deactivates the catalyst. However, the hypothesis offered by Park et al. [36] is that the coated carbon is decomposed during the heating process to produce reductive carbon monoxide gas, which directly reduces cobalt oxide species to metallic cobalt. (See Fig. 8) This process is termed auto-reduction.
Auto-reduction mechanism of the cobalt catalyst under N2 flow [36]
In studying the effect of carbon layers on the ease of reduction of catalysts, Zhao et al. [38] observed that an increase in carbon coating on the support increases the reducibility of catalysts. An increase in carbon coating improves higher hydrocarbon selectivity as well as CO conversion, perhaps because of the back-and-forth migration of electrons between carbon monoxide and the cobalt catalyst, which results in the conversion of CO molecules on the cobalt catalyst [39]. Auto-reduction has also been shown to successfully impede the strong interactions between the cobalt catalyst and the support in FTS. In a study conducted by Park et al. [36] to investigate cobalt on GC nitride-coated alumina supports, the g-C3N4 supported cobalt catalyst was reduced entirely to cobalt while the g-C3N4 unsupported cobalt catalyst exhibited strong metal-support interactions in the form of irreducible cobalt aluminates. (See Fig. 9).
Illustration of the effect of the CN-Al support on the reducibility of cobalt species [36]
2.1.5 Auto-reduction of Cobalt Supported on Carbon Produced From Glucose
To demonstrate that the carbon element in glucose can aid in reducing catalysts, Jiang et al. [40] synthesized Co-supported on carbon by carbonising glucose in an inert atmosphere to reduce strong metal-support interactions. The binding energy of Co 2p3/2 was found to be slightly lower in Co/0.5C-SiO2 than in Co/SiO2 (0.1 eV), as shown in Fig. 10. This shows that an increase in carbon content reduced the interactions between the cobalt oxides and the support. The SMSI between Co and the support was reduced by coating SiO2 with carbon, which prompted an increase in reduction.
Co2p XPS spectra for a Co/SiO2, b Co/0.5C-SiO2 and c Co/1.5C-SiO2
In conclusion, it can be asserted that if a simple sugar such as glucose can provide carbon to aid in the auto-reduction process, other simple sugars such as fructose and galactose may perform the same function. Studies can be done on other sugars to prove this phenomenon.
2.2 Auto-reduction of Iron Supported on Nanostructured Carbon Species
Auto-reduction has been successfully applied to cobalt-based catalysts, but a few studies have been done on auto-reduction of iron-based catalysts to improve their performance [20, 41, 42]. Iron is an interesting catalyst in that it is low cost [43], there are abundant reserves, it operates over a wide temperature range and it promotes the water gas shift (WGS) reaction, which is suitable when using syngas that is highly hydrogen-deficient. In areas where coal and biomass are the sources of syngas, iron supported on conventional oxides (Al2O3, SiO2, and TiO2) has been employed as a catalyst, but it has limitations such as susceptibility to deactivation by oxidation, selectivity to oxygenates, SMSIs [44] and low activity. All these attributes prompt an urgent need for studies on auto-reduction of iron catalysts.
The following sections focus on auto-reduction of iron oxides supported on carbon nanotubes, GC and CSs.
2.2.1 Auto-reduction of Iron Oxide Supported on Carbon Nanotubes
Li et al. [41] reported successful auto-reduction of iron embedded in nitrogen-doped CNTs. The facile reduction was a result of weak bonding of oxygen to iron encapsulated in carbon nanotubes. Furthermore, a small inner diameter was seen to increase the extent of auto-reduction by exposing more of the carbon to the iron. Structurally, the presence of defects on the carbon nanotubes expedited the reduction of iron by enhancing the dissociation of the bond. Moreover, the authors projected that the changes to the electronic structure of the carbon nanotubes resulted in close contact between the iron and the inner wall of the carbon nanotubes, thus enhancing auto-reduction. The authors proposed that the drift in the electron density from the inner wall to the outer well disturbed the electron density balance within the CNT wall, thereby offsetting the interaction between the inner wall and the iron oxide, and consequently weakening the iron oxide, which led to facile reduction of the catalyst.
One study investigated the auto-reduction of iron supported on N-functionalised CNTs [45]. The results showed that the functionalisation effect on CNTs led to high reduction and strong metal-support interactions, which led to stability, high FTS activity and high hydrocarbon selectivity (diesel), i.e. 52% [45]. The H2-TPR profiles revealed carbon gasification at above 600 °C, and there was no formation of hardly reducible phases at temperatures above 720 °C—these may form if iron is reduced in the absence of carbon [46]. Notably, the auto-reduced iron/CNT catalyst had higher selectivity to higher hydrocarbons. In two other studies, CO or syngas activated iron catalysts had high selectivity to higher hydrocarbons (wax) [47, 48]. However, this is dependent on the type of reactor used for FTS, as CO or syngas activation favours higher hydrocarbons when a fixed bed reactor is used, while syngas activation is more selective to light hydrocarbons in a slurry reactor compared to CO activation [48]. It is important to note that similar comparisons have not been made between conventional activation gases and auto-reduction.
It is reported that CO or syngas treated catalysts are prone to deactivation during FTS as a result of partial conversion of Fe2O3 to χ-Fe5C2, and a build-up of unreactive iron oxides and inactive carbonaceous species on the catalyst surface [49].
2.2.2 Auto-reduction of Iron Oxide Supported on GC
GC has found use as a support material for cobalt, and has also been successfully employed as a support for iron-based catalysts. Ni et al. [20] prepared Fe-based FTS catalysts using a modified sol–gel method without using an additional reduction step. To ensure the success of the process, the catalyst precursor was doped with GC and potassium and exposed to argon at 700 °C for 5 h. The auto-reduced catalyst was compared to the catalyst reduced under hydrogen and higher activity was reported for the auto-reduced catalyst. The auto-reduced catalysts (FeK0@SiO2-GC, FeK0.5@SiO2-GC, FeK1.5@SiO2-GC, FeK2.0@SiO2-GC, FeK1.5@SiO2-GC) also exhibited higher CO conversion than the hydrogen-reduced catalyst (Fe2O3@SiO2). (See Fig. 11).
Graph showing CO conversion for Fe-based FTS catalysts [20]
2.2.3 Auto-reduction of Iron Oxide Supported on CS
Auto-reduction of iron oxide supported on hollow carbon spheres (HCS) was carried out by Teng et al. [42]. Resorcinol and formaldehyde (RF) were used as carbon precursors, and the resulting auto-reduced catalyst was confined within the hollow carbon spheres, which gives good dispersion and resistance to sintering [42]. Figure 12 illustrates the synthesis stages of Fe/HCS samples and the XRD patterns that show the crystal phase composition of the Fe/HCS catalysts and reference samples. The broad peak emerging at about 23° corresponds to the amorphous structure of the carbon. As the pyrolysis temperature was raised, the reflection becomes visible at a value of 26.5°, due to a characteristic property of graphite carbon. Fe2C was identified as the dominant iron carbide species, as shown in Fig. 12.
Schematic of the synthesis XRD spectrum of Fe/HCS samples [42]
2.3 Auto-reduction of Ruthenium Supported on Nanostructured Carbon Species
2.3.1 Auto-reduction of Ruthenium Supported on Ordered Mesoporous Carbons
Apart from Co and Fe catalysts, Xiong et al. [50] successfully auto-reduced a ruthenium (Ru) catalyst using OMC at 1123 K. Carbon gasification was observed to lead to auto-reduction at higher temperatures (> 673 K). In addition, deactivation by oxidation was found to be limited in ruthenium catalysts owing to Ru embedding on OMC, probably by the hybridisation between the d orbital of ruthenium and the pz orbital of graphene. It is believed that the contact between Ru and the carbon walls creates electron-deficient sheets that promote a hydrogen spill-over from ruthenium to OMC. Shen et al. [51] state that a hydrogen spill-over is energetically favorable on carbon with defects, and that the interconnected channels found within OMCs may account for this. OMCs were the most preferred carbon source for thermal auto-reduction, owing to their high stability, as they managed to withstand high temperatures during auto-reduction [52].
3 The Role of Carbon and Functional Groups During Auto-reduction
This section deals with the role of carbon and functional groups in auto-reduction and the active phases of iron and cobalt-based catalysts.
Polyvinyl pyrrolidone is a water-soluble polymer with the formula (C6H9NO). It was used as a carbon precursor to study the role of carbon in enhancing the reducibility of Co3O4 as a result of the weak interactions between carbon and cobalt [53]. The core–shell catalyst based on carbon-modified mesoporous silica (Co3O4@C-m-SiO2) showed higher reducibility of Co3O4 compared to core–shell Co3O4@m-SiO2 without carbon modification.
Carbon loading has also been found to have an effect on dispersion and catalyst activity [27, 54]. Figure 13 shows the effect of carbide formation on the activity of the metals and carbides in FTS. In accordance with DFT calculations, an increase in carbon content has been found to weaken the bonding between iron species and CO, and thereby increase FTS activity.
Illustration of the effect of the formation of carbides on the activity [54]
Hydrogen chemisorption studies demonstrate that a weight loading of carbon that is greater than 50% does not result in an increase in cobalt dispersion [27]. This was ascribed to the increase in the carbon layer blocking the absorption of hydrogen by Co nanoparticles [27]. While hydrogen absorption may be impeded by a higher weight loading of carbon, one study [36] reported that the benefit of a carbon layer (g-C3N4) between a support such as Al2O3 and a catalyst such as Co, prevents the formation of cobalt aluminates; as the carbon decomposes, the reduction of cobalt occurs. The reduction of cobalt using g-C3N4 was validated by exposing a Co/Al2O3 catalyst to the same conditions as that of the Co/g-C3N4 /Al2O3 catalyst [36]. Reduced activity was observed with the Co/Al2O3 catalyst [36]. This reinforces the idea that carbon lays the foundation for auto-reduction to take place.
Thus, the role of carbon materials during pre-treatment of conventional catalysts appears to be preventing the formation of inert species such as cobalt aluminates, preventing sintering and facilitating auto-reduction. Careful functionalisation has to be observed during the pre-treatment stage, as a less defective carbon surface may result in less encapsulation and dispersion of cobalt nanoparticles on the supports [27].
3.1 The Role of functional Groups on Carbon Supports During Auto-reduction
Although carbon materials are inert, it appears that their chemical characteristics can be altered (tuned) by exposing them to a high temperature or an inert atmosphere or by adding functionalised groups. When compared with oxides, carbon materials have been found to possess more porosity, greater stability and a larger surface area when using a high temperature and inert conditions [26, 55].
Functional groups on carbon materials have proven to induce the reducibility of cobalt species while heteroatom (O, N and S) doping is known to improve the hydrophobicity of carbon [56]. Carbon supports can be oxidized on the surface to produce functional groups such as hydroxyl or carboxyl groups [45]. The carboxylic groups formed on the surface can be used to anchor metal species; however, the heteroatoms O and S have not been applied as widely as the N heteroatom. The catalysts are usually functionalised with nitrogen (N), in order to improve the carbon support, [45]. Liu et al. found that the electronic conductivity between the support and the active phases can be facilitated by N inserted into the CNTs matrix, which leads to high dispersion. A difference in the resulting catalyst surface area between different functional groups has been observed [57]. A catalyst synthesized from the iron nitrate precursor had a higher surface area than the one synthesized from an iron acetate precursor [57]. This may explain the ease of reduction and less aggregation of carbon supports decorated with nitrogen-containing functional groups [28].
The g-C3N4 support material is popular, as it consists of C-N bonds. The N sites provide lone pair electrons, which enhances the dispersion of active metals while hindering the aggregation of Co nanoparticles, and this leads to inactive Co2C [28]. In one study, doping nitrogen on OMCs resulted in interfacial electronic interactions, which influenced the dispersion of cobalt particles on the surface of nitrogen-doped supports [35]. In addition, the defects created have been reported [35] to act as adsorption sites, thus enhancing the interaction between small cobalt oxide particles and carbon supports. This results in a pulling effect being created within the cobalt oxides that leads to weak bonding of cobalt oxides and facile auto-reduction. research has shown that, in terms of enhancing catalytic activity, carbon nanotubes treated with concentrated HNO3 can open up pores and create a substantial amount of defects as well as acidic functional groups on the surface of the catalyst [58].
In one study [45], the research work demonstrated the effect of the functionalised carbon in reducing iron supported on CNTs compared to that of non-functionalised carbon, as shown in Fig. 14. The Fe/CNTs-190 shown in the first plot was prepared using a hydrothermal method, with urea and alkali (NaOH) modified CNTs being heated at 190 °C in a stainless-steel autoclave for 10 h. With the second sample, shown in the second plot (Fe/CNTs-NaU), the acid-treated CNTs were immersed in an aqueous solution and a mixture of urea and alkali, with the resultant mixture heated at 190 °C in a stainless-steel autoclave for 10 h. The reference sample (Fe/CNTs-Na) was prepared the same way as (Fe/CNTs-NaU), but without the urea. The first two peaks shifted to a higher temperature, which demonstrates that the interaction between the iron oxides and the support grows stronger as the CNTs are N-doped. The researchers [42] also noted the tail peak at 560–750 °C shifting to a lower temperature compared to the N-doped samples, and attributed this to structural damage to the catalyst and easy hydrogenation. Thus, it can be concluded that functional groups have an effect on the catalytic performance of a catalyst.
H2-TPR profiles of the as-prepared catalysts
3.2 Active Phases of FTS Catalysts
A deeper understanding of the activity of catalysts stems from appreciating the active phases involved in the FTS process.
3.2.1 Active Phases of Cobalt-based Catalysts in FTS
The consensus among other researchers is that elemental cobalt is the active phase in FTS [59, 60]. Wide-spread reports on XRD results suggest the presence of a cobalt carbide phase (Co2C) during carburization [61,62,63,64,65,66,67,68,69,70]. Cobalt carbide is reported to be an inactive phase in FTS [71]. Karaca et al. [72] reported that cobalt oxide easily transitions to a cobalt carbide phase under a pure CO atmosphere. Mohandas et al. [73] postulated that the formation of the cobalt carbide phase from cobalt oxide proceeds as shown in Eq. 5.
Metallic cobalt can be present in two phases, either as a hexagonal close-packed (hcp) structure or a face-centred cubic (fcc) structure [74]. The general agreement among researchers is that hcp is the cobalt crystallographic structure that is active for FTS [65, 72, 74,75,76]. Enache et al. [76] attribute the high activity of the hcp form to the dis-orderliness of the hexagonal structure, and suggest that the presence of defects presents a larger surface area for a reaction to take place. Additionally, it has been reported that, under a hydrogen atmosphere, the dominant phase is the fcc, whilst the hcp phase dominates under syngas conditions [76]. Interestingly, in their earlier studies, Kwak et al. [77] reported higher cobalt performance with the resultant cobalt metal hcp phase from cobalt carbide compared to the fcc phase resulting from the activation of the cobalt oxide catalyst. Kemner’s group [78] explained that the stacking faults exhibited by the hcp form act as convenient receptive sites for CO, which facilitates CO scission.
Thermodynamics plays an important role in determining the resultant crystallographic structure of cobalt. Ducreux et al. [65] showed that a low activation temperature favours the formation of the hcp structure, while a high temperature favours the fcc structure. The findings of Braconnier et al. [79] and Khodakov et al. [80] support the observations made by Ducreux et al.. A cobalt catalyst activated at a temperature as low as 327 °C under H2 activation favours the formation of the hcp form [80]. Interestingly, Bulavchenko et al. [81] observed that, with a cobalt catalyst supported on γ-Al2O3, the fcc form was present at 260 °C; however, other researchers [65, 81] report the hcp phase as being dominant at this temperature.
The effect of temperature and reducing agents on the crystallographic structure are discussed separately in this paper. However, together, temperature and the pre-treatment have a considerable effect on the crystallographic structure formed.
3.2.2 Active Phases of Iron-based Catalysts in FTS
When explaining the mechanism of auto-reduction in iron-based catalysts, an important point to stress is that, during the auto-reduction catalyst pre-treatment stage, the support used for the catalyst precursor is carbon. Hence, the iron carbide phase is established during the pretreatment stage, as well as during the FT process when syngas is swept through the catalyst; hence, more iron carbide phases are purportedly formed owing to the CO component in syngas [4]. However, with the traditional techniques, the norm is: most metal precursors are supported on silica or alumina supports; during the pre-treatment stage, the catalyst is activated in hydrogen; it is during the FT process when syngas is introduced that the evolution of the carbide phases is observed.
Iron carbide phases such as hexagonal iron carbide (ε-Fe3C), H¨agg carbide (χ-Fe5C2), pseudo-hexagonal iron carbide (ἐ-Fe2.2C), Eckstrom-Adcock iron carbide (Fe7C3) and cementite (θ-Fe3C), are reportedly associated with the active sites of iron-based catalysts [82]. Several researchers [4, 7, 8, 12, 83,84,85,86,87,88] have reported higher activity with FeC, while a few [86, 87] postulate that iron oxide is the active phase of the iron catalyst. However, some researchers [4, 88] report that iron oxide eventually reduces to its elemental form. A portion of iron (20%) in its elemental form was observed by Lox et al. [88] at 220 °C under hydrogen reduction. Some researchers [8, 89] report a mixture of Fe3O4 and FeC under synthesis gas reduction, as shown in Table 2.
O’Brien et al. [7] point out that the amount of water produced in the FT reaction is the sole determinant of FeC or Fe3O4 formation. According to Raupp and Delgass [90], the presence of a carbide phase in iron catalysts increases the activity of the catalyst. Results obtained by other authors [5, 90,91,92,93] matched this finding. However, while the active phase of iron is not yet fully understood, the general belief of most researchers is that FeC is the active phase for FTS.
Niemantsverdriet and Van der Kraan [91] suggested that the lower activation energy (43.9–69.0 kJ mol−1) [94] needed for carbon to be deposited into Fe in FTS as paralleled to the higher activation (138–146 and 145 kJ mol−1) [95] needed for other FTS catalysts (Ni and Co, respectively) is the reason why iron can transform into active iron carbides while the same cannot be said for the other catalysts.
Ding et al. [17] investigated the reduction and carburization behaviour of an iron-based FTS catalyst. They [17] suggest that the reduction of iron under H2 conditions follows a three-step process with an increase in reduction temperature: α-Fe2O3 → Fe3O4 → FeO → α-Fe. A different trend (α-Fe2O3 → Fe3O4 → FeC) to pre-treatment under H2 conditions was observed when the iron species were pre-treated under CO conditions, as shown in Fig. 15.
CO-TPD spectra showing reduced iron phases [17]
Pineau et al. [15] reported a higher degree of reduction of iron oxide when using H2 than when using CO. They [15] ascribed this to the presence of defects in magnetite, which lower the activation energy from 88 to 39 kJ/mol when it is being reduced to iron. Chenavskii et al.’s [96] observation shows that under syngas reduction, Hägg carbide is formed at 400 °C. A study carried out by Li et al. [97] to determine the effect of reducing agents on iron-based MOF catalysts for use in FTS, revealed that using iron catalysts reduced in H2 resulted in the formation of elemental iron, while Fe3C was produced from syngas, and a large portion of Fe3O4 was obtained from H2 reduction.
Based on the auto-reduction studies that have been done, it is important to stress that carbon remains the main element used to lay a foundation for auto-reduction to take place. However, one drawback of using the carbon support is that it eventually gets depleted over time and needs replacement as the auto-reduction of the metal takes place.This is a gap in the research area as this drawback has not been addressed.
4 The Role of Promoters on FTS Catalysts During Auto-reduction
Transition metals (Mn (manganese), Zr (zirconium), V (vanadium), Cr (chromium), Ru (ruthenium), Re (rhenium), Pt (platinum) and Mo (molybdenum)) [98] and alkali metals (K (potassium), Cu (copper)) are usually employed as promoters for FTS catalysts [89, 99]. Promoters are used as additives that facilitate the reduction of catalysts in FTS.
4.1 The Role of Promoters on Cobalt-based Catalysts During Auto-reduction
It has been reported that the presence of noble metals enhances the reduction of fractions of small cobalt oxides, which is bound to increase selectivity to light hydrocarbons [89] and agglomeration of cobalt oxides [100]. Transition metals such as Ru, Re and Pt have been found to ease the reduction of cobalt-based catalysts [101]. The metals are referred to as reduction promoters and are believed to increase the active sites, and thus reducibility, through hydrogen dissociation and spill-over [102]. Notably, studies showed that a Pt-promoted catalyst containing carbon evidenced higher reducibility (80.6%) than a Pt-promoted catalyst without carbon (75.6%) [27, 62]. A higher margin was also observed when a comparison was made with a non-promoted catalyst (50.3%) [27, 62]. The lower reducibility seen with non-promoted catalysts was attributed to the absence of Pt, which is believed to lower the reduction temperature through H2 spill-over to cobalt [27, 62]. This may indicate that a catalyst containing carbon that is also promoted with Pt may show relatively high activation. However, the price of noble metals should be taken into consideration if they are to be successfully incorporated in catalysts, in order to ease reduction. For example, it has been reported that ruthenium, platinum and rhenium are 200, 650 and 70 times more expensive than cobalt, respectively [89]. There are some other limitations of noble metal promoters, with Pt and Pd (palladium) reported to have high hydrogenation activity, which leads to higher methane selectivity [89]. The low chain probability growth increases with an increase in noble metal loading. However, when comparing Pt and Pd promoters, the extent of hydrogenation activity is reported to be higher with Pd [89].
Apart from noble metals, metal-oxides have also been used as promoters for cobalt-based catalysts. Using a cobalt catalyst supported on activated carbon, Ma and co-workers [103] investigated the effect of Ce (cerium), K and Zr promoters on the activity of a cobalt catalyst. It was found that the promoters had a significant effect on the activity of the Co/AC (activated carbon) catalyst, as the activity increased in the presence of these promoters. A comparative study was conducted on cobalt-based catalysts supported on carbon and promoted with metal oxides (MgO (magnesium oxide), CeO2 (cerium oxide), or V205 (vanadium oxide), with an unpromoted cobalt-based catalyst supported on carbon [104]. The activity of a cobalt catalyst supported on carbon was significantly increased by the addition of magnesium, cerium and vanadium oxides up to eight times as compared to that of the unpromoted one (Co/C) [104].
To sum up, noble metals such as platinum, palladium and ruthenium are commonly used to ease reduction in cobalt catalysts supported on carbon. The metal-oxide promoters have been found to exhibit a high surface area that allows highly dispersed cobalt catalytic phases and also weak promoter-carbon or cobalt-carbon interactions leading to improved cobalt-promoter integration.
4.2 The Role of Promoters on Iron-based Catalysts During Auto-reduction
Promoters such as K and Cu have mostly been used to improve the activity and selectivity of iron-based catalysts [105]. Duan et al. [106] found that the promotional effects of potassium on iron-based catalysts supported on carbon nanotubes include lower olefin selectivity, higher activity and better stability due to higher carbidisation and increased defects on carbon nanotubes to anchor the iron nanoparticles.
For a K-promoted iron catalyst supported on carbon spheres, Xiong et al. [57] reported that the addition of K led to higher hydrocarbon selectivity. A Cu-promoted iron catalyst supported on carbon spheres was found to have no significant effect on catalytic activity [57].
Chenavskii et al. [96] showed that potassium promoters affect the reducibility and carburization behaviour of iron. From their previous work [96], it was concluded that potassium aids in the formation of iron carbides. As much as potassium facilitated the formation of carbides, the researchers [96] observed that the promoter particles instigated aggregation of the iron carbides during activation under a syngas or CO atmosphere. Investigations done by Li et al. [89] revealed that the formation of iron carbide promoted by potassium led to an increase in the activity of the iron catalyst. Cano et al.’s [107] results were in agreement with the findings reported by Li et al. [89]. Xiong et al. [108] reported that adding K to an Fe-based catalyst increases catalytic activity [57]. This was ascribed to an increase in electron density, which strengthens the Fe–C bond but weakens the C–O and Fe–H bonds. Exposing the CH groups to the surface of the catalyst then led to higher hydrocarbons and olefins.
To sum up, there have been more studies on the promotional effects of potassium on iron-based catalysts supported on carbon than those of copper [105]. However, different promoters exhibit certain promotional effects on catalysts. The reduction of a catalyst at a high temperature leads to sintering of the catalyst and operating at a high temperature generally increases the cost of the FTS process. The tendency of copper to lower the activation temperature is well-established in the literature [92, 98], and K, Ce, and Cu promoters result in an increase in the activity of FTS catalysts. Therefore, it may be important to optimise promoters in order to improve the performance of the catalyst. This may be done by coupling promoters that are crucial in aiding catalyst activation with promoters that promote selectivity to high hydrocarbons.
5 Summary of FTS Performance for Auto-reduced and Conventionally Pre-treated Catalysts
Overall, auto-reduction has been successful in carbon nanotubes, carbon spheres, OMCs and GC. Table 3 provides a summary of the performance of conventionally-reduced and auto-reduced catalysts in FTS.
6 Other Applications of the Auto-reduction Technique
Successful auto-reduction to their respective elemental forms was reported for other metal oxides, like nickel oxide and palladium oxide [36]. It should be noted that thermal and thermal/carbon auto-reduction were used to produce the reducing gases (H2/CO) in situ for the activation process.
6.1 Thermal/Carbon Auto-reduction of Palladium Through Exposure to Formaldehyde
In one study, a Ti-based MOF (MIL-125-NH2) was used to encapsulate Pd nanoparticles for in-situ thermal/carbon auto-reduction [109]. Instead of using expensive harsh chemicals for the reduction process, exposing the amino groups in MIL-125-NH2 to formaldehyde results in novel reducing groups (-NH–CH2OH) that auto-reduce Pd2+ ions to Pd nanoparticles in situ. It is believed that doing auto-reduction this way restricts the agglomeration of Pd nanoparticles [109]. The broad band at 3300 cm−1 shown in Fig. 16 represents the presence of hydroxyls. The distinct strong band at 1659 cm−1 can be attributed to the bending vibration of the -NH2 group, which indicates the presence of -NH2 groups in MIL-125-NH2 [109]. The bands at 1338 and 1257 cm−1 are attributed to typical C-N stretching of aromatic amines in MIL-125-NH2. Due to the creation of the reducing groups, the bending vibration of the -NH2 group seen at 1659 cm−1 in MIL-125-NH2 grew weaker in MIL-125-NH-CH2OH [109].
FT-IR spectra of the as-prepared samples [110]
6.2 Thermal Auto-reduction of a Polymer (dodecane)-Coated Nickel–Alumina Catalyst
Dodecane is a straight-chain alkane with 12 carbon atoms that is decomposed thermally to produce H2 and CO gases to auto-reduce a nickel-alumina catalyst. Jo et al. conducted an experiment to deduce whether an auto-reduced catalyst or a H2 reduced catalyst would give the highest performance for diesel reforming catalysts [111]. The impregnated nickel-alumina catalyst (NA10-IM) was exposed to a hydrogen stream, and the nickel-alumina catalyst was prepared using the polymer-modified incipient method (NA10-PM). When compared to the H2-reduced NA10-IM and NA10-PM catalysts, the auto-reduced NA10-PM catalysts demonstrated excellent dodecane conversion and catalytic stability, followed by the H2-reduced polymer-coated nickel-alumina catalyst. (See Fig. 17).
A comparison of the stability of an auto-reduced catalyst (NA10-PM) to hydrogen-reduced catalysts (NA10- IM and NA10-PM) at S/C = 1.23, O2/C = 0.25, and GHSV = 12,000 h − 1 at 750 °C [111]
The surface area of the auto-reduced catalyst also had a larger surface area than the fresh catalyst and the H2-reduced catalysts, which lead to higher dodecane conversion. (See Table 4) Traditionally, oxidized catalysts are reduced by H2 gas before being used in a reaction [111]. In the thermal auto-reduction process, the oxidized catalysts are first reduced by heat and photochemical radiation; a reductant is derived from the thermal decomposition of the polymer, which acts as a carbon precursor [111]. Therefore, thermal auto-reduction may be economical in the sense that the reducing gas may not need to be sourced externally, as it can be derived in situ.
7 Limitations of the Auto-reduction Technique
Although several successful applications of the auto-reduction technique have been detailed in the literature [29, 30, 35, 36, 112], there are also drawbacks that limit the technique to large-scale implementation.
7.1 Catalyst Cost and Complex Preparation Procedures
Catalyst cost is an issue in terms of the viability of a technique. In addition to the oxidic support, implementation of the auto-reduction process means the addition of carbon nanostructures and doping the carbon on the supports, which increases the cost of the materials required to prepare the catalyst. Successful upscaling of carbon supports at the industrial level would require coupling with functional groups or promoters, which would increase the catalyst preparation cost. The use of noble metals has been found to promote hydrogen activation, which leads to higher methane selectivity.
Although carbon supports have been studied extensively as possible alternatives to conventional supports, their inert nature and hydrophobicity have proven less effective in promoting dispersion and in anchoring the active metal, in order to enhance the stability of the catalyst. This has led to possible solutions such as adding functional groups on the surface of carbon to enhance the anchoring effect. The extra caution that comes with tuning the conditions and the amount of functional groups to be deposited on the carbon surface without distorting the pore structure of the carbon increases the complexity of the process. Non-uniform dispersion of cobalt species supported on hydrophobic carbon materials is one of the key issues with the use of cobalt materials supported on carbon species. Carbon supports have been limited to model supports and the industry has been slow to embrace them, as they may require the use of numerous methods to alter the carbon materials to ensure they are better suited to the processes. Furthermore, the depletion of the carbon supports as auto-reduction takes place is another limitation that has not been looked at.
7.2 Relatively High Light Gas selectivity
As seen in Tables 1 and 3, light gas selectivity for the auto-reduced catalyst is high in most instances. Increasing selectivity to light hydrocarbons during auto-reduction may be due to the elevated temperature used during auto-reduction, which leads to an increase in methane selectivity. Furthermore, the formation of bulk cobalt carbide because of the incomplete formation of the active phases has been reported to influence higher methane selectivity.
The various assumptions made about high light gas selectivity suggest that no comparison study has been conducted to compare the effect of particle size of conventionally reduced catalysts and auto-reduced catalysts.
7.3 Lack of Comparison Studies Between Auto-reduction and Syngas Reduction Using Syngas
There is lack of information on how auto-reduction compares with syngas reduction as auto-reduction has mostly been compared to CO or H2 reduction.
The high temperature required for the auto-reduction technique, selectivity to light hydrocarbons and the expensive promoters required to tune the carbon support material are some of the limitations associated with the auto-reduction technique. However, these limitations can be seen as perspectives for future research work on catalysis.
7.4 Future Work
It is important that the following research perspectives are considered in order to overcome the limitations, and achieve an optimised performance of the auto-reduced catalysts.
7.5 More Emphasis on Functional Groups
Based on the outlook of functional groups and heteroatom (N) on aiding reducibility, different functional groups or heteroatoms (O and S) can be impregnated onto the carbon surface to assess for enhancement in catalyst dispersion and to counter the weak metal-support interactions between carbon and the catalyst that could lead to active metal sintering.
7.6 More Optimisation Studies on the Use of Promoters
Adding promoters that aid in reduction has been a successful strategy in conventional reduction techniques. Future studies could enhance the activity of auto-reduced catalysts by adding promoters that are specifically aligned to aid in auto-reduction. Noble metals that are used as promoters (such as Pt, Re and Ru) are expensive, which restricts their use. Therefore, studies could look at recycling catalysts promoted with noble metals, as the carbon support can easily be pyrolysed, so as to regenerate the noble metal.
7.7 More Emphasis on Carbon Nanostructures
A few carbon nanostructures have been investigated, but future studies could investigate the association between metal-support interactions and differently shaped carbon materials, such as carbon nanofibers, carbon nano-onions and carbon micro-coils, as their unique structures have found use in other fields of catalysis. Other forms of carbon-based support materials include activated carbon, carbon micro-coils, carbon dots, carbon nanofibers and other carbon allotropes. The effectiveness of these in the auto-reduction process, with regard to the activity of the catalysts and possible selectivity in FTS, has not been assessed. Thus, an in-depth comparison of an auto-reduced catalyst supported on different carbon-based materials to a conventionally reduced catalyst would be of considerable interest.
7.8 Investigation of Reasons for High Selectivity to Light Hydrocarbons
One study suggests that mass transfer limitations due to carbon deposition are the result of higher light gas selectivity [113]. Future studies could look at prolonging the length of time the catalyst is in the reactor so that the unconverted iron oxides can be converted to active iron carbides and the bulk Co (Co2C) can be converted to hcp Co.
No research on auto-reduction studies has looked at a possible correlation between particle size and selectivity. Given the effect of particle size which has considerable effects on the selectivity of conventionally reduced catalysts, this area should be taken into consideration in auto-reduction studies.
7.9 More Comparative Studies on Catalyst Activation Techniques
Comparative studies have focused mostly on comparing the auto-reduction technique with the H2 reduction technique. There is still much work to do in terms of comparing auto-reduction to syngas reduction, thus focus on the latter is required.
7.10 Catalyst Technology
Similar to the conventionally reduced catalysts, the high activity shown by auto-reduced catalysts is surface-related, which suggests that it is relative to the active sites. The particle size of the catalysts and the size distribution after reduction lend the catalysts a higher surface area, thus exposing more of the active sites and, in the process, increasing the activity of the catalysts. It has been reported [29] that a higher surface/interior atom ratio increases the activity of auto-reduced catalysts. Thus, future studies could also look at tuning the carbon pore support structure to assess the performance of auto-reduction.
8 Conclusion
This research aimed to review the advances in auto-reduction in comparison with conventional techniques. A significant step in scientific development is conducting processes efficiently. The auto-reduction technique allows for catalyst preparation processes to take place using only a few stages. Moreover, the reduction step eliminated from pre-treatment of the catalyst reduces the handling of the catalysts, which reduces the chances of contaminating the catalyst. Additionally, auto-reduction leads to energy saving, as it takes place in the absence of using expensive reducing gases such as H2. However, for a rationalised economic analysis, a cost–benefit analysis should be performed before definite conclusions are arrived at. Additionally, in order for this technique to be fully functional, it has to overcome the problem of lower gas selectivity and the high temperature employed in the pyrolysis of carbon when preparing the catalyst as well as the issue of having to replace the carbon support as it gets depleted over time. However, the limitations of the auto-reduction technique can be taken as recommendations for future studies. It is anticipated that further research on auto-reduction will generate further improvements in the field of catalysis, with special emphasis on the activity of the synthesised catalysts to ensure economic, well-established and sustainable technologies.
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Ncube, S., Moyo, M. A Review of the Use of Carbon Nanostructures and Other Reducing Agents During Auto-reduction for Fischer–Tropsch Synthesis and Other Applications. Catal Lett 154, 366–386 (2024). https://doi.org/10.1007/s10562-023-04330-1
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DOI: https://doi.org/10.1007/s10562-023-04330-1