Effect of Pre-Treatment Conditions on the Activity and Selectivity of Cobalt-Based Catalysts for CO Hydrogenation

Abstract

We investigated the effect of pre-treatment conditions on the activity and selectivity of cobalt catalysts for Fischer–Tropsch synthesis (FTS) by varying both the reduction atmosphere and the reduction temperature. Catalysts supported on SiO₂, Al₂O₃, and TiO₂, prepared via incipient wetness impregnation, were evaluated, and activation temperatures in the range 250–350 °C were considered. Activation with syngas led to a better product selectivity (low CH₄, high selectivity to liquid hydrocarbons, and low paraffin to olefin ratio (P/O)) than the catalysts reduced in H₂ at lower activation temperatures. The Co_xC species suppressed the hydrogenation reaction, and it is hypothesised that this resulted in the high selectivity of olefins observed for the syngas pre-treated catalysts. On the basis of the experimental results, we postulated that a synergistic effect between Co⁰ and Co_xC promotes the production of the long chain hydrocarbons and suppresses the formation of CH₄. In addition, for systems aimed at producing lower olefins, syngas activation is recommended, and for the FTS plants that focus on maximising the production of higher molecular weight products, H₂ activation might be considered. These results provide insights for the future FTS catalyst design and for target product-driven operations.

1. Introduction

Fischer–Tropsch synthesis (FTS) is a structure-sensitive reaction that converts syngas derived from natural gas, coal, and biomass to valuable chemicals and synthetic fuels over a metal-based catalyst [1]. Cobalt (Co) catalysts have attracted more attention in the recent years due to their high intrinsic hydrogenation activity, selectivity towards liquid hydrocarbons, and lower water gas shift (WGS) activity than iron and lower costs compared to noble metals [2,3]. Currently silica (SiO₂), alumina (Al₂O₃), and titania (TiO₂) are used for commercial FTS operations [4].

The hydrogenation activity of the cobalt metal (Co), which is recognised as the active phase, is highly dependent on its structure. Co particles that are hexagonally packed (hcp) are found to be more active than the face-centred cubic (fcc) structure [5,6]. Evidence from previous studies suggests that the Co particle size is influenced by the support pore size [7,8,9]. Borg [10] studied the dependency of the Co particle size on the Al₂O₃-support pore diameter and found that: (i) large Co particles were formed in the large pores and smaller ones formed in the narrow pores, (ii) the degree of reduction increased with the pore size, and (iii) the C₅₊ (the long chain hydrocarbons with carbon numbers equal or higher than 5) selectivity also increased with the pore size.

In some cases, the interaction between these supports and the metal can be too strong, which may leave a fraction of the cobalt chemically inactive after reduction. For example, Jacobs et al. [11] reported a lower degree of reduction for Al₂O₃- and TiO₂-supported catalysts due to high metal–support interactions compared to the SiO₂-support. Strong metal–metal oxide interactions have been demonstrated to play an important role in the reactivity of alumina-supported catalysts. A specific feature in these catalysts is incomplete reduction and possibly the insertion of Co ions into the alumina lattice to form spinel structures, and as a result, the catalysts exhibit low reducibility and FT activity [12]. To overcome this issue, several strategies such as the modification of the support to minimise deleterious support metal interactions have been put forward. Soled et al. [13] demonstrated that the use of silicon substitutions in the TiO₂ lattice and the treatment of the TiO₂ support using an irreducible oxide ZrO₂ can inhibit the formation of Co–support interactions and thereby enhance the reducibility of the catalysts. Other strategies to limit the solid-state chemistry interactions between Co and the support include the use of neutral supports such as carbon nanofibers [14] and small amounts of noble metal promoters [15].

Significant efforts have been devoted to enhancing the catalytic activity of FTS catalysts and to reducing the costs of the FTS process. Hydrogen (H₂) is used to activate the Co₃O₄ species in freshly prepared catalysts to active metallic Co. A few studies documented the use of syngas as an alternative reducing agent to H₂ [16,17,18]. The general consensus is that syngas reduction promotes the formation of cobalt carbides (Co_xC, x = 2, 3), which transform back to metallic Co(hcp) under normal FT operating conditions, thus improving the FT activity [18], or that the inactive Co_xC blocks the Co metal active sites, leading to catalyst deactivation [19]. De la Pena O’Shea et al. [16] achieved a significantly higher activity (90% CO conversion) with a Co/SiO₂ catalyst pre-treated in syngas compared to either H₂ or CO reduction. The improvement in activity was attributed to the increase in the number of Co active sites (high metal dispersion). The presence of Co₂C during FTS has been confirmed by Claeys et al. [20] and reported to act as a methanation site [21,22]. While the role of Co₂C is widely debated in FTS, the presence of Co₂C nanoprisms increase the selectivity towards alcohols and olefins [19,23].

Syngas reduction is conducted at relatively low temperatures (<280 °C) to avoid catalyst coking resulting from the degradation of liquid products, and to limit the deposition of inactive surface carbon via the Boudouard reaction [18,24]. Graphitic carbon has been reported to strongly suppress CH₄ formation; however, it cannot be removed from the surface, thus it blocks the Co active sites and results in catalyst deactivation [24]. De la Pena O’Shea [16] reported that no graphitic carbon was observed after the syngas treatment at 500 °C due to the simultaneous presence of H₂ and CO, which minimises the formation of carbon. Reducing the catalyst at a low temperature with syngas still produces a fraction of CoO, which is believed to be inactive in FTS [25], and to catalyse the WSG reaction [26]. In our earlier work, we reported that the intimate contact between CoO and Co metal as well as formation of Co–CoO interfaces under FT reaction conditions catalyses the FT reaction for a SiO₂-supported catalyst pre-treated in H₂ at 250 °C [27].

Although there is a large body of work on the effect of the reduction temperature including the support characteristics on various Co-based catalysts reduced under H₂, not as much attention has been paid to determine how temperature and support identity influences the reduction of Co species under syngas reduction. As H₂ is an expensive gas, the use of syngas, as both reaction and reducing agents, at lower reduction temperatures can potentially cut-down the start-up and running costs of an FT process. Here, we report on the activity and selectivity of Co catalysts supported on TiO₂ and Al₂O₃, reduced at two temperatures (250, 350 °C) under two reductive agents (H₂, syngas), per catalyst, to compare their influence on CO hydrogenation.

2. Experimental Set-Up

2.1. Catalyst Preparation

In this work, three kinds of supports, TiO₂, SiO₂, and Al₂O₃ were used for the preparation of the 15% Co/support catalysts. The catalysts were prepared via incipient wetness impregnation of cobalt nitrate solution (Co(NO₃)₂·6H₂O) onto these three kinds of supports. The chemicals used were outsourced from Sigma-Aldrich (St. Louis, MO, USA). Catalysts were dried at room temperature overnight, followed by mild drying at 40 °C for 3 h and calcination at 350 °C for 8 h. For, the detailed preparation procedure, please refer to our earlier publication [27].

2.2. Catalyst Characterisation

Brunauer–Emmet–Teller (BET) experiments were conducted on the fresh catalysts prior to reduction or reaction to determine the sample surface area and pore size. BET experiments followed the usual procedure. The sample was firstly subjected to a degassing chamber at 200 °C for 6 h, and treatment was performed at a relative pressure of 0.99 (P_a/P₀ = 0.99, where P_a is the actual gas pressure and P₀ is the vapor pressure of the adsorbing gas) to obtain the pore volume and −196 °C to obtain the surface area and porosity by nitrogen physisorption. Furthermore, the Barrett–Joyner–Halenda (BJH) method was used to obtain the pore sizes from the desorption branches on the isotherms.

The morphology of the catalysts was characterised by transmission electron microscopy (TEM). The samples for TEM studies were prepared by ultrasonic dispersion of the catalysts in ethanol, and the suspensions were added dropwise onto a copper grid. The TEM investigations were carried out using a JEOL-JEM-100CX II (100 kV) transmission electron microscope equipped with a NARON energy-dispersive spectrometer and a germanium detector.

X-ray diffraction (XRD) studies were performed using a Philips PW 1710 spectrometer with monochromatic Cu-Kα radiation to determine the catalyst particle size and crystalline structures. The measurements were made on calcined catalysts and the average Co₃O₄ particle size was calculated from the most intense peak, with the use of Scherrer formula [6], for each catalyst.

The reduction behaviour and the interaction between the active phase and the support of each catalyst were examined using the temperature programmed reduction (TPR) technique. The TPR experiments were carried out with a thermal conductivity detector (TCD) to determine the hydrogen consumption. The catalyst (500 mg) was placed in a quartz tubular reactor fitted with a thermocouple for continuous temperature measurements. The reactor was heated with a furnaced designed for the TPR machine, at a ramping rate of 10 ˚C/min, under a mixture of 5 vol % H₂ in an air flow of 30 cm³/min.

2.3. Reduction and Reaction Procedures and Data Analysis

Three fixed bed reactors with the same size (ID = 8 mm) were used in this study. One gram of Co/SiO₂, Co/Al₂O₃, and Co/TiO₂ catalysts was loaded into the three respective reactors. The three catalysts followed the same activation procedures: to reduce in a flow of either H₂ or syngas (H₂/CO ratio of 2) at two different temperatures, namely, 250 °C and 350 °C, and atmosphere pressure. After catalyst reduction, the catalysts were cooled to 180 °C. Thereafter, the same syngas used for catalyst reduction was introduced for FTS. The catalyst reactivity and product distribution were evaluated at 20 bar, 210 °C, and 60 mL/min with syngas.

The tail gases from the three reactors were monitored and analysed by an online GC (Agilent 7890B): the hydrocarbon products were analysed by a flame ionisation detector (FID), whist the other gases (H₂, CO, N₂, and CO₂) were analysed by two TCDs.

3. Results

3.1. Characteristics

Table 1. Physical properties of the catalysts and reactor used in this work.

Catalyst	15% Co/SiO2	15% Co/Al2O3	15% Co/TiO2
Catalyst BET pore size (nm)	6.8	43.1	38.7
Catalyst BET surface area (nm)	407.0	115.8	88.4
TEM average particle size (nm)	26	38	28
XRD crystallise size (nm)	17.0	33.0	21.5
Reactor	Fixed bed reactor
Reactor diameter (mm)	0.8	0.8	0.8
Catalyst weight loaded into the reactor (g)	1	1	1

lists the physical properties of the catalysts. It shows that the Al₂O₃-supported catalyst had a larger pore size and a larger particle size than the catalysts supported on TiO₂ and SiO₂. For further details on the catalyst physical properties, please refer to our previous publications [27].

The crystal morphology of the catalysts is illustrated in Figure 1, determined via TEM. In the cross sections, the visible darker dense areas represent Co₃O₄ particles, and the lighter areas correspond to the support. The Co₃O₄ particles seemed to be more highly dispersed on the TiO₂-support followed by SiO₂-support then the Al₂O₃-support. This might have been due to the smaller TiO₂-support particles that are observed in Figure 1. The large Co₃O₄ particles observed on the alumina support suggest that the support had a wider pore, thus distributing bigger particles than TiO₂ and SiO₂. The Co₃O₄ particles on the Al₂O₃- and SiO₂-supports seemed to be spherical shape, whilst on the TiO₂, the cobalt particles assumed the shape of the TiO₂-support particles, in this case cubic/rhombus-shaped.

The XRD patterns for the model catalysts (Co/TiO₂, Co/SiO₂, Co/Al₂O₃) are presented in Figure 2. XRD characteristics of Co₃O₄ were detected for all the calcined catalysts with Co/TiO₂ and Co/Al₂O₃, showing distinctive Co₃O₄ crystalline features, marked with a black circle (see Figure 2). The Co/SiO₂ diffractogram showed considerably broad features, which suggests that the silica support is likely amorphous, and contains smaller Co₃O₄ nanoparticles. The average Co₃O₄ crystallite size (Table 1) was calculated from the Scherrer equation [6]. The Co₃O₄ crystallite size varied slightly as a function of the pore size, with Co/Al₂O₃ showing the biggest size (33.0 nm), owing to its large pore size, followed by Co/TiO₂ (21.5 nm) and then lastly Co/SiO₂ (17.0 nm).

The TPR reduction profiles presented in Figure 3 for the three kinds of catalysts showed two reduction peaks, which were similar to those observed for bulk Co₃O₄ oxide. These profiles point to a two-step reduction process: the first one of low intensity started at approximately 200 °C and overlapped with the more intense second peak whose maximum occurred at about 300 °C for Co/TiO₂ and Co/SiO₂ catalysts, and for Co/Al₂O₃, the peak started around 300 °C and the second peak emerged at around 450 °C. Other than the fact that the second reduction peak for the Al₂O₃-supported catalyst emerged at a higher temperature than the second peak on the TiO₂- or SiO₂-supported catalysts, it also extended its shoulder to a higher magnitude, in this case 700 °C. Therefore, the reduction process of Co₃O₄ can be described by the reduction of Co³⁺ ions present in the spinel structure of a fresh catalyst into Co²⁺ with subsequent structural change to CoO, followed by the reduction of Co²⁺ ions to Co⁰ metal. The results observed over the Al₂O₃-supported sample suggests that the catalyst supported on Al₂O₃ is harder to reduce than the one supported by TiO₂ or SiO₂, which may have been due to strong metal–support interactions, which is in line with the literature [11,12].

Furthermore, a higher reduction temperature was required for the reduction of the Al₂O₃-supported catalyst compared to TiO₂- and SiO₂-supported catalysts, as observed from of the reduction profiles (Figure 3), which might have been due to the fact that the cobalt particles diffused into the Al₂O₃ lattice and formed the irreducible compounds, such as aluminates.

3.2. Catalyst Activity and Selectivity

3.2.1. Reaction Rate

The Fischer–Tropsch (FT) activity and selectivity of the supported cobalt catalysts is illustrated in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10. The catalysts pre-treated in H₂ exhibited higher CO reaction rates compared to the samples reduced in syngas, at all reduction temperatures (see Figure 4). For the syngas pre-treatment, the CO reaction rates were found to be higher at a higher reduction temperature (350 °C) for all the samples. Similar results to the syngas pre-treatment were observed, where higher CO reaction rates were achieved at a higher reduction temperature for all the three catalysts reduced in H₂ (except for the SiO₂ catalyst reduced in H₂ at 250 °C).

The Co/SiO₂ catalyst with the highest surface area and lowest particle size (Table 1) was the most active catalyst when it reduced in H₂ at 250 °C (see Figure 4B). The high surface area of SiO₂-support and the lower metal–support interaction enhanced the reducibility and the dispersion of the metal. Our previous research [27] reported that the oxidised Co/SiO₂ catalyst, in H₂ at 250 °C, formed a multiphase of CoO–Co/SiO₂, and this CoO–Co interface promoted the CO dissociation and secondary olefin hydrogenation reactions, thus leading to a higher FT reaction rate [27].

Compared with SiO₂- and Al₂O₃-supported catalysts, the TiO₂-supported catalyst presented the highest CO reaction rates, when reduced either in H₂ or syngas at all temperatures except for the H₂-reduction at 250 °C. The lower surface area, observed via BET (Table 1) for the TiO₂-supported catalyst, promoted the agglomeration of the Co₃O₄ as larger Co₃O₄ particles, which might have increased their reducibility, and in consequence enhanced their catalytic activity. The low activity over the Al₂O₃-supported catalyst with the biggest Co₃O₄ particles (XRD: 33.0 nm in Table 1) must be related to the strong metal-support interaction (see Figure 3) and low metal dispersion due to the large cobalt particles (Table 1). For further discussion, please refer to Section 4.

The CO reaction rates were found to be higher at a higher reduction temperature (350 °C) for all the samples, except for the syngas pre-treated TiO₂-supported sample and H₂-pre-treated SiO₂-supported sample. The SiO₂-supported catalyst showed higher reaction rates at 250 °C compared to 350 °C (see Figure 4B). Our previous work over the SiO₂ sample demonstrated the effect of Co–CoO bonding promoting the FT reaction, thus leading to a higher FT reaction rate at a lower reduction temperature (250 °C), when the CoO density is higher than the density observed at 350 °C [27].

3.2.2. Product Formation Rate

The overall product formation rate as a function of temperature is illustrated in Figure 5 and Figure 6. Changing the reduction medium from H₂ to syngas led to a complete change in the formation of products. A noticeable effect was the lower CH₄ formation rate observed for all the samples treated with syngas compared to H₂, excluding the SiO₂-supported catalyst treated with syngas at 350 °C. Co/TiO₂ showed the lowest CH₄ formation rate followed by Co/SiO₂ and then Co/Al₂O₃, at all reduction temperatures (see Figure 5). Another observation is that the CH₄ formation rate was higher for the syngas-treated samples at 350 °C, whereas for the H₂-treated samples, higher CH₄ formation rates were observed at 250 °C, except for the catalyst supported on SiO₂.

For long chain hydrocarbons (C₅₊), higher formation rates were observed over the H₂-treated samples compared to the catalysts reduced in syngas (see Figure 6) due to lower CO reaction rates. Figure 6 shows that: (1) for H₂ reduction, higher C₅₊ formation rates were achieved at a higher reduction temperature for both TiO₂- and Al₂O₃-supported catalysts, while the higher C₅₊ formation rates were obtained at a lower reduction temperature for the SiO₂-supported catalysts; (2) for syngas reduction, higher C₅₊ formation rates were observed at a higher reduction temperature for both Al₂O₃- and SiO₂-supported catalysts, while there was only a slight difference between the C₅₊ formation rates at the reduction temperatures of 250 and 350 °C for the catalyst supported by TiO₂.

3.2.3. Product Selectivity

The effect of syngas or H₂ pre-treatment on the selectivity of the model cobalt catalysts as a function of temperatures is shown in Figure 7 and Figure 8. All syngas-treated samples showed better selectivity (low CH₄, high C₅₊) compared to H₂-reduced samples when reduced at a lower temperature, 250 °C. Increasing the syngas reduction temperature increased the CH₄ selectivity and decreased C₅₊ selectivity, whereas for the H₂-treated samples, an increase in the reduction temperature decreased the CH₄ selectivity and increased C₅₊ selectivity for all the three catalysts (Figure 8B). An increase in the reduction temperatures caused a slight increase in the selectivity of long chain hydrocarbons.

3.2.4. Paraffin to Olefin (P/O) Ratio

P/O ratio is a very important factor that reflects the selectivity of the paraffin (P) and olefin (O) products—a higher P/O ratio represents the products that are more paraffinic and a lower value indicates a higher selectivity to olefinic products. P_n/O_n represents the parafin to olefin ratio with carbon number n. In the current work, the ratios of P₂/O₂ (ethane/ethylene) and P₄/O₄ (butane/butene) are reported in Figure 9 for the catalysts either reduced by syngas or H₂ at different reduction temperatures. For the Co/TiO₂ catalyst, pre-treatment with syngas (at both 250 and 350 °C) produced more olefins than paraffins compared to the H₂ pre-treatment (low P/O ratios). For Co/SiO₂, similar results as for the Co/TiO₂ catalyst—lower P/O ratios were obtained for syngas-reduced catalysts compared to H₂ reduction at 250 °C. However, syngas pre-treatment at 350 °C produced more paraffin products over the Co/Al₂O₃ catalyst compared to H₂ pre-treatment at a similar reduction temperature.

4. Discussion and Implications

With the aim to understand the reaction pathways observed with different pre-treatment agents at different reduction temperatures, we replotted some of the data reported in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 in order to highlight the important findings of this work—the results are shown in Figure 10. The syngas-treated catalysts afforded a lower CO reaction rate compared to H₂-treated samples (Figure 10A). This could be attributed to the lower Co site density caused by incomplete reduction of Co₃O₄ to metallic Co⁰. Metallic Co⁰ is known to be the active phase for the conversion of syngas to hydrocarbon products [20]; therefore, the lower the Co⁰ density, the lower the CO hydrogenation activity. Our findings are in line with Gnanamani et al. [28] who reported that cobalt catalysts do not reduce completely under syngas treatment.

Catalyst pre-treatment is a way to transform cobalt oxides to active sites. For H₂ reduction, cobalt oxides are reduced to metallic Co⁰; in the meantime, there is still some cobalt oxides (Co₃O₄ and/or CoO) left due to partial reduction depending on the reduction temperature or the extent of metal–support interactions. For syngas (a mixture of H₂/CO) reduction, the presence of CO in the mixture can also react with the cobalt oxides to form cobalt carbides (Co_xC), which has been confirmed by Peacock et al. [29] and Claeys et al. [30] using the in situ magnetometer. In addition, the Boudouard reaction (2CO = CO₂+ C) may occur when the operating temperature is high. Figure 11 lists the possible cobalt phases after cobalt catalyst reduction under different atmospheres.

From our experimental data (see Figure 10B–D), with syngas reduction at 250 °C, all the three catalysts had a much lower CH₄ selectivity and lower P/O ratio compared with the catalysts treated with H₂ at 250 °C, which indicates that the reactions of CO hydrogenation to paraffins were suppressed during the low temperature syngas reduction. These experimental results may provide evidence that the Co₂C phase promotes the formation of olefins by suppressing the olefin hydrogenation reaction. Furthermore, the existence of the metallic Co-hcp phase, obtained from further reduction of cobalt via the Co_xC intermediate, could catalyse the FT chain growth reaction by converting syngas to light olefins, which in turn react to form longer chain hydrocarbons for syngas-treated catalysts reduced at 250 °C. This reaction path is not the dominant mechanism for H₂-reduced catalysts at these reduction temperatures. On the basis of our results, we hypothesised that cobalt in association with the cobalt carbides enhances the production of higher hydrocarbons. Jiao et al. [31] and Gnanamani et al. [28] both supported this hypothesis in that they both suggested that Co₂C contributes to the selectivity of light olefin products and alcohol formation via a CO insertion mechanism. In addition, Jalama et al. [18] reported a higher olefin to paraffin ratio for the samples pre-treated in syngas than in H₂, which was attributed to the presence of the Co₂C phase.

The large shift in product selectivity by changing the activation conditions (as shown in Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9) led us to believe that the method of activation of cobalt as well as the corresponding temperature play vital roles in the subsequent hydrogenation of CO. A higher reduction temperature was found to increase the selectivity of C₁ in detriment to all other hydrocarbons (Figure 10), when syngas was used as a reducing agent. When reducing at 350 °C, the Boudouard reaction may occur, and the carbonaceous deposits on the surface could act as methanation sites and decrease the number of Co active sites available for FTS. Findings over these catalysts suggest that carburisation and carbon deposits are feasible under syngas reduction, and that this may cause an increase in both the C₅₊ selectivity or CH₄ selectivity depending on the amount of surface carbon available. Our findings are in line with Lee et al. [32], who reported that surface carbonaceous deposits can exist in two forms, namely, active carbon or graphitic carbon, and that the active carbon can hydrogenate to methane under normal FT conditions.

In the case of H₂ pre-treated samples, an increase in the reduction temperature from 250 to 350 °C led to the production of higher hydrocarbons. These results demonstrate that C₅₊ hydrocarbon formation is a function of temperature and that a high reduction temperature is associated with a higher reducibility of Co₃O₄ to Co⁰; therefore, it can be deduced that Co⁰ is selective to the production of C₅₊ hydrocarbons.

This study also reflects on the effect of support properties on the performance of cobalt catalysts under different pre-treatment conditions. The catalyst supported on TiO₂ exhibited the highest selectivity towards liquid products with the lowest CH₄ selectivity when treated in both H₂ and syngas. This can be attributed to the higher Co site density observed via TEM (Figure 1A) and a higher reducibility, as established by the TPR profile in Figure 3, due to weaker metal interactions. On the other hand, Al₂O₃ showed the least activity (Figure 4) when both syngas and H₂ were used as a pre-treatment feed, due to (1) the strong metal–support interaction, resulting in lower reducibility, as observed via TPR, and (2) the lower metal dispersion, observed via TEM, caused by the larger Co₃O₄ particles that formed in the large alumina pores, observed via TEM, XRD, and BET (see Table 1). In the case of the SiO₂-support, the TPR reduction profile resembled that of the TiO₂ support, suggesting that the SiO₂ support is also weakly bonded to the Co metal. However, the Co/SiO₂ catalyst showed a different reactivity to that of TiO₂, which may have been due to a large surface area and a lower metal dispersion than the catalyst supported on TiO₂ (see Figure 1C and Table 1).

The support identity therefore plays a major role in the performance of the catalyst. Two parameters seem to determine the catalytic activity of the Co₃O₄ nanoparticles: (1) the Co particle size, which is influenced by the structure of the support, and (2) the extent of the metal–support interaction, metal–metal oxide interaction, and the metal–metal carbite interaction, which determines the specific nature of the active sites and their intrinsic catalytic activity.

5. Conclusions

The purpose of this study is to show the advantages of using syngas as a reducing agent for Co-FTS catalysts. To this end, we demonstrated the effect of the pre-treatment conditions by comparing the activity and product selectivity of the catalysts treated with syngas or H₂ at different temperatures. A lower CH₄ selectivity, higher C₅₊ selectivity, and lower P/O ratio were observed for the catalysts treated with syngas at 250 °C compared to the catalysts reduced either with H₂- at 250 °C or syngas at 350 °C. The formation of the Co_xC phase during the reduction in syngas may either: (1) act as an active site for the production of lower olefins or (2) suppress the hydrogenation reaction. On the basis of the experimental results, we hypothesised that there may be synergy between Co⁰ and Co_xC to convert CO and H₂ to long chain hydrocarbons.