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Article

Revisiting the Impact of CO2 on the Activity and Selectivity of Cobalt-Based Catalysts for Fischer–Tropsch Synthesis Under Industrial-Relevant Conditions

by
Zhiyu Chen
1,
Jinbo Du
1,
Denghui Chen
1,
Fuqing Gong
1,
Yang Gao
1,
Zhen Huang
1,
De Chen
2 and
Jia Yang
1,2,*
1
College of Smart Energy, Shanghai Jiao Tong University, No. 665, Jianchuan Road, Shanghai 200240, China
2
Department of Chemical Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(4), 329; https://doi.org/10.3390/catal15040329
Submission received: 2 March 2025 / Revised: 16 March 2025 / Accepted: 27 March 2025 / Published: 31 March 2025
(This article belongs to the Section Catalysis for Sustainable Energy)
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Abstract

:
Understanding the impact of CO2 on cobalt-based Fischer–Tropsch synthesis catalysts is critical for optimizing system efficiency, particularly in scenarios employing solid oxide electrolysis cells for syngas production, given the inevitable incorporation of CO2 into syngas during the SOEC co-electrolysis process. In this study, we conducted comparative experiments using a Co-Re/γ-Al2O3 catalyst in a fixed-bed reactor under industrial conditions (2 MPa, 493 K, GHSV = 6000–8000 Ncm3/gcat/h), varying the feed gas compositions of H2, CO, CO2, and Ar. At an H2/CO ratio of 2, the addition of CO2 led to a progressive decline in catalyst performance, attributed to carbon deposition and cobalt carbide formation, as confirmed by Raman spectroscopy, XRD analyses, and TPH. Furthermore, DFT calculations combined with ab initio atomistic thermodynamics (AIAT) were performed to gain molecular insights into the loss of catalyst activity arising from multiple factors, including (sub)surface carbon derived from CO or CO2, polymeric carbon, and carbide formation.

Graphical Abstract

1. Introduction

In recent years, with the rise of CO2 level in atmosphere, the demand for achieving zero greenhouse gas emissions in all human activities has become increasingly urgent. One of the most pressing areas for action is the transportation sector. According to estimates by the International Energy Agency, the aviation industry contributes about 2.8% of global carbon dioxide emissions [1]. “Power-to-Liquid” (P-t-L) is a technology that converts electrical energy, captured CO2, and H2 produced from water electrolysis into liquid fuels (eFuels or electrofuels). One production process of e-fuels utilizes electricity generated from renewable energy sources (such as wind, solar, or nuclear power) to co-electrolyze water and carbon dioxide through solid oxide electrolysis cells (SOECs) [2,3]. This process directly produces syngas, a mixture of hydrogen and carbon monoxide, thereby reducing system complexity and improving overall energy efficiency. The syngas is then catalytically converted into liquid energy carriers through Fischer–Tropsch synthesis, which can be upgraded and used as e-diesel, e-gasoline, and e-kerosene. These liquid fuels can be used as drop-in fuels in transportation sectors (such as aviation, shipping, and automotive), replacing traditional liquid or gaseous fuels [4]. With the rise of distributed renewable energy sources such as wind and solar power, SOEC technology for syngas production is gaining increasing attention [2].
However, co-electrolysis processes cannot achieve complete conversion of CO2, resulting in residual CO2 in the syngas. The subsequent separation of CO2 from the Fischer–Tropsch syngas (CO and H2) necessitates additional equipment, which decreases the economic viability and overall energy efficiency of the system especially for small scale distributed system. Direct conversion of raw syngas from SOECs without removing CO2 is highly desirable. However, existing studies on the impact of CO2 on cobalt-based Fischer–Tropsch catalysts remain controversial. Table 1 summarizes the effects of CO2 on cobalt-based Fischer–Tropsch catalysts, classifying these effects into three main categories: (1) Catalyst Deactivation due to CO2, (2) CO2 Acting as an Inert Gas, and (3) CO2 Promoting a Shift Toward Methanation in Syngas.
Catalyst Deactivation by CO2: Kim et al. [5] suggested that CO2 in the feed gas can act as a mild oxidant, leading to partial oxidation of the cobalt metal surface, thereby reducing catalytic activity and selectivity towards C5+ products. Park et al. [6] conducted a 1000-h hydrogenation experiment with a CO2 and CO mixture in a slurry-phase continuous stirred tank reactor and observed catalyst deactivation. The observed deactivation was attributed to the deposition of polymeric carbon on the catalyst surface and the aggregation of catalysts into larger particles. At lower reaction temperatures (463 K), Riedel et al. [7] reported that CO2 negatively impacted Co/SiO2 catalysts prepared via co-precipitation, while other catalysts remained unaffected.
CO2 as an Inert Gas: When the H2/CO ratio in the feed gas is high (>4), Visconti et al. [8] considered CO2 to be an inert component. The authors observed, using FT-IR, that in the presence of both CO and CO2, CO2 hardly undergoes hydrogenation, which was attributed to competitive adsorption effects. Additionally, Riedel et al. [7] found that at 463 K, CO2 had no significant impact on three out of four catalysts studied.
Shift Towards Methanation by CO2 in Syngas: As the CO content in the feed gas gradually decreases while CO2 concentration increases, both Bredy et al. [9] and Riedel et al. [10] observed a shift from Fischer–Tropsch synthesis towards methanation over cobalt catalysts. Bredy et al. [9], using DRIFTS, found that the structure and oxidation state of the metallic cobalt surface remained unchanged during the reaction, attributing this transition primarily to surface coverage effects. Zhang et al. [11] studied the hydrogenation of mixed CO2 and CO gases over cobalt-silica catalysts. They found that CO was converted more rapidly than CO2, with CO accounting for over 90% of the total carbon oxide conversion. Chakrabart et al. [12] utilized 14CO2 to trace the conversion and selectivity of CO2 during the hydrogenation of CO2 and CO mixtures. Their findings indicated that 14CO2 was indeed converted. The only products derived from 14CO2 were C1-C3 hydrocarbons, with methane being the predominant product.
Overall, the impact of CO2 in syngas on the catalytic performance of cobalt-based Fischer–Tropsch catalysts remains a topic of debate. Current literature suggests that the effect of CO2 is highly dependent on process conditions, particularly the composition of the syngas. Moreover, in studies that argue CO2 negatively impacts cobalt-based Fischer–Tropsch catalysts, there is limited exploration of how the catalyst’s performance changes upon the introduction or removal of CO2, as well as a lack of direct comparisons with conditions where CO2 is absent. Additionally, the regeneration of the catalyst after CO2-induced deactivation has not been investigated. Therefore, further experimental studies are needed to elucidate the extent of CO2 effects and to determine the catalysts’ tolerance range. This study aims to address these knowledge gaps. Specifically, we examine the effects of CO2 on the activity and selectivity of the representative Fischer–Tropsch catalyst Co-Re/γ-Al2O3 under industrially relevant conditions (2 MPa, 493 K, XCO = 30–70%) with varying ratios of H2, CO, and CO2. For the Co-Re/γ-Al2O3 catalyst, cobalt acts as the active center, directly catalyzing CO hydrogenation to generate hydrocarbons, while a small amount of the noble metal Re enhances the reducibility of the cobalt catalyst.
Table 2 provides details of three experimental groups, each with different syngas compositions at various stages, to assess the influence of feed gas conditions on catalytic performance. In brief, Experiment 1 involved switching high concentrations of CO2 (12.5–25 mol%) on and off in Co-Re/γ-Al2O3 catalysts with 12% cobalt loading, directly comparing the catalyst performance before and after the introduction and removal of CO2. Experiment 2 compared feed gases with and without 25 mol% CO2 in two independent runs, offering a clear comparison of the effects of CO2 under similar initial conversion rates. Experiment 3 studied the effect of a low concentration of CO2 (6 mol%) by alternating the CO2 feed and investigating catalyst regeneration through hydrogen re-reduction.
Furthermore, while current research offers limited insights into deactivation mechanisms, our experiments employ DFT calculations combined with ab initio atomistic thermodynamics (AIAT) to provide molecular insights into the loss of catalyst activity.

2. Results

2.1. Experiment 1: CO Hydrogenation Performance with Increasing CO2 Concentration (12.5% and 25%)

By introducing different concentrations of CO2 (12.5 mol%, and 25 mol%) into the syngas, we investigated the effects of CO2 concentration on the activity and selectivity of a rhenium-promoted cobalt Fischer–Tropsch synthesis catalyst. 12CoRe/Al catalyst was tested for FTS at 493 K and 21 bar. The reaction was divided into four periods (Table 2), differing only in the feed gas composition during each period. In all periods, the partial pressures of H2 and CO in the feed gas remained constant.
During the initial phase of the reaction, the space velocity was gradually reduced to achieve a CO conversion rate of 55%, which was then maintained steadily for a period. As shown in Figure 1, when the feed gas composition was H2:CO:CO2:Ar = 2:1:0:1 (Period 1), both the product conversion rate and selectivity remained stable, with a C5+ selectivity of 83%, and the product distribution exhibited a typical Fischer–Tropsch pattern.
When the feed gas composition was changed to H2:CO:CO2:Ar = 2:1:0.5:0.5 (Period 2), the introduction of CO2 began to cause gradual catalyst deactivation. The CO conversion decreased from 57.6% to 51.3% over 30 h (Figure 1a), with a deactivation rate of −0.21%/h. The C5+ selectivity decreased from 83% to 80.6% (Figure 1c). Specifically, the selectivity toward C1-C4 hydrocarbons increased slightly with methane selectivity rising from 9% to 10% (Figure 1b). Additionally, the olefin-to-paraffin ratio for C3-C4 hydrocarbons gradually decreased, with the C4 olefin-to-paraffin ratio dropping from 0.74 to 0.69 over 40 h (Table S1).
When the feed gas composition was further changed to H2:CO:CO2:Ar = 2:1:1:0 (Period 3), the higher concentration of CO2 in the feed gas accelerated catalyst deactivation. Over 30 h, the CO conversion rate sharply decreased from 51.3% to 27.6%, with a deactivation rate of −0.86% per hour (Figure 1a). Accompanying this rapid decline in CO conversion, the C5+ selectivity decreased from 80% to 71% (Figure 1c), and the selectivity for C1-C4 paraffins significantly increased, particularly methane, whose selectivity rose from 10% in Period 2 to 16.6% (Figure 1b). Moreover, the olefin-to-paraffin ratio for C3-C4 hydrocarbons exhibited a more pronounced decrease compared to Period 2, with the C4 olefin-to-paraffin ratio dropping from 0.69 to 0.37 over 30 h (Figure 1d).
In Period 4, when all CO2 in the feed gas was replaced back with Ar, the residual CO2 in the reactor was completely purged within 10 h, restoring the feed gas composition to that of Period 1 (H2:CO:CO2:Ar = 2:1:0:1). The residual CO2 in the system was completely purged after 10 h, and the reaction conditions were identical to those in Period 1. As the CO2 concentration gradually decreased, the rate of catalyst deactivation markedly slowed. After all CO2 was purged, the CO conversion rate no longer decreased, and the product selectivity stabilized, with methane selectivity maintaining at 17% (Figure 1a).
Overall, Experiment 1 demonstrated that introducing 12.5 and 25 mol% CO2 into the syngas resulted in continuous deactivation of the CoRe/γ-Al2O3 catalysts, leading to a shift in product distribution towards the formation of lower-chain paraffins. Upon removal of CO2, the catalyst’s activity and selectivity did not recover but instead stabilized, maintaining high selectivity for C1–C4 paraffins, consistent with the conditions under high CO2 concentration in Period 3.

2.2. Experiment 2: Comparative Fischer–Tropsch Synthesis with and Without CO2 Addition in Syngas (25 mol%)

In Experiment 2, a higher loading CoReAl catalyst (20 wt%) was used to investigate the effect of CO2 addition in syngas on catalyst stability. Two distinct conditions were tested: Experiment 2a, where the catalyst was exposed to syngas without CO2 for a 600-h stability test, and Experiment 2b, where the syngas contained a fixed CO2 concentration of 25%. The feed gas compositions for each stage are listed in Table 2. These experiments were designed to compare the catalyst performance under syngas with and without CO2. Additionally, Experiment 2b was compared with Experiment 1 to evaluate the impact of CO2 on cobalt-based catalysts with different loadings. Both experiments used the same initial heating conditions (443–463 K at 1 K/min; 463–493 K at 12 K/h). At comparable conversion levels, the partial pressures of water in the two reactions were similar, providing a more direct comparison of the effects of CO2 on cobalt catalysts with different loadings.
Figure 2 shows the variations of CO conversion and CH4 selectivity with the reaction time in Experiment 2a (syngas) and Experiment 2b (CO2). In Experiment 2a (syngas) (Figure 2a,b), the catalyst’s activity and selectivity remained relatively stable throughout the entire 600-h stability test, with no significant deactivation observed. Similar to Experiment 1 (increasing CO2) with the 12CoRe/Al catalyst, Experiment 2b for 20CoReAl exhibited a cumulative and sustained impact of CO2 in the feed gas on the reaction. Notably, as the catalyst gradually deactivated, a sharp drop in activity was observed at approximately 280 h in Experiment 2b, characterized by a rapid decline in CO conversion and a sharp increase in methane selectivity (Figure 2c,d). Following the sharp drop, as the catalyst activity decreased to a certain level, the deactivation rate slowed down. Another observation is that the sharp drop of conversion occurs at an earlier time on stream for 12CoRe/Al catalysts as compared with 20CoRe/Al. This is possibly due to the fact that the total number of cobalt surface sites are less on 12CoRe/Al since the test experiment was performed using the same amount of catalyst instead of the same amount of cobalt.
Overall, Experiment 2a and 2b compared the effects of the presence or absence of CO2 in syngas under similar initial CO conversion conditions. The results indicate that CO2 leads to sustained catalyst deactivation, with a cumulative impact on catalyst selectivity that becomes more pronounced after reaching a certain level of accumulation.

2.3. Experiment 3: CO Hydrogenation with Switching Between Ar and Low CO2 Concentration (6 mol%) and Catalyst Regeneration

Experiment 3 was conducted using a catalyst that had undergone FT reaction followed by a subsequent reduction process. The experiment consisted of four stages (Table 2), comparing the effects of introducing Ar and CO2 into the syngas feed and exploring the regeneration of the catalyst. Based on the reaction data (Figure 3), the following observations were made:
Period 1 (H2:CO:CO2:Ar = 2:1:0:0.2): The CO conversion stabilized around 32%. Subsequently, we replaced all the Ar in the feed gas with an equivalent amount of lower concentration of CO2 6 (mol%), keeping syngas partial pressure constant. As CO2 gradually replaced Ar in the feed gas during Period 2 (H2:CO:CO2:Ar = 2:1:0.2:0), the catalyst began to deactivate. Over a span of 30 h, the CO conversion rate decreased from 32% to 24%, representing a decline of 31%. Concurrently, CH4 selectivity increased, C5+ selectivity decreased, and the olefin-to-paraffin ratio for C4 rapidly decreased.
Subsequently, in Period 3 (H2:CO:CO2:Ar = 2:1:0:0.2), we reintroduced an equivalent amount of Ar to replace all CO2 in the feed gas. Approximately 10 h after ceasing CO2 addition, the residual CO2 in the reactor was completely purged by Ar. As CO2 was gradually replaced, the catalyst activity and selectivity began to stabilize; however, they did not fully recover to the levels observed in Period 1 (without CO2), which is consistent with the phenomena observed in Experiment 1.
Subsequently, we re-reduced the catalyst under H2 at 623 K for 16 h before resume the reaction. The re-reduced catalyst activity briefly recovered to initial activity as in period 1, although it subsequently deactivated. In contrast, the catalyst’s selectivity recovered to the initial level as observed in period 1 and remained stable. This behavior, in which the regenerated catalyst exhibited gradual deactivation while maintaining relatively stable selectivity, has also been observed in experiments where cobalt carbide was formed via CO pre-treatment [13].
In Experiment 3, we compared the effects of introducing equal amounts of CO2 and Ar into the syngas feed on the reaction. The data indicate that CO2 impacts both the activity and selectivity of the catalyst. This observation differs from the findings of Visconti et al. [8], who concluded that CO2 acts as an inert gas with no impact on the catalyst. This discrepancy may be attributed to the higher H2/CO ratio of 4.9 used in their experiments (Table 1).

3. Discussion

ICP, XRD, and BET analyses were performed on the calcined and dewaxed catalysts after the reaction. The metal loadings measured by ICP were close to the expected nominal values (Table 3).
Table 3 summarizes the BET surface area, average pore diameter, and pore volume of the 20% Co-0.5% Re/γ-Al2O3 catalysts from Experiments 2a (syngas) and 2b (CO2), both calcined and after reaction. The fresh catalyst had a BET surface area of 130 m²/g, a pore volume of 0.31 cm3/g, and an average pore diameter of 9.6 nm. For the tested catalysts (reacted under conditions without CO2 and with CO2), the formation of wax and carbon deposition led to decreases in surface area, pore volume, and average pore diameter. The addition of CO2 resulted in a more pronounced reduction in the catalyst’s surface area, decreasing by approximately 40%, along with a 32% decrease in pore volume. This reduction may be attributed to increased carbon deposition, as indicated by Raman spectroscopy and TPR-H2, which will be discussed in detail later. Carbon deposition can block the catalyst pores, leading to diffusion limitations [15,16], or irreversibly block the metal surface, affecting the adsorption of neighboring species (such as CO), thereby influencing the CO dissociation rate, and even encapsulating metal particles [16,17,18].
It is also noteworthy that the effect of CO2 on the catalyst is significantly related to the reaction temperature [19,20]. At lower temperatures (463 K), Riedel et al. observed that CO2 does not have a significant impact on the activity and selectivity of most cobalt-based catalysts [7]. This may be attributed to the reduced rate of carbon formation at lower reaction temperatures [19,20]. In Experiment 1 (increasing CO2) and Experiment 2b (CO2), where the bed temperature stabilized at 493 K, the D2 Raman band at approximately 1510 cm−1 was observed, corresponding to amorphous carbon (Figure 4) [21]. In Experiment 2b (CO2), the presence of the D1 band around 1300 cm−1, which is indicative of disorder in graphene, and the G band at 1610 cm−1, which can be attributed to graphitic carbon with E2g symmetry, were both observed. These findings correlate with the observed catalyst deactivation, as such carbon species can encapsulate cobalt active sites, thereby reducing CO hydrogenation activity and altering product selectivity. In contrast, in Experiment 2a (syngas), no carbon-related peaks were detected in the Raman spectra. Similarly, no carbon-related peaks were observed in Experiments 3, which may be attributed to the re-reduction of the catalysts.
Moreover, the carbon formed on the catalyst surface can diffuse into the cobalt metal, leading to the formation of cobalt carbide [22]. To gain more information on which types of carbon was deposited during CO2 cofeeding experiment. The spent catalyst from Experiment 2b was wax extracted and tested for temperature programmed hydrogenation and monitor the evolution of methane signal. A cobalt carbide reference catalyst was used for comparison. Figure 5 presents the methane evolution curves of pre-carbonized catalysts (CO-CoReAl) and spent-dewaxed CoReAl from Experiment 2b using TPH. For the CO-20CoReAl catalyst (Figure 5a), which contained no wax, the peak observed at approximately 453 K corresponds to cobalt carbide, consistent with methane peaks reported in the literature for cobalt carbide prepared through prolonged CO treatment [23]. A second peak at approximately 698 K corresponds to polymeric carbon species [24]. In the case of the Experiment 2b-20CoReAl catalyst (Figure 5b), the methane signals revealed hydrogenation of carbon species at approximately 513 K, 613 K, and 743 K. The first peak (513 K) can be partially attributed to surface carbon species, residual wax, or cobalt carbide [23]. Based on XRD results (Figure 6), we believe this peak is likely due to cobalt carbide. The intermediate species (613 K, peak 2) are likely smaller polymeric carbon chains, while the high-temperature peak (743 K, peak 3) corresponds to polymeric carbon species [24].
The addition of CO2 facilitates the carburization of cobalt, resulting in the formation of cobalt carbide (Co2C). Figure 6 presents the XRD patterns of the Fischer–Tropsch catalysts tested in the experiments. Upon removal from the reactor, the catalysts were exposed to air, leading to oxidation of metallic phase into cobalt oxides (Co3O4 and CoO). The peaks observed at 37.0°, 41.3°, 42.5°, 45.7°, and 56.6° in Experiment 1 (increasing CO2/12CoReAl) and Experiment 2b (CO2/20CoReAl) can be attributed to the crystal planes of cobalt carbide (Co2C) (PDF#04-004-4639) [19,25,26,27]. The peak at 21.3° is attributed to high-carbon number hydrocarbon compounds, as the catalysts in those experiments were not subjected to dewaxing. The peaks corresponding Co2C were not observed in Experiment 2a (syngas). XRD analysis (Figure 6) confirms the presence of Co2C in experiments with CO2 exposure, while it is absent in syngas-only conditions. The transformation of metallic cobalt to CO2C, induced by CO2, may be a reason for catalyst deactivation [28,29]. Cobalt carbide is generally considered inactive in FTS catalysis. It exhibits lower activity under FT reaction conditions, with decreased C5+ selectivity in the products, increased selectivity toward gaseous paraffins (mainly methane), and lower olefin-to-paraffin ratios for C2–C4 hydrocarbons [19,30,31]. The activity and selectivity characteristics observed in the experiments 1 (increasing CO2), 2b (CO2), and 3 where CO2 was added to the feed gas are consistent with CO2C activity and selectivity in FTS. Previous studies have reported the presence of CO2C in cobalt-based FTS under typical Fischer–Tropsch reaction conditions (493–553 K, 3–20 bar, H2/CO ratio of 1–4), refs. [32,33,34,35,36,37,38] in which cobalt is slowly and steadily converted into CO2C [35,38]. The CO2 in the feed gas may accelerate this process.
Furthermore, in experiments 1, 2b and 3, a pronounced phenomenon of sharp drops in catalyst activity and selectivity was observed. The characteristics of these sharp drops are summarized in the Table 4. In the 20CoRe/Al system, these sharp drops typically occurred when the CO conversion reached approximately 30%. These sharp drops were characterized by a rapid decline in CO conversion, accompanied by a significant increase in methane selectivity.
Density functional theory (DFT) calculations combined with ab initio atomistic thermodynamics (AIAT) were performed to gain molecular insights into the loss of catalyst activity arising from multiple factors, including (sub)surface carbon derived from CO or CO2, polymeric carbon, and carbide formation. Computational methods, details, and several simplifications—based on both previous and current experimental observations—are provided in the Supporting Information. Table S3 summarizes the chemical states of carbon species in the gaseous phase and on the catalyst surface at different stages of the present FT reactions. The results show that gaseous CO and CO2 have higher chemical potentials (approximately −7.90 to −8.11 eV and −8.36 to −8.68 eV, respectively) than alkanes and adsorbed carbon atoms, implying that both CO and CO2 readily decompose into active carbon species (C) from a thermodynamic perspective. This prediction is consistent with our experimental results and with the observations of Park et al. [6], who reported that introducing CO2 into the feed gas promotes carbon deposition on cobalt-based F–T catalysts. The subsequent transformation of these C species is influenced by the specific surface involved. On the (111) surface, C* atoms tend to diffuse into the Co subsurface due to their lower chemical potential, initiating a carburization process that yields cobalt carbide, consistent with the previous literature. For example, Tan et al. [39] found that on Co(111), a surface carbide species is more stable than a surface CH2 intermediate, indicating a strong thermodynamic driving force for carburization on this facet. In contrast, carbon diffusion to subsurface is energetically unfavorable on the (100) and (311) surfaces, indicating that the formed carbon atoms are more prone to aggregate into carbon oligomers, polymeric carbon, or graphite-like carbon, all of which can contribute significantly to catalyst deactivation. Notably, polymeric carbon exhibits much lower chemical potentials (−9.45 to −9.48 eV) than Co2C (−8.90 eV) on the three examined surfaces. This finding is in agreement with literature data [39,40,41]. Swart et al. [41] found that the graphene-covered Co(111) is the most stable system. Likewise, Tan et al. [39] reported that extended (or infinite) graphene sheets over Co(111) were found to be more stable than the clock-reconstructed Co(111) surface. Valero et al. [39] also demonstrated that at high carbon coverages, adding additional carbon to subsurface sites becomes less favorable because it would sacrifice the formation of strong C=C bonds in surface polymeric carbon species. However, the precursors of polymeric carbon—such as isolated six-membered rings, dimers, and trimers—are relatively difficult to form, requiring an additional energy of approximately 0.87 to 1.20 eV, similar to the nucleation barriers observed in the synthesis of zeolites and metal–organic frameworks (MOFs). In other words, both the formation of cobalt carbide and carbon deposition may occur concurrently during the F–T reaction, as supported by Raman (Figure 4), TPH measurements (Figure 5) and XRD (Figure 6). Saeys et al. [39] also reported the coexistence of Co2C and amorphous polycyclic aromatic carbon deposits in spent cobalt-based F–T catalysts.
In view of these experimental and computational findings, we propose a general evolution pathway for Co-based catalysts during F–T reactions. Figure 7, constructed using CO conversion data from Experiment 2b, illustrates the relationship between cobalt surface carbon deposition and CO conversion over time. During normal Fischer–Tropsch synthesis at stage 1 (involving only syngas and Ar), the CO can dissociate to carbidic carbon and atomic oxygen, the carbidic carbon can further participate in the subsequent reactions, such as hydrogenation to form methane or chain growth to produce higher hydrocarbons. From Stage 2 to Stage 3, replacing Ar with CO2 in the syngas increases the availability of carbon in the gas phase, thereby raising the concentration of surface carbon on the catalyst. At this point, adsorbed carbon atoms can diffuse into the subsurface and bulk of the Co catalyst, leading to the formation of cobalt carbide. However, according to DFT calculations, the diffusion of surface carbon into the subsurface to form cobalt carbide is energetically less favorable. Therefore, carbide formation is considered a minor effect in the overall deactivation mechanism. Meanwhile, a nucleation-like process occurs on other Co surfaces, where a portion of the surface carbon begins to accumulate and gradually forms cyclic structures, giving rise to polymeric carbon precursors. During this interval, catalytic activity declines slowly because only a limited fraction of Co active sites becomes blocked. As the surface carbon continued to form cyclic and polycyclic structures, these ring-like structures can act as nucleation sites, facilitating the rapid growth of carbon deposits on the cobalt surface. The formation of these nuclei marked the onset of the “sharp drop” phenomenon, corresponding to position 3 in the Figure 7. Once enough carbon nuclei are formed, polymeric carbon accumulates rapidly and blocks most available sites, causing a sharp drop in activity. As CO conversion declined, the partial pressures of CO and H2 increased, while the partial pressure of H2O decreased. This shift in reaction conditions elevated the chemical potential of carbon in the system, accelerating the diffusion of surface carbon into the subsurface. This also enhanced carbon diffusion further exacerbated catalyst deactivation. However, the cobalt (111) crystal plane, due to its structural characteristics, was less affected by carbon deposition and maintained a relatively low level of activity. Cobalt (111) facet exhibits relatively weaker C–C coupling ability, resulting in a stronger tendency to catalyze the formation of short-chain gaseous alkanes, particularly methane, as well as a lower probability of carbon chain growth in its products. Consequently, carbon atoms preferentially react with hydrogen, leading to the production of methane. As the surface carbon coverage approached equilibrium, the rate of catalyst deactivation slowed down.
Upon removal of CO2 from the feed gas in Experiments 1 and 3 (Figure 1 and Figure 3), the catalyst activity and selectivity ceased to deteriorate but did not recover, maintaining a high selectivity toward gaseous paraffins. This can be attributed to the stability of carbon deposits and cobalt carbide under Fischer–Tropsch synthesis conditions [42,43]. Upon re-reduction of the deactivated catalyst with pure hydrogen (200 mL/min, 623 K, 16 h—Experiment 3) (Figure 3), both catalyst activity and selectivity were partially restored. This partial recovery is likely due to the hydrogenation of cobalt carbide [19,44] and the removal of part of carbon deposits, as evidenced by TPH analysis Figure 5.

4. Materials and Methods

4.1. Catalyst Preparation

Two catalysts with different loadings, 20 wt% Co-0.5 wt% Re/γ-Al2O3 and 12 wt% Co-0.5 wt% Re/γ-Al2O3, were prepared using a multi-step incipient wetness impregnation method. In the SOECs-FT coupled system for electronic fuel (e-fuel) synthesis, the selection of the Co-Re/γ-Al2O3 catalyst is primarily attributed to the synergistic effect between cobalt (Co) and rhenium (Re) and their excellent compatibility with the γ-Al2O3 support. Cobalt (Co) serves as the active center, directly catalyzing the hydrogenation of CO to produce hydrocarbon products. Its catalytic performance is highly dependent on its high dispersion (i.e., exposing more active sites) and high reducibility (i.e., the efficiency of CoO → Co0 transformation). The introduction of rhenium (Re) significantly enhances the reaction rate of Co-based catalysts supported on γ-Al2O3, mainly due to its ability to improve the reducibility of cobalt oxides and enhance cobalt dispersion [45,46,47,48,49]. Re facilitates hydrogen spillover, transferring active hydrogen species to the surface of cobalt oxides, thereby increasing the dispersion of active metallic Co [49]. Furthermore, in the γ-Al2O3 system, Re improves the selectivity of long-chain hydrocarbons [50,51] For example, Schanke et al. [51] reported that the introduction of Re into Co-based catalysts supported on γ-Al2O3 increased the C5+ selectivity by approximately 15%, which is crucial for the synthesis of liquid electronic fuels (e-fuels).
The 20 wt% cobalt catalyst was prepared by impregnating the support twice with an aqueous solution of Co(NO3)2·6H2O, while the 12 wt% cobalt catalyst was achieved through a single impregnation. After the addition of cobalt, the catalyst precursors were dried in an oven at 383 K for 3 h and then calcined in an air atmosphere (100 mL/min) by heating to 573 K at a rate of 2 K/min and holding for 16 h. Subsequently, an aqueous solution of HReO4 was added via incipient wetness impregnation to achieve a Re loading of 0.5 wt%. The prepared catalysts were again dried in an oven at 383 K for 3 h and finally calcined at 573 K in air for 16 h. In all cases, the temperature was raised from 298 K to the final calcination temperature over 2 h. Before the experiments, the catalysts were sieved to 53–90 μm. The prepared catalysts were designated as 12CoRe/Al and 20CoRe/Al, respectively.
The preparation of the reference carbide catalyst involved the following steps: First, the catalyst was reduced in a tubular furnace at 623 K for 3 h with a ramping rate of 1 K/min under a 100 mL/min H2 flow. Subsequently, the catalyst was carburized in a CO atmosphere at 523 K with a CO flow of 100 mL/min. Finally, it was passivated at room temperature using a diluted 0.1% O2/He mixture, resulting in a catalyst containing carbon but free of wax. This catalyst was designated as CO-20CoReAl.

4.2. Catalyst Characterization

BET surface area, average pore volume, and pore size were measured using a Micromeritics ASAP 2460 instrument (Norcross, GA, USA) via nitrogen adsorption, with data collected at liquid nitrogen temperature. Samples (0.099 g, particle size 53–90 μm) were degassed under vacuum at 423 K prior to analysis. Elemental analysis was performed using an Agilent (Santa Clara, CA, USA) ICP-OES 730 inductively coupled plasma optical emission spectrometer.
The XRD analysis of the calcined catalyst was performed using a Rigaku (Singapore) Flex 600 diffractometer. When calculating the size of cobalt particles, we assumed that the particles were spherical. The average Co3O4 crystallite size was determined from the most intense Co3O4 diffraction peak at 2θ of 36.9° using the Scherrer equation. Then, we obtained the crystallite size of metallic cobalt by multiplying the estimated grain thickness by a correction factor of ¾ [14]. Additionally, the tested catalyst was analyzed by X-ray diffraction using a Rigaku Smartlab 9 Advance diffractometer equipped with monochromatic CuKα radiation. Specifically, the tested 12CoRe/Al catalyst from Experiment 1 was analyzed without dewaxing, while all other tested catalysts underwent a dewaxing process. The dewaxing of the catalysts was conducted using a Soxhlet extractor (Shanghai, China), where xylene was used as the extraction solvent for 24 h. After the extraction, the catalysts were removed from the apparatus and dried in an oven at 383 K for 2 h to ensure complete evaporation of the solvent.
Temperature-Programmed Hydrogenation (TPH) experiments were conducted using a U-shaped quartz reactor connected to a Micromeritics 2930 instrument (Norcross, GA, USA). Prior to the TPH analysis, samples were heated to 423 K in a helium flow to remove adsorbed water and other contaminants. After cooling to 323 K, the samples were subjected to a temperature ramp of 10 K/min up to 873 K in a 10% hydrogen-in-argon mixture flowing at 50 mL/min. The effluent gas composition was continuously monitored using mass spectrometry to analyze the carbon species hydrogenation behavior of the catalysts.
All tested samples were further characterized using Raman spectroscopy. Raman spectra were obtained using a Horiba LabRAM HR Evolution spectrometer (Kyoto, Japan) with a 532 nm He-Cd laser as the excitation source. The laser light was focused on the sample for 10 s using a 50× objective lens, and a grating with 600 lines/mm was used. The scanning range was set from 1000 to 2000 cm−1

4.3. Activity and Selectivity Measurements in Fischer–Tropsch Synthesis

Fischer–Tropsch synthesis experiments were conducted in a stainless-steel fixed-bed reactor (inner diameter 10 mm) equipped with a three-zone heating furnace. The catalyst used was Co-Re/γ-Al2O3 (1 g, particle size 50–90 μm). To minimize temperature gradients, the catalyst was diluted with inert silicon carbide (particle size 50–90 μm, 5 g) and covered with a top layer of larger silicon carbide particles (particle size 0.25–0.4 mm).
The catalyst was reduced in flowing hydrogen (1 bar, 200 NmL/min), with the temperature ramped from room temperature to 623 K at a rate of 1 K/min and held for 16 h for both calcined catalyst and re-reduction for tested catalyst. After reduction, the temperature was lowered to 443 K, and the reactor was purged with argon (200 NmL/min) for 1 h. Subsequently, the pressure was increased to 20 bar with syngas. Next, the reactor temperature was slowly raised to the specified reaction temperature from 443 K to 476 K at 1 K/min, then from 476 K to 493 K at 12 K/h for all experiments. Synthesis gas with an H2/CO ratio of 2 (containing approximately 3% N2 as an internal standard) was introduced into the reactor using Bronkhorst mass flow controllers. Different syngas compositions were used at various stages of the experiments, as detailed in Table 2.
Heavy hydrocarbons were collected in a heated trap maintained at 363 K, and liquid products were collected using a cooling trap set at 273 K. The gaseous effluent from the reactor was periodically sampled and analyzed using a gas chromatograph (FL97plus) (Shanghai, China) for H2, N2, CO, CO2, and C1–C4 hydrocarbons. The gas chromatograph was equipped with a thermal conductivity detector (TCD) using molecular sieve and Porapak Q columns (Santa Clara, CA, USA) for detecting carbon monoxide, carbon dioxide, methane, and nitrogen, and a flame ionization detector (FID) equipped with an Al2O3-plot capillary column for hydrocarbon analysis. The C5+ selectivity was calculated by subtracting the amounts of C1–C4 hydrocarbons. Before the experiments, the feed gas was measured via bypass as a reference.
AI-assisted language editing was performed using Deepseek to improve readability. Final content was rigorously reviewed and approved by all authors.

5. Conclusions

To evaluate the catalytic activity of CO2 on cobalt-based Fischer–Tropsch (FT) catalysts and its effect on product distribution, we examined the activity and selectivity of a representative catalyst, Co-Re/γ-Al2O3, under industrially relevant conditions (2 MPa, 493 K, XCO = 30–70%) with varying ratios of H2, CO, and CO2.
A comparison of CO2 co-feed experiments revealed that the presence of CO2 in the feed gas at low H2/CO ratio of 2 deactivates the Co-Re/γ-Al2O3 catalyst. This deactivation leads to an increased selectivity towards gaseous paraffins (mainly methane), a decreased selectivity for C5+ hydrocarbons, and a reduced olefin-to-paraffin ratio for C3/C4 hydrocarbons, which is unfavorable for the desired FT synthesis products. The formation of carbon deposits and cobalt carbide is primarily responsible for this catalyst deactivation when CO2 is introduced while maintaining an H2/CO ratio of 2, as observed in Experiments 1 (increasing CO2-12CoReAl), 2b (CO2-20CoReAl) and 3. The presence of CO2 in the feed gas adversely affects Co-Re/Al catalysts in a cumulative manner. In PtL projects utilizing SOECs co-electrolysis to produce liquid fuel, it is essential to include a CO2 removal step during the preparation of syngas.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15040329/s1, Table S1: Effect of increasing CO2 on catalyst activity and hydrocarbon selectivity; Table S2: The heating procedures for all experiments; Figure S1: Top view of bare, and carbon-containing Co(100), Co(111) and Co(311) surface. Blue and grey balls represent Co and C atoms, respectively; Table S3: Chemical potential (eV) of carbon species in gaseous and surface carbon-containing species at different stages. Stage 1: Without CO2 addition. Stage 2: CO2 addition before sharp deactivation. Stage 3: CO2 addition after sharp deactivation. μC(HmOn) represent the chemical potential of carbon atom in the gaseous CHmOn species. Refs. [41,52,53,54,55,56,57,58,59,60,61,62,63,64] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, J.Y., J.D. and Z.C.; methodology, J.Y., J.D. and Z.C.; software, Y.G., F.G. and Z.C.; validation, Z.C.; formal analysis, Z.C.; investigation, Z.C. and J.D.; resources, J.Y. and Z.H.; data curation, Z.C.; writing—original draft preparation, Z.C. and J.D.; writing—review and editing, J.Y., D.C. (Denghui Chen) and F.G.; visualization, Z.C. and J.D.; supervision, J.Y. and D.C. (De Chen); project administration, Y.G.; funding acquisition, J.Y. and Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Shanghai Municipal Science and Technology Major Project and the National Natural Science Foundation of China (No. 52394205). The APC was funded by the National Natural Science Foundation of China.

Data Availability Statement

The data generated in this study are not publicly available due to ethical restrictions and privacy considerations. Requests for access to the dataset may be directed to the corresponding author. Supporting materials necessary for peer review are available upon reasonable request.

Acknowledgments

AI-assisted language editing was performed using Deepseek to improve readability. Final content was rigorously reviewed and approved by all authors.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
P-t-Lpower-to-liquid
P-t-Xpower-to-X
SOECssolid oxide electrolysis cells
FTSFischer–Tropsch synthesis
FT-IRFourier Transform Infrared Spectroscopy
DRIFTSDiffuse Reflectance Infrared Fourier Transform Spectroscopy
20CoReAl20 wt% Co-0.5 wt% Re/γ-Al2O3
12CoReAl12 wt% Co-0.5 wt% Re/γ-Al2O3
ICPInductively Coupled Plasma
XRDX-ray diffraction
BETBrunauer–Emmett–Teller method
CO-CoReCO-treated 20 wt% Co-0.5 wt% Re/γ-Al2O3
TPHtemperature-programmed hydrogenation
MSmass spectrometry
DFTdensity functional theory
AIATab initio atomistic thermodynamics
TCDthermal conductivity detector
FIDflame ionization detector

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Catalysts 15 00329 g001
Figure 1. Observed CO conversion (a), C1-C4 paraffin selectivity (b), C5+ selectivity (c), and C4 olefin/paraffin ratio (d) as a function of time on stream 90 h after start-up for 12 wt% Co-0.5 wt% Re/γ-Al2O3, P = 22 bar, T = 493 K, and GHSV = 7200 Ncm³/gcat/h; Period 1 = H2:CO:CO2:Ar = 2:1:0:1; Period 2 = H2:CO:CO2:Ar = 2:1:0.5:0.5; Period 3 = H2:CO:CO2:Ar = 2:1:1:0; Period 4 = H2:CO:CO2:Ar = 2:1:0:1.
Figure 1. Observed CO conversion (a), C1-C4 paraffin selectivity (b), C5+ selectivity (c), and C4 olefin/paraffin ratio (d) as a function of time on stream 90 h after start-up for 12 wt% Co-0.5 wt% Re/γ-Al2O3, P = 22 bar, T = 493 K, and GHSV = 7200 Ncm³/gcat/h; Period 1 = H2:CO:CO2:Ar = 2:1:0:1; Period 2 = H2:CO:CO2:Ar = 2:1:0.5:0.5; Period 3 = H2:CO:CO2:Ar = 2:1:1:0; Period 4 = H2:CO:CO2:Ar = 2:1:0:1.
Catalysts 15 00329 g001
Catalysts 15 00329 g002
Figure 2. (a) CO conversion and (b) CH4 selectivity for Experiment 2a (H2:CO:CO2:Ar = 2:1:0:0); (c) CO conversion and (d) CH4 selectivity for Experiment 2b with two gas-phase periods: Period 1 (H2:CO:CO2:Ar = 2:1:0:1) and Period 2 (H2:CO:CO2:Ar = 2:1:1:0) as a function of time on stream for the 20 wt%Co-0.5 wt%Re/γ-Al2O3 catalyst in Fischer–Tropsch synthesis. Reaction conditions: P = 20 bar, T = 493 K, and GHSV = 10,000 Ncm³/h/gcat.
Figure 2. (a) CO conversion and (b) CH4 selectivity for Experiment 2a (H2:CO:CO2:Ar = 2:1:0:0); (c) CO conversion and (d) CH4 selectivity for Experiment 2b with two gas-phase periods: Period 1 (H2:CO:CO2:Ar = 2:1:0:1) and Period 2 (H2:CO:CO2:Ar = 2:1:1:0) as a function of time on stream for the 20 wt%Co-0.5 wt%Re/γ-Al2O3 catalyst in Fischer–Tropsch synthesis. Reaction conditions: P = 20 bar, T = 493 K, and GHSV = 10,000 Ncm³/h/gcat.
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Catalysts 15 00329 g003
Figure 3. Observed CO conversion, methane selectivity, C5+ selectivity, and C4 olefin-to-paraffin ratio as a function of time on stream 70 h after start-up for 20 wt% Co-0.5 wt% Re/γAl2O3, P = 23 bar, T = 493 K, and GHSV = 6000 Ncm3/h/gcat; Period 1: H2:CO:CO2:Ar = 2:1:0:0.2; Period 2: H2:CO:CO2:Ar = 2:1:0.2:0; Period 3: H2:CO:CO2:Ar = 2:1:0:0.2; Period 4: H2:CO:CO2:Ar = 2:1:0:0.2.
Figure 3. Observed CO conversion, methane selectivity, C5+ selectivity, and C4 olefin-to-paraffin ratio as a function of time on stream 70 h after start-up for 20 wt% Co-0.5 wt% Re/γAl2O3, P = 23 bar, T = 493 K, and GHSV = 6000 Ncm3/h/gcat; Period 1: H2:CO:CO2:Ar = 2:1:0:0.2; Period 2: H2:CO:CO2:Ar = 2:1:0.2:0; Period 3: H2:CO:CO2:Ar = 2:1:0:0.2; Period 4: H2:CO:CO2:Ar = 2:1:0:0.2.
Catalysts 15 00329 g003
Catalysts 15 00329 g004
Figure 4. Raman spectra of the tested Co-Re/γ-Al2O3 catalysts after Fischer–Tropsch synthesis: 12 wt% Co-0.5 wt% Re/γ-Al2O3 from Experiment 1, 20 wt% Co-0.5 wt% Re/γ-Al2O3 from Experiments 2a(syngas), 2b(CO2), and 3(Re-reduction).
Figure 4. Raman spectra of the tested Co-Re/γ-Al2O3 catalysts after Fischer–Tropsch synthesis: 12 wt% Co-0.5 wt% Re/γ-Al2O3 from Experiment 1, 20 wt% Co-0.5 wt% Re/γ-Al2O3 from Experiments 2a(syngas), 2b(CO2), and 3(Re-reduction).
Catalysts 15 00329 g004
Catalysts 15 00329 g005
Figure 5. Peak deconvolution of methane profiles during TPH for (a) pre-carburized 20CO-CoReAl and (b) wax-extracted 20CoReAl catalysts after FTs (Experiment 2b). Methane signals were monitored by mass spectrometry using the fragment ion at m/z 15.
Figure 5. Peak deconvolution of methane profiles during TPH for (a) pre-carburized 20CO-CoReAl and (b) wax-extracted 20CoReAl catalysts after FTs (Experiment 2b). Methane signals were monitored by mass spectrometry using the fragment ion at m/z 15.
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Catalysts 15 00329 g006
Figure 6. XRD data of the calcined and tested Co-Re/γ-Al2O3 catalysts after Fischer–Tropsch synthesis.
Figure 6. XRD data of the calcined and tested Co-Re/γ-Al2O3 catalysts after Fischer–Tropsch synthesis.
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Catalysts 15 00329 g007
Figure 7. Schematic illustration of a possible detailed mechanism of CO2-induced deactivation, as exemplified in Experiment 2b.
Figure 7. Schematic illustration of a possible detailed mechanism of CO2-induced deactivation, as exemplified in Experiment 2b.
Catalysts 15 00329 g007
Table 1. Summary on the effect of CO2 on the performance of Cobalt Fischer–Tropsch catalysts.
Table 1. Summary on the effect of CO2 on the performance of Cobalt Fischer–Tropsch catalysts.
Catalyst (Impregnated/Precipitation)BET Surface Area (m2/g)Pore Volume (cm3/g)Pore diameter (nm)Particle Size (nm)Type of Reactor/Pressure (Mpa)/Temperate (K)/GHSV (cm3/h/gcat)CO2 AdditionCO Conversion 1Methane Selectivity 1C5+ Selectivity 1Overall Effect of CO2 on Catalyst Stability and Deactivation Ref.
20%Co/γ-Al2O3(imp)218.90.73410.19.5FBR-2 Mpa-493 K-2000H2/CO/CO2/Ar cm3/min = 32/16/0~13.3/5.4~18.7Negative Positive Negative CO2 is found to be responsible for the partial oxidation of surface cobalt metal with the coexistence of generated water.[5]
20%Co/P-Al2O3(imp)1920.284.514.9CSTR-2 Mpa-503 K-2000H2/CO/CO2/Ar mol% = 57.3/28.4/9.3/5Negative--Increased the rate of deactivation.[6]
20%Co-0.2%Ru/P-Al2O3(imp)1980.314.813.4CSTR-2 Mpa-503 K-2000H2/CO/CO2/Ar mol% = 57.3/28.4/9.3/5Negative---Increased the rate of deactivation. Greater deactivation than unprompted catalyst.[6]
20%Co-0.2%Pt/P-Al2O3(imp)1940.264.310.3CSTR-2 Mpa-503 K-2000H2/CO/CO2/Ar mol% = 57.3/28.4/9.3/5Negative--Increased the rate of deactivation. [6]
31.2%Co-Zr-Ru/SiO2(Prec)248-7.91.69CFSSR-463 KH2/CO/CO2 Mpa = 0.54/0.27/ 0~0.62NegativePositive NegativeIncreased the rate of deactivation. Irreversible after removal of CO2.[7]
27.3%Co-Zr-Pt/SiO2(Imp)116.2-12.56.57CFSSR-463 KH2/CO/CO2 Mpa = 0.35/0.18/ 0~0.7no changeno changeno changeCO2 behaves as an inert gas [7]
25.9%Co-La-Ru/Al2O3(Imp)81.6-13.82.21CFSSR-463 KH2/CO/CO2 Mpa = 0.33/0.17/ 0~0.68no changeno changeno changeCO2 behaves as an inert gas [7]
19.4%Co-La-Ru/Al2O3(Prec)124.1-12.43.21CFSSR-463 KH2/CO/CO2 Mpa = 0.54/0.29/ 0~0.52no changeno changeno changeCO2 behaves as an inert gas [7]
15%Co/γ-Al2O3(imp)1200.31--FBR-2 Mpa-493 K-4800H2/CO/CO2/N2 bar = 14.2/2.9/2.9~0/0~2.9no changeno changeno changeCO2 behaves as an inert gas.[8]
15%Co-0.45%Pt/Sirolox (imp)---18Operando DRIFTS-483 KH2/CO/CO2/He cm3/min = 45/0.25~1/15/15 -PositiveNegativeAs CO decreases and CO2 increases, the reaction shifts from Fischer–Tropsch synthesis to methanation.[9]
15%Co/Sirolox (imp)---12Operando DRIFTS-483 KH2/CO/CO2/He cm3/min = 45/0.25~1/15/15 -PositiveNegativeAs CO decreases and CO2 increases, the reaction shifts from Fischer–Tropsch synthesis to methanation.[9]
15%Co-0.56%Pt/TiO2 (imp)---15Operando DRIFTS-483 KH2/CO/CO2/He cm3/min = 45/0.25~1/15/15 -PositiveNegativeAs CO decreases and CO2 increases, the reaction shifts from Fischer–Tropsch synthesis to methanation.[9]
15%Co/TiO2 (imp)---15Operando DRIFTS-483 KH2/CO/CO2/He cm3/min = 45/0.25~1/15/15 -PositiveNegativeAs CO decreases and CO2 increases, the reaction shifts from Fischer–Tropsch synthesis to methanation.[9]
100Co-60MnO-0.15Pt/147SiO2 (Prec)----FBR-1 Mpa-463 K-1800H2/CO/CO2 mol% = 2/1~0/0~1-PositiveNegativeAs CO decreases and CO2 increases, the reaction shifts from Fischer–Tropsch synthesis to methanation.[10]
15%Co/SiO2(imp)----FBR/2.4 Mpa/493 KH2/CO/CO2---CO2 participates in the reaction during the FT process, The conversion rate of CO is four times that of CO2.[11]
25%Co-0.5%Pt/γ-Al2O3(imp)1300.281.91 FBR-2-493 K-6000H2/CO/CO2 mol% = 3/1~0/0~1 PositiveNegative14CO2 was indeed converted, The only products derived from 14CO2 were C1-C3 hydrocarbons.[12]
1: As CO2 is added to the feed gas.
Table 2. Reaction conditions and feed gas ratios at each period of the experiments.
Table 2. Reaction conditions and feed gas ratios at each period of the experiments.
EXP NumberCatalystT(K)/P(bar)/GHSV (Ncm3/h/gcat)Feed Gas Ratio: H2/CO/CO2/Ar
Period 1Period 2Period 3Period 4
1 (increasing CO2)12CoRe/Al493/21/72002:1:0:12:1:0.5:0.52:1:1:02:1:0:1
2a (syngas)20CoRe/Al493/20/10,0002:1:0:0
2b (CO2)20CoRe/Al493/20/10,0002:1:0:12:1:1:0
3 (Re-reduction)20CoRe/Al493/23/50002:1:0:0.22:1:0.2:02:1:0:0.22:1:0:0.2
Table 3. Catalyst properties: particle size, dispersion, BET surface area, and pore volume.
Table 3. Catalyst properties: particle size, dispersion, BET surface area, and pore volume.
Exp.
Number		CatalystCo/Re (ICP) [wt%]Crystallite Size (XRD) [nm] 1BET and BJH Methods
(Before/After Reaction)
BET
Surface Area (m2/g)		Pore Volume (cm3/g) Pore
Diameter
(nm)
2a (syngas)20CoRe/Al19.4/0.4812.8130/1020.31/0.279.6/8.1
2b (CO2)20CoRe/Al19.4/0.4812.8130/750.31/0.29.6/7.5
1: Cobalt metal crystallite size calculated from XRD of calcined catalyst, using d(Co) = 0.75d(Co3O4) [14]. The average Co3O4 crystallite size was calculated using the Scherrer equation from the most intense diffraction peak at 2θ = 36.9°.
Table 4. Catalyst sharp drop information under various experimental conditions.
Table 4. Catalyst sharp drop information under various experimental conditions.
ExperimentCatalystFeed Gas Ratio
H2:CO:CO2:Ar		TOS (h)XCO (%)PCO (atm)
112CoRe/Al2:1:1:0150494.15
2b20CoRe/Al2:1:1:0282254.63
320CoRe/Al2:1:0.2:0105306.11
PCO—average partial pressure in catalyst bed.
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Chen, Z.; Du, J.; Chen, D.; Gong, F.; Gao, Y.; Huang, Z.; Chen, D.; Yang, J. Revisiting the Impact of CO2 on the Activity and Selectivity of Cobalt-Based Catalysts for Fischer–Tropsch Synthesis Under Industrial-Relevant Conditions. Catalysts 2025, 15, 329. https://doi.org/10.3390/catal15040329

AMA Style

Chen Z, Du J, Chen D, Gong F, Gao Y, Huang Z, Chen D, Yang J. Revisiting the Impact of CO2 on the Activity and Selectivity of Cobalt-Based Catalysts for Fischer–Tropsch Synthesis Under Industrial-Relevant Conditions. Catalysts. 2025; 15(4):329. https://doi.org/10.3390/catal15040329

Chicago/Turabian Style

Chen, Zhiyu, Jinbo Du, Denghui Chen, Fuqing Gong, Yang Gao, Zhen Huang, De Chen, and Jia Yang. 2025. "Revisiting the Impact of CO2 on the Activity and Selectivity of Cobalt-Based Catalysts for Fischer–Tropsch Synthesis Under Industrial-Relevant Conditions" Catalysts 15, no. 4: 329. https://doi.org/10.3390/catal15040329

APA Style

Chen, Z., Du, J., Chen, D., Gong, F., Gao, Y., Huang, Z., Chen, D., & Yang, J. (2025). Revisiting the Impact of CO2 on the Activity and Selectivity of Cobalt-Based Catalysts for Fischer–Tropsch Synthesis Under Industrial-Relevant Conditions. Catalysts, 15(4), 329. https://doi.org/10.3390/catal15040329

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