Fischer–Tropsch synthesis of CO₂-rich syngas using a CoRu-KIT-6 catalyst in a 3D-printed stainless steel (SS) microchannel microreactor

Abstract

A CoRu-KIT-6 catalyst, prepared by a one-pot hydrothermal method, was used for Fischer–Tropsch synthesis (FTS) of syngas containing CO₂ in a 3D-printed stainless-steel microchannel microreactor (SSMR) at 20 bar. The catalyst was characterized using N₂ adsorption–desorption isotherm measurement, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), H₂-temperature programmed reduction (H₂-TPR), CO₂-temperature programmed desorption (CO₂-TPD) and X-ray photoelectron spectroscopy (XPS) techniques. While the surface area from the N₂ adsorption–desorption isotherm is quite high (690.4 m² g⁻¹), with low reduction temperatures (H₂-TPR) of the metals, the low-angle XRD, SEM, and TEM studies show that the ordered mesoporous structure of KIT-6 is conserved after the addition of Co and Ru metals. The bimetallic catalyst was used to investigate the effect of reaction temperature and CO₂ concentration (by volume) in feed gas mixtures on the conversion of CO and CO₂ in modified FTS. Four different volume compositions of CO₂/CO/H₂ (10 : 30 : 60, 25 : 25 : 50, 40 : 20 : 40, and 70 : 10 : 20) were used for modified FTS in the temperature range of 210 to 350 °C. While CO₂ conversion increases with rising reaction temperature, CO conversion is adversely affected by the higher CO₂ concentration in the feed. The Co-Ru-KIT-6 catalyst exhibits excellent catalytic activity, achieving higher conversion and hydrocarbon selectivity with CO₂-rich syngas. The CO₂-rich syngas composition (CO₂ : CO : H₂ = 25 : 25 : 50) at 350 °C showed the best result for CO conversion of 82.3%, CO₂ conversion of 28.1% and higher selectivity to longer chain hydrocarbons (C₅₊), 79.1%.

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1. Introduction

Carbon dioxide (CO₂), a greenhouse gas, is produced by a range of sectors including power plants, steel companies, oil factories, chemical plants, and other energy-related operations.¹ CO₂ has a major impact on the rise of global temperature. Elevated CO₂ emissions could significantly contribute to weather change and global warming, presenting a significant environmental and human challenge in the 21st century.^2,3 In this context, carbon capture and utilization (CCU) and carbon capture and storage (CCS) have been proposed to regulate atmospheric CO₂ levels. Although the CCU technique is receiving significant attention due to its ability to utilize CO₂, the molecule's thermal stability, with a standard enthalpy of formation of −396.0 kJ mol⁻¹, presents a big challenge.⁴ Nevertheless, conversion of carbon dioxide (CO₂) into valuable chemicals is gaining significant interest from the government, industry, and the investment community. While climate change mitigation remains the primary focus of interest, other contributing factors include technological advancements and the promotion of a circular economy. Thus, utilization of CO₂ to value-added products is the primary focus of researchers. CO₂ can also replace fossil fuels in the synthesis of value-added chemicals and polymers.

Catalytic conversion of CO₂-enriched syngas via the Fischer–Tropsch synthesis (FTS) process is a well-established route to produce ‘clean’ transportation fuels and chemicals.^5–7 Coal, natural gas, and biomass are primary resources to obtain CO₂-enriched syngas.⁸ However, the composition of CO₂-enriched syngas depends on several factors, including the type of gasifier, operational procedures, and gasifying agents used. Typically, the CO₂ composition in untreated syngas ranges from approximately 1 to 30% (v/v).⁹ This elevated CO₂ content often results from the high oxygen content of feedstocks such as biomass. When converted to syngas, these oxygen-rich materials generate a higher proportion of CO₂, which can lower the overall efficiency of the Fischer–Tropsch synthesis (FTS) process by diluting reactive components, H₂ and CO.^10–11 Therefore, removal of excess CO₂ from the feed stream is necessary, as it not only improves processing efficiency but also increases the cost for production of the final products.¹² The presence of CO₂ influences CO conversion and selectivity to hydrocarbons.¹³ Thus, syngas enriched with CO₂ has been gaining interest for FTS to avoid an expensive CO₂ separation process.

Different catalysts have been used for hydrogenation of CO₂ containing syngas under different operating conditions.^14–16 Iron and cobalt based catalysts are widely used for CO₂ enriched syngas for the FTS process despite the lower reactivity of CO₂ than CO.^17–21 The product distribution changes abruptly in terms of short and long chain hydrocarbons. Methane (short-chain hydrocarbon) is primarily formed in the case of CO₂ hydrogenation over Fe and Co based catalysts. In contrast, long-chain hydrocarbons are formed from CO hydrogenation over Fe and Co based catalysts.²² The effect of Co-based catalysts is still under research because of its inactivity towards water–gas shift (WGS) and reverse water–gas shift (RWGS) reactions for the CO₂-enriched syngas FTS process because of catalyst inactivity and efficiency concerns. However, when Co is used in conjunction with Fe, the combined bimetallic catalysts offer both higher FTS activity and enhanced CO₂ utilization through the WGS/RWGS pathways. Some researchers have successfully carried out CO₂ hydrogenation in the presence of CO and H₂.²³ However, the results are not competitive enough due to the following reasons: (i) the enormous stability of the CO₂ molecule is an obstacle for this process and affects the overall process efficiency in terms of conversion and selectivity; (ii) the formation of water during the RWGS reaction is another issue that reduces the efficiency of the overall process.²⁴ The addition of CO₂ can also decrease CO conversion if operating conditions are not optimized.^25,26,16 Thus, the CO₂ addition acts as a promoter, or an inhibitor based on operating conditions and process parameters.^27,28 Therefore, CO₂-rich syngas conversion is a very important area of research.

There are two different pathways for CO₂ hydrogenation: direct with intermediate steps (not shown) and indirect. For the direct synthesis, CO₂ is immediately converted to hydrocarbons (eqn (1)), while for the secondary pathway, first the RWGS reaction (eqn (2)) takes place to produce CO and then CO becomes hydrogenated by the FTS process (eqn (3)) to produce hydrocarbons.^28,29

nCO₂ + 3nH₂ → C_nH_2n + 2nH₂O (1)

CO₂ + H₂ ↔ CO + H₂O (2)

nCO + 2nH₂ → C_nH_2n + nH₂O (3)

Several researchers have used syngas for the FTS process with Fe and Co promoted catalysts.^30–36 Iron-based catalysts are known for their activity in water–gas shift (WGS) and reverse water–gas shift (RWGS) reactions, making them particularly suitable for CO₂-rich syngas by facilitating the in situ generation of CO and maintaining the H₂/CO ratio. The use of Fe–Co catalysts may present a promising approach for syngas derived from CO₂-rich feedstocks because of their activity towards WGS and RWGS reactions. The presence of CO₂ in FTS activity using Co-based catalysts was investigated by Yao et al.²⁷ They found that the intermediate composition of CO₂/CO/H₂ is an optimized ratio to produce hydrocarbons. The “optimized ratio” does not refer to equal parts of CO₂, CO, and H₂ (1 : 1 : 1) but rather to an intermediate or a balanced level of CO₂ and CO with H₂. These levels are selected to achieve effective hydrogenation and hydrocarbon production while mitigating challenges like water formation and CO₂ inhibition. In another study, Riedel and Schaub²⁸ observed that CO₂ acted as an inert substance in FTS with three of four catalysts. They also found that the reaction rate decreased for other catalysts. Similarly, Riedel et al.²⁹ investigated the FTS process employing Fe and Co-based catalysts with CO/H₂ and CO₂/H₂ syngas mixtures. Their findings highlighted the diverse impacts of CO₂ on the FTS process and hydrocarbon selectivity. The phrase “diverse impacts of CO₂” on the FTS process in that research work refers to the multiple effects of CO₂ on the reaction's efficiency, product selectivity and catalyst behavior. CO₂ acted as an inert substance, and no hydrocarbon was produced due to CO₂ for Co-based catalysts. In terms of reactor operation, microreactors offer a promising avenue for green energy production, leveraging small-scale, efficient reactor systems. Reactions are much easier to control, which minimize risk and side reactions. Microchannel reactors are characterized by their high surface-area-to-volume ratio, which enhances heat and mass transfer, ensuring more uniform reaction conditions and minimizing temperature gradients—critical for highly exothermic processes like the Fischer–Tropsch synthesis (FTS). The small dimensions of the channels also reduce diffusion limitations, improving reactant utilization and selectivity toward desired products. Micro flow phenomena are good to achieve better heat and mass transfer during a chemical reaction.

In our previous kinetic studies at 1 bar in a 3D-printed SSMR, we observed that Co-Ru-KIT-6 is an active catalyst for syngas (CO and H₂) conversion to hydrocarbons.³⁴ KIT-6 has high porosity and performs well in terms of stability for FTS. The significant surface area and well-structured hexagonal arrangement of KIT-6 have drawn researchers' interest for its potential application in catalysis. In our present study at 20 bar, we used a Co-Ru-KIT-6 catalyst for CO₂-rich syngas conversion by the FTS process in a 3D-printed SSMR. In contrast to our previous study, the key finding of this work is the shift in product distribution. No CO₂ was present in our previous work at 1 bar. In our present study, the use of high pressure (20 bar) has led to the formation of olefins (C₂⁼–C₄⁼) and higher hydrocarbons (C₅₊). The mesoporous silica support, KIT-6, was used for FTS because of its high stability and adequate porosity.³⁷ The effect of different proportions of CO₂ addition (v/v) on FTS activity and hydrocarbon selectivity was investigated. More significantly, in this study, we obtained liquid fuels using our microchannel microreactor. It's worth mentioning, in this context, that although Velocys has reported production of liquid fuels from syngas in the patent literature, to our knowledge, our microchannel microreactor is much smaller than that reported by Velocys and others used for the production of clean fuels.³⁸

2. Materials and methods

2.1. Materials

All necessary chemical reagents for catalyst synthesis, such as 98% tetraethyl orthosilicate (TEOS), Pluronic P123 (avg. MW-5800), 37% HCl, Co(NO₃)₂·6H₂O (≥98%), and RuCl₃·xH₂O (Ru content 45–55%), were procured from Sigma-Aldrich. Furthermore, 99% butanol was purchased from Fischer Scientific (NJ, USA).

2.2. Microreactor fabrication

The microchannel microreactor used for catalyst screening, with dimensions of 1000 μm in height and width, underwent modifications at the inlet and outlet to confirm secure sealing for high-pressure FTS. In Fig. 1, both the AutoCAD design and the final 3D printed SSMR are depicted. The cross-sectional view in Fig. 1 shows seven microchannels positioned between the cylindrical inlet and outlet. Each microchannel measures 5 cm in length, with dimensions of 1000 μm in height and width. The microreactor's inlet and outlet are fitted with outer diameter ¼ inch Swagelok tubing, enabling precise alignment with a Swagelok filter for leak-proof sealing during high-pressure reactions. Quartz wool was utilized at the reactor's end, and the catalyst was filled into the reaction zone of the microreactor. Then, the reactor was positioned on a microreactor heating block.

Fig. 1 3D printed stainless steel microchannel microreactor.

2.3. Catalyst synthesis

The catalyst, CoRu-KIT-6, utilizing a mesoporous silica support with a composition of 10% Co and approximately 5% Ru, was synthesized via a one-pot hydrothermal procedure. The metal loading was maintained at 10% Co and approximately 5% Ru by weight. The synthesis process involved combining the materials in a molar ratio of 1 : 0.017 : 1.83 : 195 : 1.31 (TEOS : P123 : HCl : DI water : butanol) to prepare CoRu-KIT-6.³⁹ Initially, P123 was introduced into HCl at 35 °C until a clear mixture was formed. Subsequently, a mixture of butanol holding metal precursors based on 10% Co and 5% Ru amounts was added to the former mixture and mixed till homogeneity was achieved. Then, TEOS was included dropwise and mixed at 500 rpm for 24 hours in the mixture. The resulting mixture was aged for 24 hours at 100 °C, accompanied by air drying for 24 hours under a fume hood. The material was further dried in a hot air oven at 110 °C for 24 hours and finally calcined at 550 °C for 4 hours to eliminate P123 with heating and cooling rates set at 2 °C min⁻¹.

2.4. Catalyst characterization

The N₂ adsorption–desorption isotherm of the catalyst was obtained on a 3-Flex instrument (Make: Micromeritics, USA). The N₂ adsorption–desorption isotherm analysis was done at liquid N₂ temperature (−196 °C). The pore size distribution (PSD) plot was used to determine the average pore diameter by BJH (Barrett–Joyner–Halenda). The H₂-TPR analysis was carried out using the same instrument with ∼50 mg of the catalyst filling the analysis tube. A stream of 10% (v/v) H₂/Ar flow at a rate of 110 ml min⁻¹ was continued, through this tube, while the temperature gradually increased to 1000 °C at a ramp rate of 10 °C min⁻¹. For XRD analyses, D8 Discover X-ray and Rigaku Smart Lab X-ray diffractometers were employed, respectively. Cu Kα radiation with a wavelength of 1.5418 Å was utilized. The measurements were done at an interval size of 0.05° and a time per step of 3 s per step. The modified Scherrer equation was applied to measure the average crystal size. The diffraction peaks were matched with the JCPDS (Joint Committee on Powder Diffraction Standards) database. TEM (Carl Zeiss Libra 120) at 120 keV and FESEM (JEOL) were used to obtain the morphology and the size of the catalyst. For TEM analysis, the catalyst sample was prepared by distributing a small amount of catalyst in ethanol (3 ml), followed by vortex scattering and agitation for a few minutes. Subsequently, the suspension was drop-coated onto a carbon-coated copper grid with a mesh size of 300 μm and then dried in an oven at 100 °C for 12 hours. The elemental percentage and oxidation states of the metals were examined using energy dispersive X-ray spectroscopy (EDS) (Zeiss Auriga FIB/FESEM acquired from Carl Zeiss, Oberkochen, Germany), while oxidation states were determined via XPS (Escalab Xi+-Thermo Scientific procured from Thermo Scientific, West Sussex, UK). The external standard was used as a binding energy reference. The sample was attached using carbon tape. Charge compensation was achieved using a fixed ion beam. The basic surface site of the catalyst was determined via CO₂-TPD analysis on a 3-Flex instrument (Make: Micromeritics, USA). At first, the catalyst (∼50 mg) was reduced in the presence of H₂ through the analysis tube by increasing the temperature up to 800 °C. A stream of 10% (v/v) H₂/Ar flow at a rate of 110 ml min⁻¹ was continued through the tube. Then, CO₂ (50 ml min⁻¹) was purged through the sample to adsorb at room temperature for 1 h. The temperature was increased again in the presence of He (50 ml min⁻¹) gas to desorb CO₂ from the basic site of the catalyst.

2.5. Catalyst activity test (FTS process)

FTS experiments were conducted within a custom-built microfluidic SS microreactor produced through 3D printing in our laboratory. We used a PID-feedback loop acting on the heater. The heater temperature is controlled by LabView programming. Gas flow rates were regulated utilizing flow suite software (Bronkhorst, range 0–20 sccm). The reactor consists of seven microchannels measuring 1000 μm × 1000 μm × 5 cm (as depicted in Fig. 1).⁴⁰ To manage various compositions of H₂/CO/CO₂ within the CO₂-enriched syngas mixture, three mass flow controllers (Bronkhorst) were utilized. The specific compositions of the gas mixture are detailed below.

A mass flow controller (Bronkhorst) was used to maintain the constant N₂ flow (1.5 ml min⁻¹) to provide an internal standard for GCMS. A Bronkhorst back-pressure controller (ranging from 1 bar to 20 bar) was used to maintain the reaction pressure. Before initiating the FTS experiments, the catalyst underwent ex situ reduction in a Carbolite Gero tubular furnace with 10% (v/v) H₂/Ar. To load the catalyst in the microreactor, we used quartz wool, and then we put the catalyst and again used quartz wool to hold the catalyst. A gas chromatograph (Agilent 7890B GC) with a mass spectroscopy detector (Agilent 5977 MSD) was used to analyze products (gas). A DB-1 capillary column (30 m, 0.25 mm ID and 0.5 μm) was used for product gas analysis. Approximately 0.16 g of catalyst was charged into the 3D printed SSMR for the experiment. Additionally, the reduction (in situ) of the catalyst was performed overnight at 350 °C before the reaction in the microreactor with H₂ gas at a flow rate of 3 ml min⁻¹ and 1 bar pressure. The gas hourly space velocity (GHSV) for the FTS process was maintained at 6000 h⁻¹. The FTS experiments were carried out at 20 bar. We performed the reaction at each reaction temperature for 3 h. We didn't observe any deactivation of the catalyst at the same temperature. We used a fresh catalyst for all gas mixture compositions (as per Table 1). The product selectivity was calculated based on eqn (4).²⁴ The reactant i (i = CO and CO₂) conversion was calculated based on eqn (5).⁷

where ‘n’ is the number of carbon present in different products.where F_in is the total molar flow rate of the reactor inlet gas, mol min⁻¹, F_out is the total molar flow rate of the reactor outlet gas, mol min⁻¹, x_{reactant(i),in} is the molar fraction of reactant i in the reactor inlet gas, and x_{reactant(i),out} is the molar fraction of reactant i in the reactor outlet gas.

Table 1 H₂/CO/CO₂ gas mixture composition

Sample	H2 (vol%)	H2 (ml min^−1)	CO (vol%)	CO (ml min^−1)	CO2 (vol%)	CO2 (ml min^−1)
1	66.67	2	33.33	1	0	0
2	60	2	30	1	10	0.33
3	50	2	25	1	25	1
4	40	2	20	1	40	2
5	20	2	10	1	70	7
6	66.67	2	0	0	33.33	1

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3. Results and discussion

3.1. N₂ adsorption–desorption isotherms

The textural properties of the KIT-6 support and CoRu-KIT-6 catalyst were obtained using Brunauer–Emmett–Teller (BET) analysis. Table 2 shows the surface area, pore volume, and the average pore diameter of the support and catalyst. The surface area, pore volume and pore size of the mesoporous silica (KIT-6) support were 767.6 m² g⁻¹, 1.13 cc g⁻¹ and 5.89 nm, respectively. These results are quite comparable with the studies of other groups.^39,41 The surface area, pore volume and pore size of CoRu-KIT-6 were 690.4 m² g⁻¹, 0.92 cc g⁻¹ and 5.33 nm, respectively. Fig. 2(a) depicts the N₂ adsorption–desorption isotherms of the mesoporous support and catalyst. In the case of the KIT-6 support, the isotherm of the KIT-6 support shows a type IV pattern, characteristic of mesoporous materials as defined by the International Union of Pure and Applied Chemistry (IUPAC) classification.⁴²

Table 2 Textural properties of the mesoporous silica support and catalyst

Support and catalyst	Surface area (m^2 g^−1)	Pore volume (cc g^−1)	Pore size (nm)
KIT-6	767.6	1.13	5.89
CoRu-KIT-6	690.4	0.92	5.33

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Fig. 2 (a) N₂ adsorption–desorption isotherms and (b) pore size distributions of KIT-6 and the CoRu-KIT-6 catalyst.

The capillary condensation occurred at higher relative pressure. The type H1 hysteresis loop indicates large channel-like pores in a lean range of size.³⁹ The nature of the isotherm is almost identical to that of the support for the CoRu-KIT-6 catalyst. The amount of nitrogen adsorption by the metal-incorporated support is lower compared to the support. This is because of the presence of Co and Ru nanoparticles blocking the support pores. Thus, the surface area, pore volume and pore size are also recorded for the catalyst. Fig. 2(b) shows the PSD of the KIT-6 support and CoRu-KIT-6 catalyst. The highest number of mesopores is observed in the range of 2 to 8 nm.

The larger surface area of the KIT-6 support helps to distribute active phases and allows uniformity of small nanoparticles.

3.2. FESEM-EDS analysis

Fig. 3 depicts the FESEM images of the KIT-6 support and CoRu-KIT-6 catalyst. Fig. 3(a) exhibits the flat surface of the KIT-6 support. An almost similar morphology was observed by Tuysuz et al.⁴³ The average particle size of the KIT-6 support is 62.57 nm. The morphology of the CoRu-KIT-6 catalyst, where grain-like structure particles are distributed, is shown in Fig. 3(b). However, there is some agglomeration observed on the surface of the catalyst. The catalyst particle size is evaluated using ImageJ software, with an average particle size of 39.79 nm.

Fig. 3 FESEM images of the (a) KIT-6 support and (b) CoRu-KIT-6 catalyst.

Table 3 shows the EDS results of the catalyst. The actual loading is obtained from the EDS study using 15 kV accelerating voltage. This result is almost the same as the intended loading. This reflects that the one-pot hydrothermal synthesis method is appropriate for the preparation of the catalyst for the FTS process.³⁴ The uniform distribution of the metal nanoparticles influences catalytic activity. A uniform distribution ensures that the active sites (Co and Ru) are evenly accessible to reactants, maximizing the utilization of these sites during the FTS process.

Table 3 EDS results of the CoRu-KIT-6 catalyst

Catalyst	Intended loading (wt%)		Actual loading (wt%)
	Co	Ru	Co	Ru
CoRu-KIT-6	10	5	8.4	4.5

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3.3. TEM analysis

Fig. 4 presents the TEM images of the KIT-6 support and CoRu-KIT-6 catalyst. A highly ordered mesoporous structure is observed mainly due to the KIT-6 support (Fig. 4(a)). This is in good concurrence with the XRD and N₂ adsorption–desorption analyses. The average spacing between two channels, calculated using ImageJ software, is 1.534 nm. Fig. 4(b) shows that the addition of metals did not distort the framework of the KIT-6 support. The catalyst particle size was assessed utilizing ImageJ software, resulting in an average particle size of 42.62 nm (Fig. S1†). EDS analysis was carried out with TEM analysis to find out the metal (Co and Ru) loading. Table 4 shows the EDS results of the CoRu-KIT-6 catalyst. The EDS spectrum is shown in the ESI† (Fig. S2).

Fig. 4 TEM images of the (a) KIT-6 support and (b) CoRu-KIT-6 catalyst.

Table 4 EDS results of the CoRu-KIT-6 catalyst

Catalyst	Intended loading (wt%)		Actual loading (wt%)
	Co	Ru	Co	Ru
CoRu-KIT-6	10	5	10.89	4.3

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3.4. H₂-TPR analysis

The H₂-TPR technique serves as a valuable method for understanding the reduction characteristics of supported metal oxide catalysts. One crucial aspect influencing catalyst activity is the ease of reducing the supported metal oxide catalyst.⁴¹ Catalysts with lower reduction temperatures may exhibit higher activity in Fischer–Tropsch synthesis (FTS). However, TPR results can be influenced by various experimental parameters including the heating rate, flow rate, and composition (specifically, H₂ concentration) of the reducing gas.⁴⁴ The variation of these factors will influence the reduction of temperature of the catalyst. Additionally, the reduction of temperature is influenced by factors such as the sample particle size and interactions between metal oxides and the support, assuming that other parameters remain constant.⁴⁵ Decrease in particle size led to lower temperatures, attributed to increased accessibility of the higher surface area for hydrogen gas interaction.

In this work, 10% (v/v) H₂/Ar reducing gas is used for TPR analysis. The deconvoluted TPR plot is shown in Fig. 5. Peak 2 (green line) corresponds to RuO₂ reduction (RuO₂ + H₂ → Ru + H₂O) in the presence of H₂ at a lower temperature of around 195 °C. A similar lower temperature reduction peak for RuO₂ is also observed by Panpranot et al.⁴⁶ Peak 1 (blue line) and peak 3 are associated with the two-steps reduction of Co₃O₄ (Co₃O₄ → CoO → Co⁰) at 245 °C and 151 °C.⁴⁷ However, the separate reduction temperature for each step is difficult to predict due to the nature of the support (KIT-6).

Fig. 5 H₂-TPR profile of the CoRu-KIT-6 catalyst.

Table 5 depicts the hydrogen consumption and reduction degree of the catalyst. TPR analysis yields a hydrogen consumption of ∼64.14 μmol g⁻¹. In the case of Co₃O₄ (peak 1) and RuO₂ (peak 2), the hydrogen consumptions are 20.64 and 8.24 μmol g⁻¹, respectively. The Co-based catalyst reducibility has been reported in the temperature range of 150 to 400 °C.⁴⁸ Most of the reaction temperatures used for the FTS process are in this temperature span. The overall reduction degree for this catalyst is 79.47% in this temperature range. As peak 1 and peak 2 are observed in this temperature range, we have calculated the reduction degree for the two peaks. These results show that 32.18% Co₃O₄ and 12.85% RuO₂ are reduced in this temperature range.

Table 5 H₂ consumption and reduction degree at 150–400 °C of the Co-Ru-KIT-6 catalyst

Catalyst	H2 consumption (μmol g^−1)	Reduction degree^a (%)
a The degree of reduction (expressed as a percentage) was defined, as per the literature,^48 as the calculated amount of H2 consumed between 150 and 400 °C in the TPR peak, divided by the theoretical H2 utilization (in μmol g^−1), and then multiplied by 100.
CoRu-KIT-6	64.14	79.47
Co3O4 (peak 1 & peak 3)	20.64	32.18
RuO2 (peak 2)	8.24	12.85

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3.5. XRD analysis

XRD analysis was performed to identify the crystal phases. The low-angle XRD patterns of the KIT-6 support and CoRu-KIT-6 catalyst are shown in Fig. 6(a). Mesoporous KIT-6 depicts characteristic diffraction peaks of 3D mesopores with cubic Ia3d silica, corresponding to the 2θ = 0.97° (211) plane.⁴⁹ The XRD pattern of the catalyst also exhibited a similar characteristic peak. This result indicates that after incorporation of metals (Co and Ru) into the mesoporous silica structure, the KIT-6 structure is conserved. Fig. 6(b) depicts the wide-angle XRD patterns of the KIT-6 support and CoRu-KIT-6 catalyst. In the sample of KIT-6, the peak corresponding to 2θ = 21.6° is due to amorphous silica.⁵⁰ For the catalyst, the peaks at 18.90° (111), 31.09° (220), 36.74° (311), 38.36° (222), 44.72° (400), 59.25° (511), and 65.26° (440) correlate with the Co₃O₄ cubic structure (JCPDS-42-1467).^34,51,52 The peaks at 28.18° (110) and 54.26° (211) correlate with the RuO₂ orthorhombic structure (JCPDS-88-0323).³⁴

Fig. 6 (a) Low-angle and (b) wide-angle XRD patterns of the KIT-6 support and CoRu-KIT-6 catalyst.

Table 6 shows the average crystal size (diameter) of the Co₃O₄ and RuO₂ crystals of the catalyst. The modified Scherrer equation was employed to calculate the average crystal size.⁵³ The crystal size of Co₃O₄ is quite a bit larger than the crystal size of RuO₂. This result also suggests that the noble metal crystal size is smaller than the transition metal size. The size is proportional to structural rearrangements such as substitutional, interstitial, and intermetallic alloy formation.³⁰ The crystal size obtained from the XRD analysis is validated by the FESEM and TEM analyses, ensuring the reliability of structural characterization of the catalyst.

Table 6 Crystal size calculation based on the XRD data

Catalyst	Crystal size^a (nm)
	Co3O4	RuO2
a Using the modified Scherrer equation.^53
CoRu-KIT-6	55.24	38.91

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3.6. XPS analysis

Fig. 7(a) presents the Co 2p XPS spectrum of the CoRu-KIT-6 catalyst. The data fitting was performed using a linear background subtraction and Gaussian–Lorentzian peak shapes. Two distinct peaks are observed at binding energies (BE) of 776.5 eV and 792.2 eV, corresponding to Co 2p_3/2 and Co 2p_1/2, respectively. The broad and asymmetric Co 2p_3/2 peak is further deconvoluted into two peaks at 775.6 eV and 778.4 eV, which are attributed to Co³⁺ and Co²⁺ species within the CoRu-KIT-6 catalyst.⁵⁴ Additionally, a weak satellite peak at 782.7 eV is detected, indicating the presence of Co²⁺ species and suggesting a strong interaction between Co²⁺ species and the KIT-6 support.⁵⁴ The Co 2p_1/2 peak is further deconvoluted into two peaks at 790 eV and 794.47 eV (shown as dotted lines), which are attributed to C³⁺ and C²⁺ species within the catalyst.⁵⁵Fig. 7(b) displays the Ru 3d XPS spectrum of the CoRu-KIT-6 catalyst, with a peak at 280.7 eV corresponding to the Ru 3d_3/2 core level.⁵⁶Fig. 7(c) shows the Si 2p XPS spectrum, featuring a single peak at a binding energy of 99.47 eV, confirming the presence of silicates in the CoRu-KIT-6 catalyst.

Fig. 7 XPS spectra of (a) Co 2p, (b) Ru 3d and (c) Si 2p of the CoRu-KIT-6 catalyst.

3.7. CO₂-TPD analysis

The basicity of the reduced catalyst was measured by CO₂-TPD analysis up to 800 °C, shown in Fig. 8. The desorption peak observed between 250 and 500 °C was designated to medium-strength basic sites regarded as metal–oxygen pairs.^57,58 According to the analysis, the medium basic sites are observed due to the addition of metals to the KIT-6 mesoporous silica support. The medium basic site was increased, which accelerates the oxygen mobility that can influence CO₂ activation.⁵⁹

Fig. 8 CO₂-TPD analysis of the CoRu-KIT-6 catalyst.

3.8. Fischer–Tropsch synthesis

3.8.1. Effect of temperature on catalyst activity. In general, the catalyst activity is enhanced with the increase of reaction temperature, but too high and low reaction temperatures are not suitable for any chemical reaction.⁶⁰ To find the optimal performance of the catalyst for various feed compositions, the effect of reaction temperature on catalyst activity was explored, focusing on selectivity with regard to hydrocarbons and the conversion of CO and CO₂.

To investigate the influence of temperature on catalyst activity based on CO conversion and product selectivity, reactions were carried out in the temperature range of 210 °C to 350 °C at high pressure (P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, H₂ : CO = 66.67 : 33.33 and GHSV = 6000 h⁻¹). Fig. 9 shows that the CO conversion increases with the increase of reaction temperature. This result is quite comparable to that reported by Pendyala et al.⁶¹ Shariati et al.⁶² suggested that the FTS rate depends on the available reduced Co sites on the catalyst surface. They proposed that larger Co particle sizes lead to greater selectivity for heavy hydrocarbons due to the easier adsorption of CO molecules. We have obtained higher C₅₊ selectivity in this temperature range. At 210 °C, the CO conversion (13.71%) and CH₄ formation (1.37%) were low due to inadequate energy at low temperature to activate the reactant on the catalyst surface.⁶³ At higher temperatures such as 330 °C and 350 °C, the CO conversion was almost stable, which suggests that the available thermal energy is no longer a limiting factor for the conversion. The system can be either in or out of equilibrium in this temperature range. Undesired products such as CH₄ formation are also high at higher temperature. In addition, the catalyst can be deactivated by sintering and coke formation over the catalyst surface at a higher temperature.^64,65 A very small amount of CO₂ and light olefins (C₂–C₄⁼) were also produced during CO hydrogenation. The appearance of the active site of Ru in the catalyst is very important. It also affects the increase of CO conversion in FTS by reducing the active site of Co particles.⁶²

Fig. 9 Effect of reaction temperature on CO conversion and product selectivity for the CoRu-KIT-6 catalyst (H₂ : CO = 66.67 : 33.33) (reaction conditions: P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹).

In order to investigate the effect of CO₂ concentration on FTS (Fig. 10–13), CO₂ was mixed with syngas from 10 to 70% (volume basis), keeping the H₂/CO volume ratio constant at 2; the reaction was carried out using the CoRu-KIT-6 catalyst in a stainless-steel microreactor (SSMR) at 20 bar, with a N₂ gas flow rate of 1.5 ml min⁻¹ and a GHSV of 6000 h⁻¹. The influence of temperature (200 to 350 °C) on catalyst activity was measured in terms of product selectivity and CO and CO₂ conversion. CO₂ selectivity was measured by the subtraction of the inlet amount from the outlet amount (not shown here).⁶⁶ The amount of CO₂ in inlet gas was 10, 25, 40 and 70% (volume basis). CO conversion increases with decreasing amount of CO in the feed gas, where CO₂ remains inactive towards hydrogenation at higher concentrations of CO (30 and 25% (v/v)).⁶⁷ Thus, the hydrocarbons were mainly formed from the CO hydrogenation reaction with the increase of reaction temperature. Some researchers^68,69 have also concluded that CO₂ acts as an inert gas in the presence of cobalt catalysts with a high amount of CO content in the feed stream. When the CO₂ concentration increases (>25% (v/v)), the CO conversion decreases, and thus it reduces the conversion of CO to hydrocarbons. The hydrocarbon production was observed to decrease when the CO₂ content in the feed gas increased over 25% (v/v). This behavior may be attributed to the shift in reaction pathways, where higher CO₂ concentrations thermodynamically favor the reverse water–gas shift (RWGS) reaction, which converts CO₂ to CO by consuming H₂. As a result, the availability of H₂ for Fischer–Tropsch synthesis (FTS) is reduced, potentially leading to a decrease in the hydrocarbon yield. While cobalt and ruthenium catalysts are known for their high FTS activity, they generally exhibit low intrinsic activity toward WGS and RWGS reactions. Therefore, at higher CO₂ concentration, the RWGS reaction may proceed only to a limited extent, depending on the temperature and catalyst surface interactions. The selectivity to C₅₊ escalated with the decrease of the CO₂ content (70 to 10% (v/v)) in the feed gas. These results are in good agreement with those reported by other researchers who worked on cobalt-based catalysts^67,69–71 for FTS. The selectivity to lighter olefins (C₂–C₄⁼) increased with the increase of CO₂ concentration.²² The conversion of CO₂ to hydrocarbons using its higher concentration (40 and 70% (v/v)) influenced the overall product distribution in the FTS reaction. Higher CO₂ concentration promotes the reverse water–gas shift (RWGS) reaction, which tends to form H₂O.^72,73 The presence of H₂O inhibits hydrogenation, which increases chain propagation and suppresses methane formation. The hypothesis that the presence of H₂O suppresses hydrocarbon selectivity in RWGS scenarios is valid but not exclusive to RWGS, as the FTS process itself generates water as a by-product. This is consistent with the lower CH₄ and C₂–C₄ selectivity at higher CO₂ concentration (40 and 70% (v/v)) in the feed stream. Here, the Co-based catalyst primarily acts on CO and relies on hydrogen for hydrocarbon formation.

Fig. 10 Effect of reaction temperature on CO and CO₂ conversion and product selectivity for the CoRu-KIT-6 catalyst using a lower CO₂ : CO ratio (H₂ : CO : CO₂ = 60 : 30 : 10) (reaction conditions: P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹).

Fig. 11 Effect of reaction temperature on CO and CO₂ conversion and product selectivity for the CoRu-KIT-6 catalyst using a higher CO₂ : CO ratio (H₂ : CO : CO₂ = 50 : 25 : 25) (reaction conditions: P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹).

Fig. 12 Effect of reaction temperature on CO and CO₂ conversion and product selectivity for the CoRu-KIT-6 catalyst (H₂ : CO : CO₂ = 40 : 20 : 40) (reaction conditions: P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹).

Fig. 13 Effect of reaction temperature on CO and CO₂ conversion and product selectivity for the CoRu-KIT-6 catalyst using a very high CO₂ : CO ratio (H₂ : CO : CO₂ = 20 : 10 : 70) (reaction conditions: P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹).

Fig. 14 shows the results from CO₂ hydrogenation at different reaction temperatures over the CoRu-KIT-6 catalyst under the same reaction conditions (P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹). The CO₂ conversion increases from 16.77% to 24.91% with the rise of reaction temperature from 200 to 350 °C. The hydrocarbon selectivity rises with the rise of temperature due to enhanced hydrogenation. The higher C₅₊ selectivity compared to CH₄, C₂–C₄⁼and C₂–C₄ suggests that the increase of temperature favors the carbon chain growth.⁷⁴ A considerable amount of CO is produced for the CO₂ hydrogenation process.

Fig. 14 Effect of reaction temperature on CO₂ conversion and product selectivity for the CoRu-KIT-6 catalyst (H₂ : CO₂ = 66.67 : 33.33) (reaction conditions: P = 20 bar, N₂ flow rate = 1.5 ml min⁻¹, and GHSV = 6000 h⁻¹).

Overall, the conversion of pure CO was much more than pure CO₂ conversion under the same operating conditions (from Fig. 10 and 14). This result suggests that CO is absorbed on the Co catalyst to a better extent than CO₂.⁶⁶ Whereas the CO₂ conversion increases little with temperature, the conversion of CO is almost 70% more than the total CO₂ conversion.

Overall, the activity of cobalt-based catalysts for the FTS process is affected by the addition of Ru. In terms of catalyst activity, the Co-Ru/KIT-6 catalyst retains high activity and low CH₄/CO₂ selectivity at 350 °C due to several stabilizing features. The mesoporous KIT-6 support provides structural confinement and thermal stability, while the elevated 5 wt% Ru loading enhances Co reducibility and suppresses reoxidation, as confirmed by H₂-TPR. TEM and XRD confirm that the ordered mesoporous framework and dispersed metal nanoparticles remain intact after high-temperature exposure. CO₂-TPD also indicates moderate basic sites that may assist in CO₂ activation. Taken together, these attributes explain the catalyst's robust performance and resistance to typical deactivation pathways at elevated temperatures.

4. Conclusions

This study elucidates the complex dynamics between reaction temperature, CO₂ concentration, and CO conversion in FTS. The catalyst's (mesoporous silica KIT-6 supported CoRu catalyst) ability to perform effectively under CO₂-rich conditions offers promising implications for sustainable syngas utilization. Four different compositions (volume based) of CO₂/CO/H₂ with a H₂/CO ratio of 2 were successfully studied. The other two compositions contained CO and CO₂ with H₂. The conversion of CO decreases with the rise in CO₂ concentration, while the selectivity to hydrocarbons increases with the rise in temperature. Hydrocarbon selectivity is affected by temperature for different CO₂/CO/H₂ volume mixtures, with higher temperatures favoring longer-chain hydrocarbons (C₅₊) due to enhanced chain growth and reduced methane (CH₄) formation. For CO/H₂ and CO₂/H₂ ratios, the temperature has less effect on the product selectivity due to differences in the feed composition. The conversion of CO₂ always remains low (below 30%) with the increase in temperature and its composition. These results suggest that CO₂ is produced during the WGS reaction, which can increase the amount of CO₂ during the FTS process. As a result, the excess amount of CO₂ is not able to affect the FTS process. The CO₂-rich syngas composition (CO₂ : CO : H₂ = 25 : 25 : 50) showed the best result based on the conversion and hydrocarbon selectivity. In summary, the CoRu-KIT-6 catalyst was successfully used with CO₂-rich syngas conversion in a microchannel microreactor for the FTS process. The influence of high surface area (from BET analysis) helped uniform active site distribution or the effect of lower reduction temperatures (from H₂-TPR analysis) enhanced the catalyst activity in terms of conversion and product selectivity.

Data availability

The data supporting the findings of this study are available within the article and its ESI.†

Author contributions

S. Bepari and N. Mohammad conceived and designed the experiments, carried out the catalytic activity tests, performed catalyst characterization, and drafted the manuscript. D. Kuila conceived the original idea, supervised the project and provided guidance throughout, with input from all co-authors. All authors critically reviewed and approved the final version of the manuscript.

Conflicts of interest

The authors have no conflicts of interest to declare.

Acknowledgements

This work is performed at North Carolina A&T State University and the Joint School of Nanoscience and Nanoengineering, a member of the Southeastern Nanotechnology Infrastructure Corridor (SENIC) and the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-1542174). The authors acknowledge the funding received from the National Science Foundation, NSF CREST (HRD1736173) and the University of North Carolina, Research Opportunity Initiative, UNC-ROI (#110092).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cy00673b

‡ These authors contributed equally to this work.

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