Carbon-Supported Fe-Based Catalyst for Thermal-Catalytic CO₂ Hydrogenation into C₂₊ Alcohols: The Effect of Carbon Support Porosity on Catalytic Performance

Abstract

Carbon materials supported Fe-based catalysts possess great potential for the thermal-catalytic hydrogenation of CO₂ into valuable chemicals, such as alkenes and oxygenates, due to the excellent active sites’ accessibility, appropriate interaction between the active site and carbon support, as well as the excellent capacities in C-O bond activation and C-C bond coupling. Even though tremendous progress has been made to boost the CO₂ hydrogenation performance of carbon-supported Fe-based catalysts, e.g., additives modification, the choice of different carbon materials (graphene or carbon nanotubes), electronic property tailoring, etc., the effect of carbon support porosity on the evolution of Fe-based active sites and the corresponding catalytic performance has been rarely investigated. Herein, a series of porous carbon samples with different porosities are obtained by the K₂CO₃ activation of petroleum pitch under different temperatures. Fe-based active sites and the alkali promoter Na are anchored on the porous carbon to study the effect of carbon support porosity on the physicochemical properties of Fe-based active sites and CO₂ hydrogenation performance. Multiple characterizations clarify that the bigger meso/macro-pores in the carbon support are beneficial for the formation of the Fe₅C₂ crystal phase for C-C bond coupling, therefore boosting the synthesis of C₂₊ chemicals, especially C₂₊ alcohols (C₂₊OH), while the limited micro-pores are unfavorable for C₂₊ chemicals synthesis owing to the sluggish crystal phase evolution and reactants’ inaccessibility. We wish our work could enrich the horizon for the rational design of highly efficient carbon-supported Fe-based catalysts.

1. Introduction

The resource utilization of CO₂ can alleviate the pressure of the environmental effects of CO₂ to a certain extent and, at the same time, provide a new green and sustainable pathway for high value-added chemical synthesis. The conversion of CO₂ into valuable chemicals, such as olefins, aromatics, gasoline, and alcohol, through thermocatalytic hydrogenation technology is an efficient means that stands out among various CO₂ utilization technologies due to the high conversion efficiency and promising industrial application [1,2,3,4,5,6,7,8,9,10,11,12]. Among these chemicals, C₂₊ alcohols (C₂₊OH) have received the attention of many scientists due to their wide range of uses. Most C₂₊ alcohols, including ethanol with a specific energy of 8.3 kWh/kg and an energy density of 6.7 kWh/L, hold significant economic value and an energy density comparable to gasoline (12.9 kWh/kg and 9.5 kWh/L) [13]. Many countries have used ethanol as a fuel additive or solvent for chemical products. Some C₂-C₅ alcohols can be used directly as a transportation fuel or blended with gasoline to increase the octane rating, thereby improving engine performance [14]. Furthermore, C₂₊ alcohols serve as essential raw materials for the production of various products, including plastics, plasticizers, and pharmaceuticals. However, the lack of highly efficient catalysts is still the bottleneck that limits the application of CO₂ hydrogenation into C₂₊OH technology at a large scale.

Significant progress has been made in the study of CO₂-to-C₂₊OH catalysts in recent years, mainly noble metal-based catalysts represented by Rh-based and transition metal-based catalysts represented by Co-, Cu-, and Mo-based. However, such catalysts are suffering from some inherent problems. Although the noble metal catalysts deliver high selectivity for C₂₊OH, their expensive catalyst costs and low CO₂ conversion rates have hindered their industrialization. Transition metal-based catalysts, which are relatively cost-effective, generally suffer from low C₂₊OH selectivity, and the catalytic stability needs to be further improved [15]. Many scientists are gradually studying Fe-based catalysts because of their simplicity, ease of obtaining, low price, and lack of environmental pollution. However, block Fe-based catalysts undergo significant nanoparticle agglomeration and particle size growth during the reaction process, leading to mechanical decomposition and stability loss [16]. Therefore, Fe-based species are usually immobilized on porous supports to prolong the stability. Due to the strong interaction between the metal and the support, the general silica and alumina supports are prone to irreversibly forming inactive substances during the reaction [17]. Nanostructured carbon materials are desirable for dispersing Fe-based nanoparticles due to the appropriate metal–support interaction, high surface area, tunable texture property, and controlled surface chemistry [18,19,20,21,22]. However, most of the current research is directly loading active metal onto carbon supports [23,24,25,26,27], and there is still a great challenge for the exploration of the domain-limiting effect of pore structure on the reaction properties.

This work loaded the Na-modified Fe-based species onto the carbon supports with different pore structures that were prepared by the K₂CO₃ activation of petroleum asphalt at different temperatures. The porous carbon support effectively controls the size and distribution of the Fe-based nanoparticles, in which carbon supports with larger meso/macro-pores could facilitate the sufficient penetration of the Fe-based component into the pore structure and improve the dispersion. Furthermore, the support porosity influences the differential adsorption of H₂ and CO₂ molecules on the catalyst surface, with the CO₂-rich and H₂-poor environments near the Fe-based species promoting the evolution of the Fe₅C₂ phase as the active site, thereby enhancing CO₂ hydrogenation performance. This result reveals the influence of carbon support porosity on the catalytic performance and provides an important guideline for future in-depth investigations into carbon-supported Fe-based catalysts for thermal-catalytic CO₂ hydrogenation.

2. Results and Discussion

2.1. Structural Characterization of Catalysts

Figure 1a illustrates the preparation process of the carbon-supported Fe-based catalysts. Initially, petroleum asphalt underwent pyrolysis via the one-step activation method using K₂CO₃ at varying temperatures under an N₂ atmosphere. The resulting carbon supports were denoted as MCx (x represents the pyrolysis temperature, x = 700/800/900/1000 °C). Subsequently, the Fe-based active sites were loaded onto a carbon support by the impregnation method followed by carbonization treatment at 550 °C for 3 h under an N₂ atmosphere and further impregnation with the Na promoter to obtain the catalyst named NaFe/MCx.

The N₂ adsorption–desorption test was performed on petroleum asphalt-derived porous carbon MCx to explore the effect of K₂CO₃ activation temperature on pore structure (

Table 1. Texture properties of MCx.

Catalyst	SBET (m2 g−1)	SBETmicro (m2 g−1)	SBETmeso (m2 g−1)	Vmicro (cm3 g−1)	Vmeso (cm3 g−1)	dsize (nm)
MC700	969.04	926.45	42.59	0.34	0.06	1.67
MC800	1254.20	1178.86	75.34	0.44	0.09	1.70
MC900	1732.19	1492.01	240.18	0.59	0.20	1.83
MC1000	1645.13	734.38	910.74	0.25	0.68	2.54

). MC700 and MC1000 delivered the lowest and highest N₂ adsorption–desorption capacity, respectively (Figure 1b,c and Table 1). For MC700, the micropores dominated the pore structure with a 926.45 m² g⁻¹ specific surface area of micropores. The specific surface area of mesopores accounted for only 4% of the total. Obviously, the percentage of mesopores in MCx increased with the increase in pyrolysis temperature. The specific surface area of the mesopores in MC1000 increased to 910.74 m² g⁻¹, constituting 55% of the total specific surface area. The increasing mesopores’ specific surface area indicated that the pore structure of MCx, especially the mesoporous property, can be modulated by adjusting the pyrolysis temperature. Despite the high surface area of microporous-dominant materials, their CO₂ trapping ability and kinetic performance may be unsatisfactory due to the insufficient diffusion of CO₂ molecules into the core of micropores [28]. Meso/macro-pores, conversely, possess a larger pore volume, exhibit excellent CO₂ trapping capacity under high pressure, facilitate faster mass transfer [29], and provide sufficient sites for anchoring Fe-based active sites, thus enhancing accessibility to catalytically active sites [30].

Transmission electron microscope (TEM) characterization was performed on the fresh NaFe/MCx catalysts to observe the distribution of Fe-based particles on the carbon support (Figure 2). Based on the findings from Figure 1 and Table 1, the specific surface area of the MC700 catalyst was the smallest, with micropores dominating the pore structure. This prevents most of the Fe-based particles from entering the pores of the carbon support, resulting in large particle sizes accumulated outside the carbon support with the largest average diameter of 15.37 nm (Figure 2a). With the pyrolysis temperature increased, the average pore size of the carbon support was enlarged. Therefore, the Fe-based nanoparticles were more easily dispersed into the pore structure, causing the reduced average diameter of the Fe-based active sites (Figure 2b,c). A TEM image of NaFe/MC1000 revealed uniformly dispersed Fe-based nanoparticles without an apparent accumulation on the carbon support. The Fe-based nanoparticles with an average diameter of 8.69 nm were well dispersed on the carbon supports and encapsulated in the pores of MC1000 (Figure 2d). The uniform distribution of Fe-based active sites inside the pores of carbon supports could enhance the provision of catalytic active centers that are easily accessible to gaseous reactants, thus improving the catalytic performance of thermal-catalytic CO₂ hydrogenation.

X-ray diffraction (XRD) characterization elucidated the crystal phase of NaFe/MCx catalysts (Figure 3a). The fresh NaFe/MCx catalyst was primarily composed of the Fe₃O₄ phase. With the increase in the activation temperature, the dispersion of the Fe-based component increased, accompanied by a decrease in Fe₃O₄ diffraction peak intensity, which is consistent with the TEM characterization results. Fe₃O₄ is the main active site of the reverse water–gas shift (RWGS, CO₂ + H₂ → CO + H₂O) reaction, which dissociates CO₂ into CO for the subsequent Fischer–Tropsch synthesis [31]. As the reaction proceeds, Fe sites undergo gradual carburization, transforming from Fe₃O₄ to Fe-based carbide phases. H₂-temperature programmed reduction (H₂-TPR) recorded the reduction behavior of the catalysts. As shown in Figure 3b, the H₂-TPR curves showed the multi-step reduction processes of Fe₃O₄ in catalysts [32,33]. The exposure degree of Fe-based active sites under the reducing atmosphere determines the reduction behavior of the catalyst. Benefitting from the smallest particle size, highest intra-pore dispersion, and substantial pore volume, NaFe/MC1000 exhibited superior reduction behavior among all samples, with a slight shift in the reduction peak towards lower temperatures. Conversely, larger Fe-based nanoparticles entering the pore structure diminished Fe-based active sites’ ability to contact H₂, resulting in a wider reduction process of NaFe/MC900 and NaFe/MC800. The largest catalyst particle size of NaFe/MC700 hindered the reduction behavior but primarily stacked outside pore structures, maintaining relatively strong H₂ contact, hence exhibiting a significantly shorter reduction interval [34,35,36].

Figure 3c,d represent the CO-temperature programmed reduction (CO-TPR) profiles of the NaFe/MCx catalysts and the XRD patterns of the catalysts after CO-TPR, respectively. The peaks that appeared in the CO-TPR curves represented the consumption of CO, indicating that CO can reduce the Fe-based nanoparticles for carburization and form Fe-based carbide compounds during the CO-TPR process. The appearance of Fe₃C-related peaks in the XRD patterns of the CO-TPR catalysts also confirmed this conclusion. Notably, the NaFe/MC1000 catalyst delivered the lowest CO-TPR temperature, implying that the smallest Fe-based nanoparticles endowed by the mesoporous carbon support were easier to be reduced and carbonized by CO molecules. However, for the NaFe/MC700 catalyst, no obvious CO-TPR peaks were detected due to the difficulty of the reduction and carbonization of the largest Fe-based nanoparticles that stacked outside the micropores.

2.2. Catalytic Performance

The Fe-based component served as the active site, facilitating the dissociation of CO₂, carbon chain growth, and the insertion of oxygen-containing intermediates. Consequently, the Na/MC1000 sample was inactive due to the absence of these crucial sites (

Table 2. Catalytic performance of the NaFe/MCx catalysts in CO₂ hydrogenation.

Catalyst	CO2 Conv. (%)	CO Sel. (%)	Hydrocarbons				MeOH	C2+OH	C2+OH STY mg gcat−1 h−1	Yield
			CH4	C2–4 =	C5+ =	Paraffin
NaFe/MC700	6.1	87.0	90.9	1.1	0.0	8.0	0.0	0.0	0.0	0.0
NaFe/MC800	17.5	78.2	31.2	7.6	1.5	33.6	6.6	19.5	15.2	0.7
NaFe/MC900	22.3	62.1	23.8	17.3	5.7	29.2	2.8	21.3	36.2	1.8
NaFe/MC1000	22.8	70.8	22.5	28.2	3.9	20.0	2.7	22.6	30.3	1.5
Fe/MC1000	18.4	87.05	36.8	30.3	1.3	31.2	0.1	4.5	0.8	0.8
Na/MC1000	1.6	-	-	-	-	-	-	-	-	-

Reaction conditions: 320 °C, 5 MPa (23.75% CO₂, 71.23% H₂, and 5.02% Ar), 15 mL min⁻¹, GHSV = 9000 mL gcat⁻¹ h⁻¹, 1 g quartz sand, and time on stream (TOS) = 24 h. Catalyst weight: 0.1 g.

). Na acts as a promoter, increasing the surface basicity of Fe catalysts, facilitating the formation of iron carbide phases, and inhibiting the over-hydrogenation process by strengthening the desorption of desirable products (especially alkenes) [37,38,39]. As a result, Fe/MC1000 catalysts without Na doping exhibited lower C₂₊ alcohols selectivity and higher selectivity of CH₄ and paraffins (Table 2). Although the contents of Fe and Na were consistent among different catalysts, they showed completely different catalytic performance, as shown in Table 2 and Figure 4. NaFe/MC700 exhibited a CO₂ conversion of 6.1% and the highest CH₄ selectivity of 90.9%, devoid of valuable oxygenates in the product. As the pyrolysis temperature of the carbon support increased, the CO₂ conversion rate and C₂₊OH selectivity gradually rose, while CH₄ selectivity declined. NaFe/MC1000 exhibited the highest CO₂ conversion (22.8%) and selectivity towards C₂₊OH (22.6%), alongside the lowest CH₄ selectivity (22.5%). Furthermore, over a 48 h testing period, the CO₂ conversion and product distribution of NaFe/MC1000 remained stable without being impacted by carbon deposition or pore blockage (Table 2 and Figure 4c,d). We believe that the pore structure of the catalysts endowed by different pyrolysis temperatures plays a decisive role in the reaction performance, and the increased proportion of the mesopores in the catalysts will improve the conversion of CO₂ and the selectivity of C₂₊OH.

Figure 4b exhibits the trend of the CO₂ conversion rate of NaFe/MCx. The observed gradual increase in CO₂ conversion over time can be attributed to the induction period required by the Fe-based catalyst to achieve a stable active state (Figure 4b). This process involves the remodeling of the catalyst surface, during which the Fe-based catalyst undergoes carburization during the reaction and gradually forms surface active sites represented by Fe₃O₄ and Fe₅C₂ to adsorb and activate CO₂ molecules [40]. The CO₂ conversion rate of NaFe/MC700 markedly declined after 6 h of reaction initiation. The CO₂ conversion rate of NaFe/MC700 markedly declined after 6 h of reaction initiation. This trend can be attributed to the predominantly microporous structure of NaFe/MC700. The microporous structure limits the accessibility and evolution of the Fe-based active sites, resulting in suboptimal catalytic performance. Additionally, catalysts with a primarily microporous structure are more prone to significant deactivation due to the serious carbon deposition phenomenon [41].

2.3. Influence of Catalyst Pore Structure on Catalytic Performance

Figure 5 reflects the morphology of Fe-based particles in the spent NaFe/MCx catalysts. The TEM images revealed no significant aggregation of Fe-based particles on the carbon supports, indicating that the proper interaction between carbon supports and metal particles effectively prevents agglomeration during the reaction. The particle sizes increased to varying degrees after the CO₂ hydrogenation reaction, indicating distinct evolution patterns of Fe species driven by different pore-structured supports. Specifically, for NaFe/MC1000 catalysts (Figure 5d), the prevalence of mesopores promotes significant Fe species evolution during reaction, as evidenced by the obvious growth of Fe-based particle size.

N₂ adsorption–desorption tests were conducted on fresh catalysts to explore the pore structure of the NaFe/MCx catalysts (Figure 6a,b). NaFe/MC700 and NaFe/MC900 exhibited the lowest and highest N₂ adsorption–desorption capacities, respectively. NaFe/MC700 had the lowest specific surface area of 710.77 m² g⁻¹, with micropores dominating the pore structure. The specific surface area of NaFe/MCx was gradually increased with the increase in the activation temperature, but exhibited a reduction when carbon support pyrolysis reached 1000 °C. This reduction could be attributed to the graphitic carbon layer formation in the NaFe/MC1000 catalyst as the activation temperature reached 1000 °C. Additionally, the adsorption hysteresis loop appeared in the N₂ adsorption–desorption curve of the NaFe/MC1000 (Figure 6a), which indicates that the pores retained after pyrolysis are mainly mesoporous [41]. Notably, the mesoporous specific surface area of NaFe/MC1000 reached the highest value of 357.49 m² g⁻¹ (

Table 3. Texture properties of NaFe/MCx.

Catalyst	SBET (m2 g−1)	SBETmicro (m2 g−1)	SBETmeso (m2 g−1)	Vmicro (cm3 g−1)	Vmeso (cm3 g−1)	dsize (nm)	Fe Content a (%)
NaFe/MC700	710.77	649.34	61.43	0.25	0.09	1.93	22.1
NaFe/MC800	916.97	851.07	65.90	0.33	0.10	1.80	19.9
NaFe/MC900	1224.74	1024.59	200.15	0.40	0.19	1.92	20.5
NaFe/MC1000	862.76	505.27	357.49	0.19	0.36	2.55	21.3

^a Element content in catalysts as characterized by ICP-AES.

and Figure 6b). The changes in the pore structure of the carbon support during pyrolysis were related to the activation of K₂CO₃. During the chemical activation process, a higher activation temperature intensified the K₂CO₃ activation effect, enhancing etching and pore creation in carbon material [42].

The adsorption capacity of carbon-supported Fe-based catalysts to the feedstock gas components (CO₂ and H₂) significantly influences their reaction performance. Accordingly, physical adsorption–desorption tests for CO₂ and H₂ were conducted. The results indicated that the adsorption capacities for both CO₂ and H₂ increased with the pressure rising to 3 bar (Figure 6c,d). Notably, the CO₂ adsorption capacity consistently surpassed that for H2, attributable to the inherent properties of Fe-based catalysts. Incorporating transition metal oxides like Cu and Ni into the carbon support enhances CO₂ adsorption capability [43,44,45,46]. Similarly, Fe-based species on the carbon support also enhance CO₂ adsorption capability significantly. NaFe/MC700 exhibited the lowest CO₂ adsorption capacity, while its adsorption capacity for H₂ was higher (Figure 6c,d). H₂ is more readily diffused into the micropore-dominated structure of NaFe/MC700, creating an H₂-rich environment near the active sites during reaction and leading to the over-hydrogenation phenomenon. This behavior corresponds well with the high CH₄ selectivity observed in NaFe/MC700 (Table 2). Conversely, the mesoporous NaFe/MC1000 catalyst demonstrated the highest H₂ physisorption and substantial CO₂ physisorption, and the larger meso/macro pores in the carbon support facilitated the effective diffusion of both gases, matching the rates of CO₂ activation and hydrogenation to boost the reaction performance (Figure 6c,d). Consequently, the CH₄ selectivity was reduced, and the selectivity for C₂₊OH increased to 22.6% (Table 2), underscoring the role of an enhanced mesoporous structure in promoting the formation of C₂₊OH.

The temperature-programmed desorption mass spectrometry (TPD-MS) of CO₂/H₂ was conducted on NaFe/MCx catalysts to examine the chemisorption behavior of the feed gas components. As the pyrolysis temperature increased, the position of the CO₂ desorption peak shifted toward lower temperatures (Figure 7a), with the NaFe@MC1000 catalyst exhibiting the lowest CO₂ desorption temperature due to the higher mesopore content. The H₂ desorption peak for the NaFe/MC700 catalyst shifted to higher temperatures and intensified (Figure 7b), indicating strong H₂ adsorption on the catalyst surface, likely facilitating H₂-rich environments conducive to the over-hydrogenation of -CO* intermediates and predominant production of CH₄ (Table 2). As the pyrolysis temperature increased, the catalyst’s H₂ adsorption capacity decreased, establishing a catalytic interface with moderate hydrogenation potential that effectively suppressed undesirable hydrogenation reactions and enhanced carburization effects.

Figure 7c,d show the XRD and X-ray Photoelectron Spectroscopy (XPS) profiles of the spent NaFe/MCx catalysts. For the spent catalysts, Fe₃O₄ was the mainly oxide phase, while Fe₃C and Fe₅C₂ were mainly the Fe-based carbide phases. The XPS characterization of the spent NaFe/MCx catalysts revealed the formation of Fe-C bonds in all catalysts, indicating the presence of Fe-based carbide compounds, consistent with the XRD results. There was almost no formation of the Fe-based carbide phase in the predominantly microporous NaFe/MC700 catalyst, and the signals of the Fe-based carbide phase in the spent catalysts became more pronounced with the increase in mesopores. As is known, the active phases of Fe₃O₄ and Fe-based carbide make outstanding contributions to the thermal-catalytic CO₂ hydrogenation reaction, in which CO₂ molecules are converted to -CO* intermediates by the Fe₃O₄ active site via the RWGS reaction. Subsequently, -CO* carburizes the Fe₃O₄ active phase to carbides dominated by the Fe₃C and Fe₅C₂ phases with H₂-assisted action [47,48], while the Fe-based carbides dissociate or non-dissociatively activate the -CO* intermediates to -CH_x* or -CH_yO* (x = 1, 2, or 3, and y = 0, 1, or 2), respectively, and further C-C coupling with subsequent hydrogenation steps occur to finalize the synthesis of C₂₊OH [3,49].

3. Materials and Methods

3.1. Materials

Petroleum asphalt was supplied by the Sinopec Jiu Jiang Company (Jiu Jiang, China). Potassium carbonate (K₂CO₃, AR) and sodium carbonate (Na₂CO₃, AR) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O, AR) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All reagents were used directly without further purification.

3.2. Catalyst Preparation

The carbon supports were prepared by a one-step heat-treatment method using K₂CO₃ chemical activation. First, 1 g of petroleum asphalt and 4 g of K₂CO₃ were ground and mixed thoroughly, then heated up to 700/800/900/1000 °C for 2 h in a tube furnace under an N₂ atmosphere at a heating rate of 5 °C·min⁻¹ to obtain the black samples. Subsequently, the samples were washed with deionized water for 12 h at 70 °C and filtered three times to remove unreacted salts, ensuring that no other elements in the catalyst interfered with the experiments. Then, the samples were dried in an oven at 60 °C for 12 h to obtain carbon supports named MC700/800/900/1000, respectively. In the next step, the carbon supports were impregnated with a 15% Fe(NO₃)₃.9H₂O solution, dried at 70 °C for 12 h, and then pyrolyzed at 550 °C under an N₂ atmosphere for 3 h. The Na-modified carbon-supported Fe-based catalysts, named NaFe/MCx (x represents the temperature of the calcination, x = 700/800/900/1000), were then fabricated by the impregnation method under the conditions of 3% Na₂CO₃, a 70 °C thermal treatment, and a 12 h reaction time. Finally, all catalysts were pressed, crushed, and sieved to 20–40 meshes for the CO₂ hydrogenation catalytic performance test.

3.3. Catalyst Characterization

The X-ray diffraction (XRD) patterns of the catalysts’ powder were characterized with a Rigaku RINT 2400 X-ray diffractometer using Cu K α-radiation. Transmission electron microscopy (TEM, JEM-2100UHR, JEOL) was used to observe the morphologies of the catalysts before and after the reaction. The crystal phase structures found in the experiments were determined using the PDF–4+ (2019) crystal database and then matched using Highscore Plus software. X-ray photoelectron spectroscopy (XPS, ThermoFisher (Beijing, China), ESCALAB 250Xi) analysis of the reacted catalysts was recorded to determine the elemental composition and valence changes. The specific surface areas and pore size distributions of the catalysts were measured by a Micromeritics 3Flex 2 M P instrument. Before the measurements, the catalysts were degassed at 200 °C for 6 h. The element content of NaFe/MCx catalysts was determined by an inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent ICP-720ES). The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The average pore size and pore volume were calculated using the Barrett–Joyner–Halenda (BJH) method. The pore size distribution was analyzed by the BJH (mesopores) and Horváth–Kawazoe (HK, micropores) methods.

At different temperatures and pressures, the gas adsorption isotherms for CO₂ and H₂ were determined using the volumetric method on a high-pressure gas adsorption apparatus (BSD-PH, BeiShiDe Instrument Co., Ltd., Beijing, China). The procedure involved loading approximately 0.1 g of the powder sample into the sample tube. Before initiating the gas adsorption test, the sample was placed in a heating furnace, warmed to 200 °C, and activated under vacuum conditions for 5 h. During the adsorption test, the adsorption temperature was precisely controlled using a program-controlled water bath jacket. Various pressure set points were maintained through computer program-controlled procedures.

CO/H₂ temperature-programmed reduction (CO/H₂-TPR) and CO₂/H₂-temperature-programmed desorption and mass spectra (CO₂/H₂-TPD-MS) were performed on a PCA-1200 instrument connected to an MS-200 mass spectrometer (Beijing Builder, Beijing, China). For CO₂/H₂-TPD, a 30 mg catalyst was pretreated at 150 °C for 30 min under a flow of pure Ar to remove the physically adsorbed water and organic products on the spent catalyst. Then, the sample was saturated with CO₂/H₂ at 50 °C for 1 h. After removing the physically adsorbed CO₂/H₂ by a He flow, the CO₂/H₂-TPD curve was collected under the He flow (30 mL min⁻¹) with a heating rate of 10 °C min⁻¹. The signals of the desorbed H₂ (m/z = 2) and CO₂ (m/z = 44) were detected by MS and a thermal conductivity detector (TCD). CO/H₂-TPR was carried out in a stream of 10% CO/H₂ in Ar with a heating rate of 10 °C min⁻¹.

3.4. Catalytic Evaluation

The catalytic activity tests were performed on a continuous flow type fixed-bed reactor with an inner diameter of 6 mm. For the catalytic performance tests, 0.1 g of NaFe/MC700/800/900/1000 catalysts and 1 g of quartz sand were fixed in the middle of the reactor by quartz wool, with 2 g of quartz sand used as a spoiler above the catalyst bed. Before the reaction, the catalyst was reduced by pure H₂ at 400 °C for 4 h. After cooling to room temperature, the reactant gas (23.75% CO₂, 71.23% H₂, and 5.02% Ar) was fed into the reactor until the pressure reached 5 MPa. At the same time, the temperature of the reactor was increased to 320 °C. The heavy hydrocarbons were collected by the ice trap set between the reactor and the back pressure valve, and then analyzed by an off-line gas chromatograph (GC9790II) equipped with an FID and an InerCap-5 capillary column (GL Sciences (Shanghai, China), 0.25 mm × 30 m). The exhaust gas was analyzed by two on-line gas chromatographs (Fuli 9790II, Zhejiang Fuli Analytical Instruments Co., Ltd., Taizhou, China), in which one was equipped with a TCD detector and an active charcoal column for the analysis of Ar, CO, CH₄, and CO₂, and the other was equipped with an FID detector and a Porapak-Q column for the analysis of light hydrocarbons. CO₂ or CO (denoted as CO_x, x = 1 or 2) conversion and product selectivity were calculated using the following equations.

(1) CO₂ conversion was calculated according to the following:

$${\mathrm{C}\mathrm{O}}_2 \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{{\mathrm{C}\mathrm{O}}_{2\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}-{\mathrm{C}\mathrm{O}}_{2\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}{{\mathrm{C}\mathrm{O}}_{2\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}}}\times 100\mathrm{\%}$$

where ${\mathrm{C}\mathrm{O}}_{2\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}$ and ${\mathrm{C}\mathrm{O}}_{2\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}$ represent the moles of CO₂ at the inlet and outlet, respectively.

(2) Product selectivity, the percentage of CO₂ converted into a given product, was calculated as follows:

$${\mathrm{S}\mathrm{e}\mathrm{l}}_{\mathrm{i}}={\displaystyle \frac{{\mathrm{N}}_{\mathrm{i}}\times {\mathrm{n}}_{\mathrm{i}}}{{\sum }_1^{\mathrm{i}}({\mathrm{N}}_{\mathrm{i}}\times {\mathrm{n}}_{\mathrm{i}})}}\times 100\mathrm{\%}$$

where ${\mathrm{N}}_{\mathrm{i}}$ and ${\mathrm{n}}_{\mathrm{i}}$ represent the mole percentage and carbon number of product i.

(3) CO selectivity (CO selectivity for CO₂ hydrogenation) was calculated according to the following:

$$\mathrm{C}\mathrm{O} \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}={\displaystyle \frac{{\mathrm{C}\mathrm{O}}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}{{\mathrm{C}\mathrm{O}}_{2\mathrm{i}\mathrm{n}\mathrm{l}\mathrm{e}\mathrm{t}}-{\mathrm{C}\mathrm{O}}_{2\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}}\times 100\mathrm{\%}$$

The carbon balances of the reaction data were calculated, and all were higher than 90%. Typically, the experimental data after the reaction of 24 h were used for discussion.

4. Conclusions

Porous carbon support Na-modified Fe-based catalysts were prepared by K₂CO₃ activation combined with the impregnation method using cheap and readily available petroleum asphalt as a carbon source. The fabricated catalysts were applied in the CO₂ hydrogenation reaction to obtain high value-added C₂₊OH chemicals. Varying the activation temperature of petroleum asphalt obtained NaFe/MCx catalysts with different pore structures. The NaFe/MC1000 catalyst with the highest percentage of mesopores showed the most excellent catalytic activity. Under the reaction conditions of 320 °C, 5 MPa, and GHSV = 9000 mL g_cat⁻¹ h⁻¹, the CO₂ conversion was 22.8%, and the C₂₊ alcohol selectivity was 22.6%. The characterization results showed that the pore-limiting role of carbon support influenced the size and distribution of Fe-based nanoparticles and the adsorption pattern of reactive gases, which in turn affects the formation of the Fe₅C₂ active phase by carburization and, ultimately, the performance of CO₂ hydrogenation. The catalysts with a larger mesoporous specific surface area exhibited higher CO₂ conversion and C₂₊OH selectivity due to the preferential formation of the Fe₅C₂ crystalline phase for C-C coupling. We wish our work could provide guidance for the rational design of the carbon-supported Fe-based catalyst for CO₂ hydrogenation into C₂₊OH.