Synergistic Ru–Fe–Ca interfaces enable low-temperature CO₂ methanation over γ-Al₂O₃-supported nanocomposites

Abstract

Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ nanocomposites for CO₂ methanation were prepared by wet impregnation. Catalytic performance evaluation was carried out under atmospheric pressure with an H₂ : CO₂ ratio of 4 : 1 over a temperature range of 50–300 °C. The findings showed that the Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst calcined at 1000 °C achieved 85.59% CO₂ conversion and 79.58% methane yield at 300 °C. XRD and BET analyses revealed mesoporous features, with a surface area of 34.58 m² g⁻¹. FESEM analysis further showed that the catalyst had a circular morphology, with particle sizes ranging from 65 to 87 nm. EDX confirmed the distribution of the constituent elements, while XRD identified RuO₂ as a prominent Ru-containing phase associated with enhanced catalytic performance. The mechanistic study showed that carboxylate species are important intermediates in FTIR studies, which are hydrogenated and ultimately lead to methane formation. The results indicate that catalytic performance is associated with synergistic interactions among Ru-, Fe-, and Ca-containing species and their interaction with the γ-Al₂O₃ support, enabling efficient CO₂ conversion and CH₄ formation at 300 °C. Further studies of long-term stability, carbon deposition, and catalyst regeneration are needed to assess the catalyst's durability under continuous methanation conditions.

Synergistic Ru–Fe–Ca interfaces enable efficient low-temperature CO₂ methanation over γ-Al₂O₃ supported nanocomposites, achieving 85.59% CO₂ conversion and 79.58% CH₄ yield at 300 °C through carboxylate-mediated hydrogenation.

Introduction

The increasing concentration of carbon dioxide (CO₂) in the atmosphere is the main driving force behind the ongoing and unprecedented rapid global warming.¹ The catalytic transformation of CO₂ into fuel has become an important objective, as it can help improve the global carbon balance by converting CO₂ into valuable fuel products.² The methanation of CO₂ is commonly referred to as the Sabatier reaction, which was originally introduced by Sabatier and Senderens.³ CO₂ methanation is considered an important catalytic process because of its scientific relevance and strong potential for industrial applications.⁴ It is well established that the presence of basic sites in catalysts enhances the chemisorption of CO₂, which helps accelerate the reaction rate and improves overall catalytic performance.^5–8 Calcium-based catalysts have attracted significant industrial interest because calcium is low-cost and readily available. Among them, CaO, a typical alkaline earth metal oxide, is widely recognized as an effective promoter for different catalysts due to its strong CO₂ chemisorption ability.^9,10

Studies have found that the iron surface is most suitable for CO₂ activation. This behavior is associated with a considerable charge transfer (0.83e⁻) from the iron surface to the CO₂ molecule, which leads to the elongation of C–O bonds (1.30 Å) along with bending of the CO₂ molecule.^11,12 Ruthenium catalysts are the most stable in a wide temperature range.¹³ In industrial applications, a catalyst with high activity, selectivity, and long-term stability is highly desirable. To achieve this, the catalyst support should possess sufficient surface area and strong structural properties to ensure high metal dispersion and durability. The support also plays a significant role in determining interactions between metal oxides and the support material. At present, γ-Al₂O₃ is commonly used as a support because of its large surface area and its widespread application in the preparation of metal catalysts in industry.^12,14–16

In the present study, trimetallic Ru–Fe–Ca catalysts supported on γ-Al₂O₃ was investigated to examine their structural properties, catalytic performance, and reaction mechanism. Three important parameters, including calcination temperature, dopant loading, and catalyst loading, were systematically optimized. The prepared catalysts were characterized using BET surface area analysis, XRD, field emission scanning electron microscopy coupled with EDX (FESEM-EDX), and in situ Fourier transform infrared (in situ FTIR) spectroscopy. In addition, we explored the relationship between catalytic activity and the presence of Ru species on the catalyst surface during CO₂ methanation.

Experimental section

Materials

Ruthenium(iii) chloride hydrate (RuCl₃·H₂O, 99.95% purity) was purchased from Acros Organics. Iron(iii) nitrate nonahydrate (Fe(NO₃)₃·9H₂O, ≥98% purity) was obtained from QReC. Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O, 99% purity) was supplied by Merck. γ-Al₂O₃ beads with a diameter of 3 mm were provided by Sigma-Aldrich. Deionized water was used as the solvent throughout the catalyst preparation.

Catalyst preparation

The Ru/Fe/Ca catalysts supported on γ-Al₂O₃ (diameter 3 mm) were prepared by the wet impregnation method. For the preparation of Ru/Fe/Ca (5 : 25 : 70)/Al₂O_3, precisely 0.125 g of RuCl₃·2H₂O, 2.194 g of Fe(NO₃)₃·9H₂O and 5 g of Ca(NO₃)₂·4H₂O were dissolved in 10 mL of deionized water. The precursor salt quantities were calculated from their molecular weights, elemental compositions, and the target nominal Ru/Fe/Ca mass ratio of 5 : 25 : 70. The resulting mixture was stirred for 30 min at room temperature. Subsequently, 10 g of γ-Al₂O₃ beads were immersed in the catalyst solution for 30 minutes. After impregnation, each sample was aged at 90 °C for 24 h, then calcined at 1000 °C for 5 h at a heating rate of 10 °C min⁻¹. The prepared catalyst was labeled Ru/Fe/Ca (5 : 25 : 70)/Al₂O₃, with nominal Ru, Fe, and Ca contents of 5, 25, and 70 wt%, respectively. The same calculation procedure was used to determine the exact precursor masses required for the preparation of Ru/Fe/Ca (5 : 40 : 55)/Al₂O₃,Ru/Fe/Ca (5 : 35 : 60)/Al₂O₃, Ru/Fe/Ca (5 : 30 : 65)/Al₂O₃, Ru/Fe/Ca (5 : 20 : 75)/Al₂O₃, Ru/Fe/Ca (5 : 15 : 80)/Al₂O₃ and Ru/Fe/Ca (5 : 10 : 85)/Al₂O₃.

The specific surface area was measured using the BET method with a Micromeritics ASAP 2010 instrument through N₂ adsorption–desorption isotherms. Before analysis, the catalyst samples were degassed at 120 °C for 6 hours to remove adsorbed moisture from both the surface and porous structure. The crystalline phases were examined by X-ray diffraction using a Rigaku diffractometer (λ = 1.5418 Å, current = 100 mA, voltage = 40 kV) with Cu Kα radiation over a scanning range of 2θ = 20–80°. Surface morphology was analyzed using a Zeiss Supra 35 VP FESEM operated at 5.0 kV, while elemental composition was determined by Energy Dispersive X-ray Spectroscopy (EDX).

Catalytic performance

CO₂ methanation was carried out using an online FTIR system (Brand: Nicolet Avatar 670 DGTS spectrophotometer) under atmospheric pressure within a temperature range of 50–300 °C. For the experimental study, 10 g of γ-Al₂O₃-supported catalyst was loaded into a reactor tube. Initially, the catalyst was pretreated in the presence of H₂ (40 mL min⁻¹) by heating from room temperature to 300 °C with a heating rate of 5 °C min⁻¹ and maintaining that temperature for 30 minutes. After temperature stabilization, a gas mixture of H₂ and CO₂ in a 4 : 1 ratio was introduced into the reactor. The composition of the outlet gases was then monitored using the online FTIR Nicolet Avatar 670 DGTS system. The CH₄ concentration in the outlet stream was quantified from the characteristic CH₄ absorption bands in the FTIR spectra using a calibration curve prepared from standard CH₄ concentrations under the same measurement conditions. The CH₄ formation percentage was calculated from the measured outlet CH₄ concentration relative to the inlet CO₂ concentration according to eqn (1). The schematic representation of the homemade setup is presented in Fig. 1.

1

Fig. 1. Schematic diagram of methanation of CO₂.

Results and discussion

Optimization of Ru/Fe/Ca/Al₂O₃ catalyst

Effect of calcium oxide loading

The influence of CaO loading was examined to determine the optimum amount required for efficient CO₂ conversion. Fig. 2 presents the detailed trend of CaO loading and its effect on the CO₂ conversion performance of the Ru/Fe/Ca/γ-Al₂O₃ catalyst. The CaO content varied as 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, and 85 wt%. All catalyst samples were calcined at 1000 °C for 5 h using a heating rate of 10 °C min⁻¹.

Fig. 2. Catalytic performance of CO₂ conversion over Ru/Fe/Ca/γ-Al₂O₃ catalyst calcined at 1000 °C for 5 hours with various loadings of calcium oxide.

Fig. 2 shows that all prepared catalysts exhibit low CO₂ conversion at lower reaction temperatures, followed by a sharp increase when the temperature reaches 250 °C. The dependence of CO₂ conversion on CaO loading is non-monotonic, with the catalytic performance initially increasing with CaO content and reaching a maximum at 70 wt% CaO, followed by a marked decrease at 75 wt% and partial recovery at higher CaO loadings. This indicates that CaO loading has a strong effect on the CO₂ conversion performance of the Ru/Fe/Ca/γ-Al₂O₃ catalyst.

At 300 °C, the CO₂ conversion was 8.35% for the catalyst containing 55 wt% CaO, while the maximum conversion of 85.59% was obtained at 70 wt% CaO. Increasing the CaO content beyond 70 wt% reduced conversion to 40.71% at 75 wt% CaO, followed by a partial recovery at higher CaO loadings, reaching 55.60% at 85 wt% CaO at 300 °C. Thus, increasing CaO content does not produce a monotonic improvement in CO₂ conversion, indicating the existence of an optimum CaO loading.¹⁷ The initial improvement in conversion with increasing CaO loading can reasonably be associated with increasing contribution of CaO to the basic character of the catalyst, which can facilitate CO₂ adsorption and activation. However, excessive CaO loading may adversely affect the accessibility and dispersion of the Ru–Fe-containing active species and alter the textural characteristics of the supported catalyst. The sharp decrease observed at 75 wt% CaO therefore suggests that the beneficial effect of CaO reaches an optimum, beyond which additional CaO does not proportionally enhance CO₂ activation and may instead reduce the accessibility of the Ru–Fe active interfaces. Because detailed structural characterization was not performed for every CaO loading, this interpretation is presented as a plausible explanation rather than a definitive mechanistic assignment. As shown in Fig. 2, catalytic activity is strongly influenced by the composition of the Ru–Fe–Ca active phase. Therefore, the Ru/Fe/Ca/γ-Al₂O₃ catalyst with a 5 : 25 : 70 ratio exhibited the best catalytic performance under the investigated CaO-loading conditions and was selected for further investigation of the effect of calcination temperature.

Influence of calcination temperatures

During the calcination process, several changes could occur on the surface of the catalyst, including alteration of structure and texture through the sintering process as well as the formation of active phases.¹⁸ Therefore, the effect of calcination temperature was studied on calcium oxide-based catalyst supported on alumina towards CO₂ conversion. The prepared catalysts were calcined at 900 °C, 1000 °C, and 1100 °C for 5 h at a heating rate of 10 °C min⁻¹ after being dried at 90 °C for 24 h in an oven. The catalytic performance of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ at different calcination temperatures was evaluated, as shown in Fig. 3. The catalyst calcined at 1000 °C demonstrated the highest CO₂ conversion of 85.59%, which was significantly higher than the catalysts calcined at 900 °C (59.24%) and 1100 °C (3.59%) at a reaction temperature of 300 °C. The very low performance observed for the catalyst calcined at 1100 °C can be attributed to particle agglomeration, as reported by Oh and Bang.¹⁹

Fig. 3. Influence of calcination temperature on the catalytic performance of CO₂ conversion reaction over Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst calcined for 5 hours at 900 °C, 1000 °C, and 1100 °C.

The percentage CH₄ formation trend plot over Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst calcined for 5 hours at 900, 1000, and 1100 °C is shown in Fig. 3. The potential catalyst calcined at 1000 °C showed the highest formation of methane, which yielded 92.8%, which is higher than that of other catalysts calcined at 900 and 1100 °C. The results have shown that the methanation reaction was favorable at high reaction temperature.

Effect of Ru/Fe/Ca (5 : 25 : 70)/Al₂O₃ catalyst dosage

The effect of catalyst dosage on CO₂ methanation was investigated using 3, 7, and 10 g of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃, and the results are presented in Fig. 4. At a fixed total feed flow rate, increasing the catalyst mass increased the overall CO₂ conversion and CH₄ formation, which can be attributed to the increased amount of catalyst available for the reaction, together with the corresponding increase in gas–solid contact time. At 275 °C, the CO₂ conversion increased from 35% for 3 g to 45.21% for 7 g and 71.47% for 10 g of catalyst. At 300 °C, the CO₂ conversion reached 51.52% for 3 g and increased further with catalyst dosage, reaching approximately 68% for 7 g and 85.59% for 10 g of catalyst. Thus, CO₂ conversion increased with catalyst dosage under the fixed-feed-flow conditions used in this experiment. However, this increase should be interpreted with consideration of the corresponding decrease in GHSV and increase in gas–solid contact time at higher catalyst loading; therefore, the higher conversion does not necessarily indicate higher intrinsic catalytic activity.

Fig. 4. Catalytic performances of CO₂ conversion and CH₄ formation during methanation reaction with different dosages of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalysts calcined at 1000 °C for 5 h.

Characterization of the catalytic systems

Nitrogen adsorption method

Table 1 presents the textural properties of all studied samples. The BET surface area of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 900, 1000, and 1100 °C, along with Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃ calcined at 1000 °C, showed a decreasing trend from 73 to 18 m² g⁻¹ as the mean pore radius increased from 5.99 to 10.66 nm, indicating the typical inverse relationship between pore size and surface area. In contrast, the pore radius of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 1100 °C sharply decreased to 1.41 nm, indicating substantial pore contraction at the highest calcination temperature. The surface area of Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃ calcined at 1000 °C was 37.74 m² g⁻¹, which was slightly higher than that of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at the same temperature (34.58 m² g⁻¹). The lower amount of adsorbed nitrogen observed for Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 1000 °C, compared with Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃ calcined at 1000 °C, was mainly related to the reduced surface area and the loading of FeO and CaO.

Table 1. Effect of synthesis temperature on the textural properties of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst.

Catalysts	Calcination temp. (°C)	Surface areaa (m2 g−1)	Pore volumeb (cm3 g−1)	Pore radiusc (nm)
Ru/Fe/Ca (5 : 25 : 70)/γ-Al2O3	900 °C	73.14	0.3811	5.9984
Ru/Fe/Ca (5 : 30 : 65)/γ-Al2O3	1000 °C	37.74	0.2636	9.43
Ru/Fe/Ca (5 : 25 : 70)/γ-Al2O3	1000 °C	34.58	0.2431	10.668
Ru/Fe/Ca (5 : 25 : 70)/γ-Al2O3	1100 °C	18.45	0.1247	1.41

^aObtained from the BET method.

^bBJH pore volume.

^cBJH pore diameter.

Although the 900 °C catalyst exhibited a substantially higher BET surface area (73.14 m² g⁻¹) than the catalyst calcined at 1000 °C (34.58 m² g⁻¹), its higher surface area did not translate directly into higher CH₄ formation. This indicates that catalytic performance is not governed solely by the accessible surface area. Calcination temperature can influence decomposition of precursor salts, the distribution and crystallinity of the Ru-, Fe-, and Ca-containing phases, and their interaction with the γ-Al₂O₃ support. At 1000 °C, these thermal transformations may generate a more favorable arrangement of the active components and metal–support interfaces for CO₂ activation and subsequent hydrogenation, despite the concomitant decrease in surface area. Thus, the superior methanation performance at 1000 °C is attributed to a balance between textural properties and the chemical/structural characteristics of the active phases rather than to BET surface area alone.

The further decrease in surface area to 18.45 m² g⁻¹ at 1100 °C, together with the pronounced particle aggregation observed by FESEM, suggests excessive thermal sintering and loss of accessible active sites at the highest calcination temperature. Therefore, 1000 °C is a more favourable calcination temperature for this catalyst system, as it achieves sufficient thermal transformation of the precursor-derived phases without the extensive agglomeration observed at 1100 °C.

The reduction in BET surface area at 1100 °C is consistent with thermal sintering and aggregation of the catalyst components, as further supported by the FESEM observations. A moderate surface area is beneficial because it supports better dispersion of metal oxides across the alumina support surface. However, the pore size of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 1000 °C (10.668 nm) was slightly larger than Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃ calcined at 1000 °C (9.43 nm). This variation in pore size caused by different loadings on the support leads to significant wall contraction and, as a result, pore expansion.

Furthermore, the IUPAC classification of N₂ adsorption–desorption isotherms in Fig. 5a can be identified as a Type-IV isotherm, which is characteristic of materials containing mesopores and provides evidence for mesoporous features in the catalyst calcined at 1000 °C. Type-IV isotherms represent stepwise multilayer adsorption on solid materials containing narrow slit-like pores with irregular size and shape. The hysteresis features of the isotherms further reflect changes in the pore structure with calcination temperature.²⁰

Fig. 5. (a) Isotherms of N₂-adsorption/desorption (b) pore size distributions of catalyst Ru/Fe/Ca (5 : 25 : 70)/Al₂O₃ calcined at different temperatures for 5 h.

Fig. 5b shows that the pore radius of catalyst Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 900 °C was shifted to the right when the calcination temperature increased to 1000 °C for Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ and Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃. The pore radius of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 1100 °C was 1.41 nm, indicating that the pore radius of the catalyst decreases with calcination temperature. Hence, this analysis has shown that as the temperature increases from 1000 to 1100 °C, metal oxide particles under study can shrink and agglomerate. The agglomeration of metal oxide is supported by FESEM results depicted in Fig. 6. The Type-IV isotherm and the BJH pore-size distribution provide evidence of mesoporous features for the samples calcined at 900 and 1000 °C, whereas the marked decrease in pore size at 1100 °C indicates substantial pore contraction associated with thermal sintering.

Fig. 6. XRD diffractograms for the catalyst Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined for 5 hours at (a) 900 °C, (b) 1000 °C, and (c) 1100 °C. Note: c = cubic, r = rhombohedral, t = tetrahedral.

XRD analysis

XRD analysis was carried out to better understand the structural properties of the catalysts studied. XRD measurements identified the phases present in the catalysts. Fig. 6 presents the X-ray diffraction patterns for Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 900, 1000, and 1100 °C. The XRD pattern of the catalyst calcined at 900 °C (Fig. 6a) contains diffraction peaks that can be assigned to several crystalline phases. The diffraction peaks at 2θ = 25.51° and 35.06° are attributed to rhombohedral Al₂O₃ (JCPDS No. 01-076-7775). Additional diffraction peaks at 2θ = 25.51°, 28.95°, 32.52°, 33.02°, 34.39°, and 67.29° are assigned to monoclinic calcium tetra-aluminate (JCPDS No. 01-089-3851). The diffraction peaks at 32.29°, 54.02°, and 67.59° correspond to cubic CaO (JCPDS No. 01-082-1691), whereas the weaker diffraction peaks at 24.17°, 33.19°, 35.68°, 39.32°, 54.13°, 62.52°, and 64.09° are assigned to rhombohedral Fe₂O₃ (JCPDS No. 01-079-0007). The diffraction peaks at 28.06°, 35.09°, 40.10°, 45.08°, 54.33°, 65.65°, and 67.01° are consistent with tetragonal RuO₂ (JCPDS No. 01-088-0322). Thus, the 900 °C catalyst contains identifiable crystalline Al₂O₃, calcium tetra-aluminate, CaO, Fe₂O₃, and RuO₂ phases. Although the diffraction peaks are relatively weak at 900 °C, the pattern does not support describing the catalyst as completely amorphous because distinct crystalline reflections from the above phases are clearly observed.

Fig. 6b shows the XRD patterns of Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃ calcined at 1000 °C. The intensity of peaks in Fig. 6b is higher and more crystalline in shape compared to Fig. 6a. For the catalyst calcined at 1000 °C (Fig. 6b), the diffraction peaks are sharper and more intense than those observed at 900 °C, indicating increased crystallinity. The diffraction peaks at 25.51°, 35.06°, 37.68°, 41.56°, 43.24°, 46.06°, 52.41°, 57.34°, 59.58°, 61.13°, 66.34°, and 68.03° are assigned to rhombohedral Al₂O₃ (JCPDS No. 01-076-7775). The diffraction peaks at 25.38°, 27.56°, 28.95°, 31.04°, 32.52°, 33.02°, 34.39°, 36.86°, 38.68°, 41.36°, 45.28°, 46.23°, 47.63°, 48.68°, 50.66°, 51.77°, 57.16°, 59.57°, 60.05°, 61.13°, 66.44°, 67.29°, 70.53°, and 71.72° correspond to monoclinic calcium tetra-aluminate (JCPDS No. 01-089-3851). The diffraction peaks at 32.29°, 37.46°, 54.02°, and 67.59° are attributed to cubic CaO (JCPDS No. 01-082-1691). The diffraction peaks at 24.17°, 33.19°, 43.56°, 54.13°, 57.53°, 62.52°, and 66.12° are assigned to rhombohedral Fe₂O₃ (JCPDS No. 01-079-0007). The diffraction peaks at 28.06°, 35.09°, 54.33°, 59.46°, and 67.01° are consistent with tetragonal RuO₂ (JCPDS No. 01-088-0322). The temperature-dependent increase in the intensity and sharpening of the diffraction peaks indicates progressive crystallite growth upon calcination. No additional crystalline diffraction peaks attributable to other major phases were detected within the detection limit of the XRD measurement. The identified phases therefore remain Al₂O₃ (JCPDS No. 01-076-7775), calcium tetra-aluminate (JCPDS No. 01-089-3851), CaO (JCPDS No. 01-082-1691), Fe₂O₃ (JCPDS No. 01-079-0007), and RuO₂ (JCPDS No. 01-088-0322).

For the Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst calcined at 1100 °C (Fig. 6c), the same major crystalline phases can be identified. The diffraction peaks corresponding to Al₂O₃, calcium tetra-aluminate, CaO, Fe₂O₃, and RuO₂ are assigned using JCPDS No. 01-076-7775, 01-089-3851, 01-082-1691, 01-079-0007, and 01-088-0322, respectively. The increased peak intensity, together with the FESEM-observed particle aggregation, indicates enhanced crystallite growth and thermal sintering at the highest calcination temperature. The presence of RuO₂ diffraction peaks confirms the formation of a crystalline Ru-containing oxide phase; however, the degree of dispersion of RuO₂ cannot be determined solely from XRD peak intensity. Likewise, XRD alone does not establish the oxidation state of surface Ru under reaction conditions. The higher CH₄ formation observed for the catalyst calcined at 1000 °C is therefore not attributed solely to a higher Ru content or RuO₂ crystallinity. Rather, the XRD results, together with the BET, FESEM, EDX, and FTIR results, suggest that calcination at 1000 °C produces a favorable balance between phase development, catalyst-support interactions, accessible active interfaces, and textural properties. Further heating to 1100 °C promotes particle growth and aggregation, which can reduce the accessibility of catalytically relevant sites.

FESEM and EDX analysis

Fig. 7a–f displays the surface morphology of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 900,1000 and 1100 °C with magnifications of 25.0k and 50.0k. According to FESEM analysis, all micrographs showed a polycrystalline structure, as mentioned in the X-ray diffractograms. As shown in Fig. 7, different calcination temperatures yield different surface morphologies and grain sizes for each catalyst. The FESEM image of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 900 °C (Fig. 7a and b) showed well-developed, typical circular particles with smooth surfaces, with sizes between 46 and 70 nm. In the FESEM images of Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃ calcined at 1000 °C, additionally, a mixture of rod-, circular-, and plate-shaped morphologies with larger particles appeared, as reported in Fig. 7c and d. For Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ calcined at 1000 °C, a circular-shaped morphology with larger crystal particles was observed due to crystal growth (Fig. 7e and f), in the range of 65 to 87 nm. However, an aggregation of the particles occurred for the catalyst calcined at 1100 °C (Fig. 7g and h), whose size ranges between 130 and 190 nm.²¹

Fig. 7. (a–f) FESEM micrographs of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalysts calcined at 900, 1000 and 1100 °C at magnifications of 25.0k and 50.0k. (g–i) Corresponding particle size distribution histogram of catalysts calcined at (g) 900 °C, (h) 1000 °C, and (i) 1100 °C.

Elemental analysis using the EDX method (weight ratio, %) for the catalyst Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃, calcined at 900, 1000, and 1100 °C, and Ru/Fe/Ca (5 : 30 : 65)/γ-Al₂O₃, calcined at 1000 is shown in Fig. 8. EDX analysis confirmed the presence and variation of Ru, Fe, Ca, Al, and O among the investigated catalysts, with Ru content ranging from approximately 0.3 to 4.4 wt%. The variation in surface elemental composition is consistent with the influence of calcination temperature and catalyst composition on the distribution of the active components. XRD-identified crystalline Ru-containing phases provide complementary evidence for the structural evolution of the Ru species. However, EDX alone cannot distinguish Ru oxidation state; therefore, interpret the observed differences in Ru content in terms of elemental distribution rather than Ru oxidation state.

Fig. 8. (a–d) EDX mapping profile for elements (Ca, Fe, Ru) in Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst calcined at 900, 1000, and 1100 °C for 5 hours.

The higher CH₄ formation observed for the 1000 °C catalyst, despite its lower BET surface area relative to the 900 °C sample, can therefore be rationalized by considering the combined effects of textural and chemical properties. The 900 °C sample provides a larger surface area but may retain precursor-derived phases or a less developed interfacial structure. Increasing the calcination temperature to 1000 °C can promote more complete precursor decomposition and restructuring of the Ru-, Fe-, and Ca-containing phases, potentially improving the accessibility and functionality of the active interfaces involved in CO₂ adsorption and hydrogenation. Further heating to 1100 °C, however, causes pronounced particle growth and aggregation, as observed by FESEM, accompanied by a substantial loss of surface area. These results indicate that an optimum calcination temperature exists in which the active phase structure and interfacial characteristics compensate for the loss of surface area.

Mechanistic study

Fig. 9 presents the FTIR spectra of the species formed over the catalyst surface of the Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst at different reaction temperatures. The band observed at approximately 2358 cm⁻¹ is attributed to the asymmetric stretching vibration of CO₂ rather than to the C O stretching vibration of a carboxylate species. Carboxylate species are typically characterized by vibrational bands in the lower-wavenumber region, generally around 1500–1650 cm⁻¹.

Fig. 9. FTIR spectra for Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ at reaction temperature, (a) 50 °C, (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C, (f) 275 °C, and (g) 300 °C calcined at 1000 °C for 5 hours.

The peak at approximately 680 cm⁻¹ is attributed to CO₂ bending vibrations. Moreover, the intense peak at 2351 cm⁻¹ corresponds to its asymmetrical stretching mode.²² The weak features observed at 3600–3800 cm⁻¹ is attributed to combined bands due to the excitation of two modes. The CH₄ was determined in the gas phase, starting with a reaction temperature of 250 °C, characterized by roto-vibration modes centred at 3016 cm and 1305 cm⁻¹ assigned to stretching and deformation of C–H. CO species were not detected at 2000–1700 cm⁻¹ region. The absence of detectable CO under the applied conditions suggests that CO does not accumulate as a major gas-phase intermediate, although its transient formation cannot be excluded.

Mechanistic study on the surface of Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst

During catalytic hydrogenation of CO₂ over Ru/Fe/Ca catalyst, several reaction intermediates have been studied. No spectral lines of monodentate carbonate (1504 cm⁻¹), bidentate carbonate (1567, 1289, 1014, 856 cm⁻¹), polydentate carbonates (1462, 1351 cm⁻¹) and hydrogen carbonate (1413, 1218 cm⁻¹) species were detected over Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst surface.^23,24 Likewise, bands attributed to hydrogen carbonates (3616, 1594, 1220, and 1060 cm⁻¹) were also absent on the catalyst surface.²⁵

The band near 1640 cm⁻¹ is mainly assigned to the O–H bending vibration of adsorbed water, which becomes more pronounced from 100 °C and increases with increasing reaction temperature. This observation is consistent with the formation of water during CO₂ hydrogenation. At 100 °C, a band appeared at approximately 1641 cm⁻¹ with a shoulder near 1701 cm⁻¹, which may be associated with surface carboxylate/carbonyl-containing species.²⁶ This indicates that carboxylates are likely formed through the reduction of carbon dioxide (Fig. 9), while dissociated hydrogen atoms are produced on the ruthenium particles. The persistence of these features with increasing temperature suggests that CO₂-derived surface intermediates form readily and participate in subsequent hydrogenation steps. Meanwhile, Ru sites are expected to facilitate H₂ dissociation, providing surface hydrogen species for the hydrogenation of CO₂-derived intermediates.

The C–H stretching band was detected at a reaction temperature of 250 °C, coinciding with the onset of detectable methane formation.²¹ When the temperature increased from 250 to 275 °C, the increasing C–H signal indicates enhanced formation of hydrogenated carbon-containing species and ultimately CH₄. These observations support a surface-mediated CO₂ hydrogenation pathway involving CO₂-derived intermediates and sequential hydrogenation.

Mechanism

Fig. 10 presents the proposed pathways for CO₂ activation and methanation over Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃. During CO₂ methanation, Ru-containing sites are proposed to facilitate H₂ dissociation, while CaO- and Fe-containing oxide sites can contribute to CO₂ adsorption and activation.²⁷ The proximity of these components on γ-Al₂O₃ can facilitate hydrogen transfer between the sites and promote subsequent hydrogenation of CO₂-derived intermediates. The proximity of these components on γ-Al₂O₃ may therefore create complementary sites for CO₂ activation and H₂ dissociation/hydrogenation.

Fig. 10. Proposed reaction mechanism over Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst calcined at 1000 °C.

The FTIR observations suggest that CO₂-derived surface species, including carboxylate/carbonyl-containing intermediates, are formed at relatively low temperatures, whereas C–H vibrations become evident from approximately 250 °C. This temperature-dependent evolution of these bands is consistent with progressive hydrogenation of adsorbed carbon-containing species and the formation of methane. The basic CaO component may enhance CO₂ adsorption, while Ru-containing sites provide hydrogen activation, and Fe-containing sites may modify the electronic and interfacial environment of the active phase. Accordingly, the Ru–Fe–Ca/γ-Al₂O₃ interface is proposed to provide complementary functions for CO₂ activation and hydrogenation.

The FTIR spectra indicate three principal classes of surface species: (i) CO₂-derived oxygenated carbon species, including carboxylate/carbonate-type species; (ii) surface hydroxyl/water-related species; and (iii) C–H containing species assigned to methane. The band observed at approximately 3016 cm⁻¹ is consistent with the C–H stretching vibration of CH₄, in agreement with previous operando FTIR studies of CO₂ methanation over Ru-based catalysts.²⁸ The appearance of the methane-related band at elevated reaction temperature, together with the evolution of the CO₂-derived species, supports a surface-mediated hydrogenation process. However, the available FTIR measurements do not establish that a particular carboxylate species is the sole or obligatory intermediate.

Accordingly, the mechanism shown in Fig. 10 is proposed as a plausible surface reaction pathway. Following CO₂ adsorption and activation at the CaO-containing basic sites and adjacent Ru–Fe interfaces, hydrogen is supplied by dissociative H₂ activation on Ru.²⁹ These observations support a surface-mediated hydrogenation pathway rather than establishing a single exclusive reaction route. The catalyst interface provides multiple sites for the adsorption of reaction intermediates. In this study, the first one suggests the formation of carboxylate species as a reaction intermediate (*CO₂ → *CO₂⁻ (carboxylate)), then carboxylate involves the dissociation reaction to form *C and *O (*CO₂⁻ → *C + *O), followed by hydrogenation to form *HC (*C + *H → *CH), which is subsequently hydrogenated to produce *CH₂ (*CH + *H → *CH₂), *CH₃ (*CH₂ + *H → *CH₃) and finally the desired product of CH_4(g) (*CH₃ + *H → *CH_4(gas)) as similarly proposed by Weatherbee and Bartholomew.³⁰ As a by-product, water is formed during methanation reaction (*O + *H → *OH,*OH + *H → *H₂O).

Conclusion

The calcination temperature strongly influenced the structural characteristics and catalytic performance of the Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst. Among the investigated samples, the catalyst calcined at 1000 °C showed the best catalytic performance for CO₂ methanation. The Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst, calcined at 1000 °C, has a moderate surface area (34.58 m² g⁻¹) and exhibited mesoporous features based on Type-IV N₂ adsorption–desorption isotherm and BJH pore size analysis. The CO₂ conversion (85.59%) and methane formation of 80.15% were achieved at 300 °C and atmospheric pressure. Reaction temperature, calcination temperature, and catalyst amount influenced the observed catalytic performance. XRD and EDX results indicate the presence and distribution of the constituent elements, while XRD identified RuO₂ as a prominent Ru-containing phase. Higher Ru content affected both catalyst reducibility and H₂ adsorption capacity. Also, IR measurements were performed under CO₂ : H₂ = 1 : 4 conditions on the Ru/Fe/Ca/γ-Al₂O₃ catalyst (5 wt% Ru, 25 wt% Fe, and 70 wt% Ca).

It appears that, on the Ru/Fe/Ca (5 : 25 : 70)/γ-Al₂O₃ catalyst, CO₂ adsorbs more strongly because iron provides more active sites. Carboxylate species were observed from 150 °C onward. Therefore, the CO₂ hydrogenation mechanism is likely to proceed through a pathway involving the dissociation of CO₂ into *C and *O, followed by the hydrogenation of *C to form *HC, which is then successively hydrogenated to produce *CH₂, *CH_3, and finally CH_4(g). These observations suggest that the catalyst's surface basicity plays an important role in CO₂ adsorption and transformation, with carboxylate species serving as key intermediates during hydrogenation to methane. Although the catalyst showed promising catalytic activity under the investigated conditions, the present study did not evaluate long-term stability, carbon deposition, or regeneration behaviour. Therefore, time-on-stream stability and post-reaction characterization are necessary to establish catalyst durability and practical applicability under continuous CO₂ methanation conditions.

Author contributions

All authors affirm their substantial contributions to this research and collectively assume full responsibility for its content, encompassing conceptualization, computational analysis, manuscript preparation, and revision processes.

Conflicts of interest

The authors declare that there are no financial or personal relationships that could have influenced the outcomes, interpretation, or conclusions presented in this study.

Data availability

The data will be provided by the corresponding author upon request.