Promoting CO₂ Methanation Performance over NiO@TiO₂ Nanoparticles via Oxygen Vacancies Enriched Fe-Oxide Modifiers Assisted Surface and Interface Engineering

Abstract

Surface and interface engineering play a crucial role in enhancing the CO₂ methanation performance of heterogeneous catalysts. In this study, we present NiO-TiO₂ nanoparticles modified with oxygen vacancy-rich Fe₃O₄ clusters, significantly improving CO₂ methanation performance. The as-prepared catalyst (referred to as NiO@Fe₃O₄) achieves an impressive CH₄ selectivity of 91.2% and a methane production yield of 6400.50 μmol/g at 573 K, an approximately 83% increase compared to unmodified NiO nanoparticles (3154.2 μmol/g). The results of physical characterizations and gas chromatography confirm that the outstanding activity and selectivity of the NiO@Fe₃O₄ catalyst arise from the synergistic interaction between its surface-active sites. Notably, the high concentration of oxygen vacancies within Fe₃O₄ enhances CO₂ activation, while adjacent NiO sites efficiently promote H₂ dissociation. These findings provide valuable insights into the rational design of heterogeneous catalysts, highlighting the advantages of Fe₃O₄ as an efficient promoter over conventional metal oxides for catalytic applications. Additionally, we envision that the obtained results will help to design transition metal-based industry viable catalysts for a diverse range of applications.

1. Introduction

The increasing levels of atmospheric carbon dioxide (CO₂) resulting from industrial activities and fossil fuel combustion have raised significant environmental concerns, particularly in relation to global warming and climate change. To mitigate these issues, CO₂ utilization technologies have gained immense attention as a means to convert CO₂ into value-added products. Among the various CO₂ conversion approaches, CO₂ methanation, also known as the Sabatier reaction (CO₂ + 4H₂ → CH₄ + 2H₂O), has emerged as a promising strategy for energy storage and synthetic fuel production [1,2]. This reaction is thermodynamically favorable at elevated temperatures and can be efficiently catalyzed by transition metal-based catalysts, particularly those containing nickel (Ni), due to their excellent hydrogenation activity and cost-effectiveness [3,4]. However, traditional Ni-based catalysts often suffer from low selectivity, poor stability, and carbon deposition, which hinder their practical application. Consequently, optimizing catalyst design through surface and interface engineering has become a key research focus in enhancing CO₂ methanation performance. Nickel-based catalysts have been widely investigated for CO₂ methanation due to their high activity and relatively low cost. However, their catalytic performance is strongly influenced by factors such as metal dispersion, support interactions, and surface defects [5]. Several strategies have been explored to improve the efficiency and durability of Ni-based catalysts, including support modification, alloying with other metals, and defect engineering [6]. Among these strategies, the incorporation of metal oxides, such as TiO₂, Al₂O₃, and CeO₂, has shown promising results in enhancing catalytic stability and selectivity by modifying the electronic properties of Ni sites and facilitating CO₂ adsorption and activation [7]. Recent studies have highlighted the role of oxygen vacancies in metal oxide-supported catalysts in promoting CO₂ methanation [8]. Oxygen vacancies serve as active sites for CO₂ activation by facilitating electron transfer and enhancing the adsorption of CO₂ molecules. TiO₂, in particular, is known for its ability to generate oxygen vacancies under reductive conditions, making it an excellent support material for Ni-based catalysts [9]. However, further enhancement of oxygen vacancy density is necessary to maximize catalytic activity. To address this challenge, incorporating additional oxygen-deficient metal oxides has emerged as a promising approach. Particularly, iron oxide (FeOx) has attracted significant interest as a promoter in catalytic applications due to its unique redox properties and the ability to generate a high density of oxygen vacancies [10]. The presence of FeOx in Ni-based catalysts can modulate the electronic structure of Ni, leading to enhanced catalytic performance. Additionally, FeOx can act as a co-catalyst, facilitating the activation of CO₂ through its abundant surface oxygen vacancies [11]. Several studies have demonstrated the effectiveness of oxygen vacancy engineering in improving the performance of Ni-based catalysts. For instance, it has been reported that Ni/TiO₂ catalysts with abundant oxygen vacancies exhibited significantly higher CO₂ methanation activity compared to their defect-free counterparts [12]. Similarly, the role of Fe₃O₄ has been highlighted for enhancing the reducibility and dispersion of Ni species, leading to improved catalytic performance [13]. In addition to FeOx, other metal oxides such as CeO₂ and ZrO₂ have been explored for their ability to introduce oxygen vacancies [14,15]. CeO₂, in particular, is known for its excellent oxygen storage capacity and has been widely used in CO₂ methanation catalysts [14]. However, recent studies suggest that FeOx offers comparable, if not superior, performance due to its stronger interaction with NiO and its ability to facilitate both CO₂ activation and hydrogenation. Building upon these insights, this study aims to develop an advanced NiO@Fe₃O₄ catalyst with enhanced oxygen vacancy density for improved CO₂ methanation performance. By integrating Fe₃O₄ as a promoter, we seek to optimize the catalytic activity and selectivity of NiO-based catalysts. The as-prepared catalyst demonstrates remarkable CH₄ selectivity of 91.2% and a methane production yield of 6400.50 μmol/g at 573 K, representing an 83% improvement over unmodified NiO nanoparticles. Through detailed physical characterizations and gas chromatography analysis, we elucidate the synergistic interactions between NiO and Fe₃O₄ that contribute to the catalyst’s superior performance.

2. Experimental Section

2.1. Sample Preparation

The TiO₂-supported NiO@Fe₃O₄ catalyst was synthesized through a multi-step process (Scheme 1). In the first step, 2 g of TiO₂ (3 wt.% solution in deionized (D.I.) water, equivalent to 60 mg of actual TiO₂) was dispersed in 3.06 g of an aqueous solution containing 0.1 M nickel(II) chloride hexahydrate (NiCl₂·6H₂O, Showa Chemical Co., Ltd., Tokyo, Japan). The mixture was stirred at 800 rpm for 6 h, allowing Ni²⁺ ions to adsorb onto the TiO₂ surface, forming Ni²⁺-adsorbed TiO₂ (Ni²⁺_ads-TiO₂). This process resulted in the adsorption of 0.102 mmoles (6 mg) of Ni²⁺ ions, corresponding to a NiO@TiO₂ weight ratio of 10 wt.%. In the second step, 5 mL of an aqueous solution containing 0.11 g of sodium borohydride (NaBH₄; 99%, Sigma-Aldrich Co., Burlington, MA, USA) was introduced into the Ni²⁺_ads-TiO₂ solution under continuous stirring at 800 rpm for 10 s. This reaction facilitated the reduction of Ni²⁺ ions, forming metastable nickel metal nanoparticles (Ni-TiO₂), which subsequently oxidized to NiO. In the third step, 0.51 g of an iron precursor solution containing 0.051 mmoles (5 wt.% to Ti; i.e., the exact amount of Fe is 3 mg) of Fe³⁺ ions (0.1 M iron(III) chloride hexahydrate (FeCl₃·6H₂O, Showa Chemical Co., Ltd., Tokyo, Japan) was added to the NiO-TiO₂ solution. The Fe³⁺ ions were reduced by the excess NaBH₄ introduced in the previous step, leading to the formation of Fe-modified NiO-TiO₂ (NiO@Fe₃O₄). The final products were sequentially washed with acetone, isopropanol (IPA), and D.I. water, followed by centrifugation and drying at 70 °C. Control samples, including Ni-TiO₂ and Fe-TiO₂, were prepared by immersing TiO₂ in either a Ni or Fe precursor solution at 25 °C for 8 h, followed by reduction with NaBH₄. These control samples underwent the same washing, ion removal, and drying procedures as NiO@Fe₃O₄ to obtain the final powder samples.

2.2. Structural Characterizations

To investigate its structural and electronic characteristics, a combination of advanced microscopy and X-ray techniques was utilized. High-resolution transmission electron microscopy (HRTEM) analyses were conducted at the Electron Microscopy Center of National Sun Yat-sen University, Taiwan. Prior to TEM imaging, plasma cleaning was performed to remove surface contaminants. X-ray diffraction (XRD) analysis was carried out at the BL-01C2 beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, using an incident X-ray wavelength of 0.6888 Å (18.0 KeV) to gain insights into the structural properties of the material. X-ray absorption spectroscopy (XAS) was employed to probe the electronic states and atomic arrangements, with spectra recorded in fluorescence mode at the Ni K-edge, and Fe K-edge at NSRRC beamlines BL-17C and 01C1.

2.3. Catalytic Activity Measurement

Gas chromatography (GC) analysis was carried out using an Agilent 7890 GC system equipped with a Valco pulsed discharge helium ionization detector (PDHID, model D-3-I-7890, VICI, USA). The catalyst sample for the reaction comprised 12 mg of catalyst, which was physically blended with 23 mg of silicone gel. This mixture was then placed inside a glass reaction tube with dimensions of 100 mm in length, 2 mm inner diameter, and 3 mm outer diameter. The reaction operated under constant pressure mode, regulated by a pressure control module (PCM) from Agilent. Ultra-high-purity helium (99.9995%) was employed as the carrier gas, maintaining a steady flow rate of 30 mL min⁻¹. The gas products generated during the reaction passed through the packed catalyst bed into the GC system for analysis, utilizing a temperature-programmed desorption (TPD) method. The GC oven temperature was progressively increased from 273 K to 573 K at a rate of 20 K min⁻¹ to facilitate the effective separation of gas components, which were subsequently detected by the PDHID. Prior to initiating the reaction, nitrogen gas was introduced at a flow rate of 50 mL min⁻¹ for one hour at room temperature to eliminate any residual moisture within the system. After this pretreatment, a CO₂-H₂ gas mixture with a molar ratio of 1:3 was continuously supplied to the reaction bed at a flow rate of 20 mL min⁻¹ across a temperature range of 323 K to 573 K. At each target temperature, the system was held in an isothermal state for 30 min to ensure stable reaction conditions. The resulting gas-phase products were subsequently analyzed using the GC system, providing valuable insights into the catalytic performance and composition of the reaction products.

3. Results and Discussion

3.1. Structural Characterizations

High-resolution transmission electron microscopy (HRTEM) was utilized to examine the surface morphology and crystal structure information of the as-synthesized NiO@Fe₃O₄ catalyst. As shown in Figure 1a and Figure 1b, respectively, represent the low and high-resolution TEM images of the NiO@Fe₃O₄ catalyst. Notably, the NiO nanoparticles are grown on TiO₂ support with the surface (denoted in the white box) and interface (denoted in the red box) decorated sub-nanometer FeOx-clusters. For confirming the state of constituting elements, the d-spacing values at specific locations are measured. Accordingly, the d-spacing values for Ti, Ni, and Fe are 0.348 nm, 0.240 nm, and 0.215 nm, respectively, and can be assigned to the Anatase TiO₂ (101), amorphous NiO (111) and Fe₃O₄ (400) planes [16,17,18]. These observations are consistent with our experiment design, confirming that the Fe₃O₄ modifiers are successfully decorated at the surface of amorphous NiO nanoparticles as well as the NiO-to-TiO₂ interface via the precise control in our synthesis approach.

Figure 2 presents the X-ray diffraction (XRD) pattern of the NiO@Fe₃O₄ catalyst, alongside the reference patterns of control samples, including TiO₂, Ni-TiO₂, and Fe-TiO₂. The TiO₂ sample exhibits a combination of anatase (denoted by stars) and rutile (denoted by squares) phases, with the anatase phase being predominant. Upon the deposition of Ni (in Ni-TiO₂) and Fe (in Fe-TiO₂) nanoparticles onto the TiO₂ support, the characteristic TiO₂ peaks remain unchanged, indicating that the growth of Ni and Fe nanoparticles occurs on the TiO₂ surface without altering its crystal structure. Furthermore, the presence of a broad and weak diffraction peak centered at approximately 14.67° in Ni-TiO₂, corresponding to the amorphous NiO (111) facet (mp-19009), confirms that Ni is present in an amorphous NiO phase. Similarly, Fe-TiO₂ exhibits a relatively intense diffraction peak at 19.55°, associated with the Fe₃O₄ (400) facet (mp-19306), verifying that Fe is present as Fe₃O₄. These findings are consistent with the observations made using high-resolution transmission electron microscopy (HRTEM). Additionally, the significant reduction in TiO₂ peak intensities, without any peak shift, suggests surface disordering induced by the oxidation of Ni and Fe into NiO and Fe₃O₄ nanoparticles. In the case of the NiO@Fe₃O₄ catalyst, the characteristic diffraction signals of NiO and Fe₃O₄ are absent, indicating that both Ni and Fe exist in an amorphous state and are homogeneously distributed throughout the sample [19].

To gain deeper insight into the local atomic and electronic structure of the elements in the NiO@Fe₃O₄ catalyst, X-ray absorption near-edge spectroscopy (XANES) was conducted at the Ni and Fe K-edges. Figure 3a presents a comparative analysis of the XANES spectra for the NiO@Fe₃O₄ catalyst, Ni-TiO₂, and Ni foil. The Ni K-edge spectra reveal three key features: the pre-edge feature (denoted as X), the inflection point (I), and the absorption edge intensity (H_A). These features provide crucial information about the local coordination geometry, the relative oxidation (valence) state in comparison to the reference sample, and the degree of unoccupied states resulting from electron transfer to neighboring atoms [20]. As depicted in Figure 3a, the NiO@Fe₃O₄ catalyst exhibits a flattened pre-edge feature compared to Ni-TiO₂, indicating that the incorporation of Fe atoms has significantly altered the local coordination geometry of Ni atoms from tetrahedral to a distorted octahedral configuration [20]. Additionally, the inflection point (I) remains unchanged, suggesting that Ni retains the same valence state as in Ni-TiO₂. The lower absorption edge intensity (H_A) in NiO@Fe₃O₄ suggests an increased occupancy in the 4s/4p orbitals. This phenomenon arises due to electron transfer from neighboring Fe atoms to Ni, which can be attributed to the significant electronegativity difference between Ni and Fe. Figure 3b displays the XANES spectra at the Fe K-edge for the NiO@Fe₃O₄ catalyst alongside the control samples. In comparison to Fe-TiO₂, the NiO@Fe₃O₄ catalyst exhibits a more intense and expanded pre-edge region (denoted as X) and a nearly flattened post-edge region (C). These spectral changes indicate a transformation in the local coordination geometry of Fe atoms from an octahedral to a distorted tetrahedral configuration, likely due to the partial intermixing of Fe with Ni domains. Notably, despite having the same inflection point (I) as Fe-TiO₂—implying an unchanged oxidation state—the NiO@Fe₃O₄ catalyst exhibits a higher white-line intensity (H_B), indicating an increased number of unoccupied states in the 4s/4p orbitals [21]. When considered alongside the Ni K-edge XANES results, which reveal electron transfer from Fe to Ni, these findings confirm a substantial electron relocation from Fe atoms to Ni atoms within the NiO@Fe₃O₄ catalyst. This electronic redistribution is a crucial factor influencing the catalytic properties of the material, further supporting the role of Fe incorporation in modifying the electronic environment of Ni. Given that the Fe/Ni weight ratio is 1:2 in the NiO@Fe₃O₄ catalyst and HRTEM images confirm the presence of FeOx atomic clusters decorating both the Ni surface and the Ni-TiO₂ interface, it can be inferred that approximately 20–25% of the Ni surface sites are covered by Fe domains. This interpretation is further supported by the slight variation observed in the XANES spectra at the Ni and Fe K-edges. Furthermore, the oxygen vacancies in Fe-TiO₂ and NiO@Fe₃O₄ are confirmed by the X-ray photoelectron spectroscopy (XPS) analysis at O-1s orbitals. As shown in Figure 3c, the Ni-TiO₂ does not show the oxygen vacancies (OVs) peak, whereas the Fe-TiO₂ (Figure 3d) and NiO@Fe₃O₄ (Figure 3e) catalysts exhibit the peaks corresponding to OVs. These results confirm that decorated Fe₃O₄ clusters contain abundant Ovs [22].

3.2. Catalytic Performance

The CO₂ conversion efficiency of the NiO@Fe₃O₄ catalyst, along with the control samples (Ni-TiO₂ and Fe-TiO₂), was systematically evaluated across a temperature range of 323 K to 573 K in a flow reactor system operating under atmospheric pressure. The reaction was carried out in a hydrogen-rich environment (H₂/CO₂ ratio of 3:1) to investigate the catalytic activity and reaction mechanism. To gain deeper insight into the fundamental reaction pathways, the catalytic behavior of each sample was initially assessed in a pure CO₂ atmosphere without hydrogen. Figure 4a,b illustrate the CO and CH₄ production yields for the NiO@Fe₃O₄ catalyst and the control samples under pure CO₂ conditions. The results indicate that Ni-TiO₂ does not generate any detectable CO across the entire temperature range, while both Fe-TiO₂ and NiO@Fe₃O₄ exhibit significant CO production. This strongly suggests that the Fe-containing domains play a crucial role in promoting CO₂ dissociation, facilitating the conversion of CO₂ to CO through an oxygen removal mechanism. However, no CH₄ formation is observed for any of the catalysts under pure CO₂ conditions, which is expected due to the absence of H₂ in the reaction environment, preventing the subsequent hydrogenation of CO₂-derived intermediates. The catalytic performance in the presence of both CO₂ and H₂ was further examined, as shown in Figure 4c,d. Under these reaction conditions, Fe-TiO₂ exhibits a substantially higher CO production yield, while Ni-TiO₂ demonstrates a greater CH₄ yield. This trend indicates that Fe domains primarily facilitate CO₂ activation and subsequent CO formation, whereas Ni domains play a dominant role in H₂ dissociation and CH₄ synthesis [20]. These findings align well with the known catalytic behaviors of Fe and Ni in CO₂ methanation, where Fe tends to enhance CO₂ reduction to CO, while Ni is highly effective in facilitating H₂ splitting and subsequent CH₄ formation. Notably, the NiO@Fe₃O₄ catalyst achieves the highest CH₄ production yield of 6400.50 μmol/g with an outstanding CH₄ selectivity of 91.2% at 573 K, representing an 83% enhancement compared to monometallic Ni-TiO₂, which lacks Fe₃O₄ incorporation. The superior catalytic performance of NiO@Fe₃O₄ can be attributed to the synergistic interaction between Ni and Fe domains, where the oxygen vacancies within Fe₃O₄ significantly enhance CO₂ activation, and adjacent NiO sites effectively promote H₂ dissociation [20]. This cooperative effect leads to improved adsorption, activation, and conversion of CO₂ into CH₄, demonstrating the critical role of Fe₃O₄ in optimizing Ni-based catalysts for CO₂ methanation applications. The aforementioned discussion is further confirmed by the Arrhenius plots, where Fe-TiO₂ and NiO@Fe₃O₄ catalysts, respectively, show the lowest slopes for CO and CH₄ production, confirming the lowest activation energy. By cross-referencing the results of physical characterizations and GC analysis, the CO₂ methanation pathways on the surface of NiO@Fe₃O₄ catalyst have been proposed and depicted in Figure 4e. Finally, the CO₂ methanation performance of the NiO@Fe₃O₄ catalyst has been compared with the literature in

Table 1. Benchmark table for the CO₂ methanation performance.

Sample	Temperature (K)	YieldCH4 (μmol/g)	References
NiO@Fe3O4	573	6400.50	This work
CNP-10		1719.87	[23]
CNP-1		1651.91
CNP-		1353.68
NiPd-TMOS (NiOTPd-T)		1905.1	[24]
NiOT-T		1083.2
Pd-T		92.2
LaSrO3	1273	124.1	[25]
NiFe2O4	1273	125.9	[26]
La0.3Sr0.7Co0.7Fe0.3O3 (LSCF)	973	705	[27]
Co1.3Fe1.7O4	1673	750	[28]

, confirming its superiority as compared to previously reported materials.

4. Conclusions

In summary, this study demonstrates the significant enhancement in CO₂ methanation performance achieved through surface and interface engineering of NiO-TiO₂ nanoparticles modified with oxygen vacancy-rich Fe₃O₄ clusters. The optimized NiO@Fe₃O₄ catalyst exhibits remarkable CH₄ selectivity of 91.2% and an exceptional methane production yield of 6400.50 μmol/g at 573 K, representing an 83% improvement compared to unmodified NiO nanoparticles (3154.2 μmol/g). Comprehensive physical characterization and gas chromatography analysis reveal that this superior catalytic performance is attributed to the synergistic interaction between surface-active sites. The presence of oxygen vacancies within Fe₃O₄ plays a crucial role in enhancing CO₂ activation, while adjacent NiO sites effectively facilitate H₂ dissociation, thereby promoting efficient methanation. This study underscores the potential of Fe₃O₄ as an effective promoter in heterogeneous catalysis, surpassing conventional metal oxides in boosting catalytic efficiency. These findings provide valuable insights into the rational design of advanced catalysts for CO₂ conversion applications, paving the way for the development of sustainable and efficient catalytic systems for greenhouse gas mitigation and energy production.