CO₂ Methanation over Ni-Based Catalysts: Investigation of Mixed Silica/MgO Support Materials

Abstract

Catalytic CO₂ methanation represents a promising process route for converting carbon dioxide into methane, a valuable energy carrier. This study investigates the performance of Ni-based catalysts on mixed silica and MgO support materials for CO₂ methanation. Silica was derived from rice husk (SiO₂(RH)), representing a sustainable, cost-effective source for catalyst support, and MgO was used as a reference and to enhance the catalytic activity of the Ni-based catalysts through admixture with SiO₂(RH). The results were compared to CO₂ methanation over Ni-based catalysts on reduced iron ore from natural siderite (siderite_reduced), providing another abundant source for catalyst support. The experiments were conducted in a tubular reactor with a feed gas composition of H₂:CO₂:N₂ = 56:14:30, feed gas flow rates ranging from 4.01 to 14.66 m³·kg⁻¹·h⁻¹ (STP), and reaction temperatures of 548–648 K. The highest CO₂ conversion with the Ni/SiO₂(RH) catalyst was 39.01% at a methane selectivity of 92.64%. The use of mixed silica and MgO supports (SiO₂(RH)/MgO) for nickel revealed a beneficial effect, enhancing CO₂ conversion and methane formation. In this case, methane selectivities consistently exceeded 91.57%. Superior methane selectivity and CO₂ conversion were obtained with Ni/MgO catalysts and Ni/SiO₂(RH)/MgO catalysts with high MgO fractions, highlighting the fundamental effect of MgO in the catalyst support for CO₂ methanation.

1. Introduction

Rising carbon dioxide (CO₂) emissions pose significant environmental challenges and drive the search for innovative ways to mitigate greenhouse gas emissions arising from CO₂. A significant amount of research has focused on the high-value conversion of CO₂ into a range of useful products, such as CO, methanol (CH₃OH), methane (CH₄), and more [1,2,3,4]. Among CO₂ hydrogenation pathways, CO₂ methanation is especially promising due to its ability to produce methane, the main component of natural gas. Methane can be directly injected into existing gas infrastructure, making it ideal for large-scale energy storage, especially when paired with renewable hydrogen. When powered by renewable energy, it also enables carbon-neutral or even carbon-negative fuel production, supporting a sustainable energy future.

CO₂ methanation relies primarily on effective catalysts to facilitate the conversion of CO₂ and hydrogen (H₂) into methane and its by-product water (H₂O), as shown in Equation (1). With hydrogen, CO₂ is also reduced to carbon monoxide (CO) via the reverse water–gas shift reaction (Equation (2)). In turn, CO is hydrogenated to methane via Equation (3). The reactions are highly exothermic; therefore, high temperatures are unfavorable for high carbon oxide conversion [5,6,7].

CO₂ + 4H₂ ⇌ CH₄ + 2H₂O  ∆_RH⁰ = −165.0 kJ mol⁻¹

(1)

CO₂ + H₂ ⇌ CO + H₂O  ∆_RH⁰ = −41.2 kJ mol⁻¹

(2)

CO + 3H₂ ⇌ CH₄ + H₂O  ∆_RH⁰ = −206.2 kJ mol⁻¹

(3)

Various catalysts have been studied for CO₂ methanation and are described in the literature [4,8,9]. Noble metals, such as platinum (Pt) and palladium (Pd), exhibit high catalytic performance. However, their applicability is limited by cost-related barriers [10,11]. Meanwhile, metallic and bimetallic catalysts have been documented to exhibit synergistic improvements in activity, selectivity, and resistance to deactivation. In addition, ruthenium-based (Ru) [3,12], rhodium-based (Rh) [12,13,14], cobalt-based (Co) [15], and nickel-based (Ni) [16,17] catalysts effectively catalyze CO₂ methanation [13,18]. Rh- and Ru-based catalysts exhibit superior activity and selectivity. Nevertheless, the application of Rh- and Ru-based catalysts is limited by their high costs, prompting the search for low-cost alternative catalytic systems [19,20,21]. Cobalt is currently emerging as a viable alternative; however, Co-based catalysts have only moderate activity [3,22,23,24,25,26].

Currently, Ni-based catalysts are the most widely researched catalysts for CO₂ methanation. They significantly enhance the reaction kinetics and CO₂ methanation efficiency [27,28,29]. The catalytically active metal is crucial for the activity and selectivity of a catalyst, but the support material adopts a decisive role too [17,30,31,32]. The nature of the support material may facilitate increased dispersion of the nickel particles and increase the surface area and stability of a catalyst [18]. The high surface area of support materials, predominantly oxides, makes them suitable for the use in CO₂ methanation. Examples of support materials are Al₂O₃, CeO₂, ZrO₂ [1], MgO, FeO, and SiO₂. Alumina (Al₂O₃) is a highly utilized support material due to its high surface area, adaptable porous structure, and chemical properties, as evidenced by several studies [33,34,35]. Song et al. developed CeZrO_x/Ni inverse catalysts that significantly improved low-temperature CO₂ methanation by engineering the metal–oxide interface, leading to enhanced >99% CH₄ yields at 473 K [8].

Magnesium oxide (MgO) is also a well-known, highly effective support material for Ni-based catalysts for CO₂ methanation. Its cost-effectiveness and enhanced basicity help prevent catalyst deactivation, sintering, and carbon deposition. Furthermore, MgO activates CO₂ through chemisorption, whereas nickel provides the adsorbent capacity for hydrogen and is highly selective for methane, as stated by Varun et al. and Loder et al. [36,37]. Loder et al. investigated Ni/MgO catalysts under optimized conditions, which yielded remarkable methane selectivities of 98–99% and CO₂ conversions of 85–87%.

Furthermore, iron-based catalysts are mentioned as a cost-effective alternative for catalysts and catalyst support. Pandey et al. demonstrated that the presence of a Ni-Fe alloy, along with multiple metal sites, contributed to enhanced CO₂ conversion and increased methane production [38]. Suksumrit et al. investigated the use of reduced siderite ore as support material for nickel-based catalysts for CO₂ methanation [39]. The study showed that siderite ore reduced in a hydrogen atmosphere exhibits only minor catalytic activity for CO₂ hydrogenation. Nevertheless, when combined with MgO as support material for nickel, enhanced catalytic performance was observed, with CO₂ conversions approaching equilibrium conversions and CH₄ selectivities of ≥95%, with most values exceeding 99.9%.

Another noteworthy abundant source of support material is silica (SiO₂), which can be obtained from natural silica sand, silica mines, synthetic processes (through the reaction of silicon tetra-chloride with water), and rice husk ash (RHA) [40,41]. Bakar et al. demonstrated a way to produce high-purity amorphous silica from rice husk. In their study, two acid-leaching processes for rice husk, one using hydrochloric acid (HCl) and the other using sulfuric acid (H₂SO₄), were compared. The obtained amorphous silica exhibited an average particle size range of 0.50 to 0.70 μm with a high degree of purity (99%) [42]. Therefore, silica derived from rice husks displays promising characteristics for being an effective support material for CO₂ methanation, as evidenced by numerous research studies. Aziz et al. investigated the influence of various silica support materials for Ni-based catalysts for CO₂ methanation over a temperature range of 423–723 K at atmospheric pressure. Among others, they employed support materials of mesostructured silica nanoparticles (MSNs), mobile crystalline material (MCM-41), and silica (SiO₂) [29,43]. Mesoporous silica nanoparticles are a commonly utilized support material for nickel catalysts in CO₂ methanation. These catalysts exhibit high activity and selectivity due to the distinctive attributes of MSNs, including their nanoscale dimensions, ordered structure, elevated surface area, substantial pore volume, and adjustable pore sizes (1.5–10 nm) [44,45]. Studies demonstrated that MSN exhibits the highest performance and does not deactivate within 200 h. The strong basicity of the Ni/MSN catalysts played a pivotal role in enhancing the reaction activity and promoting greater CO₂ adsorption on the catalyst’s surface. At a reaction temperature of 573 K, Ni/SiO₂ exhibited superior catalytic activity when compared to Ni/γ-Al₂O₃. The results indicated that all catalysts exhibit comparable CO₂ conversion efficiencies (with an average of approximately 50%) and showed a high selectivity for methane (96.6%), as observed for Ni/SiO₂ [44]. Guo et al. investigated MgO-modified Ni/SiO₂ catalysts via an impregnation technique. They concluded that modified MgO has the potential to enhance the dispersion of Ni nanoparticles while concurrently inhibiting the sintering and oxidation of metallic nickel [46].

While SiO₂ as a Lewis acid shows a slightly acidic behavior, MgO is characterized by its alkaline properties, which render it beneficial for CO₂ activation through chemisorption. Mixed MgO/SiO₂ catalysts are, for instance, used for the conversion of ethanol to 1,3-butadiene [47]. In the context of catalytic CO₂ methanation, several studies have also examined the combination of SiO₂ and MgO as a support for nickel catalysts. Guo and Lu (2014) compared the different roles of the alkaline-earth metal oxides MgO, CaO, SrO, and BaO on silica-supported nickel catalysts for CO₂ methanation and revealed a distinct effect of the different alkaline-earth metal oxides [48]. They used 10 wt% nickel catalysts and added 4 wt% of the alkaline-earth metal oxide to the SiO₂ in the support. The catalysts were prepared by wet impregnation. Interestingly, the addition of 4 wt% MgO to Ni/SiO₂ showed remarkable inhibition effects on the catalytic activity of the nickel catalysts. In another study, however, which delivered promising results, Guo and Lu (2014) investigated the effect of the impregnation strategy on the structural characters and the CO₂ methanation properties over MgO-modified Ni/SiO₂ catalysts [46]. Therefore, Mg-modified Ni/SiO₂ catalysts with different MgO contents (1, 2, and 4 wt%) were prepared by two impregnation methods, by co-impregnation and by sequential impregnation. The Mg-modified Ni/SiO₂ catalysts showed better activity and stability than those prepared by the sequential impregnation method. Modification of MgO was identified as a key factor for enhancing the capacity of CO₂ adsorption onto the support and for accelerating the activation of CO₂. Moreover, it was shown that modified MgO could also increase Ni species dispersion and suppress the metallic Ni sintering and oxidation. Taherian et al. focused on the role of yttria and magnesia on CO₂ methanation using promoted nickel-based catalysts on modified mesoporous silica support. They reported that the incorporation of magnesia and yttria promoters is beneficial for CO₂ hydrogenation [49].

Table 1. Selected experimental studies with Ni-based catalysts on different SiO₂ and MgO support materials for CO₂ methanation.

Catalyst Composition	Preparation Method	Operation Conditions	Performance	Ref.
Ni/MgO (wNi = 0–27 wt%)	Wet impregnation	T = 533–648 K GHSV = 3.7 m3·kg−1·h−1 H2:CO2:N2 = 4:1:5	$X_{{\mathrm{CO}}_2}$ = 87% $S_{{\mathrm{CH}}_4}$ = 99%	[37]
Ni-Fe/olivine ((MgxFe1−x)2 SiO4)	Wet impregnation	T = 673 K H2:CO2 = 6:1 GHSV = 11,000 h−1	$X_{{\mathrm{CO}}_2}$ = 98% $S_{{\mathrm{CH}}_4}$ = 99%	[38]
10% Ni-MgO/SiO2 (wMgO = 1, 2, and 4 wt%)	Co-impregnation, sequential impregnation	T = 573–673 K H2:CO2 = 4:1 GHSV = 15,000 cm3·h−1·g−1	$X_{{\mathrm{CO}}_2}$ = 23.9–66.5% $S_{{\mathrm{CH}}_4}$ = 91.7–96.8%	[46]
10% Ni-MgO/SiO2 (wMgO = 4 wt%)	Sequential impregnation	T = 573–723 K H2:CO2 = 4:1 GHSV = 15,000 cm3·h−1·g−1	$X_{{\mathrm{CO}}_2}$ = 8.7–62.0% $S_{{\mathrm{CH}}_4}$ = 87.1–90.1%	[48]
2Y2O3-Ni/MgO-MCM-41 (wyttria = 2 wt%)	Direct synthesis	T = 473–873 K H2:CO2 = 4:1 GHSV = 9000 cm3·h−1·g−1	$X_{{\mathrm{CO}}_2, 673\mathrm{K}}$ = 65.55% $S_{{\mathrm{CH}}_4, 673\mathrm{K}}$ = 84.44%	[49]
1% Cu and 9–10% Ni/SiO2	Wet impregnation	WHSV = 60,000 cm3·g−1·h−1 H2:CO2 = 4:1	XCO2 = 39.5%, SCH4 = 44.4% at 673 K (1% Cu and 10% Ni/SiO2)	[50]
Ni/MCM-41 with VOx-modified	Impregnation method and treated by glow discharge plasma	T = 673 K WHSV = 60,000 cm3·g−1·h−1	$X_{{\mathrm{CO}}_2}$ = 81.4% $S_{{\mathrm{CH}}_4}$ = 72.8%	[51]
Ni/SiO2	Impregnation	T = 523 K	$X_{{\mathrm{CO}}_2}$ = 99% $S_{{\mathrm{CH}}_4}$ = 100% (decreased by 15.54% after 100 h)	[52]
Ni-Fe/S16	Mesoporous silica molecular sieve	T = 473–573 K H2:CO:N2 = 3:1:1 WHSV = 15,000 cm3·g−1·h−1	XCO = 100% (at 503 K) $S_{{\mathrm{CH}}_4}$ > 90%	[53]
10% Ni/MgO-SiO2	Successive impregnation	T = 423–623 K CO2:H2 = 1:5.2 GHSV = 4650 h−1	$X_{{\mathrm{CO}}_2}$ ≈ 31–33% $S_{{\mathrm{CH}}_4}$ ≈ 85–87% (at 523 K)	[54]
2Y2O3-Ni/MgO-MCM-41	One-pot synthesis	T = 673 K H2:CO2 = 4:1 GHSV = 9 L·gcat−1·h−1	$X_{{\mathrm{CO}}_2}$ = 65.55% $S_{{\mathrm{CH}}_4}$ = 84.44%	[49]

gives a representative selection of experimental studies of CO₂ methanation over nickel-based catalysts with SiO₂ and MgO support materials. As evident, there is a multitude of effective and capable support materials and combinations thereof. The selection of a suitable catalyst support is of great importance as it has a significant impact on the dispersion of the active nickel sites and the overall efficiency of the catalyst. The advantageous impact of employing silica derived from agricultural sources as support material could prompt the utilization of natural resources as plentiful catalyst support materials in different applications.

In a previous study, reduced siderite ore was investigated as a support material for nickel-based catalysts, and its potential for CO₂ methanation was evaluated [39]. In this study, the performance of silica-supported nickel catalysts is tested and compared to nickel on reduced siderite ore as well as on Ni/MgO catalysts, which serve as reference catalysts. This comparative study aims at assessing the viability of rice-husk-derived silica as a support material and further enhancement of the understanding of nickel-based catalysts on various support materials. In other words, the focus is on nickel catalysts with three distinct support materials: (i) silica derived from rice husk, (ii) mixed rice-husk-derived silica and MgO, and (iii) MgO as a reference support material. Additionally, the results are compared to reduced iron ore derived from natural siderite ore as support material for nickel.

2. Mechanisms of CO₂ Methanation on SiO₂ and MgO Support Materials

2.1. SiO₂ as a Catalyst Support

As demonstrated by Roque-Malherbe et al., who investigated several surface-modified supports, CO₂ adsorption on SiO₂ may involve both physical adsorption and the formation of weakly bonded adducts with surface hydroxyl groups [55]. Physical adsorption occurs predominantly within silica’s micropores, driven by dispersive forces and quadrupole interactions, with consistent isosteric heats across various silica types supporting this mechanism. Additionally, a secondary interaction, wherein CO₂ forms a weakly bonded adduct with surface hydroxyl groups (Hδ⁺···δ–O=C=Oδ⁻), was identified by DRIFT spectroscopy (Figure 1), which marked a novel contribution to the understanding of CO₂ adsorption mechanisms.

Mechanistic investigations of CO₂ methanation over Ni/SiO₂ catalysts from Zhao et al. [56], integrating theoretical and experimental data, confirmed the occurrence of three distinct pathways for CH₄ formation: (i) the reverse water–gas shift + CO hydrogenation pathway, (ii) the formate pathway, and (iii) the C–O bond cleavage pathway (Figure 2).

The reverse water–gas shift reaction, followed by CO hydrogenation, is the dominant pathway for methane synthesis, particularly at lower temperatures. Density functional theory (DFT) calculations confirm that this pathway has the lowest energy barrier compared to alternative pathways. The formate pathway involves the initial adsorption of CO₂ as a monodentate carbonate intermediate that transforms into a formate species before hydrogenation to methane. In contrast, the C–O bond cleavage pathway implies the dissociation of CO₂ into surface carbon and oxygen species, followed by hydrogenation to methane.

2.2. MgO as a Catalyst Support

Ni/MgO catalysts have demonstrated bifunctional catalytic activity during CO₂ hydrogenation to methane, with nickel serving as the active site for hydrogen adsorption and MgO facilitating CO₂ activation through chemisorption. Meixner et al. studied the interaction mechanisms of CO₂ with MgO surfaces, with respect to the kinetics of adsorption, desorption, and surface diffusion. The adsorption of CO₂ on MgO powder occurs through both physisorption (van der Waals forces) and chemisorption (strong chemical bonds). Infrared studies showed the formation of monodentate and bidentate carbonate. The efficiency of adsorption is influenced by the temperature and the CO₂ partial pressure. The formation of carbonate during the adsorption of CO₂ was also found on various other oxide surfaces, such as ZnO, ThO₂, TiO₂, and Cr₂O₃ [57,58,59,60].

Loder et al. provided a detailed kinetic analysis of CO₂ methanation over Ni/MgO catalysts at atmospheric pressure and developed a Langmuir–Hinshelwood model that elucidated the bifunctional role of Ni/MgO catalysts regarding H₂ and CO₂ activation. Loder proposed a reaction mechanism for CO₂ methanation over Ni/MgO catalysts involving five key steps: (i) initial dissociative adsorption of H₂ onto Ni sites, (ii) subsequent adsorption of CO₂ at the Ni-MgO interface, where a CO₂ molecule interacts with two Mg and one Ni site, (iii) the reaction between adsorbed CO₂ and H₂ to form methane and water, (iv) desorption of the product methane, and (v) desorption of the by-product water. This multi-step mechanism underscores the bifunctional catalytic nature of Ni/MgO, with Ni facilitating H₂ activation and MgO playing a crucial role in CO₂ activation [37]. The proposed reaction mechanism aligns with the findings of a study from Huang et al. They reported synergistic effects through the use of density functional theory calculations. The advantages of Ni/MgO catalysts arise from the synergistic interaction between Ni and MgO, leading to enhanced Ni reducibility with increased electron transfer to adsorbed CO₂, thus promoting the formation of the H₂COO* intermediate, which is a key precursor to methane via the formate pathway. This contrasts with the HCOOH* pathway observed on pure Ni(111). Furthermore, hydrogen spillover from Ni to MgO, coupled with the strong OH adsorption on MgO, accelerates water desorption, thereby enhancing the overall reaction kinetics and improved catalytic activity [61].

In addition to the studies with nickel-based catalysts on MgO, Park and McFarland and Kim et al. studied CO₂ methanation over palladium-based Pd-MgO/SiO₂ catalysts. Their results showed that Pd actively dissociates hydrogen molecules, enabling the stepwise hydrogenation of magnesium carbonate, formed through CO₂ chemisorption, into methane. The primary steps of the catalytic reaction are (i) CO₂ adsorption onto MgO, where carbonate species are exposed to hydrogen, leading to water formation via hydrogenation of oxygen species, and (ii) hydrogenation of the residual carbon on the MgO surface, culminating in methane production. MgO facilitates the formation of magnesium carbonate on its surface, which is a critical intermediate in methanation, as shown in Figure 3 [11,62].

3. Results and Discussion

3.1. Catalyst Characterization

In this study, silica from different sources was investigated as a primary support material for nickel. Commercial silica SiO₂(Com) and rice husk silica SiO₂(RH) were used. The preparation of SiO₂(RH) was conducted using an acid refluxing method, as outlined in Section 4.2.1. The yield of SiO₂(RH) from two batches is shown in

Table 2. Silica yields from rice husks using the acid refluxing method.

Raw Material Weight (g)	Silica Product (g)	Silica Yield (%)
30.4	6.67	21.94
21.43	4.11	19.18
Total silica	10.78	20.56

.

The rice husk raw material (30.4 g and 21.43 g) yielded 21.94% and 19.18% silica, respectively. The total silica production from both batches amounted to 10.78 g, with an overall silica yield of 20.56%. This result demonstrates the efficiency of the acid refluxing method for silica extraction, which corresponds to the work of Korotkova et al. [63]. The range of silica content and overall composition within the rice husks is presented in

Table 3. Chemical composition of rice husks [63].

Composition	SiO2	Lignin	Cellulose	Protein	Fat	Other Nutrients
wt%	18.8–22.3	9–20	28–38	1.9–3.0	0.3–0.8	9.3–9.5

.

The chemical composition of the rice husk silica SiO₂(RH) was determined by XRF spectroscopy and is described in Section 4.1. Rice husk material gives high-purity silica at 99.86% SiO₂, with only minor contamination. Minor impurities include Al₂O₃ (0.07 wt%) and P₂O₅ (0.06 wt%), while other oxides such as TiO₂, Fe₂O₃, MnO, MgO, CaO, K₂O, and Na₂O are only present in trace amounts, each below 0.05 wt%.

The X-ray diffraction results of the high-purity SiO₂(RH) and nickel-doped rice husk silica Ni/SiO₂(RH) (before activation) are show in Figure 4. The XRD pattern of pure SiO₂(RH) shows a broad peak that is characteristic of an amorphous structure, indicating the non-crystalline nature of the silica derived from rice husks. In contrast, the XRD pattern of Ni/SiO₂(RH) exhibits additional sharp peaks corresponding to crystalline phases of nickel oxide (NiO) and metallic aluminum (Al). These nickel oxide peaks confirm the successful loading of nickel onto the silica support. The aluminum (Al) peaks originate from minor impurities and are not expected to have a pronounced influence on the catalytic activity.

Moreover, the nickel-based catalysts on mixed rice husk silica and magnesium oxide support Ni/SiO₂(RH)/MgO (before and after use) were analyzed by XRD analysis. The ratio of silica to MgO was SiO₂:MgO = 5:95 (w/w). The phase composition and crystallinity of the freshly prepared catalyst and the catalyst after use are shown in Figure 5. Prominent diffraction peaks can be attributed to the crystalline phases of NiO and MgO. Additionally, a diffraction peak corresponding to ZrO₂ is observed for Ni/SiO₂(RH)/MgO (after use) as ZrO₂ was used as a packing material during the hydrogenation reaction in the reactor. Due to the minor SiO₂ content, aluminum impurities were not detected in the catalysts with the mixed-support material.

3.2. CO₂ Methanation over Ni-Based Catalysts on Silica and Mixed SiO₂/MgO Support

The performance of the silica-supported nickel catalysts was evaluated and compared with previous findings of Ni/siderite_reduced and Ni/MgO catalysts that were used as reference catalysts. The experimental study followed the methodologies of Sommerbauer et al. [64] and Loder et al. [37,65], using a stoichiometric feed gas ratio of CO₂:H₂ = 4:1. To balance the feed gas stream, inert nitrogen was added, resulting in a gas composition of H₂:CO₂:N₂ = 56:14:30. The catalytic performance for CO₂ methanation was tested in a temperature range of 533–648 K with feed gas flow rates of 4.01, 8.02, 11.32, and 14.66 m³·kg⁻¹·h⁻¹ (STP). The potential of three different silica support materials for Ni-based catalysts for CO₂ methanation (i) commercial silica (SiO₂(Com)), (ii) rice husk silica (SiO₂(RH)), and (iii) mixed rice husk silica and MgO (SiO₂(RH)/MgO) are discussed in this section.

In all experimental runs, only methane and CO were found as gaseous products in the product gas stream. No hydrocarbons were detected in the condensate. In the following, the methane selectivity is mainly discussed and specified. However, this always means that the remaining converted amount of CO₂ is present as CO in the product gas. This was confirmed on the basis of the mass balance.

3.2.1. Commercial Silica

The results of CO₂ conversion and CH₄ selectivity for varying flow rates (4.01, 8.02, 11.32, and 14.66 m³·kg⁻¹·h⁻¹) and temperatures (548, 598, and 648 K) with commercial silica SiO₂(Com) as support material are shown in Figure 6. The commercial silica support was doped with 10 wt% nickel for the methanation experiments.

For each temperature, the CO₂ conversion and the CH₄ selectivity decreased with increasing feed gas flow rate, which was to be expected for CO₂ conversion due to the decreasing residence time in the reactor with increasing flow rate. At the lowest flow rate of 4.01 m³·kg⁻¹·h⁻¹ and the highest reaction temperature of 648 K, the highest CO₂ conversion with 29.33% and the highest CH₄ selectivity of 61.13%, were achieved. The lowest CO₂ conversion and CH₄ selectivity were 1.63% and 13.18%, respectively, at a feed gas flow rate of 14.66 m³·kg⁻¹·h⁻¹ and 548 K. Increasing methane selectivity with longer residence time in the reactor indicates a subsequent reaction in which methane is formed in a secondary step from originally formed CO.

Furthermore, as expected, both CO₂ conversion and methane selectivity increased with increasing temperature from 548 K to 648 K.

3.2.2. Rice Husk Silica

Next, rice-husk-derived silica SiO₂(RH) was loaded with 10 wt% nickel to produce Ni/SiO₂(RH) catalysts. The reaction was also carried out with a feed gas composition of H₂:CO₂:N₂ = 56:14:30, varying feed gas flow rates ranging from 4.01 to 14.66 m³·kg⁻¹·h⁻¹ (STP), and reaction temperatures of 548 K, 598 K, and 648 K. The CO₂ conversions and CH₄ selectivities with Ni/SiO₂(RH) for the various process conditions are shown in Figure 7.

As expected, with increasing temperature, the CO₂ conversion increased for all flow rates with the Ni/SiO₂(RH) catalysts. In contrast to the findings for the Ni/SiO₂(Com) catalysts, with the Ni/SiO₂(RH) catalysts, the selectivity for CH₄ slightly decreased with increasing temperature. This trend was more pronounced for higher flow rates and negligible for the lowest feed gas flow rate of 4.01 m³·kg⁻¹·h⁻¹ (STP). The highest CH₄ selectivity was 92.64% with the Ni/SiO₂(RH) catalysts (at 548 K), which was higher than the 61.13% achieved with the Ni/SiO₂(Com) at 648 K.

3.2.3. Comparison of Commercial Silica and Rice Husk Silica as Support

When comparing the two sources of silica, commercial silica and silica derived from rice husks (Figure 8), at the lowest reaction temperature of 543 K, the CO₂ conversion lay in a similar range for both support materials (0.01–8.41%). At higher reaction temperatures (598 and 648 K), the use of Ni/SiO₂(RH) resulted in higher CO₂ conversions (9.27–39.01%) than when Ni/SiO₂(Com) was used (6.9–29.33%) for all feed gas flow rates. Moreover, a notable difference was observed for the CH₄ selectivity. Whereas the use of Ni/SiO₂(RH) showed a high CH₄ selectivity in the range of 72.45–92.64%, Ni/SiO₂(Com) exhibited significantly lower CH₄ selectivities, ranging from 13.18% to 61.13%. The higher methane selectivities with Ni/SiO₂(RH) lead to the conclusion that Ni/SiO₂(RH) catalysts are more capable of catalyzing the methanation of initially formed CO compared to the catalysts Ni/SiO₂(Com).

Low feed gas flow rates allowing for sufficient reaction times are essential for maximizing CO₂ conversion for both Ni/SiO₂(Com) and Ni/SiO₂(RH) catalysts. From the experimental results, it is also evident that SiO₂(RH) significantly outperforms SiO₂(Com) in terms of CH₄ selectivity while maintaining comparable performance in terms of CO₂ conversion at 548 K and 598 K, as well as an even superior CO₂ conversion at 648 K. This may be dedicated to the morphological structure of the silica that was obtained via acid refluxing from rice husks. Morphological differences are discussed in Section 3.2.6.

These findings clearly highlight the distinct influence of the support material’s properties, such as surface morphology and metal-–upport interaction, on the catalytic performance of Ni-based catalysts for CO₂ methanation. The higher CO₂ conversion and methane selectivity that was achieved with Ni/SiO₂(RH) catalysts demonstrates their potential to address the limitations observed with commercial silica as a support material.

3.2.4. Mixed Rice Husk Silica and Magnesium Oxide Support

From several studies, it became evident that MgO is an efficient support material for nickel for CO₂ methanation as it is characterized by the chemisorption capacity of CO₂ and thus CO₂ activation through magnesium carbonate formation [37,39,64,65]. Therefore, the next step was to investigate whether a synergistic effect can be achieved by combining MgO with SiO₂ from rice husks as a support for nickel that exceeds the catalytic effect of the nickel-based catalysts on a single-support material. This time, the amount of nickel loading was increased to 29 wt%. The 29 wt% nickel catalysts were prepared in five different weight ratios of SiO₂(RH):MgO (5:95, 30:70, 50:50, 70:30, and 95:5). As before, the experiments were conducted with a feed gas composition of H₂:CO₂:N₂ = 56:14:30, feed gas flow rates ranging from 4.01 to 14.66 m³·kg⁻¹·h⁻¹ (STP), and reaction temperatures between 533 K and 648 K. The results for the CO₂ conversions and CH₄ selectivities are shown in Figure 9.

As expected, the CO₂ conversion increased with increasing temperature and decreasing flow rate, meaning higher residence in the reactor. For the mixed-support material catalysts, the ratio SiO₂(RH):MgO = 5:95 consistently demonstrated the highest CO₂ conversion across all temperatures and feed gas flow rates, achieving 59.91–77.24% CO₂ conversion at 648 K. In contrast, the 30:70 and 50:50 ratios exhibited moderate performance, with CO₂ conversions improving at higher temperatures but remaining lower than the ones with the 5:95 ratio. The lowest CO₂ conversions were observed for the 70:30 and 95:5 SiO₂(RH):MgO ratios, with values of 3.93% and 1.43%, respectively, at a temperature of 543 K and a feed gas flow rate of 14.66 m³·kg⁻¹·h⁻¹ (STP).

For a temperature of 648 K and a feed gas flow rate of 14.66 m³·kg⁻¹·h⁻¹ (STP), the catalytic performance, CO₂ conversion, and CH₄ selectivity of the 29 wt% Ni/SiO₂(RH)/MgO catalysts with varying weight ratios of SiO₂(RH):MgO (5:95–95:5) was compared to the catalytic performance of nickel catalysts on SiO₂(RH) and MgO supports only (Figure 10). It has to be noted that the nickel loading was only 10 wt% for the Ni/SiO₂(RH) and Ni/MgO catalysts in contrast to the 29 wt% nickel loading of the mixed-support catalysts. The 29 wt% Ni/SiO₂(RH)/MgO catalysts with the highest fractions of MgO in the support material (SiO₂(RH):MgO = 5:95, 30:70, and 50:50) showed the highest catalytic performance regarding CO₂ conversion. Compared to the 10 wt% Ni/MgO catalyst, the higher catalytic performance can of course be dedicated to the higher nickel loading. The two Ni/SiO₂(RH)/MgO catalysts with a low MgO content (30 and 5 wt%) in the carrier material performed worse than the Ni/MgO catalyst despite the low nickel loading of 10 wt% on MgO. The 10 wt% Ni/SiO₂(RH) showed the lowest catalytic performance. From this result, it can be deduced that Ni/MgO catalysts show the highest catalytic activity in terms of CO₂ conversion, even at comparatively low nickel loadings of 10 wt%. This once again highlights the important role of MgO as support material for nickel-based CO₂ methanation catalysts. MgO can be used together with SiO₂(RH) as support material for nickel; however, the use of SiO₂ is associated with reduced catalytic activity. In this case, the reduced catalytic activity can only be counteracted by a higher nickel loading. In this study, this was achieved by tripling the nickel loading. However, this effect is cancelled out if the SiO₂ loading is too high.

Looking at the CH₄ selectivity, it can be seen that a lower selectivity was achieved with both the Ni/MgO and the Ni/SiO₂(RH) catalysts. In this case, a mixed SiO₂(RH)/MgO support seems to be advantageous. At all temperatures, the catalysts with SiO₂(RH):MgO ratios of 5:95, 30:70, 50:50, and 70:30 achieved the highest CH₄ selectivities, consistently exceeding 91.57%. The catalyst with a SiO₂(RH):MgO ratio of 95:5 exhibited a slightly lower CH₄ selectivity, with values of 96.67, 89.12, 83.40, and 78.89% at flow rates of 4.01, 8.02, 11.32, and 14.66 m³·kg⁻¹·h⁻¹ (STP), respectively. A comparison of the two single-support material catalysts Ni/MgO and Ni/SiO₂(RH) shows that the Ni/SiO₂(RH) catalyst is clearly more selective for CH₄ than the Ni/MgO catalyst.

3.2.5. Comparison of Different Support Materials: SiO₂(RH), SiO₂(RH)/MgO, MgO, and MgO/siderite_reduced

It has become evident that MgO is an efficient support material for nickel for CO₂ methanation. Figure 11 gives an overall comparison and depicts the CO₂ conversion for different catalyst systems, including the 29 wt% Ni/SiO₂(RH)/MgO catalysts from this study, the 28% wt Ni/siderite_reduced/MgO catalysts from a previous study [39], the 28% wt% Ni/siderite_reduced [39], and the 31 wt% Ni/MgO [39] catalysts for comparison. Additionally, the equilibrium conversion is represented as a solid line, providing a theoretical benchmark for comparison.

As expected, for all catalyst systems, the CO₂ conversion increased when the temperature increased from 533 K to 648 K, proving that higher reaction temperatures enhance the catalytic performance.

Both the highest CO₂ conversion (79.36%) and the highest CH₄ selectivity (99.67%) were obtained with the 31 wt% Ni/MgO catalyst, reaching a CO₂ conversion close to the thermodynamic equilibrium conversion (83.56%, 90.09%, and 94.69% at 648, 598, and 548 K, respectively). This highlights the superior catalytic performance of Ni/MgO, likely due to the strong interaction between nickel and the basic MgO support, with MgO providing the capacity for CO₂ chemisorption and enhancing nickel dispersion and catalytic activity. In the case of a 28 wt% Ni catalyst supported on mixed reduced siderite ore and MgO (Ni/MgO/siderite_reduced), an increase in the ratio of reduced siderite to MgO led to enhanced CO₂ conversion and CH₄ selectivity that was comparable to the Ni/MgO catalyst at comparable or equal nickel loading, as stated in a previous study by Suksumrit et al. [39]. The optimal performance was observed at a siderite_reduced:MgO ratio of 70:30, achieving the highest CO₂ conversion of 77.98% and CH₄ selectivity of 99.66%.

Conversely, for Ni-based catalysts supported on SiO₂(RH)/MgO, increasing the SiO₂(RH) content resulted in a decline in both CO₂ conversion and CH₄ selectivity. The best catalytic performance for this mixed-support material was obtained at a SiO₂:MgO ratio of 5:95 and feed gas flow rate 8.02 m³·kg⁻¹·h⁻¹ (STP), with a CO₂ conversion of 69.66% and a CH₄ selectivity of 98.01%. Nevertheless, the SiO₂/MgO support consistently maintained high CH₄ selectivities (≥95%) across all tested flow rates and support ratios.

The results indicate that the catalysts with both MgO and SiO₂ alone as a support material have a lower selectivity for CH₄ than the combination of the two or the combination of MgO with reduced siderite ore. From this, it can be concluded that there is a synergistic effect with regard to CH₄ selectivity.

In summary, it can be said that with comparable nickel loadings, the Ni/MgO catalyst achieved the highest catalytic activity, closely followed by Ni/siderite_reduced/MgO catalysts. The catalytic activity of Ni/SiO₂(RH)/MgO catalysts with the same nickel loading is comparatively low, but the mixed-support material appears to have a positive effect on the CH₄ selectivity. The Ni/siderite_reduced catalyst shows only minor catalytic performance, both regarding CO₂ conversion and CH₄ selectivity.

3.2.6. Characterization of Silica from Rice Husks as Catalyst Support and Ni-Based Catalysts Ni/SiO₂(RH) and Ni/SiO₂(RH)/MgO

Transmission electron microscopy images of the silica derived from rice husks as well as the Ni-based catalysts Ni/SiO₂(RH) and Ni/SiO₂(RH)/MgO are shown in Figure 12a–l. The images in Figure 12a–c display loosely agglomerated clusters of nanoparticles, which are characteristic for an amorphous structure. In contrast, Figure 12d–f exhibit a more structured morphology with evidence of uniform deposition, indicating a tendency towards crystallinity. Consequently, rice husk silica may demonstrate a variety of morphological structures, which can influence the dispersion of Ni particles on the SiO₂(RH) surface. The observed variation in light and dark contrast within the images suggests the presence of interparticle voids that are indicative of a mesoporous or macroporous texture.

Regarding the distribution of nickel on the support materials, Figure 12g–i show that Ni particles are uniformly dispersed on the SiO₂(RH) surface. The Ni/SiO₂(RH)/MgO catalyst also exhibits a uniform dispersion of Ni particles, although a minor agglomeration of Ni can be observed, as shown in Figure 12j–l. This reveals that MgO addition significantly affects the dispersion behavior of nickel on the support, which may be attributed to the strongly basic nature of MgO. As a catalyst support for nickel, MgO enhances the surface alkalinity of the support and thereby promotes CO₂ adsorption—a favorable condition for CO₂ hydrogenation. However, if the strong metal–support interaction (SMSI) is insufficient or disrupted, it may hinder effective anchoring of Ni particles, resulting in their migration and coalescence, and ultimately leading to particle agglomeration.

The energy-dispersive X-ray spectroscopy element mapping proves that silica from rice husks mainly contains Si and O elements, as shown in Figure 13a. This is consistent with the XRF results (

Table 4. Average crystallite size of the Ni-based catalysts, the 10 wt% Ni/SiO₂(RH), the 28 wt% Ni/SiO₂(RH)/MgO (freshly prepared), and the Ni/SiO₂(RH)/MgO (after use for CO₂ methanation) as determined from XRD analysis.

Catalyst	Average Crystallite Size (nm)
Ni/SiO2(RH)	10.3
Ni/SiO2(RH)/MgO (freshly prepared)	10.8
Ni/SiO2(RH)//MgO (after use)	12.2–13.0 *

* Particle size of NiO 13.0 nm and MgO 12.2 nm.

), indicating a high silica purity of 99.86%. For Ni/SiO₂(RH), Si and O appear simultaneously with the peaks of Ni, meaning that there is an intimate contact between Ni, Si, and O, as shown in Figure 13b. Similarly, Figure 13c reveals the co-existence of Si, O, Ni, and Mg, indicating close interaction among these elements in the Ni/SiO₂(RH)/MgO catalysts. In addition, the EDS illustrations present the element distribution for Si (green), O (pink), Ni (yellow), and Mg (white). In both cases, the Ni particles appear uniformly and homogeneously distributed on the SiO₂(RH)/MgO and SiO₂(RH) supports, as evidenced by the even yellow coloration in Figure 13b,c. The difference in the yellow color between SiO₂(RH)/MgO and SiO₂(RH) results from the higher nickel loading on SiO₂(RH)/MgO (28 wt% vs. 10 wt% on SiO₂(RH)). The results indicate that the Ni/SiO₂(RH)/MgO catalyst exhibits a higher nickel dispersion compared to the Ni/SiO₂(RH) catalyst. This observation is consistent with the findings from Ye et al. [66], who demonstrated that Ni/SiO₂ catalysts lack strong metal support interaction (SMSI), which results in reduced catalytic activity. In the present study, the EDS elemental mapping shows a significant overlap between Mg, Ni, and Si signals in the Ni/SiO₂(RH)/MgO sample, suggesting strong intermixing of the support components. The partial clustering of Ni observed in the sample may suggest the presence of SMSI-like behavior, where the MgO support material helps to stabilize the Ni particles and possibly changes their electronic properties, which could improve the catalyst’s performance. The average crystallite sizes of Ni/SiO₂(RH)/MgO (the freshly prepared one and the catalyst after use) compared to Ni/SiO₂(RH) are shown in Table 4.

The crystallite size of 10.3 nm observed for Ni/SiO₂(RH) indicates that nickel is initially well dispersed on the rice-husk-derived silica support. With the addition of MgO, the average crystallite size increases slightly to 10.8 nm. However, after catalytic use, the crystallite sizes further increase to 12.2–13.0 nm, which is indicative of sintering or agglomeration processes occurring under the reaction conditions used. This growth in crystallite size is typically associated with a decrease in active surface area, which may contribute to a decline in catalytic performance over time. These findings suggest further study of long-term stability of the catalysts and possibly the incorporation of structural stabilizers to improve the thermal and structural stability during prolonged operation.

3.2.7. Evaluation of Long-Term Stability of Ni/MgO and Ni/SiO₂(RH) Catalysts

The catalytic performance of the 30 wt% Ni/MgO and 10 wt% Ni/SiO₂(RH) catalysts was evaluated over three operation cycles (=catalytic CO₂ methanation at 648 K in the tubular reactor), with each cycle lasting for six hours. To assess the catalyst stability for repeated use at steady-state operation as well as during heat-up und cool-down phases, the catalyst material was stored in inert nitrogen atmosphere for 24 h between operation cycles. The corresponding results are presented in Figure 14.

The Ni/MgO proves to be not only a more stable but also more active catalyst for CO₂ methanation, maintaining high CO₂ conversion (>79%) and high CH₄ selectivity (>99%) over three reaction cycles. In contrast, the silica-support catalyst from rice-husk-derived silica (RH) lacks the robustness to stabilize the dispersed Ni species over time, especially under the investigated cycling conditions, which is shown by a significant decline in CO₂ conversion over the cycles (

Table 5. CO₂ conversion and CH₄ selectivity of 30 wt% Ni/MgO and 10 wt% Ni/SiO₂(RH) catalysts over three operation cycles with cool-down, storage in nitrogen, and heat-up in between the cycles; feed gas ratio H₂:CO₂:N₂ = 56:14:30, feed gas flow rate 8.02 m³·kg⁻¹·h⁻¹ (STP), 648 K.

	tsteady-state (h)	tstorage (h)	Tcat (K)	${\mathit{X}}_{{\mathbf{C}\mathbf{O}}_2}$ (%)	${\mathit{S}}_{{\mathbf{C}\mathbf{H}}_4}$ (%)
10 wt% Ni/SiO2 (RH)
Run 1	6	24	648 ± 1.4	31.57	81.74 ± 0.03
Run 2	6	24	648 ± 1.2	30.52	81.53 ± 0.06
Run 3	6	0	648 ± 1.5	28.09	81.21 ± 0.11
30 wt% Ni/MgO
Run 1	6	24	648 ± 1.5	79.36	99.58 ± 0.21
Run 2	6	24	648 ± 1.4	79.44	99.55 ± 0.20
Run 3	6	0	648 ± 1.5	79.39	99.56 ± 0.20

). The reduction in catalytic activity could be attributed to partial sintering. The results underscore the critical influence of the catalyst support for long-term performance and stability of catalysts under practical operating conditions.

4. Materials and Methods

4.1. Raw Materials and Chemicals

All catalysts were prepared by wet impregnation with nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99%, p.a., Lactan, Karlsruhe, Germany). The MgO support was prepared from MagGran© (4 MgCO₃·Mg(OH)₂·4H₂O). Silica, as the support material was obtained from two resources: (i) commercial silica (Silicon(IV) oxide, 99.8%, S.A. 5 m³·g⁻¹, LOT D13Y012, Alfa Aesar, Ward Hill, MA, USA) and (ii) rice husk from agricultural resource (Nakhon Sawan, Thailand). The composition of the silica that was obtained from risk husk (RHS, rice husk silica) was determined by XRF spectroscopy (

Table 6. Chemical composition of the silica obtained from risk husks (RHS, rice husk silica) determined by XRF spectroscopy.

Component	wt%
SiO2	99.86
TiO2	0.01
Al2O3	0.07
Fe2O3	<0.05
MnO	<0.05
MgO	<0.05
CaO	<0.05
K2O	<0.05
Na2O	<0.05
P2O5	0.06

), where minor P₂O₅ impuritiesare not expected to have a pronounced influence on the catalytic activity. Gul et al. reported that catalysts with low Ni/P atomic ratios (0.5 and 1) exhibited no activity for the reverse water–gas shift reaction across the entire temperature range studied (573–1073 K). Even at higher Ni/P ratios (2–2.4), only minimal methanation activity was observed [67].

The siderite ore that was used in a previous study and added for comparison purposes originated from the Styrian Erzberg in Austria (particle size 0.5–1 mm). Its mineral composition, as well as the catalyst preparation procedure and the obtained results, is described in Suksumrit et al. [39].

4.2. Catalyst Preparation

4.2.1. Preparation of the Support Material

(i)

Silica support (rice husks): The rice husk was washed with deionized water to remove impurities, and, subsequently, it was dried overnight at 383 K in an air oven. Acid leaching was conducted by refluxing the cleaned dry rice husk in a solution of HCl (molar concentration of 0.5 mol·L⁻¹) at 353 K for two hours. The procedure was carried out in a round bottom flask equipped with a heating mantle, magnetic stirrer, and reflux condenser. The stirring rate was 450 rpm, and the solid-to-liquid ratio was 0.1. Following filtration, the rice husk was rinsed with deionized water until pH reached 7 and then dried overnight at room temperature. Finally, it was further dried and calcined in a muffle furnace (Heraeus M 110) at 383 K for four hours and 873 K for two hours in order to obtain pure amorphous white silica as shown in Figure 15. The procedure was adapted from [68].

(ii)

Reduced siderite ore support (as described in [39]): In a previous study, original siderite ore was reduced in a hydrogen atmosphere consisting of 90% hydrogen (with a feed gas ratio of H₂:N₂ = 9:1 (v/v)) at a flow rate of 0.048 m³·h⁻¹ in a tubular reactor at 973 K until the composition of the exit gas equaled that of the feed gas. Subsequently, the reduced siderite ore (siderite_reduced) was exposed to ambient air at room temperature, resulting in partial reoxidation.

(iii)

Magnesium oxide support: Magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O) powder was calcined in a muffle furnace (Heraeus M 110) with air at 723 K for two hours, followed by an additional calcination at 823 K for five hours.

4.2.2. Impregnation

Impregnation of silica from rice husks with nickel: Amorphous white silica (99.86%; analyzed by XRF analysis; Figure 16) was impregnated with nickel by immersing the silica in a nickel nitrate hexahydrate solution through continuous stirring on a hot plate until complete dryness was achieved. For 5 g of the amorphous silica, 60 mL of a 17.52 g·L⁻¹ nickel nitrate hexahydrate solution was used. The prepared catalyst precursors were dried overnight at 383 K and further calcined in air at 973 K for four hours.

Impregnation of MgO (and reduced siderite ore) (as described in [39]): A nickel nitrate solution with a nickel concentration of 53–60 g·L⁻¹ was prepared by dissolving Ni(NO₃)₂·6H₂O in ultrapure water in a flask. The resulting 70 mL nickel solution was then cooled in a water bath (with a temperature range of 293–298 K). A total of 10 g of calcined MgO and/or reduced siderite ore was incrementally introduced to the nickel solution at a rate of 0.167 g·min⁻¹. The resulting slurry-phase solution was agitated for two hours and subsequently filtered using a vacuum pump, which facilitated the separation of the slurry phase from the residual water phase. The filtered (green) slurry phase (Figure 17) was then dried overnight at room temperature.

Impregnation of mixed MgO and silica from rice husks: A nickel nitrate solution with a nickel concentration of 56–60 g·L⁻¹ was prepared by dissolving Ni(NO₃)₂·6H₂O in ultrapure water in a flask. The resulting 70 mL nickel solution was heated on a hot plate (with a temperature range of 343–353 K). A total of 10 g of calcined MgO and amorphous white silica was incrementally introduced to the nickel solution at a rate of 0.167 g·min⁻¹ through continuous stirring on the hot plate for 1.5–2 h and subsequently filtered using a vacuum pump, facilitating the separation of the slurry phase from the residual water phase. After that, the prepared catalyst was dried overnight at room temperature.

4.2.3. Thermal Deposition

All freshly prepared pre-dried catalyst precursors with the SiO₂ and MgO support material were further dried in a muffle furnace (Heraeus M 110) in air—first, at 393 K for two hours and subsequently at 673 K for five hours. Following this drying procedure, the resulting catalyst precursors exhibited a light-gray hue.

4.2.4. Activation/Reduction with Hydrogen

As final step, NiO on the support materials was reduced to elemental nickel. A total of 4 g of the catalyst precursor powder was reduced with hydrogen (maintaining a feed gas ratio of H₂:N₂ = 9:1 (v/v) at a gas flow rate of 0.048 m³·h⁻¹) in the tubular reactor at a temperature of 773 K (with temperature readings taken at the T₃ thermocouple position, as described in detail in Section 4.4.1) for four hours. Subsequently, the catalysts were retained in the tubular reactor and were applied for the CO₂ methanation experiments.

4.3. Catalyst Characterization Procedure

Catalyst characterization involved transmission electron microscopy (TEM), atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF) spectrometry, and X-ray diffraction (XRD). These analyses were conducted in accordance with the methodology outlined by Suksumrit et al. [39].

4.3.1. Transmission Electron Microscopy (TEM)

The surface morphology, interface structure, elemental composition, and distribution of silica derived from rice husk, as well as of the Ni-based catalysts (Ni/SiO₂(RH) and Ni/SiO₂(RH)/MgO), were characterized using a JEM-2100 transmission electron microscope equipped with energy-dispersive X-ray spectroscopy performing an elemental analysis by the JED-2300 Analysis Station™ Plus., manufactured by JEOL Ltd., Tokyo, Japan. The instrument operated at an accelerating voltage of 200 kV, with a probe current of 1 nA, an energy detection range of 0–40 keV, a dwell time of 0.2 ms, and a sweep time of 50 s.

4.3.2. X-Ray Diffraction

The crystalline structure of the catalyst samples was characterized by X-ray diffraction (XRD) on a Rigaku SmartLab^® diffractometer (Rigaku Corporation, Tokyo, Japan). Scans were performed over a 2θ range of 5.0° to 80.0° with a step size of 0.01° and a scan rate of 2.0° min⁻¹.

4.3.3. X-Ray Fluorescence Spectrometry

To confirm the nickel content in the catalyst samples, X-ray fluorescence (XRF) analysis was conducted using a wavelength-dispersive spectrometer (S8 TIGER Series 2, Bruker AXS GmbH, Karlsruhe, Germany). Prior to analysis, the samples were homogenized with lithium tetraborate (Li₂B₄O₇) and lithium metaborate (LiBO₂) and then fused in a high-temperature furnace at 1273 K for one hour. The measurements were performed using the Best Detection-Vac34mm configuration, applying a fully quantitative evaluation method to determine the elemental composition.

4.3.4. Atomic Absorption Spectrometry

Initial determination of nickel loading in the catalyst samples was carried out using atomic absorption spectrometry (AAS) with a Perkin Elmer AAnalyst 400 spectrometer (Perkin Elmer Instruments LLC, Shelton, WA, USA). The instrument was operated with a nickel-specific hollow cathode lamp at a wavelength of 232 nm and a current of 25 mA, utilizing a compressed air/ethylene flame. During catalyst synthesis, where various support materials were impregnated with nickel nitrate, solution samples were taken both prior to and following the impregnation step. These samples were diluted in a solution comprising 10 mL of HNO₃ (Carl Roth GmbH, Karlsruhe, Germany) and 990 mL of deionized water. Nickel concentrations were measured, and the corresponding nickel loadings were calculated based on the concentration differences.

4.4. Experimental Setup and Experimental Procedure of the Methanation Experiments

Hydrogen (99.999%), carbon dioxide (99.998%), and nitrogen (99.999%) supplied by Air Liquide were used for the CO₂ methanation experiments. Nitrogen was used as an inert gas for heat transport and balancing purposes.

4.4.1. Experimental Setup

CO₂ methanation experiments were conducted using a fixed-bed tubular reactor system constructed from stainless steel (Parr Instrument GmbH, Moline, IL, USA), as shown in Figure 17. The reactor tube measured 0.82 m in length with an internal diameter of 25 mm. Feed gas flow rates for H₂, CO₂, and N₂ were precisely regulated using three mass flow controllers (MFC1–MFC3). Prior to entering the catalyst bed, the gas mixture was preheated through a preheating coil (PHC) to ensure uniform inlet temperature. The reactor was externally heated using an electric furnace equipped with three independently controlled heating zones (HT1–HT3) along the reactor tube, allowing fine-tuned temperature management. Internal temperature distribution was monitored by six thermocouples (T1–T6), strategically positioned at two locations within each heating zone, realized by a thermocouple string (6 mm diameter) that was positioned along the centerline of the reactor to provide accurate temperature readings at the catalyst bed. A 4 g quantity of the Ni-based catalysts was centrally loaded into the reactor, supported by stainless steel spacers and a sieve to maintain bed integrity (Figure 18). Post-reaction, the effluent gas passed through a two-stage condensation system. The first cooling stage occurred in a heat exchanger (HE-1), where condensate was collected in a trap (CT-1). Further cooling was carried out in a second exchanger (HE-2), followed by collection of residual moisture in an additional condensate trap (CT-2) to prevent water vapor from entering the gas analysis system. The dried product stream was analyzed using a multi-component gas analyzer (GA). The hydrogen concentration was determined using a Caldos27 thermal conductivity detector, featuring dual measurement ranges (0–0.5 vol% and 0–100 vol%) with a maximum output error of ±0.5% (2σ) within the lower range. CO₂, CO, and CH₄ concentrations were quantified using a Uras26 infrared photometer, with an output error of ±0.2% (2σ) relative to the measurement span and capable of operating across 0–10 vol% and 0–100 vol% ranges.

4.4.2. Experimental Procedure

Prior to each experimental run, the reactor system was purged with high-purity nitrogen at ambient pressure to remove any residual gases. Once purging was complete, 4 g of the selected catalyst was placed in the center of the reactor tube. The predefined feed gas mixture, with a volumetric ratio of H₂:CO₂:N₂ = 56:14:30 (v/v/v), was then introduced into the system. The reactor was gradually heated to the target reaction temperature, which was monitored at the end of the catalyst bed (T₃) and designated as the catalytic temperature (T_cat). CO₂ methanation experiments were conducted under atmospheric conditions, with feed gas flow rates ranging from 8.02 to 14.66 m³·kg⁻¹·h⁻¹ (STP), and reaction temperatures varied between 548 K and 623 K. The CO₂ conversion and CH₄ selectivity values are reported as the averages of three independent measurements conducted at each reaction condition. After the gas mixture passed through the catalyst bed, the reaction products were directed through a two-stage condensation system to remove moisture. The liquid phase collected in the condensate traps was analyzed using gas chromatography–mass spectrometry (GC–MS) with an Agilent Technologies 6890N system. Meanwhile, the resulting dry gas stream was analyzed using a multi-gas analyzer to quantify the concentrations of H₂, CO₂, CO, and CH₄.

Catalyst performance was assessed based on CO₂ conversion (as defined by Equation (4)) and CH₄ selectivity (Equation (5)). Additionally, the volumetric expansion coefficient was determined using Equation (6), while the concentrations of nitrogen, hydrogen, methane, and carbon monoxide were calculated according to Equations (7)–(9).

$$X_{{\mathrm{C}\mathrm{O}}_2}={\displaystyle \frac{c_{{\mathrm{C}\mathrm{O}}_2,0}-{c_{\mathrm{C}\mathrm{O}}}_2}{c_{{\mathrm{C}\mathrm{O}}_2,0}+{{\epsilon }_{\mathrm{C}\mathrm{O}}}_2·{c_{\mathrm{C}\mathrm{O}}}_2}}$$

(4)

$$S_{{\mathrm{C}\mathrm{H}}_4}={\displaystyle \frac{{c_{\mathrm{C}\mathrm{H}}}_4}{{c_{\mathrm{C}\mathrm{H}}}_4+{c_{\mathrm{C}\mathrm{O}}}_2}}$$

(5)

$${\epsilon }_{{\mathrm{C}\mathrm{O}}_2}=y_{{\mathrm{C}\mathrm{O}}_2,0}·({\displaystyle \frac{\mathrm{c}}{\left|\left.\mathrm{a}\right|\right.}}+{\displaystyle \frac{\mathrm{d}}{\left|\left.\mathrm{a}\right|\right.}}-{\displaystyle \frac{\mathrm{b}}{\left|\left.\mathrm{a}\right|\right.}}-1)$$

(6)

$$c_{{\mathrm{N}}_2}={\displaystyle \frac{{c_{\mathrm{N}}}_{\mathrm{2,0}}}{1+{{\epsilon }_{\mathrm{C}\mathrm{O}}}_2·{X_{\mathrm{C}\mathrm{O}}}_2}}$$

(7)

$$c_{{\mathrm{H}}_2}={\displaystyle \frac{{c_{\mathrm{H}}}_{\mathrm{2,0}}-\mathrm{b}·{X_{\mathrm{C}\mathrm{O}}}_2·c_{{\mathrm{C}\mathrm{O}}_2,0}}{1+{{\epsilon }_{\mathrm{C}\mathrm{O}}}_2·{X_{\mathrm{C}\mathrm{O}}}_2}}$$

(8)

$$c_{\mathrm{P}}={\displaystyle \frac{\left(\left|\mathrm{c}\right|\mathrm{o}\mathrm{r}\left|\mathrm{d}\right|\right)·{X_{\mathrm{C}\mathrm{O}}}_2·c_{{\mathrm{C}\mathrm{O}}_2,0}}{1+{{\epsilon }_{\mathrm{C}\mathrm{O}}}_2·{X_{\mathrm{C}\mathrm{O}}}_2}}$$

(9)

5. Conclusions

Rice husk silica SiO₂(RH) was investigated for its potential as an alternative and sustainable option as a support material for nickel catalysts for CO₂ methanation. Compared to commercial SiO₂, silica derived from rice husks via acid refluxing showed superior catalytic performance regarding CO₂ conversion and CH₄ selectivity. Nevertheless, compared to Ni/MgO catalysts with equal nickel loadings, the Ni/SiO₂ performed worse. With higher nickel loadings, this effect could be counteracted. With both Ni/MgO catalysts and Ni/SiO₂(RH)/MgO catalysts, high selectivities for CH₄ were achieved, with slightly higher selectivities when SiO₂ was present in the MgO support material. To conclude, the important role of MgO in the support material for nickel-based CO₂ methanation catalysts was substantiated, which is based on the pronounced capacity of CO₂ chemisorption and therefore CO₂ activation. Furthermore, minor portions of SiO₂(RH) had a positive effect on the CH₄ selectivity, revealing a minor synergistic effect of SiO₂ and MgO in the support. Despite these satisfying results, the long-term stability of both Ni/MgO and Ni/SiO₂(RH)/MgO catalysts remains a critical issue that requires further investigation in future studies.