Ni-Based Catalysts Coupled with SERP for Efficient Power-to-X Conversion

Abstract

The industrial application of CO₂ methanation in Power-to-X (P2X) systems requires the development of highly active catalysts capable of operating at milder temperatures to ensure energy efficiency, while exhibiting high activity, stability and selectivity. This study reports the synthesis and optimization of Ni-based catalysts on Al₂O₃ supports, guided by a Design of Experiments (DoE, 2⁴ factorial design) approach. Initial optimization afforded a robust catalyst achieving 80% CO₂ conversion and >99% CH₄ selectivity at 325 °C. Remarkably, the incorporation of CeO₂ traces to the Ni-based catalyst substantially boosted catalytic activity, enabling higher conversions at temperatures up to 75 °C lower than the unpromoted catalyst. This improvement is attributed to Ni–CeO_x synergy, which facilitates CO₂ activation and Ni reducibility. Both formulations exhibited exceptional long-term stability over 100 h. Furthermore, process intensification via the Sorption-Enhanced Reaction Process (SERP) with the Ni-based catalyst demonstrated even superior efficiency, rapidly increasing CO₂ conversion beyond 95% with the same selectivity range. Our findings establish a clear and consistent pathway for industrial CO₂ valorization through next-generation P2X technology for high-purity synthetic natural gas (SNG) production. This process offers an efficient and sustainable route toward industrial defossilization by converting captured CO₂ and green H₂ into SNG that is readily usable within the existing energy infrastructure.

1. Introduction

The persistent increase in atmospheric carbon dioxide (CO₂) concentrations remains a primary cause of global warming and climate change [1,2,3]. In response, carbon capture and utilization (CCU) technologies have emerged as a crucial two-sided strategy, not only for mitigating emissions but also for valorizing captured CO₂ as a valuable single carbon (C1) feedstock [4,5,6,7].

As fossil resources were naturally produced via carbon hydrogenation, synthetic CO₂ hydrogenation provides a chemically reliable route for hydrocarbon regeneration. While the inherent thermal stability of CO₂ makes its hydrogenation challenging, direct hydrogen reduction has driven significant advancements in converting CO₂ to various C1 products (such as methanol, formic acid, carbon monoxide and methane). That is why thermocatalytic hydrogenation of CO₂ to methane (also known as methanation or Sabatier reaction) can be easily achieved at atmospheric pressure and high gas hourly space velocity (GHSV), reaching CO₂ conversions and CH₄ selectivity near the theoretical equilibrium [4,8,9].

The industrial interest in the methanation reaction comes from its crucial role in Power-to-X (P2X), an emergent technology capable of converting “excess” of renewable electricity into chemicals typically sourced from fossil fuels [8,10,11,12,13,14]. P2X achieves this by integrating the production of green H₂ (via renewable energy) and subsequent methanation of captured CO₂ to yield synthetic methane. This integrated process offers the valorization of a waste greenhouse gas into a valuable C1 feedstock, while simultaneously facilitating the storage of intermittent renewable energy [15]. The resulting products and energy carriers simultaneously enhance grid stability and offer significant versatility in the pathways toward defossilizing the current energy infrastructure—namely, by reducing emissions through the utilization of captured CO₂ and green H₂, and by replacing natural gas of fossil origin.

The Sabatier reaction is exothermic (ΔH°₂₉₈ = −165 kJ mol⁻¹) and thermodynamically favorable at moderate temperatures (200–400 °C) [8,16]. Noble metals such as Ru, Rh, and Pt exhibit excellent catalytic activity, stability and resistance to deactivation, but their large-scale application is very restricted by high cost and limited availability [14]. In contrast, Ni-based catalysts offer a promising alternative for CO₂ methanation, as they display comparable intrinsic activity with highly improved cost-efficiency due to their relatively low price. Even so, there are still some drawbacks to overcome, since Ni catalysts are prone to sintering, carbon deposition, and insufficient stability when working at high temperatures [8,17,18].

So far, improved performance in Ni-based catalysts has mainly been achieved by tailoring the catalyst via either support modification or the incorporation of structural promoters [19,20,21,22]. First, alumina (Al₂O₃) is conventionally used as a support due to its high surface area, mechanical strength, and thermal stability. However, strong metal–support interactions often lead to the formation of NiAl₂O₄ spinel phases, which are difficult to reduce, ultimately lowering the concentration of active metallic Ni sites [14,19]. Consequently, alternative supports such as ZrO₂, SiO₂, TiO₂, and carbon-based materials have been explored [23,24,25,26]. For instance, Ni/ZrO₂ catalysts typically display enhanced CO₂ adsorption and activation, attributed to the presence of oxygen vacancies in the zirconia lattice and moderate metal-support interactions [19]. Furthermore, rare-earth oxides such as CeO₂, La₂O₃, and Y₂O₃ are among the most widely studied promoters due to their ability to create oxygen vacancies, thus enhancing Ni reducibility and CO₂ adsorption [21,27]. CeO₂ in particular has shown remarkable promoting effects in CO₂ hydrogenation due to its high oxygen storage capacity and reversible Ce⁴⁺/Ce³⁺ redox cycle, which generates oxygen vacancies that facilitate CO₂ adsorption and activation. In addition, strong Ni–CeO₂ interactions enhance Ni reducibility, promote hydrogen dissociation, and stabilize well-dispersed metallic nanoparticles [7,28,29,30,31,32,33,34,35,36,37,38,39].

Beyond catalyst design, other process intensification strategies have been explored to further improve CO₂ methanation. Notably, sorption-enhanced reaction processes (SERP) incorporate a selective sorbent material (commonly molecular sieves or other porous oxides) into the catalytic bed, which immediately captures the water produced during methanation [40,41,42,43,44]. This continuous removal of H₂O shifts the reaction equilibrium toward higher CH₄ selectivity and CO₂ conversion, effectively overcoming thermodynamic limitations at moderate temperatures. As a result, nearly complete CO₂ conversion is achieved, and downstream gas separation is simplified, since high-purity SNG is formed as water is continuously removed by the sorbent [44].

In this contribution, four Ni-based catalysts supported on two Al₂O₃ phases were prepared via incipient wet impregnation. To optimize both catalyst composition and reaction parameters, we employed a Design of Experiments (DoE, 2⁴ factorial design) approach, enabling efficient and systematic evaluation of variable interactions. Two complementary strategies to enhance catalytic activity were explored: (i) doping Ni/Al₂O₃ catalysts with CeO₂, and (ii) coupling Ni catalysts with a SERP. The novelty of this study lies in the integration of a systematic Design of Experiments (DoE) approach with Ce promotion and the Sorption-Enhanced Reaction Process (SERP) concept, employing a novel mixed γ/θ–Al₂O₃ as support to optimize Ni-based catalysts for CO₂ methanation, thereby providing an innovative route for catalyst design. As a result, the 10% Ni/Ce@γ/θ–Al₂O₃ catalyst exhibited a great mid-temperature performance, reaching 84% CO₂ conversion with methane selectivity above 99% at only 250 °C under atmospheric pressure. Furthermore, when applying the SERP configuration with the Ni-based catalyst, CO₂ conversions up to 95% were achieved at short reaction times, confirming the strong potential of combining catalyst design with process intensification strategies for efficient CO₂ methanation.

2. Results and Discussion

2.1. Catalysts Characterization

The morphology and physical properties of the catalysts were investigated. X-ray fluorescence (XRF) analysis was used to quantify the Ni and Ce loadings, revealing values closely matching the nominal compositions (

Table 1. Physicochemical properties of the synthesized catalysts.

Sample	LM (wt. %) a	SABET (m2/g) b	Dp (nm) c	Vp (cm3/g) d	DNi (%) e	SNi (m2/g) f	dNi (nm) g
g/q-Al2O3	-	154	8.8	0.4	-	-	-
g-Al2O3	-	218	9.9	0.7	-	-	-
Ni10@g/q-Al2O3	11	136	7.9	0.4	5.6	4.3	18
Ni20@g/q-Al2O3	20	121	8.1	0.3	2.5	3.3	41
Ni10@g-Al2O3	12	182	10.3	0.6	1.8	2.4	56
Ni20@g-Al2O3	20	180	10.1	0.5	1.1	0.9	90
Ni/Ce@g/q-Al2O3	Ni: 10 Ce: 8	115	8.1	0.3	4.0	2.6	26

^a Metal (Ni and Ce) loading (L_M) determined by X-Ray fluorescence analysis. ^b Specific surface area of the catalyst (SA_BET) determined by Brunauer–Emmett–Teller (BET) method. ^c Average pore diameter (D_p) determined by applying the Barrett-Joyner-Halenda (BJH) model. ^d Total pore volume (V_p) determined by applying the Barrett-Joyner-Halenda (BJH) model. ^e Ni dispersion (D_Ni) determined by H₂-TPD. ^f Metal surface area (S_Ni) determined by H₂-TPD. ^g Ni mean diameter (d_Ni) determined by H₂-TPD.

). The textural properties of the catalysts, including specific surface area (SA_BET), average pore diameter (D_p), and total pore volume (V_p), calculated from the desorption branch of the nitrogen isotherm are also summarized in Table 1, along with the corresponding values for the bare alumina supports. To enable a fair comparison, the supports were subjected to the same thermal treatment used during the catalyst synthesis. This step was essential to account for any structural changes induced by the calcination process. A consistent trend observed across all samples is a moderate decrease in both D_p and V_p following the incorporation of Ni. However, this reduction remains relatively moderate, even in catalysts with higher metal loadings, indicating that the porous network is largely preserved and that the active sites remain accessible to reactant molecules. The addition of Ni also leads to a reduction in specific surface area (SA_BET). This can be attributed to the deposition of Ni species, which slightly narrow or partially fill the pores, thereby explaining the decreasing trend in pore metrics. The 4 catalysts are comparable to the IV-type isotherm, according to the IUPAC principles. Catalysts on g-Al₂O₃ support display an H3-type hysteresis loop, indicating a heterogeneous pore network composed mainly of slit-like mesopores formed by aggregated particles. Meanwhile the ones supported on g/q-Al₂O₃ show an H1-type loop, characteristic of uniform cylindrical pores. Thus, all materials have the mesoporous structure typically observed in alumina-based catalysts, but g-Al₂O₃ has a more disordered pore structure than g/q-Al₂O₃ (Figure 1a) [45].

The H₂-TPD results stated in Table 1 were carried out to determine the dispersion (D_Ni), specific surfaces area (S_Ni), and particle size (d_Ni) of Ni on the different catalysts. As a result, a clear trend is observed: increasing Ni loading leads to a decrease in D_Ni and S_Ni, accompanied by an increase in d_Ni. This is attributed to particle agglomeration at higher metal contents. Comparing the two supports, g-Al₂O₃ consistently shows lower dispersion and larger Ni particles than g/q-Al₂O₃ at both 10 and 20 wt.% loadings. For example, at 10 wt.%, D_Ni is 1.8% and d_Ni is 56 nm for g-Al₂O₃, while for g/q-Al₂O₃, D_Ni is 5.6% and d_Ni is only 18 nm. This suggests that the mixed phase g/q-Al₂O₃ support promotes better Ni dispersion and smaller particle formation. The Ni/Ce catalyst on g/q-Al₂O₃ shows intermediate values, indicating that Ce incorporation does not hinder Ni dispersion. This observation is in agreement with previous studies on similar promoted catalytic systems [46].

To further study the reduction behavior of the catalysts, H₂-TPR profiles were obtained and shown in Figure 1b. The results reveal that the Ni loading clearly influences the reducibility of the catalysts. Catalysts with a higher Ni loading (20 wt%), regardless of the support, display more intense reduction peaks, indicating a greater amount of reducible Ni species. Specifically, Ni₂₀@g-Al₂O₃ and Ni₂₀@g/q-Al₂O₃ show sharp peaks centered around 550–600 °C. This suggests that higher metal loading leads to the formation of larger NiO particles with weaker interactions with the support, which are more readily reduced. In comparison, the 10 wt% Ni catalysts show broader and less intense reduction peaks, consistent with a lower total amount of Ni and potentially stronger metal–support interactions that stabilize Ni species, thereby hindering reduction. Interestingly, the Ni/Ce@g/q-Al₂O₃ catalyst shows a broader reduction peak compared to Ni₁₀@g/q-Al₂O₃, indicating that Ce improves the reducibility of Ni species, which is aligned with previous literature observations [35].

The crystalline phases of our catalysts and bare alumina were measured by XRD. First, Figure 2 presents the XRD patterns of the bare g-Al₂O₃ and g/q-Al₂O₃ supports (Figure 2a), along with those of the Ni-based catalysts synthesized on these materials, including the Ce-doped catalyst and the Ni₁₀@g/q-Al₂O₃ sample after reduction (Figure 2b). The bare supports were subjected to the same thermal treatment as the Ni-containing catalysts to allow for a fair comparison. In Figure 2a, the diffractogram of the commercial g-Al₂O₃ confirms the presence of a pure g-phase, consistent with the cubic structure proposed by Rudolph et al. [47]. After Ni impregnation (Ni₁₀@g-Al₂O₃), additional reflections appear at 2q ≈ 37.3°, 43.3°, and 62.9°, which are characteristic of crystalline NiO (PDF 01-071-1179), indicating the successful deposition of NiO on the g-Al₂O₃ support. In Figure 2b, the XRD pattern of the g/q-Al₂O₃ commercial support reveals a mixture of g and q alumina phases. Rietveld refinement analysis determined a composition of 70.8% g-Al₂O₃ and 29.2% q-Al₂O₃. The Ni₁₀@g/q-Al₂O₃ catalyst exhibits clear NiO reflections (PDF 01-071-1179), confirming the presence of crystalline NiO on the mixed-phase alumina. After H₂ reduction (Ni₁₀@g/q-Al₂O₃ reduced), new peaks at 2q ≈ 44.5° and 51.8° appear, corresponding to metallic Ni (PDF 00-004-0850). This confirms the effective reduction in NiO to Ni⁰ and demonstrates that a reduction temperature of 450 °C is sufficient to fully reduce the Ni species. In the Ce-doped sample (Ni/Ce@g/q-Al₂O₃), additional reflections at 2θ ≈ 28.4°, 32.9°, 47.2° and 56.0° are observed, which are attributed to CeO₂ with a fluorite-type structure (PDF 00-067-0121), confirming the successful incorporation of ceria into the catalyst.

Figure 3 presents HRTEM images of the different Ni-based catalysts. Figure 3a shows the Ni₁₀@g/q-Al₂O₃ catalyst, where well-dispersed Ni nanoparticles can be observed over a porous alumina matrix. Clear lattice fringes corresponding to metallic Ni are visible, with interplanar distances matching the (111) planes of face-centered cubic (fcc) Ni, indicating a high degree of crystallinity. Figure 3b depicts the Ni₁₀@g-Al₂O₃ catalyst. Compared to Ni₁₀@g/q-Al₂O₃, this sample exhibits more aggregated and oriented Ni nanoparticles, along with an increase in particle size. Lattice fringes remain well defined, suggesting strong metal–support interactions that may promote the growth of larger crystalline domains. These observations are consistent with the H₂-TPD results, which indicate larger Ni particle sizes for the Ni₁₀@g-Al₂O₃ catalyst. Figure 3c shows the Ni/Ce@g/q-Al₂O₃ bimetallic catalyst. This image reveals a more heterogeneous distribution of crystalline domains, likely corresponding to both Ni and CeO₂ nanoparticles. The darker contrast regions are attributed to Ce-rich particles, due to their higher electron density. The coexistence of Ni and CeO₂ phases suggests possible synergistic effects at the metal–oxide interface, which are particularly relevant for redox catalytic processes [46].

Figure 4 presents the STEM-EDS elemental mapping of the different Ni-based catalysts supported on alumina. In Figure 4a,b, the EDS maps show a homogeneous distribution of Ni (blue) across the Al (green) support, suggesting good metal dispersion. In Figure 4c, the elemental maps display the co-localization of Ni (blue), Al (green), and Ce (red), confirming the successful incorporation of Ce species into the catalyst.

2.2. Catalytic Performance: Activity, Selectivity and Stability

The performance of the catalysts was initially evaluated by analyzing CO₂ conversions and CH₄ selectivity. It is noteworthy that the only by-product detected in the reactor effluent was CO, with carbon molar balances closing within the range of 98–102%. The influence of catalytic and reaction parameters on CO₂ methanation performance was systematically studied using a 2⁴ factorial design of experiments (DoE) implemented in the JMP software (version 18.0). The DoE matrix consisted of 16 experimental runs (

Table 2. DoE results of the methanation reaction.

Exp.	Ni (w%)	Support	Flow (mLN/min)	T (°C)	Conv. (%)	CH4 sel. (%)
1	10	g-Al2O3	100	300	41	>99
2	20	g-Al2O3	100	300	72	>99
3	10	g/q-Al2O3	100	300	41	>99
4	20	g/q-Al2O3	100	300	76	>99
5	10	g-Al2O3	300	300	70	>99
6	20	g-Al2O3	300	300	46	>99
7	10	g/q-Al2O3	300	300	81	>99
8	20	g/q-Al2O3	300	300	80	>99
9	10	g-Al2O3	100	400	56	>99
10	20	g-Al2O3	100	400	77	>99
11	10	g/q-Al2O3	100	400	67	>99
12	20	g/q-Al2O3	100	400	56	>99
13	10	g-Al2O3	300	400	64	>99
14	20	g-Al2O3	300	400	76	>99
15	10	g/q-Al2O3	300	400	72	>99
16	20	g/q-Al2O3	300	400	79	>99

) that explored the effects of Ni content, support material, reaction temperature, and total gas flow (GSHV). For each parameter, specific low and high levels were selected (Table 2), allowing for the evaluation of their main effects and interactions on CO₂ conversion and CH₄ selectivity. All experiments were conducted over a 3-hour reaction period.

The analysis of the 16 experimental runs revealed that total gas flow and its interaction with catalyst metal loading had the most significant impact on CO₂ conversion, whereas none of the studied parameters substantially influenced CH₄ selectivity. Based on these results, Ni₁₀@g/q-Al₂O₃ was selected as the most promising catalyst. Since there was no significant difference between using 10% or 20% Ni, opting for the Ni₁₀@g/q-Al₂O₃ catalyst reduces the Ni amount by half without affecting the reaction performance. The best reaction conditions among those tested were 300 °C and a GHSV of 18,000 h⁻¹, resulting in a CO₂ conversion of up to 80% and a CH₄ selectivity above 99%.

Then, the catalytic activity of Ni₁₀@g/q-Al₂O₃ was explored over a range of temperatures from 150 to 400 °C. The gas phase composition at the reactor outlet was analyzed every 15 min at each temperature. As shown in Figure 5a, the catalyst exhibited CO₂ conversions below 10% between 150 and 250 °C. Conversion increased along with temperature, reaching a maximum of 84% at 325 °C with CH₄ selectivity > 99%. At temperatures above 325 °C, CO₂ conversion began to decline, while CH₄ selectivity remained consistent > 99% across all conditions, in line with reports for most Ni-based catalysts under similar reaction conditions [23].

To improve the performance of the Ni₁₀@g/q-Al₂O₃ catalyst, a modified catalyst incorporating CeO₂ was prepared and evaluated (Ni/Ce@g/q-Al₂O₃). It is well established that CeO₂ exhibits excellent oxygen storage capacity due to its ability to reversibly switch between Ce⁴⁺ and Ce³⁺ oxidation states. This facile redox behavior is directly associated with the formation of oxygen vacancies in the CeO₂ lattice when Ce⁴⁺ is partially reduced. Specifically, these oxygen vacancies play a critical role in CO₂ activation, since upon interaction with the catalyst, CO₂ can be absorbed at the vacancies and subsequently transformed into CO, which is then readily hydrogenated to form CH₄ [48,49,50].

The catalytic activity of Ni/Ce@g/q-Al₂O₃ catalyst was evaluated following the same considerations as for the Ni-based catalyst (over a temperature range of 150 to 400 °C at a GHSV of 18,000 h⁻¹). The outlet gas composition was analyzed every 15 min at each temperature step. As shown in Figure 5b, the addition of CeO₂ notably enhanced the catalytic activity compared to the non-promoted Ni₁₀@g/q-Al₂O₃ catalyst. In detail, CO₂ conversions of approximately 80% were achieved over the Ni/Ce@g/q-Al₂O₃ catalyst at 250 °C, and this conversion was maintained up to 400 °C. Importantly, CH₄ selectivity remained > 99% across the entire temperature range, indicating that the reaction proceeded efficiently and selectively toward the desired product, with no formation of CO or other by-products. In contrast, the reference catalyst without CeO₂ required temperatures above 325 °C to reach similar CO₂ conversion levels. This shift of up to 75 °C toward lower reaction temperatures clearly confirms the promoting effect of CeO₂ in the methanation process. This notable mid-temperature performance is significantly superior to reported catalysts. In the literature, Ce-promoted Ni catalysts typically achieve CO₂ conversions ranging from 55% to 73% at 350 °C [23]. However, this efficiency drops drastically under milder conditions, with studies reporting conversions of only around 20% at 250 °C [24].

Based on the catalytic performance that we observe for Ni₁₀@g/q-Al₂O₃ and Ni/Ce@g/q-Al₂O₃, long-term stability tests were conducted under continuous operation for 100 h. The experiments were performed at 325 °C for Ni₁₀@g/q-Al₂O₃ and 250 °C for Ni/Ce@g/q-Al₂O₃, corresponding to their respective optimal operating conditions. As shown in Figure 5c, both catalysts exhibited remarkable stability throughout the entire testing period. Ni₁₀@g/q-Al₂O₃ maintained a steady CO₂ conversion of approximately 80% at 325 °C, while Ni/Ce@g/q-Al₂O₃ achieved a stable CO₂ conversion of about 84% at 250 °C. In both cases, methane selectivity consistently exceeded 99%.

Besides incorporating CeO₂ as a dopant, preliminary SERP experiments were conducted using the Ni₁₀@g/q-Al₂O₃ catalyst in combination with a 3Å molecular sieve as the sorbent [44,51]. Two catalyst-to-sorbent weight ratios (1:2 and 1:3) were evaluated at two operating temperatures (300 and 325 °C). As shown in Figure 5d, a substantial enhancement in CO₂ conversion was observed, particularly during the initial stages of the reaction at 325 °C. Within the first 10 min of operation, CO₂ conversion exceeded 95%, while CH₄ selectivity remained consistently above 99% in all tested conditions. As adsorption sites progressively saturate and conversion decreases, SERP ensures continuous operation by using multiple fixed-bed reactors that cycle between reaction and regeneration—With sorbent capacity restored through mild heating or inert gas purging [40,41,43,52].

Preliminary findings highlight SERP as an effective and complementary approach to catalyst doping for improving CO₂ methanation and producing high-purity CH₄ with minimal water content. Future work will focus on a comprehensive evaluation of the SERP system, including systematic studies of sorbent capacity, regeneration behavior, and long-term operational stability under continuous reaction conditions. The ultimate objective comprises two main goals: to develop efficient and scalable catalysts for integration into the semi-industrial SERP pilot plant currently under development by EURECAT, and to successfully demonstrate this defossilization technology as a potential next-generation route for synthetic natural gas production [53].

3. Experimental

3.1. Catalysts Preparation

High-purity (99.999%) reagent-grade Ni(NO₃)₂·6H₂O and Ce(NO₃)₃·6H₂O were purchased from Sigma-Aldrich (Burlington, MA, United States) and used as Ni and Ce precursors, respectively. Two commercial alumina with particle diameters of 0.9–1.1 mm were used: g-Al₂O₃ pellets from SASOL (Sandton, South Africa; ref: 610110), and g/q-Al₂O₃ spheres from NorPro (Stow, OH, United Sates; ref: SA62240).

A series of five Ni catalysts supported on various Al₂O₃ phases were prepared through incipient wet impregnation. Before the synthesis, Al₂O₃ pellets were dried at 120 °C overnight under reduced pressure. Afterwards, an aqueous suspension of Ni(NO₃)₂·6H₂O precursor (with Ni loadings of 10 and 20 wt%) and Ce(NO₃)₃·6H₂O (Ce loading of 10 wt%) when convenient, was prepared. The Ni salt was dissolved in minimal MilliQ^® water at 80 °C until complete dissolution. The suitable amount of alumina support (either g-Al₂O₃ or g/q-Al₂O₃) was heated at the same temperature and added to the nitrate suspension while stirring vigorously with a spatula to entirely impregnate the support (which resulted in a brilliant green solid). Right after, the impregnated catalyst was calcined in a muffle furnace at 500 °C (160 °C·h⁻¹) for 4 h. Four catalysts containing only Ni were prepared and named as follows: Ni₁₀@g/q-Al₂O₃ and Ni₂₀@g/q-Al₂O₃; Ni₁₀@g-Al₂O₃, Ni₂₀@g-Al₂O₃, together with one catalyst containing 10 wt% Ni and 10 wt% Ce and named as Ni/Ce@g/q -Al₂O₃.

3.2. Catalyst Characterization

Prepared catalysts were characterized by X-ray fluorescence (XRF), nitrogen adsorption, hydrogen temperature programed reduction (H₂-TPR), hydrogen temperature programed desorption (H₂-TPD), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM).

The Ni and Ce content were determined by X-ray fluorescence (Marven Panalytical Model Zetium). The specific surface area of the catalysts was assessed using N₂ adsorption–desorption isotherms measured in a Micromeritics Triflex equipment (Norcross, GA, United States), applying the Brunauer–Emmett–Teller (BET) method. The total pore volume (V_p), average pore diameter (D_p), and pore size distributions were determined by applying the Barrett-Joyner-Halenda (BJH) model, using the desorption isotherm.

X-ray diffraction (XRD) analysis was conducted on a Bruker-AXS D8-Advance (Billerica, MA, United States) diffractometer using monochromated CuK_α radiation, over a 2θ range of 5–80°.

High-resolution Transmission electron microscopy (HRTEM) images were obtained through a JEOL F200 TEM ColdFEG (Tokio, Japan) operated at 200 kV. STEM-EDS mapping was recorded from an EDS Centurion (Richardson, TX, United States) detector (silicon drift) with an effective area of 100 mm² and 133 eV of energy resolution.

Ni reducibility and metal dispersion were studied through hydrogen temperature-programmed reduction (H₂-TPR) and desorption (H₂-TPD) experiments, conducted in a Micromeritics Triflex reactor (Norcross, GA, United States) equipped with a thermal conductivity detector (TCD). For H₂-TPR, the catalyst was pretreated under pure N₂ at 350 °C (10 °C/min) for 30 min to remove impurities. After cooling to 50 °C (10 °C/min) during 30 min, a 5% H₂/N₂ mixture was introduced, and the temperature was ramped to 800 °C at 10 °C min⁻¹ while recording the TCD signal.

For H₂-TPD, the catalyst was first reduced at 500 °C (10 °C/min) for 2 h under 5% H₂/Ar. After cooling to 50 °C (10 °C/min), it was purged with Ar for 30 min to remove physisorbed H₂. A 5% H₂/Ar mixture was then introduced for 30 min, followed by another Ar purge for 30 min to remove weakly adsorbed H₂. The temperature was subsequently raised to 550 °C at a rate of 10 °C min⁻¹ while monitoring the TCD signal. All gas flows were maintained at 50 mL min⁻¹ during the H₂-TPR and H₂-TPD experiments.

Nickel dispersion (D_Ni), specific metal surface area (S_Ni), and mean particle diameter (d_Ni) were calculated using Equations (1)–(3), respectively [48]. Where V_ad (L) denotes the volume of chemisorbed H₂ obtained from the H₂-TPD analysis. M is the molar mass of Ni (58.69 g·mol⁻¹), and SF is the stoichiometric factor for the Ni/H atomic ratio during chemisorption, assumed to be 1. m (g) refers to the mass of the catalyst used in the H₂-TPD analysis, and w represents the weight percentage of Ni in the catalyst. V_m is the molar volume of H₂ (22.4 L·mol⁻¹), and d_r is the reduction degree of Ni, typically considered as 100%. N_A is Avogadro’s constant (6.02 × 10²³·mol⁻¹), while ρ_Ni and δ_Ni represent the density (8.902 g·cm⁻³) and atomic cross-sectional area (0.0649 nm²) of Ni, respectively [48].

$${\mathrm{D}}_{\mathrm{N}\mathrm{i}}\left(\mathrm{\%}\right)={\displaystyle \frac{2\times {\mathrm{V}}_{\mathrm{a}\mathrm{d}}\times \mathrm{M}\times \mathrm{S}\mathrm{F}}{\mathrm{m}\times \mathrm{w}\times {\mathrm{V}}_{\mathrm{m}}\times {\mathrm{d}}_{\mathrm{r}}}}\times 100\mathrm{\%}$$

(1)

$${\mathrm{S}}_{\mathrm{N}\mathrm{i}}\left({\mathrm{m}}^2·{\mathrm{g}}^{-1}\right)={\displaystyle \frac{2\times {\mathrm{V}}_{\mathrm{a}\mathrm{d}}\times {\mathrm{N}}_{\mathrm{A}}\times {\mathsf{\delta }}_{\mathrm{N}\mathrm{i}}}{\mathrm{m}\times {\mathrm{V}}_{\mathrm{m}}}}$$

(2)

$${\mathrm{d}}_{\mathrm{N}\mathrm{i}}\left(\mathrm{n}\mathrm{m}\right)={\displaystyle \frac{6\times {10}^3\times \mathrm{w}}{{\mathsf{\rho }}_{\mathrm{N}\mathrm{i}}\times {\mathrm{S}}_{\mathrm{N}\mathrm{i}}}}$$

(3)

3.3. Catalytic Tests

3.3.1. Conventional Methanation Reaction

Catalytic performance of each catalyst was evaluated in a fixed-bed reactor (inner diameter of 9.1 mm; volume of 18.5 cm³; and length of 285 mm; from Micromeritics, Norcross, GA, United States) under atmospheric pressure (1 barg). 1.0 g of catalyst was placed into the isothermal region of the tubular reactor each time, which was previously filled with inert quartz wool up to that central region to ensure good space velocity around the catalyst. The temperature of the reaction was monitored using a thermocouple placed in the middle of the catalyst bed. H₂, CO₂ and N₂ flows were controlled by calibrated Bronkhorst mass flow meters. All gases were supplied by Linde (Valencia, Spain), with purities ≥ 99.999%.

Before the reaction, the catalyst was pre-treated in situ under pure H₂ flow (100 mLN/min, GHSV of 6000 h⁻¹, 1 barg) and the temperature of the system was set at 450 °C. Once the temperature was stable (± 10 °C for 10 min), the catalyst was reduced under the same conditions for 2 h. The reactor was then cooled down to reaction temperatures under N₂ (22 mLN/min). After being steady at reaction temperature ± 5 °C for 5 min, the reactant gas mixture (ratio CO₂/H₂: 1/4) was injected into the reactor at a total flow rate of 300 or 100 mLN/min (GHSV of 18,000 or 6000 h⁻¹) [34,54]. In the screening tests, the catalysts’ performance was evaluated over a 3-hour period to obtain stable performance data, while the stability tests were conducted over 100 h.

Reagents and product mixtures passed through a liquid–gas separator (at 5 °C) from Micromimetics (Norcross, GA, United States), where water was separated from the mixture, and the dry output gas flow was measured by using a mass flow meter from Alicat Scientific (Tucson, AZ, United States). The composition of the output gas mixture was analyzed with an online gas chromatograph from Agilent Technologies (7890A GC System; Barcelona, Sapin) equipped with a thermal conductivity detector (TCD), and CO₂, CH₄ and CO were quantified with calibration curves. Samples were analyzed every 15 min along the screening experiments, and every 2 h during stability tests. The conversion of CO₂ (X_CO2), and both CH₄ and CO selectivity (S_CH4 and S_CO) were calculated using the following equations:

$$X_{{CO}_2}\left(\%\right)={\displaystyle \frac{F_{{CO}_2,in}-F_{{CO}_2,out}}{F_{{CO}_2,in}}}\times 100$$

(4)

$$S_{{CH}_4}\left(\%\right)={\displaystyle \frac{F_{{CH}_4,out}}{F_{{CH}_4,out}+F_{CO,out}}}\times 100$$

(5)

$$S_{CO}\left(\%\right)={\displaystyle \frac{F_{CO,out}}{F_{{CH}_4,out}+F_{CO,out}}}\times 100$$

(6)

where F_x,out is the output flow of the specific gas, while F_CO2,in is the input flow of CO₂.

3.3.2. SERP

The catalytic performance of the Ni-based catalyst was evaluated in a SERP using the same Micromeritics (Norcross, GA, United States) tubular reactor setup as for conventional methanation, with specific modifications. For each experiment, 1.0 g of catalyst was mechanically mixed with the appropriate amount of sorbent (3Å molecular sieves, Fisher Scientific), previously triturated and sieved to ≤ 500 µm, and placed into the isothermal zone of the reactor. Prior to reaction, the sorbent was activated under N₂ flow (500 mLN/min) at 250 °C and 1 barg for 180 min, followed by cooling to room temperature under identical conditions. Catalyst reduction was then performed under pure H₂ (100 mLN/min) at 450 °C for 2 h. The reactor was subsequently cooled to the target reaction temperature under N₂ flow (22 mLN/min). Once stabilized (± 5 °C for 5 min), the reactant mixture (CO₂/H₂ = 1/4 molar ratio) was introduced at the total flow rate of 300 mLN/min. Catalytic performance was assessed by continuous monitoring of the outlet stream using the online micro-GC 990 from Agilent (Barcelona, Spain). CO₂, CH₄, and CO concentrations were quantified against calibration curves, with data acquisition at 100 s intervals during the experiments.

4. Conclusions

To sum up, we have demonstrated two complementary strategies (CeO₂ promotion and sorption-enhanced reaction processing) with remarkable potential for advancing efficient CO₂ methanation at mild temperatures. The integration of these approaches paves the way for addressing the need for energy-efficient and scalable P2X applications, supporting industrial defossilization efforts.

Our initial findings confirmed the high efficiency and low cost of Ni-based catalysts for large-scale CO₂ conversion, even when minimizing Ni loading. Remarkably, the incorporation of CeO₂ as a promoter (Ni/Ce@γ/q−Al₂O₃) in the Ni₁₀@γ/q−Al₂O₃ catalyst substantially enhanced its activity, yielding high conversions at temperatures 75 °C lower than the unpromoted catalyst. Both catalysts exhibited exceptional long-term stability over 100 h of continuous operation.

Beyond catalyst refinement, we have also validated the superior efficiency of the SERP configuration over traditional methanation. When applied using the Ni₁₀@γ/q−Al₂O₃ catalyst and 3Å molecular sieves, CO₂ conversion rapidly increased beyond 95% while maintaining a CH₄ selectivity > 99%. This confirms SERP as a highly compelling and effective process intensification strategy outstanding conventional methanation configurations.

Given the promising findings, we believe that the potential synergy across these results establishes a robust framework for the rational design of highly efficient, stable and selective methanation systems, thereby demonstrating significant potential for industrial CO₂ valorization under milder and more energy-efficient conditions.