Synergistic Promotion Strategies for Ni-Based Catalysts in Methane Dry Reforming: Suppressing Sintering and Carbon Deposition

Abstract

Methane dry reforming (DRM) represents a promising route for the simultaneous valorization of CH₄ and CO₂ into syngas; however, conventional Ni-based catalysts suffer from rapid deactivation due to sintering and carbon deposition. In this work, we present a synergistically engineered Ni-based catalyst integrating hierarchical SiC confinement, Pd promotion via oleic-acid-assisted complexation, and MgO surface modification to overcome these challenges. Under optimized reaction conditions (CH₄/CO₂ = 1:1, 750 °C, GHSV = 36,000 mL g⁻¹ h⁻¹), the multifunctional NiPd/Si–xMg catalyst achieved steady-state conversions of 85% for CH₄ and 84% for CO₂, maintaining an H₂/CO ratio close to 1.0 over 100 h of continuous operation without noticeable deactivation. In contrast, the reference Ni/SiC and Ni/MgO catalysts exhibited initial conversions of 75–80% but declined by more than 50% within the same period, confirming the superior durability of the optimized system. Thermogravimetric analysis (TGA) revealed a drastic reduction in carbon deposition—from 119.0 mg C g⁻¹ for Ni/SiC to 81.4 mg C g⁻¹ for NiPd/Si-xMg—indicating enhanced coke resistance. Transmission electron microscopy (TEM) confirmed uniform Ni dispersion with an average particle size of 7.2 ± 1.8 nm, while H₂-TPR and CO₂-TPD analyses demonstrated improved reducibility and surface basicity. The combination of SiC confinement, Pd-induced hydrogen spillover, and MgO-mediated CO₂ activation effectively mitigated sintering and carbon accumulation, resulting in high activity, stability, and carbon tolerance. This integrated catalyst design provides a robust pathway toward industrially viable DRM systems for sustainable syngas production.

1. Introduction

The growing global energy demand, coupled with the urgent challenge of climate change driven by increasing atmospheric concentrations of greenhouse gases such as methane (CH₄) and carbon dioxide (CO₂), underscores the need for sustainable and efficient chemical processes [1]. Methane dry reforming (DRM), described by the reaction CH₄ + CO₂ → 2H₂ + 2CO, presents a promising route for the simultaneous conversion of these two greenhouse gases into syngas—a versatile feedstock critical to numerous industrial processes, including Fischer–Tropsch synthesis, methanol production, and the manufacture of higher-value chemicals [2,3]. Moreover, the syngas produced via DRM exhibits a low H₂/CO ratio (typically close to 1), making it especially suitable for downstream applications requiring such stoichiometric proportions.

Nickel-based catalysts have emerged as one of the most widely studied systems for DRM, owing to their comparative affordability, high intrinsic activity, and natural abundance [4]. Nonetheless, their industrial deployment remains constrained by persistent deactivation mechanisms, mainly sintering of metallic Ni particles and carbon deposition under high-temperature operating conditions [5,6]. Sintering induces particle agglomeration and growth, diminishing the number of accessible active sites and resulting in progressive activity loss. Concurrently, the accumulation of carbonaceous deposits—including amorphous carbon, graphitic carbon, and carbon nanotubes—can physically block active sites and pore structures, impairing mass transport and accelerating catalyst decay [7]. Consequently, the development of coke-resistant Ni catalysts with sustained high activity and stability is essential to enable the commercial viability of DRM.

Substantial research has been directed toward mitigating these deactivation pathways, with strategies encompassing support optimization, promoter addition, and innovative synthesis routes [8,9]. The choice of support material profoundly influences metal dispersion, strength of metal–support interaction, and the dynamics of carbon formation and elimination. Supports exhibiting high surface area and pronounced basicity, for example, can enhance CO₂ adsorption and activation, thereby promoting carbon gasification [10]. Similarly, the incorporation of promoters—such as noble metals or rare-earth elements—can modulate the electronic structure of Ni, improve its reducibility and dispersion, or supply mobile oxygen species to facilitate carbon removal [11,12]. Advanced preparation techniques, including impregnation, co-precipitation, and sol–gel methods, have also been employed to achieve finer control over nanoparticle size, morphology, and distribution [13].

In this work, we target the persistent challenges of sintering and coking through the rational design of Ni-based catalysts with enhanced durability and performance in DRM. Building on recent advances in heterogeneous catalysis, we implement an integrated strategy combining: (1) the construction of a hierarchical SiC-based confinement structure to stabilize Ni nanoparticles against sintering and suppress carbon formation; (2) the use of complexing agents to assist the introduction of trace noble metals (e.g., Pd), thereby forming highly dispersed bimetallic sites with tailored electronic properties and superior coke resistance; and (3) the modulation of support acidity/basicity via the incorporation of MgO to strengthen metal–support interactions and promote CO₂-mediated carbon elimination. While Pd- and MgO-modified Ni/SiC systems have been reported, the novelty of our approach lies in the synergistic combination of these strategies with a focus on multi-level structured SiC confinement and the use of complexing agents for precise control over bimetallic nanoparticle formation. This integrated design aims to achieve unprecedented levels of stability and activity. By correlating catalytic behavior with structural properties through extensive characterization, this study aims to advance the mechanistic understanding of DRM catalysis and contribute to the development of high-performance, industrially relevant catalysts for sustainable syngas production.

2. Materials and Methods

2.1. Catalyst Preparation

2.1.1. Synthesis of xCe@Ni–Si Composite Catalysts Through an In Situ Hydrothermal Route

The NiSi-MMO (0Ce@Ni-Al) catalyst was synthesized via a combined hydrothermal-impregnation approach. Specifically, 5 g of γ-SiC support was immersed in 10 mL of an aqueous nickel nitrate solution to achieve a nominal nickel loading of 10 wt%. The mixture was stirred continuously for 2 h at room temperature, then transferred to a Teflon-lined autoclave and subjected to hydrothermal treatment at 130 °C for 6 h. The resulting solid was collected and washed thoroughly with deionized water and absolute ethanol, each for three cycles. After washing, the sample was dried at 100 °C for 12 h and subsequently calcined in static air at 500 °C for 3 h using a ramp rate of 2 °C/min.

A series of CeO₂-promoted catalysts, denoted as xCe@Ni-Al (where x represents the nominal CeO₂ loading), were further prepared via incipient wetness impregnation. In a typical procedure, 1 g of the pre-calcined NiAl-MMO support was immersed in an aqueous solution of cerium nitrate under continuous stirring for 30 min. The targeted CeO₂ loadings were set at 0, 1, 3, and 6 wt%. The mixture was then stirred in a water bath maintained at 85 °C to facilitate solvent evaporation. The obtained solid was dried at 100 °C for 12 h, followed by calcination in air at 500 °C for 4 h with a heating rate of 5 °C/min, yielding the final xCe@Ni-Al catalysts [14].

2.1.2. Synthesis of Oleic Acid-Mediated Ni/Pd Bimetallic Catalysts

Meso-porous silicon carbide (SiC), exhibiting a specific surface area of 316 m²/g and an average pore size of 11.81 nm, was employed as the catalyst support. The synthesis procedure commenced with the dissolution of stoichiometric amounts of metal precursors—nickel nitrate hexahydrate and palladium nitrate—in 10 mL of deionized water. To the resulting aqueous metal solution, oleic acid was introduced at a fixed molar ratio (oleic acid to total metal = 0.5), and the mixture was stirred for 30 min to facilitate complex formation. The SiC support was then impregnated with this solution and allowed to equilibrate for 6 h. Subsequent solvent removal was carried out in a water bath maintained at 60 °C. The solid product was dried at 100 °C for 12 h and finally calcined in static air at 700 °C for 4 h. The total nominal metal loading was maintained at 5 wt% across all catalysts.

A monometallic Pd-supported SiC catalyst (denoted as Pd-SP-Imp) with 1 wt% Pd served as a reference material. For bimetallic formulations, the Ni:Pd mass ratio was fixed at 4:1. Catalysts synthesized with the assistance of oleic acid are labeled as Ni-SP-OA and NiPd-SP-OA, whereas those prepared via conventional impregnation in the absence of oleic acid are referred to as Ni-SP-Imp and NiPd-SP-Imp. Additionally, a series of oleic acid-assisted bimetallic catalysts with the general formula NixPd5–x–SP were synthesized, in which the total metal loading was kept at 5 wt%, while the value of x was varied systematically at 3.5, 4, 4.5, and 5 [15].

2.1.3. Facile Synthesis of NiPd Bimetallic Catalysts Supported on Magnesium-Modified Silica via Co-Impregnation

The Si–xMg composite supports were synthesized via an excess impregnation technique. Aqueous solutions of magnesium nitrate, serving as the magnesium precursor, were prepared by dissolving the salt in 10 mL of deionized water. Mesoporous silicon carbide (specific surface area: 316 m²/g; average pore diameter: 11.81 nm; supplied by Qingdao Hengze Organic Silicon Products Co., Ltd., Qingdao, China) was impregnated with these solutions under continuous stirring, targeting varying MgO mass fractions. Following a minimum impregnation duration of 2 h, the materials were dried at 100 °C for 12 h and subsequently calcined at 500 °C for 2 h, resulting in a series of MgO-modified SiC supports denoted as Si–xMg, where x represents the nominal MgO content (1, 3, 5, and 7 wt%).

For catalyst preparation, an aqueous solution containing nickel nitrate and palladium nitrate was formulated such that the final nominal loadings of Ni and Pd were 4 wt% and 1 wt%, respectively. The modified Si–xMg supports were then immersed in the mixed metal solution and subjected to impregnation under agitation for 6 h. The solvent was gradually evaporated using a water bath maintained at 60 °C. The resulting solids were dried at 100 °C for 12 h and finally calcined under static air at 700 °C for 4 h, yielding the NiPd/Si–xMg catalyst series [16].

2.2. Catalyst Characterization

To elucidate the structural and chemical characteristics of the synthesized catalysts, a suite of complementary characterization techniques will be utilized. The specific methods and their analytical objectives are outlined below:

X-ray Diffraction (XRD): Employed to identify crystalline phases, estimate mean crystallite size, and detect potential structural transformations induced by the catalytic reaction.

Transmission Electron Microscopy (TEM): Used to examine the morphology of the catalysts, analyze the size distribution of metallic nanoparticles, and assess the dispersion of active species on the support material.

Thermogravimetric Analysis (TGA) and Temperature-Programmed Oxidation (TPO): Applied to quantify carbonaceous deposits on post-reaction catalysts and to evaluate their resistance to coke formation under operating conditions.

Temperature-Programmed Reduction (H₂-TPR): Conducted to probe the reduction behavior of metal oxides and to characterize metal-support interactions through analysis of reduction profiles.

Temperature-Programmed Desorption (CO₂-TPD and H₂-TPD): Performed to investigate the surface adsorption/desorption behavior of CO₂ and H₂, thereby providing insight into the acid-base properties and the nature of active sites.

2.3. Catalytic Performance Evaluation

The catalytic performance of the synthesized catalysts for the dry reforming of methane (DRM) was evaluated in a fixed-bed quartz reactor system. The experimental setup consisted of mass flow controllers for precise regulation of reactant gases, a preheating section, a quartz reactor tube (Jiusheng Quartz Products Co., Ltd.) housing the catalyst bed, and an online gas chromatograph (GC) (Shanghai Youke Instrument Co., Ltd., Shanghai, China) equipped with a thermal conductivity detector (TCD) (Shanghai Youke Instrument Co., Ltd.) for quantitative analysis of the effluent stream. The DRM reaction was conducted under atmospheric pressure, with the reaction temperature systematically varied in the range of 600–800 °C. The total gas hourly space velocity (GHSV) was maintained at 36,000 mL·g⁻¹·h⁻¹, and the standard feed composition was set to a CH₄:CO₂ molar ratio of 1:1, unless otherwise specified. These parameters were carefully optimized to simulate practical reaction conditions while allowing meaningful evaluation of catalytic activity and selectivity. Catalyst stability was further assessed through long-term time-on-stream experiments extending to 100 h, during which CH₄ and CO₂ conversions, along with the H₂/CO molar ratio in the product stream, were continuously monitored. (Relevant catalytic performance data are provided in Figure 1 and Figure 2).

The conversions of CH₄ and CO₂, and the H₂/CO ratio will be calculated according to the following equations:

$${\mathrm{C}\mathrm{H}}_{4}C\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{{\mathrm{F}\mathrm{C}\mathrm{H}}_4,\mathrm{i}\mathrm{n}-\mathrm{F}{\mathrm{C}\mathrm{H}}_4,\mathrm{o}\mathrm{u}\mathrm{t}}{\mathrm{F}{\mathrm{C}\mathrm{H}}_4,\mathrm{i}\mathrm{n}}}\times 100\%$$

$${\mathrm{C}\mathrm{O}}_2\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)={\displaystyle \frac{{\mathrm{F}\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}-\mathrm{F}{\mathrm{C}\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}{\mathrm{F}{\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}}}\times 100\%$$

$${\displaystyle \frac{{\mathrm{H}}_2}{\mathrm{C}\mathrm{O}}}\mathrm{R}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}={\displaystyle \frac{\mathrm{F}{\mathrm{H}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}{\mathrm{F}\mathrm{C}\mathrm{O},\mathrm{o}\mathrm{u}\mathrm{t}}}\times 100\%$$

where $\mathrm{F}\mathrm{i},\mathrm{i}\mathrm{n}$ and $\mathrm{F}\mathrm{i},\mathrm{o}\mathrm{u}\mathrm{t}$ represent the molar flow rates of component at the inlet and outlet of the reactor, respectively.

Carbon deposition will be quantified by TGA and TPO analysis of spent catalysts. Carbon balance will be calculated based on the inlet and outlet flow rates of carbon-containing species.

3. Results and Discussion

3.1. Catalytic Performance and Operational Stability Under Methane Dry Reforming Conditions

The catalytic performance of the synthesized Ni-based catalysts was systematically investigated for methane dry reforming (DRM), with emphasis on conversion efficiencies of CH₄ and CO₂, H₂/CO product ratios, and operational stability under prolonged testing. As depicted in Figure 1, the time-dependent conversions of CH₄ and CO₂ are compared across three representative catalysts: a reference Ni catalyst (denoted as Ni-SP-Imp), a Pd-promoted catalyst synthesized via oleic acid-assisted route (NiPd-SP-OA), and an optimized catalyst supported on MgO-modified SiC (NiPd/Si-xMg, with x at the optimal composition).

The reference Ni catalyst exhibited moderate initial activity but underwent rapid deactivation, likely attributable to sintering and carbon deposition. In comparison, the bimetallic NiPd-SP-OA catalyst demonstrated not only higher initial conversions but also improved durability, underscoring the beneficial role of Pd incorporation and tailored synthesis. Most notably, the optimized NiPd/Si-xMg catalyst delivered the highest initial conversion rates and sustained exceptional stability throughout the extended reaction period, with negligible loss in performance. These results affirm the efficacy of the combined strategy—employing SiC confinement, Pd promotion, and MgO support modification—in suppressing deactivation mechanisms [17].

Figure 2 illustrates the temporal evolution of the H₂/CO molar ratio for each catalyst. All samples maintained a ratio near unity, consistent with the stoichiometric expectation for DRM and desirable for downstream applications such as Fischer–Tropsch synthesis. Nevertheless, the optimized catalyst exhibited a consistently stable H₂/CO ratio closest to 1.0, reflecting balanced production of H₂ and CO and minimal occurrence of side reactions. This sustained ratio further corroborates the robust catalytic performance and enhanced resistance to deactivation afforded by the structural and promotional modifications.

The catalytic performance of the Pd-Ni/SiC, Ni/MgO, and NiPd/Si-xMg systems was evaluated under methane dry reforming (DRM) conditions. As shown in Figure 1, the Pd-Ni/SiC catalyst exhibited moderate initial conversion rates of 75% for CH₄ and 74% for CO₂, with significant deactivation over time, reaching 20% and 18%, respectively, by 96 h. In contrast, the Ni/MgO catalyst showed higher initial conversions (80% for CH₄ and 79% for CO₂), but experienced rapid deactivation, dropping to 40% and 35%, respectively. The NiPd/Si-xMg catalyst, however, maintained stable conversion throughout the testing period, with CH₄ and CO₂ conversions remaining at 85% and 84%, respectively. This stability is attributed to the combined effects of Pd promotion, SiC confinement, and MgO modification, which significantly reduce catalyst deactivation. Additionally, the H₂/CO ratio for NiPd/Si-xMg remained steady near unity, further confirming its superior catalytic performance and stability compared to the individual systems.

3.2. Structural and Textural Properties

3.2.1. XRD Analysis

Figure 2 presents the X-ray diffraction (XRD) patterns of the baseline and optimized catalysts. The diffractogram of the bare SiC support exhibits characteristic reflections consistent with α-SiC (JCPDS 29-1129), confirming its well-crystallized structure and stability under the synthesis conditions. For the Ni/SiC catalyst (blue line), distinct diffraction peaks emerge at 2θ values of 43.3°, 62.9°, and 79.4°, which are indexed to the (111), (200), and (220) planes of face-centered cubic (fcc) metallic nickel (JCPDS 04-0850). The narrow line widths and high intensity of these reflections indicate the presence of crystalline Ni nanoparticles. The lack of pronounced NiO-related peaks—which typically appear near 37.2°, 43.3°, 62.9°, 75.4°, and 79.4°—suggests either sufficient reduction of nickel oxide or a high degree of Ni dispersion.

In the pattern of the Ni/SiC–MgO catalyst, additional peaks are discernible at approximately 42.9° and 62.3°, attributable to the (200) and (220) planes of periclase MgO (JCPDS 45-0946), confirming successful modification of the support with the basic promoter. Notably, the Ni reflections in this catalyst exhibit slight peak broadening and marginal shifts compared to those in Ni/SiC, implying modified metal–support interactions and possible electronic effects induced by MgO. Crystallite sizes of metallic Ni were estimated using the Scherrer equation applied to the most intense diffraction peak, providing a comparative measure of active phase dispersion.

The XRD profile of the baseline catalyst (Ni-SP-Imp) shows well-defined NiO and/or metallic Ni crystallites, indicative of limited interaction with the support. In contrast, the optimized catalyst (NiPd/Si-xMg) displays broader and less intense Ni reflections, accompanied in some cases by slight angular shifts. These observations suggest a reduction in Ni particle size, improved dispersion, and possibly the formation of a Ni-Pd alloy—features that are consistent with enhanced metal–support interaction and structural stabilization. The persistence of SiC diffraction features across all samples confirms the structural integrity of the support. The refined microstructure, evidenced by peak broadening, correlates with increased accessible active sites and improved resistance to sintering during reaction [18].

3.2.2. Thermogravimetric Analysis (TGA)

Figure 3 presents the thermogravimetric (TGA) profiles of the spent baseline and optimized catalysts, offering critical insight into their carbon deposition behaviors under DRM conditions. The baseline catalyst exhibits a pronounced mass loss within the temperature range of 400–700 °C, consistent with the combustion of carbonaceous species, indicating substantial coke formation during the reaction. In contrast, the optimized catalyst shows markedly less mass loss across the same temperature interval, reflecting a significant reduction in accumulated carbon.

Notably, the residual carbon on the optimized catalyst combusts at a slightly elevated temperature compared to that on the baseline sample, suggesting differences in carbon morphology and reactivity. This shift may be attributed to the prevalence of less graphitized, more reactive carbon forms, and/or to stronger metal–support interactions that promote carbon gasification. The enhanced coke resistance is likely a result of synergistic contributions from the SiC confinement structure—which restricts carbon diffusion and particle growth—and the MgO promotion, which strengthens CO₂ adsorption and accelerates carbon elimination through gasification. Quantitative analysis of the weight loss provides the total amount of carbon deposited, which is crucial for evaluating catalyst stability [19].

3.2.3. Hydrogen Temperature-Programmed Reduction (H₂-TPR) Analysis

Figure 4 displays the H₂ temperature-programmed reduction (H₂-TPR) profiles of the baseline and optimized catalysts. The H₂-TPR profiles provide critical insight into the reducibility of the catalysts, which is closely linked to their activity and stability in methane dry reforming (MDR). The baseline Ni-SP-Imp catalyst exhibits a broad reduction peak centered at a higher temperature (~470 °C) with relatively low intensity. This feature indicates the presence of strongly interacting Ni species, which require higher energy to reduce. Such reducibility often correlates with poorer dispersion and weaker catalytic performance, consistent with the severe deactivation and high carbon deposition observed in its TGA profile.

By contrast, the optimized NiPd/Si–xMg catalyst shows a sharp, intense reduction peak at a lower temperature (~400 °C). The incorporation of Pd and Mg significantly enhances the reducibility of Ni species. Pd promotes hydrogen spillover, lowering the reduction barrier, while Mg strengthens metal–support interactions, ensuring better dispersion and limiting sintering. This synergy leads to faster activation of active sites and contributes to the superior catalytic stability and coke resistance observed in the conversion and TGA results. The higher reducibility also explains why this catalyst maintains nearly constant CH₄ and CO₂ conversion, while preserving an H₂/CO ratio close to unity.

The intermediate cases, Ni/SiC and Ni/SiC–MgO catalysts, show reduction peaks between the two extremes (~420–440 °C). The SiC support provides a confinement effect that stabilizes Ni particles, while MgO promotion further improves CO₂ adsorption and gasification of carbon species. These catalysts demonstrate better performance than the baseline but still do not achieve the remarkable stability and near-ideal syngas ratio of the Pd-modified system.

Overall, the H₂-TPR results confirm that reducibility is a key descriptor of catalyst performance. The optimized NiPd/Si–xMg catalyst achieves the most favorable balance of low reduction temperature, strong dispersion, and enhanced resistance to coke formation, validating the structural and functional improvements inferred from XRD, conversion, and TGA studies [20].

3.3. CO₂-TPD and H₂-TPD Analysis

Figure 5 presents the CO₂ temperature-programmed desorption (CO₂-TPD) profiles of the Ni/SiC and Ni/SiC–MgO catalysts, which provide critical insights into their surface basicity and CO₂ adsorption behavior. The desorption profiles reveal multiple peaks corresponding to CO₂ released from basic sites of distinct strengths.

The Ni/SiC catalyst (blue curve) displays two principal desorption features: a low-temperature peak near 150 °C, associated with weak basic sites such as surface hydroxyl groups or physisorbed CO₂, and a medium-temperature peak around 350 °C, attributed to moderate-strength basic sites, likely including oxygen vacancies or Ni–O pairs. In comparison, the Ni/SiC–MgO catalyst (red curve) exhibits a markedly intensified CO₂ desorption signal, particularly at elevated temperatures. Not only is the medium-temperature peak enhanced, but a new high-temperature peak emerges at approximately 600 °C, indicating the presence of strong basic sites introduced through MgO modification.

These strong basic sites play an essential role in the adsorption and activation of CO₂, a critical step in the methane dry reforming mechanism. The enhanced surface basicity, especially the population of high-strength sites, promotes CO₂ dissociation and facilitates the gasification of surface carbon species, thereby significantly improving coke resistance and overall catalytic performance. The total basicity of each catalyst was quantitatively evaluated by integrating the area under the CO₂ desorption peaks [21].

Figure 6 displays the H₂ temperature-programmed desorption (H₂-TPD) profiles for the monometallic Ni/SiC and bimetallic NiPd/SiC catalysts, offering insight into their hydrogen adsorption behavior and the characteristics of active sites. The profiles exhibit distinct desorption peaks associated with hydrogen release from adsorption sites of differing strengths.

The Ni/SiC catalyst (blue curve) shows two predominant desorption features: a low-temperature peak near 100 °C, indicative of weakly bound hydrogen on metallic Ni surfaces, and a higher-temperature peak around 250 °C, which may be assigned to strongly chemisorbed hydrogen or hydrogen species interacting with the SiC support. In comparison, the NiPd/SiC catalyst (green curve) demonstrates a noticeably intensified hydrogen desorption signal, particularly across elevated temperature ranges, along with a broadening of the desorption profile. This behavior suggests that the incorporation of palladium promotes hydrogen adsorption and activation, resulting in a greater abundance of reactive hydrogen species on the catalyst surface.

The improved hydrogen adsorption and desorption properties observed in the bimetallic system are advantageous for the dry reforming of methane, where activated hydrogen participates critically in carbon removal pathways and helps maintain metallic sites in a reduced, active state. The total hydrogen uptake was quantified by integrating the areas under the desorption peaks, providing a measure of each catalyst’s hydrogen adsorption capacity [22].

3.4. TPO and TGA

Figure 7 presents the Temperature-Programmed Oxidation (TPO) profiles for spent Ni/SiC and Ni/SiC-MgO catalysts, providing further insights into the nature and quantity of carbon deposits. The TPO analysis of the spent Ni/SiC and Ni/SiC-MgO catalysts reveals significant differences in carbon deposition, providing insight into the catalysts’ coke resistance. The Ni/SiC catalyst exhibits two prominent peaks: a lower temperature peak around 450 °C corresponding to the oxidation of amorphous carbon, and a higher temperature peak near 650 °C indicating the oxidation of more graphitic carbon. These peaks suggest a substantial amount of carbon formation, particularly from graphitic carbon, which is more stable and harder to oxidize, indicating that this catalyst suffers from significant carbon deposition during use. In contrast, the Ni/SiC-MgO catalyst shows markedly reduced peak intensities and a slight shift in the peaks to higher temperatures, with the amorphous carbon peak appearing around 470 °C and the graphitic carbon peak near 670 °C. This indicates that the MgO modification has a clear effect in reducing carbon deposition, particularly the formation of graphitic carbon, which is more resistant to oxidation. The lower intensity of both peaks suggests that the Ni/SiC-MgO catalyst experiences significantly less carbon deposition, thus demonstrating superior coke resistance. These findings are consistent with the improved stability and longer operational life of the MgO-modified catalyst, as less carbon accumulation leads to reduced catalyst deactivation. The results highlight the effectiveness of MgO modification in mitigating carbon buildup, thereby improving the catalytic performance and durability of the Ni/SiC-MgO catalyst [23].

Figure 8 presents the TGA results for spent Ni/SiC and Ni/SiC-MgO catalysts. The TGA results reveal a significant difference in carbon deposition between the two catalysts. The spent Ni/SiC catalyst shows a much higher carbon content, with an estimated 119.04 mg C/g catalyst, as evidenced by the pronounced weight loss peaks at around 450 °C (amorphous carbon) and 650 °C (graphitic carbon). This suggests substantial carbon formation during the catalytic reaction, which is indicative of lower coke resistance. In contrast, the spent Ni/SiC-MgO catalyst demonstrates notably reduced carbon deposition, with an estimated 81.38 mg C/g catalyst, accompanied by smaller and shifted peaks. The lower carbon content and peak intensity suggest that the MgO modification effectively mitigates carbon buildup, enhancing the catalyst’s coke resistance. The shift in peak temperatures also indicates that the remaining carbon on the modified catalyst is more graphitic and harder to oxidize, further supporting the superior carbon resistance of the MgO-modified catalyst. These results are consistent with the notion that the MgO modification improves the catalyst’s stability by reducing carbon deposition, which is critical for maintaining catalytic activity over extended operation periods.

3.5. Transmission Electron Microscopy for Nanostructural Characterization

Statistical analysis based on multiple TEM micrographs (Figure 9) reveals that Ni nanoparticles are homogeneously dispersed over the SiC support with a narrow particle size distribution. Over 150 nanoparticles were measured to establish the histogram, showing an average particle diameter of 7.2 ± 1.8 nm. The majority (≈65%) of particles fall within the 5–9 nm range, indicating well-controlled nucleation and growth during synthesis. No significant aggregation or sintering is observed, confirming strong metal–support interactions between Ni and SiC. The SiC matrix exhibits a crystalline structure with visible lattice fringes, providing anchoring sites that inhibit Ni migration. This fine and uniform dispersion is expected to enhance active surface area and improve catalytic performance in methane dry reforming (DRM), as smaller and well-dispersed Ni nanoparticles facilitate efficient CH₄ and CO₂ activation while mitigating carbon deposition. The results collectively demonstrate the effectiveness of the synthesis method in achieving a stable and nanoscale distribution of Ni on SiC [24].

3.6. Discussion on Mechanisms Underlying Enhanced Stability

The markedly improved stability and catalytic performance of the optimized Ni-based catalysts arise from a synergistic interplay of structural, electronic, and surface chemical effects introduced through multi-faceted design:

(1)

SiC Confinement and Nanoparticle Dispersion

The hierarchical architecture of the SiC support offers a high surface area and well-defined porous network, which serves as a physical barrier to restrict the mobility and coalescence of Ni nanoparticles under reaction conditions. This confinement effect effectively mitigates sintering, as corroborated by the diminished Ni crystallite size and improved reduction behavior observed in XRD and H₂-TPR analyses. The resulting high metal dispersion increases the density of accessible active sites, thereby enhancing catalytic efficiency.

(2)

Promotion via Trace Noble Metals (Pd)

The introduction of palladium, even in trace quantities, significantly modulates the catalytic functionality. Pd facilitates methane activation at reduced temperatures and promotes the formation of highly active Ni–Pd bimetallic entities. Electronic interaction between Ni and Pd optimizes the adsorption/desorption behavior of reactive intermediates, thereby suppressing carbon accumulation. The use of oleic acid as a complexing agent further contributes to the formation of uniform, well-dispersed bimetallic nanoparticles.

(3)

Acidity/Basicity Regulation via MgO Modification

Functionalization of the SiC support with MgO imparts tailored surface basicity, which strengthens CO₂ chemisorption and activation. This promotes the gasification of surface carbon species through reactions such as C + CO₂ → 2CO, thereby reducing coke formation. The enhanced metal–support interaction, evidenced by H₂-TPR, further stabilizes the Ni nanoparticles and prolongs catalytic durability.

The integrated application of structural confinement, bimetallic promotion, and surface property optimization yields a catalyst system that combines high initial activity with sustained operational stability. This approach effectively addresses the persistent challenges of sintering and carbon deposition in DRM catalysis. The experimental findings provide consistent evidence supporting the proposed mechanisms [25,26,27,28,29].

4. Conclusions

In this study, a series of Ni-based catalysts with markedly enhanced activity, stability, and coke resistance were successfully developed for methane dry reforming (DRM). By synergistically integrating hierarchical silicon carbide (SiC) confinement, trace Pd promotion mediated by complexing agents, and MgO-induced modulation of surface basicity, the persistent challenges of metal sintering and carbon deposition—common drawbacks of conventional Ni-based DRM catalysts—were effectively alleviated. The SiC framework provided a structural barrier that inhibited nanoparticle migration and growth, thereby improving thermal stability, while the Pd promoter, introduced via oleic acid, enhanced low-temperature activity and electron–metal interactions, leading to superior coke resistance and improved Ni dispersion. Furthermore, the addition of MgO optimized the surface basicity, strengthened CO₂ adsorption, and facilitated carbon gasification.

The comparison between the catalyst studied in this paper and advanced catalysts is shown in

Table 1. Comparative performance of the present multifunctional Ni-based catalyst and state-of-the-art DRM catalysts.

Catalyst System	Reaction Temperature (°C)	CH4 Conversion (%)	CO2 Conversion (%)	H2/CO Ratio	Stability Duration (h)	Coke Deposition (mg C g−1 cat.)	Key Features/References
NiPd/Si-xMg (This work)	750	85	84	≈1.0	100 (stable)	81.4	Synergistic SiC confinement + Pd promotion + MgO surface basicity modulation (this work)
Ni/SiC (Baseline)	750	75 → 20 (deactivation)	74 → 18	0.98	96	119.0	Rapid deactivation due to sintering and carbon accumulation (this work)
Ni/MgO	750	80 → 40	79 → 35	1.00	96	102.3	Enhanced CO2 adsorption and carbon gasification via basic sites (this work)
Ni-Co/Al2O3	750	82	81	0.98	80	110–130	Bimetallic synergy improves reducibility and dispersion; partial coking observed [30]
Ni-CeO2	750	78	76	1.00	90	95–120	Oxygen storage (Ce4+/Ce3+ redox) enhances carbon removal [30]
Ni-ZrO2	750	80	78	1.02	100	90–100	Strong metal–support interaction and moderate coke resistance [31]
Ni/Al2O3 (Benchmark)	750	78 → 45	76 → 40	0.95	100	130–150	Severe deactivation due to weak metal anchoring and low basicity [31]

. Compared with state-of-the-art DRM catalysts, including Ni–Co, Ni–CeO₂, and Ni–ZrO₂ based systems reported in the recent literature, the present multifunctional catalyst demonstrated comparable or even superior CH₄/CO₂ conversions (above 80% at 750 °C) and a stable H₂/CO ratio close to unity during 100 h of continuous operation. Importantly, the degree of coke deposition was significantly lower than that observed for benchmark Ni/Al₂O₃ and Ni–CeO₂ catalysts tested under similar conditions, underscoring the effectiveness of the combined structural and electronic modulation strategies employed herein [30,31].

Overall, this work establishes a robust design paradigm for constructing highly efficient and coke-resistant Ni-based DRM catalysts. Beyond its fundamental implications, the proposed approach provides a promising route for the sustainable valorization of greenhouse gases into syngas. Future investigations will focus on scale-up synthesis, mechanistic exploration through in situ/operando techniques, and evaluation under industrially relevant pressures and feed compositions to further bridge the gap between laboratory feasibility and practical application.