Effect of NiO Composition on the Structural and Morphological Properties of NiO-YSZ and NiO-SDC Particles Synthesized via the Impregnation Route

Abstract

Carbon dioxide (CO₂) and methane (CH₄) are major greenhouse gases, and their increasing emissions contribute significantly to global warming. Dry reforming of methane (DRM) offers a promising route to mitigate these emissions by simultaneously utilizing both CO₂ and CH₄ and converting them into syngas, a valuable intermediate for producing fuels and chemicals. Nickel-based catalysts are widely used in DRM due to their high activity and cost-effectiveness. However, their performance depends strongly on metal loading and support properties. This study aims to investigate the effect of different NiO loadings (40, 50, and 60 wt%) on the structural and morphological characteristics of NiO-YSZ and NiO-SDC catalysts synthesized via the impregnation method. In this method, yttria-stabilized zirconia (YSZ) and samarium-doped ceria (SDC) powders were dispersed into a nickel precursor solution to form supported catalysts, which were then characterized to evaluate their structural integrity, crystallinity, and surface morphology. The results showed that higher NiO loadings generally improved the structural and morphological features, with NiO-SDC demonstrating better characteristics than NiO-YSZ. These findings provide essential insights that will guide future work on fabricating membranes using these catalysts for the CO₂-CH₄ dry reforming process.

1. Introduction

Carbon dioxide (CO₂) and methane (CH₄) are major greenhouse gases (GHGs) and effective mitigation strategies are required to minimize their contribution to global warming and environmental degradation. In fact, these gases can potentially be utilized as a high-value feedstock for producing useful chemicals. As of 2024, the global GHG emissions have reached 53.2 Gt CO_2eq, representing a 1.3% increase compared to the previous year as reported by EDGAR [1]. Therefore, converting the GHGs into valuable products offers a promising pathway to support other chemical processes while simultaneously mitigating their harmful environmental impacts. Compared to storage-based approaches, utilization has gained greater preference due to the high cost, long-term uncertainties, and logistical challenges associated with storing GHGs [2].

CO₂ and CH₄ can be converted into syngas, which serves as an important intermediate that is widely used across the chemical industry. It serves as the basis for multiple downstream applications, including hydrogen generation for refinery operations, Fischer–Tropsch synthesis, fuel for gas turbines, an anode for solid oxide fuel cells, and a key raw material for producing synthetic natural gas and methanol [3,4]. There are various methods available for CO₂ and CH₄ conversion, such as steam reforming of methane (SRM), partial oxidation, autothermal reforming (ATR), and dry reforming of methane (DRM). Among available conversion routes, dry reforming of methane (DRM) is particularly attractive as it simultaneously utilizes two major greenhouse gases to produce syngas with a H₂/CO ratio close to unity. However, DRM is a highly endothermic process and faces significant challenges, including severe carbon deposition via methane decomposition and the Boudouard reaction, as well as metal sintering under high-temperature operation. Carbon accumulation on Ni active sites can lead to catalyst deactivation and performance degradation, highlighting the importance of catalyst design strategies that emphasize strong metal–support interaction, optimal metal loading, and structural stability [5,6].

The effectiveness of the DRM process depends on the catalyst employed, as DRM is a highly endothermic reaction that requires an active and stable catalytic material to achieve efficient conversion. Catalysts play a crucial role in lowering the activation energy, enhancing reaction rates, and maintaining long-term stability under high-temperature operating conditions. Among the various catalytic systems investigated, nickel-based catalysts are the most widely used due to their excellent activity in breaking C-H bonds in methane, relatively low cost, and good availability [7,8,9]. However, their performance is highly sensitive to metal loading, dispersion, and interaction with the support materials.

Nickel-based catalysts supported on oxide materials such as yttria-stabilized zirconia (YSZ) and samarium-doped ceria (SDC) have been reported in reforming reactions owing to their high catalytic activity, thermal stability, and cost effectiveness compared to noble metals [9]. Ni-YSZ is conventionally used as an anode material in solid oxide fuel cells (SOFCs) owing to its excellent electrical conductivity and strong catalytic activity for fuel oxidation [10]. Meanwhile, Ni-SDC has attracted increasing attention as it exhibits superior oxygen ion conductivity and enhanced carbon tolerance, which makes it promising for dry reforming of methane (DRM) [11]. YSZ with 8 mol% yttria stabilization (8YSZ) and SDC with 20 mol% samarium doping (SDC20) are commonly employed compositions, as they offer a balance between structural stability and ionic conductivity. In particular, the higher oxygen vacancy concentration in SDC-based systems is reported to enhance redox behavior and carbon resistance, which are desirable for DRM-related applications.

The structural and morphological properties of Ni-based catalysts are strongly influenced by metal loading and the synthesis method [12]. While higher NiO content can increase active site density, excessive loading often leads to particle agglomeration, pore blockage, and weakened metal–support interaction [13]. Therefore, optimizing NiO loading is essential to balance dispersion, stability, and reducibility. In this context, NiO loadings of 40, 50, and 60 wt% are selected to represent low, intermediate, and high metal contents commonly reported for Ni-based cermet catalysts used in reforming and SOFC-related applications [14]. Loadings below 40 wt% are often associated with insufficient catalytic activity, whereas loadings above 60 wt% may promote sintering and structural instability.

Various synthesis routes such as co-precipitation, sol–gel, and hydrothermal methods have been reported for Ni-based catalysts. While these approaches can provide high dispersion of Ni in the catalysts, they are often involved in complex procedures, strict synthesis control, or costly precursors [15]. In contrast, the impregnation method offers simplicity, scalability, and precise control over metal loading, making it particularly suitable for systematic investigations on composition-dependent properties [15,16].

Although numerous studies have investigated Ni-based catalysts for DRM, most reports focus either on catalytic performance or on a single synthesis route without systematically comparing the influence of NiO composition across different oxide supports. In particular, comparative studies examining the structural and morphological evolution of NiO–YSZ and NiO–SDC catalysts synthesized via the impregnation method at varying NiO loadings remain limited. Therefore, a clear understanding of how NiO composition influences crystallinity, surface texture, and reduction behavior is still lacking. Addressing this gap is essential for rational catalyst and membrane design for DRM-related applications.

Therefore, this work aims to systematically investigate the effect of NiO composition (40, 50, and 60 wt%) on the structural, morphological, textural, and redox properties of NiO-YSZ and NiO-SDC catalysts synthesized via impregnation. A systematic characterization was performed using XRD, FESEM, TEM, EDX, BET, and H₂-TPR to evaluate the phase composition, morphology, surface area, and reducibility. This study represents a preliminary materials characterization effort intended to establish structure–property relationships that will guide future DRM catalytic and membrane-based applications.

2. Materials and Methods

2.1. Materials

The chemicals included in this work were predominantly for the synthesis of the NiO-YSZ and NiO-SDC catalysts, which included nickel (II) nitrate hexahydrate (purity > 99%), which was purchased from R&M Chemicals (Semenyih, Negeri Sembilan, Malaysia), while zirconium (IV) oxide-yttria stabilized (8 mol% oxide-yttria, Y₂O₃) (purity > 99%) and cerium (IV) oxide-samarium doped (20 mol% oxide-samarium, Sm₂O₃) (particle size < 0.5 *m) were acquired from Sigma Aldrich (Petaling Jaya, Selangor, Malaysia).

2.2. Synthesis of NiO-YSZ and NiO-SDC Catalysts

NiO-YSZ and NiO-SDC catalysts were synthesized by following the impregnation method reported by Martins et al. (2009) [17]. Firstly, a metal precursor solution was prepared by dissolving Ni(NO₃)₂.6H₂O in 20–40 mL of deionized water at 35 °C under constant stirring for 1h. Then, the YSZ powder was immersed in the Ni(NO₃)₂ aqueous solution, and the resulting mixture was heated and stirred for 2.5 h until it reached 100 °C. Upon heating, a green paste was formed and subsequently dried in the oven for 7 h. The resulting powder was ground in the agate mortar and calcined at 900 °C for 5 h. Samples with different NiO compositions (40, 50, and 60 wt%) were prepared by changing the concentration of Ni(NO₃)₂ aqueous solution, as shown in

Table 1. Synthesized catalysts with different compositions.

Ni:YSZ (wt%)	Ni(NO3)2.6H2O (g)	YSZ/SDC (g)	Catalyst Name
40:60	3.96	1.20	40 NiO-YSZ
50:50	4.95	1.00	50 NiO-YSZ
60:40	5.95	0.80	60 NiO-YSZ
40:60	3.96	1.20	40 NiO-SDC
50:50	4.95	1.00	50 NiO-SDC
60:40	5.95	0.80	60 NiO-SDC

.

The synthesis process was repeated by exchanging the YSZ powder with SDC powder. Samples prepared are denoted hereafter as NiO-YSZ and NiO-SDC, respectively. The sample composition was expressed in the weight percentage of the Ni in the metallic state in the cermet, and the Ni content is attached to the sample notation, for example, 40NiO-YSZ and 40NiO-SDC.

2.3. Characterization of NiO-YSZ and NiO-SDC Catalysts

The synthesized NiO-YSZ and NiO-SDC catalysts underwent various characterizations. Firstly, the morphologies of the synthesized NiO-YSZ and NiO-SDC catalysts were determined using a field-emission scanning electron microscope (FESEM, ZEISS brand, Gemini 500 model, Oberkochen, Germany) and a transmission electron microscope (TEM, Hitachi model HT7830, Tokyo, Japan). Next, the catalysts were analyzed using energy-dispersive X-ray spectroscopy (EDX, Bruker Nano GmbH brand, Bruker Quantax 70 model, Berlin, Germany). In addition, the crystallinities of the synthesized NiO-YSZ and NiO-SDC catalysts were analyzed using X-ray diffraction (XRD, Panalytical brand, Xpert3 Powder model, Melvern, UK). Furthermore, through the BET theory, the specific surface area and pore characteristics were investigated using a surface area analyzer (SAP, Micromeritics, ASAP 2020 model, Norcross, GA, USA). Lastly, the reducibility of the synthesized catalysts was evaluated by hydrogen temperature-programmed reduction (H₂-TPR, Thermo Scientific brand, TPDRO 1100 model, Waltham, MA, USA).

3. Results and Discussion

The morphologies of the synthesized catalysts, including NiO-YSZ and NiO-SDC with Ni compositions of 40, 50, and 60 wt%, were characterized by FESEM and TEM, and the results are shown in Figure 1 and Figure 2, respectively. Firstly, the FESEM images of all NiO-YSZ and NiO-SDC particles with different NiO compositions exhibit an interconnected granular structure characteristic of impregnated NiO supported on ceramic oxides, which is in compliance with the results obtained from the literature [17]. Based on the FESEM and TEM images, both NiO-YSZ and NiO-SDC samples exhibit nanoscale particles at identical NiO loadings. At 40 wt% NiO loading, both NiO-YSZ and NiO-SDC samples exhibited relatively loose particle packing and localized particle clustering, suggesting limited surface coverage of the support by NiO species. As the NiO loading increased to 50 and 60 wt%, a more continuous and uniform granular network was observed for both supports, indicating improved surface coverage and structural connectivity of NiO on the oxide supports. Although both catalyst systems showed improved morphologies at higher NiO loadings, subtle differences between the supports were observed. At identical NiO loadings, NiO-SDC samples generally exhibited smaller average particle sizes and a narrower particle size distribution compared to NiO-YSZ. This behavior suggests that the SDC support is more effective in stabilizing NiO dispersion and inhibiting particle growth [17]. However, at the highest NiO loading (60 wt%), signs of particle growth were observed for both catalyst systems, indicating that excessive NiO content may promote agglomeration regardless of support type.

Meanwhile, energy-dispersive X-ray (EDX) analysis was conducted to examine the distribution of Ni on both YSZ and SDC supports at NiO loadings of 40, 50, and 60 wt%, and the results are shown in Figure 3 and Figure 4. The EDX elemental mapping results confirm the presence of Ni, Zr, Y (for YSZ-based samples), Ce, Sm (for SDC-based samples), and O, which indicates successful incorporation of Ni onto both oxide supports. For both NiO-YSZ and NiO-SDC catalysts, the Ni signal intensity increases progressively with increasing NiO loading, which is consistent with the nominal compositions. At lower loading (40 wt%), Ni is uniformly distributed over the support surface, suggesting good dispersion of NiO species. As the loading increases to 50 and 60 wt%, higher Ni surface coverage is observed while maintaining a relatively homogeneous spatial distribution with no severe Ni-rich agglomeration detected. Overall, both NiO-YSZ and NiO-SDC catalysts exhibit comparable elemental distribution characteristics across the investigated NiO loading range. The EDX mapping results also indicate that NiO is well distributed on both supports without obvious phase separation. This consistent overlap between Ni and the respective support elements suggests good contact between NiO and YSZ and SDC matrices. These observations confirm that the impregnation method provides effective NiO dispersion for both support systems, which is in agreement with the morphological features observed from FESEM and TEM analyses [18,19].

The XRD patterns of the synthesized NiO-YSZ and NiO-SDC catalysts are presented in Figure 5. For NiO-YSZ catalysts, characteristic diffraction peaks were identified at 2θ values of 37.3°, 43.3°, and 63.1°, corresponding to the cubic NiO, while additional peaks observed at 2θ values of 30.2°, 35.3°, 50.4°, and 60.3° were assigned to the structure of YSZ, confirming successful catalyst synthesis without a secondary phase. These reflections are in good agreement with values reported in previous studies [17,20]. Similarly, the NiO-SDC catalysts exhibited NiO peaks at the same 2θ positions, whereas the SDC support was identified at 2θ values of 28.5°, 33.1°, 47.4°, and 56.3° [11]. The presence of these characteristic reflections in all samples confirms the successful incorporation of NiO onto both YSZ and SDC supports and indicates the absence of unwanted secondary phases. With increasing NiO loading from 40 to 60 wt%, the NiO diffraction peaks became sharper and more intense, indicating enhanced crystallinity and increased NiO crystallite size [21]. A slight shift in NiO peak positions was also observed with increasing NiO content for both supports. This shift is attributed to lattice strain induced by metal–support interactions and interfacial stress between NiO and oxide supports, consistent with the reported literature [22]. The peak shift was more pronounced for NiO–SDC samples, suggesting stronger interfacial interaction and lattice distortion, whereas NiO-YSZ exhibited relatively stable peak positions, reflecting a more structurally constrained oxide framework.

Table 2. BET surface area analysis.

Catalysts	BET Surface Area (m2/g)	Pore Size (nm)
40 NiO-YSZ	4.8545	12.7030
50 NiO-YSZ	5.0842	14.1550
60 NiO-YSZ	5.1148	11.3000
40 NiO-SDC	6.9996	19.0692
50 NiO-SDC	7.3007	15.2548
60 NiO-SDC	6.5835	16.0708

shows the BET surface area analysis of the NiO-YSZ and NiO-SDC catalysts. The BET surface area results showed opposite trends for the two catalyst samples. For NiO-YSZ catalysts, a gradual increase in surface area was observed with increasing NiO loading, indicating the effective dispersion of NiO species without severe pore blockage [23]. In contrast, NiO-SDC catalysts exhibited a decrease in surface area at the highest NiO loading (60 wt%), which can be attributed to partial pore obstruction and particle agglomeration resulting from excessive NiO deposition [24,25]. Despite this decrease, NiO-SDC catalysts maintained higher surface areas and larger pore sizes compared to NiO-YSZ across all compositions, indicating a more open porous structure that may benefit gas diffusion during DRM. For NiO-YSZ, the pore size increased when NiO loading increased from 40 wt% to 50 wt% and then decreased when NiO loading increased from 50 wt% to 60 wt%. This trend suggests that moderate NiO loading promotes pore widening due to improved particle dispersion, whereas higher NiO content leads to partial pore blockage or densification of the pore structure. In contrast, NiO-SDC catalysts exhibited consistently larger pore sizes than those of NiO-YSZ across all compositions, with pore sizes ranging from 15.25 to 19.07 nm. The relatively larger pore sizes observed for NiO-SDC indicate a more open porous structure, which is beneficial for reactant diffusion and mass transport during catalytic reactions. Overall, the pore size results highlight the strong influence of both support type and NiO loading on the textural properties of the catalysts.

From the isotherm results, all samples exhibited Type IV isotherms, indicating the presence of predominantly mesoporous structures. Compared to NiO-YSZ with an adsorbed N₂ volume of 10–15 cm³/g, the NiO-SDC catalysts demonstrated significantly higher nitrogen uptake of 20–25 cm³/g, reflecting a larger pore volume and more developed mesoporosity associated with the SDC support. For NiO-YSZ, the sample loaded with 50 wt% NiO exhibited the highest adsorption capacity of ~14 cm³/g, suggesting an optimal balance between NiO dispersion and pore accessibility. In contrast, the NiO-SDC catalysts showed a decreasing trend in nitrogen uptake with increasing NiO loading, with the 40 wt% NiO sample displaying the highest adsorption capacity of 25 cm³/g. This behavior indicates that excessive NiO loading on the SDC support leads to pore obstruction and agglomeration, reducing accessible surface area and pore volume. Overall, the results highlight the strong influence of support type and NiO composition on the textural properties of the catalysts, with NiO-SDC exhibiting significant porosity compared to NiO-YSZ, which is expected to enhance gas diffusion and catalytic performance in the dry reforming of methane.

Figure 6 and Figure 7 show the H₂-TPR profiles of the catalysts. From these figures, the results demonstrate distinct reduction behaviors influenced by the support material and NiO composition in the catalysts. All samples exhibited a primary reduction peak corresponding to the conversion of NiO to metallic Ni, which generally appears within the 350–600 °C range [17,21].

With increasing NiO loading from 40 wt% to 60 wt%, a gradual shift in the reduction peaks towards higher temperatures is observed for both catalysts’ samples, suggesting improved reducibility due to larger NiO domains and reduced metal–support interaction strength at higher loadings. When comparing the two supports, at 40 wt% of NiO, the NiO-YSZ catalyst exhibits lower reduction temperatures compared to NiO-SDC at both peaks. As for 50 wt% of NiO, NiO-YSZ exhibits a lower reduction temperature at Peak I, while NiO-SDC exhibits a lower reduction temperature for Peak II. However, for 60 wt% of NiO, NiO-SDC shows a higher reduction in temperature compared to NiO-YSZ. The low-temperature reduction peak (Peak I) is attributed to the reduction in free or weakly interacting NiO species, typically located on the catalyst surface, while the high-temperature peak (Peak II) corresponds to strongly interacting NiO species embedded within or interacting intensively with the YSZ or SDC support lattice.

For NiO-SDC samples, the reduction peaks occurred at slightly higher temperatures and shifted further with increasing NiO loading. This indicates a stronger metal–support interaction that stabilizes NiO particles and makes them more difficult to reduce [21,23]. On the other hand, NiO-YSZ displayed reduction peaks at lower temperatures, reflecting weaker NiO–support bonding and more easily reducible NiO. These results confirm the superior anchoring effect of YSZ compared to SDC, which significantly influences the reducibility characteristics of the catalysts.

NiO-YSZ catalysts generally exhibited lower reduction temperatures, indicating easier reducibility, and facilitated the formation of metallic Ni active sites. This behavior is advantageous for catalyst activation under DRM operating conditions. In contrast, NiO-SDC samples showed reduction peaks at higher temperatures, suggesting stronger metal–support interaction. While this may hinder reducibility, it is beneficial for stabilizing Ni particles and suppressing sintering and carbon deposition. The coexistence of low- and high-temperature reduction peaks indicates a balance between catalytic activation and structural stability, which is essential for high-temperature DRM operation.

From a catalytic perspective, a TPR profile dominated by a pronounced low-temperature reduction peak is more desirable, as it indicates easier formation of metallic Ni active sites under reaction conditions, which is crucial for enhancing catalytic activity in the dry reforming of methane. However, the presence of a high-temperature peak remains beneficial, as it reflects sufficient metal–support interaction that helps stabilize Ni particles and suppress sintering during high-temperature operations.

NiO-YSZ and NiO-SDC catalysts with NiO loadings of 40, 50, and 60 wt% were successfully synthesized via the impregnation route, and their physicochemical properties were systematically evaluated. The results demonstrate that both support material and NiO composition significantly influence the structural, textural, and redox characteristics of the catalysts. Among the two catalysts, NiO-SDC exhibited superior intrinsic properties, including a higher surface area and a higher reduction temperature, indicating stronger metal–support interaction and potentially enhanced catalytic activity. For both catalyst supports, the 60 wt% NiO composition showed the most favorable balance between dispersion and structural stability. Nevertheless, considering economic factors and practical implementation, NiO-YSZ remains a relevant and widely adopted support material. These findings indicate that while NiO-SDC offers improved catalytic characteristics, further comparative evaluation is required to assess its suitability relative to NiO-YSZ in membrane-based catalytic applications.

Overall, the combined morphological, structural, textural, and reduction analyses demonstrate that NiO composition and support type jointly govern catalyst characteristics. NiO-SDC exhibits advantages in dispersion, surface area, and metal–support interaction, whereas NiO-YSZ offers improved reducibility and structural robustness. These complementary properties suggest that neither support can be considered universally superior; instead, their suitability depends on the intended application and operating conditions.

Future work will focus on the fabrication of monolith catalytic membranes using both NiO-YSZ and NiO-SDC with 60 wt% NiO, identified as the optimal composition in this study. A systematic comparison will be conducted to clarify the effects of the support material on the membrane morphology, gas transport properties, and catalytic performance during the dry reforming of methane. In addition, the trade-off between catalytic efficiency, structural stability, and material cost will be evaluated to determine the most suitable catalyst system for practical membrane reactor and SOFC-related applications.

4. Conclusions

In the present work, NiO-YSZ and NiO-SDC catalysts with varying NiO loadings (40, 50, and 60 wt%) were successfully synthesized via the impregnation method and systematically characterized to evaluate the structural and morphological properties of the catalysts. The results demonstrate that both support material and NiO composition significantly influence catalyst characteristics. NiO-SDC exhibited a higher surface area and stronger metal–support interaction, as reflected by a higher reduction temperature. This may be beneficial for suppressing carbon formation. Meanwhile, NiO-YSZ showed comparatively easier reducibility and stable structural features, which are advantageous for high-temperature operation and Ni activation. Increasing NiO loading generally enhanced crystallinity and structural continuity for both supports. Among the investigated compositions, 60 wt% NiO provided a favorable balance between metal dispersion and structural stability. However, potential agglomeration at high loading highlights the importance of careful optimization.

It should be emphasized that this study represents a preliminary materials characterization investigation, and direct catalytic performance evaluation under DRM conditions was beyond the scope of this work. Nevertheless, the established structure–property relationships provide valuable guidance for selecting suitable catalyst compositions for subsequent catalytic and membrane fabrication studies. Future work will focus on DRM catalytic performance testing and monolith catalytic membrane fabrication using optimized NiO-YSZ and NiO-SDC compositions, with additional surface-sensitive analyses to further elucidate metal–support interactions and carbon resistance.