The Role of Ga Promoter in Enhancing the Performance of Ni/ZrO₂+SiO₂ Catalysts for Dry Methane Reforming

Abstract

The potential of dry reforming methane (DRM) to convert two greenhouse gases concurrently is drawing interest from around the world. This research focused on developing supported nickel catalysts for the DRM, utilizing stabilized zirconia (SZ31107), which contains 5% SiO₂, as the support material. To promote the catalysts with a 5 wt.% Ni concentration, we used varying gallium loadings, specifically 0.1, 0.25, 0.5, 0.75, and 1 wt.%. After a detailed analysis, characterization was performed using X-ray diffraction, N₂-physorption, temperature-programmed reduction/desorption techniques, thermogravimetry, and Raman spectroscopy. The optimal DRM performance, achieved at 700 °C with a 1:1 CH₄:CO₂ feed, was recorded for the catalyst that has 0.25 wt.% Ga. The catalyst demonstrated remarkable average conversion rates of 56% for CH₄ and 66% for CO₂ after 300 min at 700 °C, with an H₂:CO ratio of 0.84. Activity was further enhanced by raising the temperature to 800 °C, which resulted in an 87% CO₂ conversion and an 80% CH₄ conversion. Studies on the catalyst’s long-term stability revealed a slow deactivation. With computed activation energies of 28,009 J/mol for CH₄ conversion and 21,875 J/mol for CO₂ conversion, temperature-programmed reaction tests conducted over the best catalyst demonstrated the DRM reaction’s endothermic character. Small additions of Ga encouraged the creation of more graphitic carbon structures, according to Raman spectroscopy of spent catalysts; the ideal catalyst had the lowest I_D/I_G ratio. These results suggest that the 5Ni+0.25Ga/SZ31107 catalyst is a promising candidate for large-scale syngas and hydrogen production.

1. Introduction

The detrimental effects of global warming are being seen on every continent as greenhouse gas concentrations, such as CO₂ and CH₄, have risen above critical thresholds. Global warming disrupts seasonal cycles, significantly affecting agricultural productivity and biodiversity [1,2]. Since dry reforming of methane (DRM) processes can simultaneously modify greenhouse gases like carbon dioxide and methane, developing catalysts for these reactions has received a large amount of interest at this crucial stage. Moreover, DRM produces carbon monoxide and hydrogen as byproducts, which can be used in a variety of synthetic processes and help provide clean hydrogen energy. The DRM reaction scheme is a complex system consisting of several side reactions, as given below [3]:

$${\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2\leftrightarrow 2{\mathrm{H}}_2+2\mathrm{C}\mathrm{O} {\Delta H}_{298\mathrm{K}}^{°}=+247\mathrm{kJ/mol}$$

(1)

$${\mathrm{C}\mathrm{H}}_4\leftrightarrow 2{\mathrm{H}}_2+\mathrm{C} {\Delta H}_{298\mathrm{K}}^{°}=+75\mathrm{kJ/mol}$$

(2)

$$2\mathrm{C}\mathrm{O}\leftrightarrow \mathrm{C}+{\mathrm{C}\mathrm{O}}_2 {\Delta H}_{298\mathrm{K}}^{°}=-172\mathrm{kJ/mol}$$

(3)

Noble metals outperform non-noble metals like Ni in terms of activity and durability when it comes to DRM. High costs pose a significant challenge to industrializing catalysts based on noble metals. The main focus of modern catalytic development is on catalyst systems based on Ni. Experiments have extensively employed Ni-based catalysts in catalysis due to their low cost, high selectivity, and high activity [4,5,6,7]. Mokrzycki et al. conducted a study on the catalytic activity of Ni-AlSBA-15 with 3, 5, or 10 wt.% Ni, utilizing the impregnation method [8]. Their results displayed that all catalysts were active in the entire temperature range tested and exhibited high H₂ and CO yields. On the other hand, syngas was produced by DRM using 3 wt.% Ni supported by hierarchical ZSM-5 and USY zeolites catalysts [9]. The findings showed that as the number of accessible Ni active sites increased, stability increased, carbon formation decreased, and metal sintering decreased. The use of suitable support and the addition of other metals as promoters could significantly enhance the catalytic performance and simultaneously lower the carbon deposition over Ni-based catalysts [10,11,12,13]. Bin Jumah’s work investigated the impact of Ce addition on Ni-based catalysts, specifically CBV3020E (ZSM-5) for DRM. This resulted in improved catalytic performance and enhanced NiO reducibility [14]. The addition of Ga metal to a Ni-based catalyst can significantly enhance its activity, achieving levels comparable to those of noble metal-based catalysts. Gallium’s unique properties, including its ability to improve dispersion, stability, and electronic structure, make it a promising promoter for Ni catalysts [15]. Ga is known to modify the acidic properties of the catalyst [16,17], while low Ga additions also increase the surface area [18]. The promotion of Ga of a SiO₂-supported Cu catalyst in the hydrogenation of CO₂ to methanol was studied [19]. The results indicate that Ga promotes Cu and increases methanol selectivity, probably by forming new active sites during the methanol formation without altering the Cu oxidation state, which, under reaction conditions, stays mostly metallic. The stability and catalytic activity of mesoporous Ni/MCM-41 as a novel catalyst for the DRM to produce syngas were investigated. This catalyst was promoted with wt.% of 0.0, 1.0, 1.5, 2.0, 2.5, and 3.0 for the Ga loading [17]. Incorporating Ga into the catalyst resulted in a decrease of medium and strong basic sites, which also reduced the amount of carbon deposited. According to Baj et al., high activity and stability are demonstrated by the Ni/CeO₂ catalyst with 3 wt.% Ga loading, which also has a H₂/CO ratio close to 1 and little carbon buildup [20]. Ga can produce enough oxygen species to remove carbon buildup and maintain catalytic activity, according to the CH₄–CO₂–CH₄ cycle experiments. Improved stability and activity are due to enhanced CO₂ adsorption and activation over the catalyst’s basic site, which is influenced by the introduction of Ga [21]. The catalyst support is important because it gives Ni catalysts a place to spread out, and its surface chemistry makes CO₂ activation much easier [22]. One way to stop the sintering and coking of Ni-based catalysts is to use basic support like ZrO₂ [23,24]. Steam reforming of methane assessed the loading of Ni onto several supports, including ZrO and Ce–ZrO [25]. The result showed that the reactor effluent had a very high H₂ concentration, and Ni/Ce–ZrO₂ had the most stable and active. Additionally, the Ni/Ce–ZrO₂ catalyst showed outstanding activity in the steam reformation of ethanol to generate hydrogen [26,27]. Because of its strong metal–support interaction, the silica support exhibits high catalytic activity and stability. This interaction can lead to the creation of metal-silicate species such Ni₃Si₂O₅(OH)₄, as has been seen with Ni [28]. To enhance the quantity of interfacial oxygen accessible for the reaction and to decrease the formation of coke, the creation of the Ni–O–Si bond can activate CO₂. Furthermore, the formation of mixed oxides like NiAlO₄ shows how silica stabilizes on basic supports [29], which can also enhance coke resistance. The use of silica-modified-alumina support for the mainstream catalyst (Ni + Ga metal) may be more productive towards DRM. In this study, a Ni catalyst is enhanced with Ga and supported on zirconia that is stabilized with silica. The catalysts, designated as 5Ni+xGa/SZ31107 (where x represents different weight percentages: 0.1, 0.25, 0.5, 0.75, 1.0 wt.%), are analyzed. The catalyst system is characterized by several techniques, including thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, surface area and porosity studies, and multiple temperature-programmed reduction and desorption methods. The relationship between the characterization results and catalytic activity is also explored.

2. Results

2.1. BET

Figure 1 displays the porosity distribution and N₂ adsorption isotherm of 5Ni+xGa/SZ31107 catalysts with x = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0 wt.%. Every catalyst has an H1 hysteresis loop in its type IV isotherm. This suggests that cylindrical mesopores are present. For the non-promoted sample, the pore size distribution plot (dV/dlogW vs. W) displays a monomodal pore size distribution of roughly 15.7 nm, while for the promoted samples, the average pore size falls between 30.8 and 34.7 nm.

Table 1. Surface Textural Properties.

Samples	BET (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
5Ni/SZ31107	65.1	0.27	15.7
5Ni+0.1Ga/SZ31107	18.5	0.16	34.5
5Ni+0.25Ga/SZ31107	21.7	0.18	33.0
5Ni+0.5Ga/SZ31107	19.3	0.15	34.7
5Ni+0.75Ga/SZ31107	23.4	0.18	30.4
5Ni+1.0Ga/SZ31107	22.6	0.2	34.7

displays the textual properties of the catalyst. The addition of gallium appears to have a substantial impact on the textural properties of the Ni/SZ31107 material. It significantly reduces the surface area and pore volume while increasing the pore diameter.

2.2. TPR

Figure 2 provides the TPR profiles of the catalysts, comparing their reducibility with varying Ga content. Two reduction peaks are visible in the 366–414 °C and 554–583 °C areas for all samples. The first peaks in the 366–414 °C temperature range are attributed to the weak interaction between NiO and the support. The peaks in the 554–583 °C temperature range are attributed to the moderate interaction between NiO and the support. Compared to the non-promoted sample, the early reduction peaks in the 0.1 and 0.25 Ga loading samples are less intense. Conversely, the moderate metal support interaction peaks exhibit more intensity. As the Ga content rises over 0.1 loading, peaks seem to expand, and the reduction peaks move to higher temperatures. This may be explained by Ga’s interaction with nickel, which results in more stable Ni–Ga complexes that are harder to decrease or change structurally, making the Ni-containing phase less accessible to hydrogen.

Table 2. Consumption of hydrogen during H₂-TPR.

Catalysts	Total Quantity (cm3/g STP)
5Ni/SZ31107	19.70
5Ni+0.1Ga/SZ31107	12.11
5Ni+0.25Ga/SZ31107	16.53
5Ni+0.5Ga/SZ31107	11.28
5Ni+0.75Ga/SZ31107	14.51
5Ni+1.0Ga/SZ31107	26.60

Theoretical Quantity (cm³/g STP) = 19.09.

displays the hydrogen consumption during H₂-TPR, where the temperature range for maximum hydrogen consumption varies across the catalysts. The total hydrogen consumption increases with increasing Ga content up to 0.25 wt.%. The increase in total hydrogen consumption with increasing Ga content suggests that the presence of Ga might lead to an increase in the reducible Ni species. Ga-incorporated catalysts with intermediate loading 0.5–0.75 wt.% exhibit a lower total hydrogen consumption. It is possible that at intermediate Ga content, some less reducible Ni–Ga species might form, leading to a decrease in overall hydrogen consumption. The TPR profile of the 5Ni+1.0Ga/SZ31107 sample exhibits a notably greater area under the curve and a shift towards the temperatures. It should be noted that 1 wt.% Ga loading is the highest loading in the present study, where the reduction of gallium oxide under hydrogen may also be probable. In the literature, reduction of different polymorphs of Ga₂O₃ was reported below 400 °C [30], whereas reduction of Ga₂O₃ over Al₂O₃ support is somewhat more difficult, and it is reduced to about 400 °C [31]. The reduction temperature of Ga₂O₃ is varied to the support [32]. In the present case, as well (over the 5Ni+1.0Ga/SZ31107 catalyst), an impression of an additional peak is observed at about 450 °C, which can be attributed to the reduction of gallium oxide over the SZ31107 support.

2.3. CO₂-TPD

The basicity of catalysts is studied using the CO₂-TPD approach. Figure 3’s plots display CO₂ desorption peaks from various basic sites. Stronger basic sites are associated with higher temperature peaks, whereas weaker basic sites are associated with lower temperatures. The concentration of those sites is indicated by the area under each peak, which is equivalent to the quantity of CO₂ desorbed from that kind of site. At lower temperatures, the 5Ni/SZ31107 shows a broad peak, suggesting the existence of weak basic sites. The amount of CO₂ desorption over different catalysts is mentioned in Table S1. The higher amount of CO₂ desorption upon increasing Ga-loading up to 0.75 wt.% indicates the growing concentration of basic sites. This phenomenon can be ascribed to the formation of a greater number of Ga–O–Ni or Ga–O–support species. These newly formed species are likely more basic than the original sites present on the 5Ni/SZ31107 catalyst without gallium. At the highest Ga loading (1 wt.%), the amount of the CO₂ desorption peak is decreased. In H₂-TPR, the reduction peaks for Ga₂O₃ over SZ31107 are noticed. That means that at high loading, isolated Ga₂O₃ species are also organized, which are acidic [33], and they deplete the concentration of basic sites markedly. To understand the type of CO₂-surface intermediate species, the infrared spectra of 5Ni/SZ31107, 5Ni+0.25Ga/SZ31107 and 5Ni1Ga/SZ31107 are presented in Figure S6. All catalysts are capable of adsorbing CO₂ from the atmosphere (due to the basicity of catalysts), and these adsorbed CO₂ species give a specific vibration band in the infrared spectra. The non-promoted catalyst (5Ni/SZ31107) shows a characteristic vibration peak of about 1370 cm⁻¹ for bidentate carbonate species [34]. Upon the promotional addition of 0.25 wt.% Ga over 5Ni/SZ31107, the vibration peak about 1530 cm⁻¹ for unidentate carbonate appears. At the highest Ga-loading (1 wt.%) over 5Ni/5Ni/SZ31107, the vibration peak for carboxylate species is also observed at about 1424 cm⁻¹ [35]. Clearly, Ga-loading induces the formation of different CO₂-surface intermediate species for catalyzing DRM.

2.4. XRD

The XRD patterns provide information about the crystalline structure of the catalysts. All six XRD patterns exhibit distinct peaks, indicating the presence of crystalline phases in all the catalyst samples. These phases correspond to the support material (SZ31107) and possibly nickel-containing species. The X-ray diffraction pattern of fresh 5Ni+xGa/SZ31107 (x = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0 wt.%) catalysts are displayed in Figure 4. The 5Ni supported over the SZ31107 catalyst has a hexagonal SiO₂ phase (at Bragg’s angle 2θ = 4.1 (100), JCPDS reference 96-152-1315), a monoclinic ZrO₂ phase (at Bragg’s angle 2θ = 17.4 (100), 24.1(011), 24.5(110), 28.2(111), 31.4(111), 34.1(002), 35.4(200), 38.8(021), 41.1(211), 44.7(112), 46.6(−202), 49.2(022), 50.1(220), 54.0(202), 55.6(013), 57.5(311), 59.9(302), 65.7(222), 71.3(104), JCPDS reference number 96-230-0545), and a cubic NiO phase (at Bragg’s angle 2θ = 37.0(111), 43.3(020), 62.9(022), 75.5(131), JCPDS reference number 96-101-0096). There are prominent peaks observed in all the patterns, particularly in the 2-theta range of approximately 25–35 degrees. These strong peaks correspond to the crystalline structure of the SZ31107 support. The positions of the major peaks appear to be largely consistent across all samples, suggesting that the fundamental crystalline structure of the support remains the dominant phase, and the addition of gallium at these concentrations does not significantly alter the lattice parameters of the major phases. There are no obvious new peaks appearing, or existing peaks disappearing, with the addition of gallium. This suggests that Ga might be highly dispersed or incorporated into existing phases without forming distinct new crystalline phases detectable by XRD at these concentrations. The overall patterns look quite similar, suggesting that adding Ga at these levels has a relatively minor impact on the bulk crystalline phases detected by XRD. The XRD of the reduced 5Ni+0.25Ga/SZ31107 catalyst is also carried out (Figure S3), where the metallic Ni peak is evident at about 44.5° and 51.8° (JCPDS reference number 00-004-0850). The metallic catalyst sites in the DRM reaction are created during reduction. The aim of reductive pretreatment of the catalysts before the DRM reaction) is to create active sites for the DRM reaction.

2.5. Catalytic Activity Result and Discussion

Figure 5 shows the catalytic activity of 5Ni+xGa/SZ31107 (x = 0, 0.1, 0.25, 0.5, 0.75, and 1%) catalysts in terms of CH₄ and CO₂ conversions and H₂/CO ratio operated at 700 °C. Graph 5A shows the change in CH₄ conversion (%) versus time on stream (TOS in minutes) for different catalysts. The graph shows that the CH₄ conversion decreases over time for all catalysts. The catalyst with the highest CH₄ conversion at the beginning is 5Ni+0.5Ga/SZ31107, but it also has the fastest decrease in CH₄ conversion. The 5Ni+1Ga/SZ31107 catalyst also has the slowest decrease in CH₄ conversion. The catalyst with 0.25 wt.% Ga showed the best performance. It achieved high average conversion rates of 56% for CH₄ and 66% for CO₂, with a hydrogen to carbon monoxide ratio of 0.84 after 300 min. In Graph 5B, all catalysts show high initial CO₂ conversion and a gradual decline in CO₂ conversion over time, suggesting deactivation. This could be due to various factors, including carbon deposition and sintering of the catalyst. Adding Ga to the Ni catalyst (5Ni+xGa/SZ31107) generally leads to a decrease in initial activity but may improve stability in some cases. The catalyst with 0.25 wt.% Ga showed the highest average conversion rate of 66% for CO₂. Graph 5C for the H₂/CO ratio depicts that all catalysts start with a high H₂/CO ratio, indicating a preference for H₂ production. The rate at which the H₂/CO ratio decreases varies depending on the catalyst. 5Ni/SZ31107 shows the most rapid decrease, while the Ga-doped catalysts exhibit a slower decline. The catalyst with 0.25 wt.% Ga showed the highest average ratio of 0.84 for H₂/CO. The performance of the 5Ni+0.25Ga/SZ31107 catalyst over 20 h is depicted in Figure 6. At the end of the 20 h, CH₄ conversion, CO₂ conversion, and the H₂/CO ratio remain ~50%, ~5%, and >0.8, respectively. These curves’ decreasing trends confirm that catalytic activity is gradually reducing, which is suggestive of a deactivation process taking place throughout the prolonged reaction period. A comparative table of catalytic activity is presented in Table S2. A highly competitive catalyst system for methane dry reforming seems to be the 5Ni+0.25Ga/SZ31107 catalyst. Considering its relatively modest Ni loading, it strikes a fair compromise between a respectable conversion rate, a moderate working temperature, and reasonable stability over time (TOS). When compared to a number of current catalysts, its performance is noticeably better, indicating its potential for real-world use.

2.6. Impact of Reaction Temperature

Figure 7 shows how the catalytic performance of the 5Ni+0.25Ga/SZ31107 catalyst is affected by the reaction temperature. As the picture illustrates, CH₄ conversion increases from 38% to 80% when the reaction temperature is raised from 600 to 800 °C, while CO₂ conversion increases from 49% to 87%. This is because the DRM reaction depicted in Figure 4 is endothermic. The conversion comparison between CH₄ and CO₂ shows that CH₄ conversion is always lower than CO₂ conversion for the studied temperature range, which indicates the coexistence of the reverse water gas shift (RWGS) reaction under these reaction conditions. Using the reaction temperature information and associated conversion rates, we can use the Arrhenius equation (Equation (4)) to determine the activation energy (E_a) for the conversion of methane and carbon dioxide. Temperature (T), universal gas constant (R), activation energy (E_a), pre-exponential factor (A), and rate constant (k) are all related in this equation.

$$k=A^{\ast }\mathrm{e}\mathrm{x}\mathrm{p}\left({-E}_a/RT\right)$$

(4)

Rearranging the Arrhenius equation, taking the natural logarithm (Equation (5)), and plotting ln(k) against 1/T produces a slope of −E_a/R, which is a linear connection. The activation energy (E_a) can be computed from this slope using the formula E_a = −slope × R.

$$Ln\left(k\right)=-{\displaystyle \frac{E_a}{R}}\left({\displaystyle \frac{1}{T}}\right)+Ln(A)$$

(5)

The rate constants obtained from the specified conversions at various temperatures are summarized in

Table 3. The experiment’s rate constants at different temperatures.

T (°C)	T(K)	1/T(K)	Conversion CH4	Ln(k)	Conversion CO2	Ln(k)
600	873	0.00114548	0.3774	−0.9744	0.4920	−0.7093
650	923	0.00108342	0.4845	−0.7246	0.5586	−0.5823
700	973	0.00102775	0.6210	−0.4764	0.6660	−0.4065
750	1023	0.00097752	0.7190	−0.3299	0.7830	−0.2446
800	1073	0.00093197	0.8088	−0.2122	0.8706	−0.1386

. The activation energy for the conversion of CH₄ and CO₂ can be found using these numbers in the previously outlined technique. The Arrhenius curve in Figure 8 was used to estimate the activation energies for the conversion of CH₄ and CO₂. Based on the information shown in Figure 8, approximate values of 30,120 J/mol and 23,029 J/mol, respectively, were calculated. Table 3 provides the experiment’s rate constants at different temperatures.

2.7. TGA

The TGA curve for the spent 5Ni+0.25Ga/SZ31107 catalyst (Figure 9) shows the weight loss of the catalyst as a function of temperature for a long-time stream for 20 h, indicating thermal stability. Up to about 400 °C, the curve displays a very tiny initial weight loss (less than 1%). This implies that within this temperature range, the spent catalyst is comparatively stable. The small loss results from the removal of physiosorbed water or other volatile species that were only loosely attached to the catalyst surface. Between around 400 and 700 °C, there is a noticeable decrease in weight. This area has seen a significant overall weight loss of 19.7%. This shows that the carbon that formed on the catalyst during the higher temperature range has been removed. The weight loss reaches a plateau above 700 °C, signifying the removal of the majority of volatile or decomposable components. The stable elements of the spent catalyst, such as the Ni–Ga metal and the support material, make up the remaining 83.3%.

2.8. Raman Spectroscopic Analysis of Spent Catalysts

Spectroscopic analysis of the used catalysts can evaluate the quality and crystallinity of the carbon nanomaterials developed on the catalyst surface [36]. The typical Raman spectra from these spent catalysts are shown graphically in Figure 10. The D and G bands, which are represented by two distinct peaks that are conspicuously located at approximately 1346 cm⁻¹ and 1575 cm⁻¹, respectively, are present in all catalysts. The D band generally indicates the presence of disordered carbon structures, such as amorphous carbon and defects in the walls of carbon nanotubes. Conversely, the G band signifies the existence of highly structured, graphitic crystalline carbon structures [37]. Therefore, valuable insights between the D band (I_D) and the G band (I_G) by examining the intensity ratio (I_D/I_G) can be extracted. The analysis results reveal that the D-band’s intensity is relatively low for the low Ga loading samples, such as 5Ni+0.1Ga/SZ31107 and 5Ni+0.25Ga/SZ31107, favored by the crystallinity and graphitic structure of the samples. The catalyst samples with an intensity ratio I_D/I_G of about one indicates a significant level of disorder in the materials. In particular, the determined I_D/I_G ratio for the 5Ni/SZ31107, 5Ni+0.1Ga/SZ31107, 5Ni+0.25Ga/SZ31107, 5Ni+0.75Ga/SZ31107, and 5Ni+1.0Ga/SZ31107 stands at 1.05, 0.89, 0.92, 0.99, 1.02, and 0.97, respectively. The best performance catalyst (5Ni+0.25Ga/SZ31107) exhibited the lowest I_D/I_G ratio of approximately 0.92, indicating a high degree of crystallinity and graphitization. On the other hand, the 5Ni/SZ31107 catalyst had the highest I_D/I_G ratios, which suggests that it had less crystallinity because it did not have as much graphitic carbon. This suggests that throughout the carbon production process, the small additions of Ga encourage a higher degree of graphitization. Figure 11 displays the Raman spectra of the top catalyst (5Ni+0.25Ga/SZ31107) after it was exposed to a long-term stream for 20 h. The D, G, and 2D bands are represented by the catalyst’s three distinct peaks, which are notably located at about 1350 cm⁻¹, 1591 cm⁻¹, and 2683 cm⁻¹. The catalyst’s intensity ratio I_D/I_G of 1.94 indicates a significant level of disorder in the catalyst.

The TEM image of the fresh and used 5Ni+0.25Ga/SZ31107 catalyst is shown in Figure S4. The particles of the fresh catalyst appear to be relatively well-dispersed, which is suitable for catalytic activity. There’s no obvious evidence of pre-existing carbon or significant agglomeration in the fresh catalyst. The used catalyst shows the formation of carbon nanofibers or nanotubes on the catalyst surface after the DRM reaction (Figure S5). The C 1s XPS data clearly show that Ga promotes the reduction of carbon deposition during methane dry reforming. This is a key component in enhancing catalyst stability for this DRM reaction. Together with the TEM pictures of carbon nanotubes, this information paints a complete picture of the deactivation mechanism and how Ga aids in its alleviation.

3. Materials and Methods

3.1. Materials Used

The supports used were supplied by the Saint-Gobain NorPro firm (Stow, OH, USA) and contained 5% SiO₂ in zirconia. Alfa Aesar (Heysham, UK) provided the nickel nitrate hexahydrate, Ni (NO₃)₂·6H₂O. Ultrapure water was created and used for all experimental requirements using a Milli-Q water purification system (MilliporeSigma; Burlington, MA, USA).

3.2. Catalysts Preparation

The catalysts were synthesized via the wet impregnation technique. First, nickel nitrate (5 wt.% Ni) was dissolved in distilled water in a crucible to form a nickel precursor solution. After that, this mixture was heated on a hot plate to 80 °C while being agitated to achieve total dissolution. The nickel solution was then mixed with gallium nitrate, corresponding to Ga loadings of 0.1, 0.25, 0.5, 0.75, and 1 wt.%. The SZ31107 support (stabilized zirconia with 5 wt.% SiO₂) was added to the mixed solution after both metal precursors had completely dissolved. The impregnated material was dried at 120 °C overnight and calcined at 800 °C for 3 h. To make the catalyst material suitable for use in chemical reactions, it was ground into a fine powder. The synthesized catalysts are designated 5Ni+xGa/SZ31107 (0.1, 0.25, 0.5, 0.75, and 1 wt.%) (Table 1), where x is the loading of the Ga promoter metal present in addition to the Ni active metal. Under the Sections S1 and S2, the Supplemental Material includes thorough explanations of the catalyst’s characterizations and performance assessments.

4. Conclusions

In this study, a series of Ni–Ga catalysts supported on stabilized zirconia were successfully synthesized via wet impregnation and evaluated for dry reforming of methane. The addition of Ga significantly impacted the textural properties, reducibility, and basicity of the catalysts. Catalytic activity tests revealed that the 5Ni+0.25Ga/SZ31107 catalyst exhibited the best performance, achieving high average conversion rates of 56% for CH₄ and 66% for CO₂, with an H₂/CO ratio of 0.84 after 300 min. Several mechanisms, such as improved dispersion of nickel species, modified basic sites, and optimized metal-support interactions, are responsible for this improved performance. In 20 h of reaction time, the CH₄ conversion drops from 56% to 50%, and the H₂/CO ratio remains above 0.5 over the 5Ni+0.25Ga/SZ31107 catalyst. Long-term stability experiments, however, showed a progressive deactivation of the ideal catalyst, indicating the presence of sintering and/or carbon deposition. The endothermic character of the DRM reaction was validated by temperature-dependent investigations conducted over the 5Ni+0.25Ga/SZ31107 catalyst. Additional information about the reaction kinetics was revealed by the computed activation energies. The inclusion of Ga, especially at the 0.25 wt.% loading, encouraged the development of more graphitic carbon, which is typically less deactivating, according to Raman spectroscopy of the spent catalysts. The 0.25 wt.% Ga loading showed the most promising catalytic performance: CH₄ and CO₂ conversions of 80% and 87%, respectively, at 800 °C. This study highlights Ga’s beneficial function in the DRM as a promoter for Ni/SZ31107 catalysts. To improve the long-term stability and industrial viability of these catalysts, more research is required, to focus on reducing catalyst deactivation through catalyst composition and reaction condition optimization.