Tailored Ni-MgO Catalysts: Unveiling Temperature-Driven Synergy in CH₄-CO₂ Reforming

Abstract

This study examines nickel catalysts on two different supports—magnesium oxide (MgO) and modified MgO (with 10 wt.% MOx; M = Ti, Zr, Al)—for their effectiveness in the dry reforming of methane. The reactions were conducted at 700 °C in a tubular microreactor. The study compares the best-performing catalyst with a reference catalyst (5Ni/MgO) by conducting dry reforming of methane at different reaction temperatures. The catalysts are evaluated using surface area, porosity, X-ray diffraction, infrared spectroscopy, transmission electron microscope, thermogravimeter, and temperature-programmed techniques. The 5Ni/MgO + ZrO₂ catalyst demonstrates inferior catalytic activity due to insufficient active sites. On the other hand, the 5Ni/MgO + TiO₂ catalyst shows limited catalytic excellence due to excessive coke deposits, which are six times higher than other catalysts. The 5Ni/MgO and 5Ni/MgO + Al₂O₃ catalysts have the richest basic and acidic profiles, respectively. The 5Ni/MgO + Al₂O₃ catalyst is superior to other catalysts due to its stronger metal–support interaction on the expanded surface and the efficient diffusion of carbon on its less crystalline surface. At 700 °C, this catalyst achieves 73% CH₄ conversion, and at 800 °C, it reaches 83% conversion. This study emphasizes the crucial role of the reaction temperature in reducing carbon deposition and enhancing the efficiency of the reforming process.

1. Introduction

Technologies for carbon capture and utilization are crucial for diminishing CO₂ emissions and preventing global warming [1]. The CO₂ reformation of methane (DRM), also referred to as “dry”, is an attractive process for producing syngas (CO/H₂) that directly uses greenhouse gas emissions of CO₂ and CH₄ [2]. The produced syngas has multiple uses, for example as feedstock, the Fischer-Tropsch process, fuel synthesis, chemical synthesis, carbonylation, and hydroformylation [2]. DRM has several advantages, including yielding a syngas ratio of unity that allows for selective modification of feed concentrations for additional chemical synthesis [3]. However, it is important to note that DRM is a strong endothermic process, which means that the reaction requires high temperatures. These high temperatures can cause sintering and agglomeration, which can deactivate the catalysts [4,5]. Despite this, numerous studies have found that Ni-based catalysts are effective and promising DRM candidates due to their high availability, low cost, and high activity when compared to noble metals like Pd, Rh, and Ru [6]. Nevertheless, although Ni particles show the greatest activity in the DRM response, they are hampered by sintering Ni and carbon buildup [3,4,5]. There must be a robust metal support relationship to halt carbon formation and Ni agglomeration, particularly over the DRM [6]. For the DRM method to be viable on a wide scale, it is essential to improve catalytic performance with suitable support materials [7,8,9,10]. Numerous attempts have been made [11,12] to reduce the amount of carbon that deposits on Ni catalysts. Reducing the size of the nickel particles is beneficial. It is established that the combination of NiO and support helps shrink the size 1of the nickel particles even at high temperatures [13]. To achieve a strong metal–support contact, it is essential to choose appropriate metal–oxide combinations and synthesis techniques. Since Mg and Ni have analogous atomic sizes, MgO can easily form a solid solution with NiO. Other advantages include good thermal stability and an inexpensive cost. The Ni-Mg interaction is beneficial to improve the nickel’s dispersion and fine-tune its particle size. Many researchers have established that catalysts supported by MgO show outstanding performance [14]. MgO support can be further enhanced by modifying it with other oxides such as Al₂O₃, TiO₂, and ZrO₂. Al₂O₃ modifier support is characterized by a large surface area and excellent stability, providing highly dispersed Ni active sites with a confinement effect [15]. A NiAl₂O₄ catalyst was developed and tested for DRM reactions at 800 °C by Zhang et al. [16]. The catalyst demonstrated a high level of long-term stability and resistance to coking and sintering. Similarly, Roger et al. [17] also elaborated on nickel aluminate’s stability and activity by examining catalysts (NiAl₄O₇, NiAl₂O₄, and Ni₂Al₂O₅) with varying concentrations. Due to their regenerating characteristics, NiAl₂O₄ and Ni₂Al₂O₅ reductions had high and consistent activity under challenging DRM conditions. The benefits of the ZrO₂ support modifier include high acidic/basic, oxygen mobility, oxidizing/reducing, and heat stability [18,19]. Androulakis et al. found that the methane conversion turnover frequency (TOF) of TiO₂-supported Rh catalysts gave optimum performance [20]. TiO₂-supported catalysts have proven to be good for improving the performance of Rh-based catalysts in the partial oxidation of methane because they help maximize and retain rhodium dispersion by avoiding its re-oxidation [21]. TiO₂-supported Rh was among the various supports investigated by Yokota et al. [22]. It exhibited optimum activity contrary to Al₂O₃-supported Rh, which presented the lowest turnover frequency (TOF). The former catalyst was able to retain Rh in a reduced state. According to the authors, electronic interactions between Rh and support were responsible for maintaining Rh’s most active DRM reaction metallic state. Equations (1)–(4) represent the DRM and the associated side reactions. The main equation for dry reforming is Equation (1), and the reverse water shift process is represented by Equation (2). The creation of coke is caused by Equation (3) of methane decomposition and Equation (4) of the Boudouard reaction.

$${\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2\leftrightarrow 2\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298\mathrm{K}}=+247 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}$$

(1)

$${\mathrm{H}}_2+{\mathrm{C}\mathrm{O}}_2\leftrightarrow \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298\mathrm{K}}=+41 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}$$

(2)

$${\mathrm{C}\mathrm{H}}_4\leftrightarrow \mathrm{C}+2{\mathrm{H}}_2\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298\mathrm{K}}=+75 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}$$

(3)

$$2\mathrm{C}\mathrm{O}\leftrightarrow \mathrm{C}+{\mathrm{C}\mathrm{O}}_2\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\Delta {\mathrm{H}}_{298\mathrm{K}}=-127 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}$$

(4)

In this study, a Ni catalyst supported on stabilized MgO with Al₂O₃, ZrO₂, and TiO₂ was prepared and applied as the catalyst for DRM at 700 °C and 800 °C. The catalytic performance, carbon resistance, and stability of the prepared catalysts with different modified magnesium oxide supports were investigated systematically. To achieve high conversion performance at reaction temperatures of 700 °C and 800 °C and to prevent metal agglomeration and coke formation, it is crucial to develop active as well as sintering and coke-resistant catalysts. This will open up new possibilities for the DRM reaction’s limited commercialization [23]. Various characterization techniques, such as X-ray diffraction, N2 physisorption, temperature-programmed reduction Raman spectroscopy, TGA, etc., were used for both fresh and used samples.

2. Characterization and Activity Results

2.1. Overview of Textural Analysis

N₂ adsorption–desorption isotherms and the BET study of catalysts are shown in Figure S1 and

Table 1. Summarizes the results of adsorption–desorption characterization, CH₄ conversion (${\mathrm{X}}_{{\mathrm{CH}}_4}$), deactivation factor (DF), turnover frequency (TOF), frequency, and weight loss (WL).

Samples	SA m2/g	${\mathit{P}}_{\mathit{v}}$ cm3/g	${\mathit{P}}_{\mathit{D}}$ Nm	Total H2 Quantity cm3/g	Total CO2 Quantity cm3/g	Total NH3 Quantity cm3/g	${\mathbf{X}}_{{\mathbf{C}\mathbf{H}}_{\mathbf{4}}}$ %		D.F a	TOF b h−1	WL %
							I	F
5Ni/MgO	55	0.52	39.8	11.11	4.49	8.40	71.3	70.0	1.82	46.30	11.0
5Ni/MgO + Al2O3	85	0.53	23.6	9.72	2.90	12.40	73.8	73.1	0.95	48.43	13.0
5Ni/MgO + ZrO2	53	0.39	33.0	2.22	2.95	8.60	57.5	50.8	11.65	34.88	12.5
5Ni/MgO + TiO2	58	0.48	36.3	13.32	3.83	8.12	67.9	63.9	5.89	44.58	63.7

SA = Surface area, $P_v$ = Pore volume, $P_D$ = Pore diameter, I = initial, F = Final, D.F ^a = (Initial CH₄ conversion-Final CH₄ conversion) × 100/(Initial CH₄ conversion), WL = weight loss.; TOF ^b = turnover frequency.

. According to the IUPAC classification, the catalysts showed type-IV isotherms with an H1-type hysteresis loop, which describes a mesoporous structure. Upon incorporating 20 wt.% titania (or zirconia) along with 80 wt.% MgO, the surface area of the catalyst sample (5Ni/MgO + TiO₂) is not affected much. However, the incorporation of 20 wt.% alumina along with 80 wt.% MgO results in a 50% rise in the surface area of the catalyst sample (5Ni/MgO + Al₂O₃). This aspect shows that combining oxides influences the catalysts’ structural characteristics.

2.2. X-ray Diffraction (XRD) Analysis

Figure 1 shows the XRD patterns of NiO, MgO, TiO₂, ZrO₂, MgNiO₂, NiTiO₃, and MgTiO₃. The diffraction peaks of the cubic phase of MgO are located at 2θ = 36.92°, 42.82°, 62.09°, 74.48°, and 78.43° (reference number 01-075-0447), whereas the cubic NiO phase characteristic is found at 2θ = 37.00° and 43.02° (reference number 00-047-1049). The anatase phase TiO₂ diffraction peaks are located at 2θ = 27.61° and 36.25° (reference number 01-073-1764), whereas the rutile phase TiO₂ diffraction peaks are located at 2θ = 28.51° and 54.52° (reference number 01-078-1510). The location of ZrO₂’s monoclinic phase is at 2θ = 24.04°, 24.53°, 28.24°, 31.54°, 34.19°, 35.39°, 50.16°, 55.44°, and 59.96° (reference number 00-007-0343), while the rhombohedral phase of NiTiO₃ is located at 2θ = 24.20°, 33.09°, 35.62°, 40.83°, 49.36°, 53.82°, and 63.88° (reference number 00-033-0960), and the rhombohedral phase of MgTiO₃ is located at 2θ = 21.24°, 24.20°, 33.09°, 35.62°, 40.83°, 49.36°, 53.82°, and 63.88° (reference number 00-002-0901). The diffraction peaks for the cubic phase of MgNiO₂ are situated at 2θ = 36.92°, 42.82°, 62.09°, and 74.48° (reference numbers 00-024-0712, 00-003-0999). Overall, the 5Ni/MgO and 5Ni/MgO + Al₂O₃ catalysts are populated with cubic MgO phases as well as cubic MgNiO₂ phases. Over 5Ni/MgO + TiO₂, the intensity of Mg-related phases is decreased, and Ti-related phases (rhombohedral NiTiO₃ phase, rutile TiO₂ phase, and anatase TiO₂ phase) appear. In the same way, the 5Ni/MgO + ZrO₂ catalyst is populated with the monoclinic zirconia phase.

2.3. Temperature-Programmed Reduction and Desorption Study

The temperature-programmed reduction (H₂-TPR) profile of catalysts is shown in Figure S2. H₂-TPR was used to understand the interaction between the active catalyst and its support. The catalysts showed broad negative peaks between 200 and 300 °C, possibly caused by hydrogen seeping into the mesopores [24]. The catalysts exhibit modest interactions between the NiO and the support matrix, with a peak temperature of around 500 °C. The strong interaction between NiO and the supports is responsible for the absorption peaks observed between 600 and 800 °C. The formation of Mg₂NiO₃, MgNiO₂, and MgNi₂O₃ as a result of a solid solution between NiO and MgO could be the cause of the high-temperature peaks for all catalysts [25]. In Table 1, the catalyst with 5Ni/MgO + ZrO₂ displayed a minimum hydrogen consumption of 2.22 cm³/g, whereas the catalyst with 5Ni/MgO + TiO₂ displayed a maximum hydrogen consumption of 13.32 cm³/g. Additional reduction features over 5Ni/MgO + TiO₂ may be due to the reduction of TiO₂ and rhombohedral NiTiO₃ phases [26]. Among 5Ni/MgO and 5Ni/MgO + Al₂O₃, 5Ni/MgO + Al₂O₃ has a higher amount of reducible NiO which is under strong metal support interaction. The basicity profile of catalysts is studied by CO₂ temperature-programmed desorption (CO₂-TPD) (Figure 2A). The CO₂-TPD profile of catalysts shows desorption peaks at about 260 °C and 330 °C, which are attributed to weak basic sites and medium strength basic sites, respectively [25]. Interestingly, 5Ni/MgO has the highest quantity of desorbed CO₂, indicating the optimum number of basic sites over the catalyst surface. Over the MgO + ZrO₂-supported Ni catalyst, the intensity of medium strength basic sites is depleted faster than weak basic sites, whereas over the MgO + TiO₂-supported Ni catalyst, the population of both types of basic sites decreased proportionally. The basic profile of 5Ni/MgO + Al₂O₃ is the worst and it had minimum CO₂ desorption. Overall, the catalyst surfaces of 5Ni/MgO, “5Ni/MgO + ZrO₂ and 5Ni/MgO + TiO₂”, and 5Ni/MgO + Al₂O₃ have high, moderate, and low concentrations of basic sites. The acidic profile of the catalysts was studied by NH₃ temperature-programmed desorption (NH₃-TPD) (Figure 2B). It is found that 5Ni/MgO and 5Ni/MgO + ZrO₂ have a low concentration of acid sites, 5Ni/MgO + TiO₂ retains a moderate concentration of acid sites and 5Ni/MgO + Al₂O₃ attains a high concentration of acid sites. The minimum basicity of the MgO + Al₂O₃-supported Ni catalyst can be justified by neutralization of basic sites over MgO by acidic alumina oxide. Again, the highest concentration of acid sites over 5Ni/MgO + Al₂O₃ is solely due to the presence of acidic aluminum oxide in the support.

2.4. Infrared (IR) Spectroscopy of Fresh and CO₂-Treated Catalyst

The infrared spectra of fresh and the CO₂-treated catalysts are shown in Figure 3. Infrared spectra of the CO₂-treated catalysts (for 2 h) show CO₂-surface-interacting species grown over the catalyst surface. Over 5Ni/MgO, the bidentate carbonate species (about 1087 cm⁻¹), carboxylate species (about 1417 cm⁻¹), ionic carbonate species (1460 cm⁻¹), and bidentate formate species (about 2850–2925 cm⁻¹) are cultivated upon treatment with CO₂ [27]. Over 5Ni/MgO + TiO₂, IR vibration peaks for bidentate carbonate are not observed, whereas over 5Ni/MgO + ZrO₂ and 5Ni/MgO/Al₂O₃, only carboxylate and ionic carbonates are grown.

2.5. Raman Analysis of Spent Catalysts

The Raman spectra of the spent catalysts are shown in Figure 4. The method displays the type and degree of graphitization on the catalysts that are being employed. The temperature range between 1250 and 1800 cm⁻¹ displays D and G band characteristic peaks. The D band located at 1341 cm⁻¹ is represents imperfect, haphazard carbon deposits (amorphous), while the G band located at 1574 cm⁻¹ represents graphite. The degree of carbon crystallinity that was produced during the reaction can be accurately determined thanks to the relative intensity sizes of the D and G bands (I_D/I_G). Smaller ratios suggest that the graphitized carbon is primarily responsible for the crystallinity. The I_D/I_G values for our spent catalysts were 0.72, 1.13, 1.70, and 1.72 for the 5Ni/MgO + ZrO₂, 5Ni/MgO, 5Ni/MgO + TiO₂, and 5Ni/MgO + Al₂O₃. 5Ni/MgO + Al₂O₃ had the highest (I_D/I_G) ratios, indicating low crystallinity due to a deficiency in graphitized carbon. Consequently, the degree of carbon graphitization over the 5Ni/MgO + Al₂O₃ catalyst was lower than that of the other catalysts. The catalyst did not deactivate early due to amorphous or poorly crystalline carbon deposits; however, the catalytic activity declined by the crystalline graphitic carbon, as seen in the 5Ni/MgO + ZrO₂ sample.

2.6. TGA Analysis of Spent Catalysts

In Table 1, the weight loss of spent catalysts is shown. 5Ni/MgO, 5Ni/MgO + Al₂O₃, and 5Ni/MgO + ZrO₂ catalysts had weight losses ranging between 11 and 13%, while the 5Ni/MgO + TiO₂ sample had a high weight loss of 64%. These TGA profiles might be connected to the catalysts’ reactivity. Indeed, the 5Ni/MgO + TiO₂ catalyst exhibited lower performance than the 5Ni/MgO and 5Ni/MgO + Al₂O₃ catalysts, thus generating higher carbon deposition.

2.7. Transmission Electron Microscopy

Figure S3 displays the 5Ni/MgO + Al₂O₃ catalyst’s fresh and spent TEM images and the fresh and used TEM images of 5Ni/MgO. The fresh catalysts exhibit small Ni particles of semispherical and cylindrical shapes, with varying sizes. The particles are uniformly distributed on the support, and their boundaries are extremely clear. However, the used catalysts show agglomerated particles that lead to the formation of interparticle voids. The formation of nanotubes of different diameters and lengths is found, particularly for the non-modified supported MgO.

2.8. Catalytic Activity Results

Before the developed catalysts were examined, the catalytic performance was investigated in a blank experiment carried out in a stainless-steel reactor with the same input ratio and reaction temperatures. The blank’s CH₄ and CO₂ conversions were 1.0% and 0.5%, respectively. The profiles of the CO₂ and CH₄ conversions as well as the H₂/CO ratio are displayed in Figure 5. The performance of the catalyst was assessed at 700 °C. Compared to the equivalent CH₄ conversions, the CO₂ conversions are greater. The existence of the reverse water gas shift reaction in Equation (2) may be the cause of this phenomenon. The catalyst 5Ni/MgO + Al₂O₃ had the best dry reforming performance, with an H₂/CO ratio of 0.94 and CH₄ and CO₂ conversions of 73.8% and 79.1%, respectively.

The reduction of the active metal is proportional to hydrogen consumption. The turnover frequency (TOF), which indicates how many times the catalytic cycle occurs on a single site per unit time, is computed as shown in Table 1. The 5Ni/MgO + Al₂O₃ catalyst has the highest value of 48.4 h⁻¹; the 5Ni/MgO + ZrO₂ catalyst has the lowest value of 34.88 h⁻¹. The low TOF over 5Ni/MgO + ZrO₂ may be related to its low hydrogen intake, which results in the least amount of reduced active Ni. Table S1 presents a comparative analysis of the current catalyst and those found in previous studies that were used for methane dry reforming. The outcome demonstrates the catalyst’s applicability and the need for more development.

2.9. Catalytic Activity Results at Different Reaction Temperatures

The effect of reaction temperature was further examined using the pristine supported catalyst 5Ni/MgO and the best-obtained modified catalyst 5Ni/MgO + Al₂O₃. Both 600 °C and 800 °C reaction temperatures were tested. Figure 6 exhibits the conversions of both methane and carbon dioxide versus time on the stream. The performance increased 2.5 times when the reaction temperature was increased from 600 °C to 800 °C. The increase could be attributed to the endothermic nature of the process. The best performance was attained using an 800 °C reaction temperature. The methane and carbon dioxide conversions acquired were 83% and 87%, respectively.

2.10. Temperature-Programmed Oxidation (TPO) Study over the Spent Catalyst

Temperature-programmed oxidation (TPO) analysis was utilized to examine the type of carbon collected on the surface of the DRM catalysts being used. In the DRM process, the TPO experiments were performed for seven hours of reaction time using spent catalysts, as shown in Figure 7 and Figure S4. Structurally organized carbon with a reduced oxidation reactivity is responsible for the peaks corresponding to the evolution of CO₂ at high temperatures. According to the TPO characterization analysis, inert carbon was not present when the catalysts were operated at 800 °C and the low-temperature peak was attributed to the existence of amorphous/active carbon. However, at 600 °C or 700 °C, the catalysts’ peaks appeared due to structurally ordered carbon with lower oxidation reactivity. The extra 2% rise in carbon deposition seen on the modified supported catalyst could be attributed to its higher reactivity.

2.11. Impact of Reaction Temperature on the TGA Analysis

The impact of reaction temperature on the TGA analysis of 5Ni/MgO and 5Ni/MgO + Al₂O₃ catalysts is shown in Figure 8. The methane dry reforming reaction appears to reach its maximum carbon deposition at 600 °C, moderate carbon deposition at 700 °C, and least carbon deposition at 800 °C. This behavior was noted in an earlier study [28]. The decrease of carbon deposition upon increasing reaction temperature can be justified thermodynamically. At low reaction temperatures, CH₄ decomposition reactions, hydrogen-consuming reactions, and other coke deposition reaction pathways (like CO + H₂ → H₂O + C and CO₂ + 2H₂ → 2H₂O + C) are thermodynamically feasible [29]. At about 700 °C, even coke removal reactions like C + H₂O → CO + H₂ become feasible, and the above coke deposition pathways have lost their thermodynamic feasibility. At 800 °C, spontaneous thermal cracking of CH₄ and their consequent oxidation results in the least coke deposit.

2.12. Raman Analysis of Spent Catalysts

Figure 9 shows the Raman spectra of 5Ni/MgO and 5Ni/MgO + Al₂O₃ catalysts at different reaction temperatures. The relative intensity indices (I_D/I_G) increase with the decrease in reaction temperature, favoring the formation of low-crystallinity carbon.

3. Discussion

Based on the characterization results, it can be said that combining oxides with support modifies the catalyst texture, crystallinity, reducibility, and acid–basic property of 5Ni/MgO + MO_x (M = Zr, Ti, and Al) catalysts. Further, these surface properties govern catalytic activity and coke deposition accordingly. Under the CO₂ stream, 5Ni/MgO + ZrO₂ was found to be populated with carboxylate and ionic carbonates (as per the IR results). The feeble active sites (2.22 cm³/g) and unstable monoclinic ZrO₂ phase over 5Ni/MgO + ZrO₂ make the catalyst inferior (50% CH₄ conversion) in the dry reforming of methane reactions. 5Ni/MgO + TiO₂ has a comparable surface area (to 5Ni/MgO) and the highest number of reducible species over the catalyst surface. However, all reducible species are not active sites. Some reduced species may be formed by the reduction of TiO₂ phases, which are not active sites for DRM. Under the CO₂ stream, the 5Ni/MgO + TiO₂ catalyst has one additional surface-interacted CO₂ species (bidentate formate species). Overall, surface area increases, increasing the number of catalytic active sites and additional CO₂-surface interacting species (than 5Ni/MgO + ZrO₂), which make the catalyst more active (64% CH₄ conversion) for DRM. However, the excessive coke deposition, which is over six times more than other catalysts, and the presence of a higher amount of graphitic carbon, limit the catalyst’s performance further. 5Ni/MgO attains the highest amount of basicity by which various types of CO₂-interacting species, such as bidentate carbonate, carboxylate species, ionic carbonate, and bidentate formate species, are populated over the catalyst surface (Figure 10A). It has the highest crystallinity where Ni sites are derived from cubic MgNiO₂ phases. The 5Ni/MgO achieves as high as 70% CH₄ conversion during 430 min on stream.

It is interesting to note that the 5Ni/MgO + Al₂O₃ catalyst has relatively fewer catalytic active species and fewer CO₂-interacting surface species than 5Ni/MgO, but it attains the highest catalyst activity (Figure 10B). In DRM, CH₄ decomposition (into CH_4−x) is the rate-determining step. After that, the oxidation of dissociated-CH₄ (or CH_4−x) by “surface interacting CO₂ species” or “gases CO₂” does not affect the reaction rate. Therefore, over the 5Ni/MgO + Al₂O₃ catalyst, oxidation of dissociated CH₄ species by gaseous CO₂ under the Eley–Rideal model [30,31] can be expected (Figure 10B, route (b)). In terms of population of catalytic active sites over 5Ni/MgO + Al₂O₃, one possible answer can be drawn that the 5Ni/MgO + Al₂O₃ catalyst has a higher number of Ni sites under strong interaction (than 5Ni/MgO), which are derived at 700–1000 °C under H₂-stream (Figure S3). This means that the 5Ni/MgO + Al₂O₃ catalyst has more stable active sites in the DRM reaction temperature than 5Ni/MgO. Again, in the acid–base profile, the 5Ni/MgO catalyst has the highest number of basic sites, whereas the 5Ni/MgO + Al₂O₃ has the highest number of acid sites. Acid sites are responsible for coke formation, which may affect the catalytic activity if coke covers the catalytic active sites. The coke deposition over the 5Ni/MgO + Al₂O₃ catalyst is slightly higher than the 5Ni/MgO. Again, the O₂-TPO study shows the presence of more inert carbon over 5Ni/MgO + Al₂O₃ than 5Ni/MgO at about 700 °C. However, Raman analysis shows the presence of the least degree of carbon graphitization over the 5Ni/MgO + Al₂O₃ catalyst. The inert and less graphitic carbon over 5Ni/MgO + Al₂O₃ is not found to affect the activity, even though the activity of the 5Ni/MgO + Al₂O₃ catalyst is higher than 5Ni/MgO. It is noticeable that the 5Ni/MgO + Al₂O₃ catalyst has the highest surface area (50% more than 5Ni/MgO) and less crystallinity, whereas the 5Ni/MgO catalyst has the highest crystalline surface. That means that the 5Ni/MgO + Al₂O₃ catalyst has more space and fewer crystalline constraints for diffusion of carbon deposit than 5Ni/MgO. Therefore, over 5Ni/MgO + Al₂O₃, the rate of CH₄ deposition is counter-supported by the rate of carbon diffusion (away from the active sites) over a “less crystalline and expanded” surface [32] (Figure 10B, route c). However, over a highly crystalline 5Ni/MgO catalyst, the rate of carbon formation may not properly match the rate of carbon diffusion (Figure 10A, route c). So, even after high coke deposition over 5Ni/MgO + Al₂O₃, carbon deposit is transferred away from the active sites properly and leaves the active sites exposed to the reaction. This means it can be said that stable catalytic active sites and better diffusion of carbon deposit away from the active site over a more expanded surface make 5Ni/MgO + Al₂O₃ a better catalyst than 5Ni/MgO. It conveys about 73% CH₄ conversion at 700 °C and 83% CH₄ conversion at 800 °C in about 430 min on stream.

4. Experiments

4.1. Materials

Nickel nitrate hexahydrate (Ni (NO₃)₂.6H₂O, 98%, Alfa Aesar, Haverhill, MA, USA), titania (TiO₂-P25, 99.9%, from Degussa p25, Nanoshel, Cheshire, UK), alumina (Al₂O₃, 97.7% Norton Chemical Process Products Corp, Northboro, MA, USA), magnesia (MgO, 99.5%. BDH, Dubai, United Arab Emirates), and zirconia (ZrO₂, 99.8%, Anhui-Elite, Hefei, China) were obtained commercially and were used as received.

4.2. Catalyst Preparation

The impregnation procedure was adopted to prepare the catalysts. A pristine support of MgO and 80% MgO combined with 20%m (m = Al₂O₃, ZrO₂, and TiO₂) mixed oxide supports were prepared by mixing 30 mL of distilled water and the appropriate amount of Ni (NO₃)₂·6H₂O to get a 5.0 wt.% loading of nickel oxide, and the needed amount of support was mixed and stirred at room temperature in a crucible. For 30 min, the solution was dried while being stirred at 80 °C. The catalysts were calcined at 600 °C for three hours. The catalyst characterization is described in Supplementary Information S1.

4.3. Catalytic Activity Test

The catalytic activity was performed in a fixed-bed, continuous-flow reactor (PID Eng. & Tech, Madrid, Spain). At atmospheric pressure, the dry reforming reaction was carried out over a sample of 0.1 g of catalyst in a 0.94 cm internal diameter and 30 cm long stainless-steel tube. The reaction temperature was monitored using a thermocouple placed in the center of the catalyst bed. Before the reaction, hydrogen (H₂) was introduced at a flow rate of 30 mL/min, at 800 °C, to convert nickel oxide to nickel metal, the active form of the catalyst. Afterward, the reactor was purged under nitrogen (N₂) flow (20 mL/min) to remove any H₂ left in the reactor. Then, the reactor temperature was adjusted under the flow of N₂ until the reaction temperatures of 600, 700 °C and 800 °C were reached. A 70 mL/min feed flow rate comprised 30, 30, and 10 mL/min of CH₄, CO₂, and N₂, respectively. The space velocity was set at 42,000 mL (h·g_cat)⁻¹. The reactor outlet gases, containing the product of syngas and unconverted feed gases, were analyzed using online gas chromatography (Shimadzu 2014, Kyoto, Japan), equipped with a TCD. The entire separation of reaction products was achieved by using two columns in series/bypass connections: Porapak Q and Molecular Sieve 5A. The following equations were used in this work to compute the H₂/CO mole ratio and methane and carbon dioxide conversions:

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

(5)

$${\mathrm{C}\mathrm{O}}_2 \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)=\frac{{\mathrm{C}\mathrm{O}}_{2,\mathrm{i}\mathrm{n}}-{\mathrm{C}\mathrm{O}}_{2,\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{C}\mathrm{O}}_{2,\mathrm{i}\mathrm{n}}}\times 100$$

(6)

$$\frac{{\mathrm{H}}_2}{\mathrm{C}\mathrm{O}}=\frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}}_2 \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{C}\mathrm{O} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d}}$$

(7)

5. Conclusions

The dry reforming of methane (DRM) generated syngas utilizing Ni catalysts supported on MgO and stabilizing MgO with TiO₂, ZrO₂, and Al₂O₃. Various tests were conducted on Ni catalysts supported on different oxides, exploring the impact of reaction temperature. The 5Ni/MgO + ZrO₂ catalyst has a shortage of sufficient active sites, resulting in its inferior catalytic performance (~50% CH₄ conversion at 430 min TOS). The 5Ni/MgO + TiO₂ catalyst is enriched with active sites and CO₂-interacting surface species, but excessive coke deposition (six times more than other catalysts) limits the CH₄ conversion below 65% at 430 min TOS. The 5Ni/MgO catalyst has the richest basic profile, and this catalyst is populated by reducible NiO-interacted species (having modest and strong interaction with support) as well as a wide population of CO₂-interacting surface species. The catalyst attains 70% CH₄ conversion. The 5Ni/MgO + Al₂O₃ catalyst has the richest acidic profile and worst basic profile, but it exhibits the highest catalytic activity for DRM, achieving CH₄ and CO₂ conversions of 73% and 79% over seven hours at 700 °C. The superiority of 5Ni/MgO + Al₂O₃ over 5Ni/MgO is due to stronger metal–support interaction, oxidation of carbon deposit by mostly gaseous CO₂, and timed diffusion of carbon over a “less crystalline and expanded” surface. Regarding reaction temperatures, the least quantity of carbon was deposited at 800 °C, with CH₄ and CO₂ conversion rates reaching 83% and 87% over 5Ni/MgO + Al₂O₃, respectively.