Influence of Supports on the Catalytic Activity and Coke Resistance of Ni Catalyst in Dry Reforming of Methane

Abstract

The dependence of the catalytic activity and coke resistance of Ni-based catalysts on the support type was investigated in the dry reforming of methane (DRM). Catalysts were prepared using incipient wetness impregnation and analyzed using ICP-OES, BET-BJH, XRD, H₂-chemisorption, H₂-TPR, and CO₂-TPD. DRM was performed at 600–750 °C at 144,000 mL/g_cat∙h of GHSV (CH₄/CO₂/N₂ = 1/1/1). Ni/Al₂O₃ and Ni/MgO catalysts formed NiAl₂O₄ and NiO-MgO solid solutions, respectively, owing to strong binding between the metal and support. In contrast, MgO-Al₂O₃ and MgAl₂O₄ supports suppressed NiAl₂O₄ and NiO-MgO solid solution formation, due to Mg addition, with high metal dispersions of 4.6 and 6.6%, respectively. In the DRM reaction, the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts showed high CH₄ conversions of 78.1 and 76.8%, respectively, compared with Ni/Al₂O₃ and Ni/MgO at 750 °C. A stability test was performed at 600 °C for 20 h. A coke study of the spent catalysts was performed using SEM and TGA. Alkaline-earth metal-containing catalysts Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ with strong CO₂ adsorption properties showed 20 wt% reduction in carbon deposition compared to commercial catalysts. Therefore, the support and basic properties of the catalyst significantly influenced the catalyst performance and coke resistance in the DRM.

1. Introduction

Recently, there has been an increased focus on problems related to climate change and the need for reducing carbon dioxide (CO₂) emissions. The development of carbon dioxide utilization (CDU) technologies is necessary to reduce CO₂ emissions over the long term. Among CDU technologies, the dry reforming of methane (DRM) reaction is an important technique that could be used to reduce the total annual anthropogenic greenhouse gas emissions (multi-Gt_CO2/year) [1,2]. An advantage of the DRM reaction is its low operational cost, as it does not require complicated gas separation [3]. The DRM reaction (Equation (1)) produces the beneficial syngas (H₂ + CO) from CH₄ and CO₂ [4]. Syngas is a raw material for high value-added chemicals, such as oxo-alcohol, formaldehyde, and methanol. Notably, the product H₂/CO ratio of the DRM reaction is 1, which renders the produced syngas suitable for the synthesis of methanol and dimethyl ether or Fischer–Tropsch synthesis [5]. The DRM reaction is strongly endothermic [6] and proceeds at a relatively high temperature (650–850 °C) [7]. The reverse water gas shift (RWGS) reaction (Equation (2)) occurs as a side reaction up to 820 °C under the DRM reaction conditions, due to the endothermic nature of the reaction. Since the RWGS reaction produces CO and H₂O using CO₂ and H₂, the H₂/CO ratio is lower than 1.

CH₄ + CO₂ → 2H₂ + 2CO (ΔH_298K = +247 kJ/mol)

(1)

CO₂ + H₂ → H₂O + CO (ΔH_298K = +41 kJ/mol)

(2)

CH₄ → 2H₂ + C (△H_298K = +74.9 kJ/mol)

(3)

2CO → CO₂ + C (△H_298K = −172.4 kJ/mol)

(4)

Ni-based catalysts are the most widely used in the DRM reaction because of their high activity and low cost [8]. However, they can cause catalyst deactivation due to processes such as carbon deposition and sintering. Moreover, they have been reported to have three phases: Ni metallic state, Ni oxide, and Ni carbide phase, which influence carbon deposition. Among these, Ni metal atoms of the Ni carbide phase bind strongly with carbon; thus, easily causing carbon deposition [9]. In addition, at high reaction temperatures the active metal forms aggregates, owing to the sintering effect. The agglomeration of particles occurs randomly, causing low metal dispersion and low specific surface area at high temperatures. Moreover, the DRM reaction is more vulnerable to carbon deposition because of the higher carbon ratio in the reactants compared with other methane reforming reactions. Carbon deposition mainly occurs through CH₄ decomposition (Equation (3)) and CO disproportionation (Equation (4)) reactions. As CH₄ decomposition is endothermic, it mainly occurs above 557 °C, whereas the CO disproportionation reaction occurs below 700 °C, due to the exothermic nature of the reaction. Therefore, carbon deposition occurs in the temperature range of 557–700 °C [10]. The morphology of carbon exhibits variations in properties, which cause differences in the activity of the catalyst. In particular, filamentous carbon influences catalyst deactivation during the DRM reaction. Filamentous carbon is formed through diffusion, nucleation, and growth through Ni particles at temperatures > 450 °C. While filamentous carbon does not block the active sites of the Ni metal, it causes catalyst destruction and a pressure drop, due to pore plugging [11]. For this reason, the DRM reaction has limited commercial viability. In this context, the development of catalysts with high catalytic activity and coke resistance in the DRM reaction is essential.

In the preparation of DRM reaction catalysts, the type of support used has a significant influence on the catalytic activity. Among the various supports used in the DRM reaction, Zhang et al. reported that a ZrO₂ support had a low metal dispersion, owing to its low specific surface area of 30 m²/g, and also had a weak interaction between NiO and the support [12]. Whereas, amorphous ZrO₂ showed a high specific surface area of 200–300 m²/g and had weak thermal stability. However, a tetragonal phase was formed by calcination at temperatures above 450 °C, which remained stable at high temperatures, owing to its strong thermal stability [13]. Depending on the calcination temperature used, Al₂O₃ supports are divided into α-Al₂O₃, θ-Al₂O₃, and γ-Al₂O₃. In comparison to other Al₂O₃ supports, the γ-Al₂O₃ support has high catalytic activity and anti-coking properties, owing to its smaller particle size [14]. In the Ni/γ-Al₂O₃ catalyst, relatively fine Ni particles are formed, owing to the high specific surface area of γ-Al₂O₃. However, NiAl₂O₄ forms a spinel structure during the heat treatment process. The high thermal stability of NiAl₂O₄ spinel results in an increase in the reduction temperature and a reduction in the number of active sites, owing to the low metal dispersion [12]. The MgO support has a low specific surface area and forms a NiO-MgO solid solution in the presence of the Ni catalyst, which limits reduction and causes low metal dispersion [15]. In contrast, the MgO support enhances the adsorption of acidic CO₂ gas, owing to the basic properties of MgO. As the adsorbed CO₂ releases oxygen species upon dissociation, the MgO support enhances the coke resistance by promoting the oxidation of carbon formed on the catalyst surface [16,17]. In addition, owing to the basic properties of the Mg-Al mixed oxide support, the Mg-Al mixed oxide support improves the amount of CO₂ adsorption, compared with the Al₂O₃ support [18]. A Mg-Al mixed oxide has a layered double hydroxide (LDH) structure, in which ion exchange between the cation and anion layers is possible. The Mg²⁺ and Al³⁺ ions are uniformly dispersed between the hydroxide layers, therefore the LDH structure can effectively disperse the nanoparticles, without particle agglomeration. In addition, the MgO-Al₂O₃ (LDH) support is transformed into a spinel structure during the heat treatment process and has high thermal stability. In particular, MgO-Al₂O₃ as a support for a Ni catalyst suppresses the formation of the NiAl₂O₄ and NiO-MgO solid solution, which is advantageous for high metal dispersion, owing to its large specific surface area [12,17]. Moreover, highly dispersed active metals on the MgO-Al₂O₃ support have strong metal-support interactions (SMSIs). Consequently, the sintering effect is inhibited [19,20].

Ni-based catalysts have a low coke resistance, but can improve the catalytic activity and durability by dispersing the active metals or enhancing their basic properties [21]. The γ-Al₂O₃ support has a large specific surface and forms fine particles, while the MgO support has excellent basic properties. The bimetallic MgO-Al₂O₃ and MgAl₂O₄ supports can, thus, overcome the disadvantages of the γ-Al₂O₃ and MgO supports. Therefore, in this study, the influence of various supports: γ-Al₂O₃, MgO, MgO-Al₂O₃, and MgAl₂O₄, was evaluated to improve the catalytic activity and coke resistance of Ni-based catalysts for the DRM reaction. In addition, the prepared catalysts were compared with commercial catalysts (Ni/Al₂O₃ and Ni/CaAl₂O₄) in the dry reforming of methane (DRM) reaction.

2. Results and Discussion

2.1. Characteristics

The characteristics of the catalysts are listed in

Table 1. Characteristics of supports and Ni-based catalysts.

	Ni Content (wt%)	Specific Surface Area 1 (m2/g)	Metal Dispersion 2 (%)	H2 Uptake 3 (μmol/g)	CO2 Desorption Amount 4 (μmol/g)
Al2O3	-	132.2	-	-	-
MgO		124.0
MgO-Al2O3		177.6
MgAl2O4		135.1
Ni/Al2O3	9.2	103.5	2.1	94	26
Ni/MgO	9.2	17.7	N/A 5	39	170
Ni/MgO-Al2O3	8.9	108.7	4.6	98	110
Ni/MgAl2O4	9.1	113.3	6.6	96	125
Commercial A	15.6	3.7	0.4	105	9
Commercial B	15.9	24.6	0.5	182	81

¹ Estimated from N₂ adsorption at −196 °C. ² Estimated from H₂-Chemisorption at 50 °C. ³ Estimated from H₂-TPR in the temperature range 30–1000 °C. ⁴ Estimated from CO₂-TPD analysis in the temperature range 50–800 °C. ⁵ N/A: Not available.

. The Ni metal content of the catalysts was confirmed using ICP-OES analysis. The loading amounts of the prepared and commercial catalysts were approximately 10 and 15 wt%, respectively. Ni-based catalysts have a reduced specific surface area compared to that of the fresh support, due to the pore-blocking during the process of preparing the supported Ni metals. In particular, the Ni/MgO catalyst exhibited a significant decrease in the specific surface area of 17.7 m²/g after the Ni metal was supported. Zanganeh et al. reported that the crystallite size increased and the specific surface area decreased with increasing calcination temperature when NiO-MgO was calcined at 600–800 °C [22]. The Ni/MgAl₂O₄ catalyst maintained a relatively high specific surface area of 113.3 m²/g. Since the Ni/MgO-Al₂O₃ catalyst exhibited changes in the textural properties of the MgO-Al₂O₃ support during the heat treatment, its specific surface area was lower by 108.7 m²/g, compared to that of the Ni/MgAl₂O₄ catalyst. This was attributed to the lack of calcination of the MgO-Al₂O₃ (LDH) support used for the Ni catalyst preparation, which contained MgO and Al₂O₃ in a 30:70 (wt%) ratio. Among the prepared Ni-based catalysts, the Ni/Al₂O₃ catalyst had the lowest metal dispersion of 2.1%, and the metal dispersion of the Ni/MgO catalyst could not be measured because of the high reduction temperature. The Ni/MgO-Al₂O₃ catalyst, which exhibited changes in the textural properties of the support during heat treatment, showed a lower metal dispersion (4.6%) than the Ni/MgAl₂O₄ catalyst (6.6%). In addition, the commercial catalysts had very low metal dispersions, of less than 0.5%, compared those of the prepared catalysts.

The composition and crystalline phases of the catalysts were confirmed by XRD analysis. XRD patterns are shown in Figure 1. The Ni/Al₂O₃ catalyst formed a NiAl₂O₄ spinel crystalline phase. Nurunnabi et al. reported that the NiO peak shifted from 2θ = 43.26° to 2θ = 42.90–43.00° with the formation of a NiO-MgO solid solution [23]. The NiO peak of the prepared Ni/MgO catalyst also shifted to 2θ = 42.76°, owing to the formation of the NiO-MgO solid solution. In particular, Ni/MgO-Al₂O₃, Ni/MgAl₂O₄ catalysts displayed peaks corresponding to MgAl₂O₄ and NiO, and those corresponding to the NiAl₂O₄ and NiO-MgO solid solutions were not detected. The catalysts without alkaline earth metals presented peaks of NiAl₂O₄, but the Ca-containing commercial catalyst B showed a reduced formation of NiAl₂O₄ compared with catalysts using Al₂O₃ as a support. Therefore, the addition of alkaline earth metals effectively suppressed the formation of NiAl₂O₄ and NiO-MgO solid solutions.

H₂-TPR analysis was conducted to analyze the reducibility of the catalysts (Figure 2). Pure NiO (unsupported) can be reduced in the 300–550 °C temperature range, while pure NiAl₂O₄ (unsupported) was reduced in the 730–900 °C temperature range [24,25]. In addition, a reduction peak associated with surface NiO weak interactions was formed in the temperature range of 500–550 °C, while the reduction peak formed in the temperature range of 700–800 °C was attributed to strong interactions between NiO and the support [26,27]. As bulk NiO has a weak interaction with the support, bulk NiO was mainly reduced at 250–500 °C [28]. A peak corresponding to the reduction of the commercial catalyst A was observed at 300–500 °C. The reduction peak corresponding to the commercial catalyst B was observed at 400–800 °C. Therefore, commercial catalysts were confirmed to have NiO with relatively weak interactions and bulk NiO. As confirmed by the XRD results, the Ni/Al₂O₃ catalyst, which was difficult to reduce due to the formation of NiAl₂O₄, underwent reduction at temperatures above 900 °C. In particular, because the Ni/MgO catalyst formed a stable NiO-MgO solid solution, the Ni/MgO catalyst was resistant to reduction up to 1000 °C. The Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts were mainly reduced at 800 °C. As the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts had SMSIs, the Ni active metals were highly dispersed, and both Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ were reduced at higher temperatures compared to the commercial catalysts. In addition, because the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts had a stable spinel structure (MgAl₂O₄) after calcination, the formation of NiAl₂O₄ or NiO-MgO solid solutions was suppressed. Therefore, the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts displayed improved reducibility compared to the Ni/Al₂O₃ and Ni/MgO catalysts.

CO₂-TPD analysis was conducted to determine the basic properties of the catalysts (Figure 3). In general, the catalytic CO₂ adsorption significantly influenced the catalytic activity, as well as the oxygen species formed by the dissociation of the CO₂ molecules, and suppressed carbon deposition [16]. The Ni/Al₂O₃ catalyst employing the acidic Al₂O₃ support showed low CO₂ desorption characteristics of 26 μmol/g (Table 1). The Ni/MgO catalyst presented a CO₂ desorption peak in the 100–700 °C temperature range, and exhibited the highest CO₂ desorption amount (170 μmol/g), owing to the basic properties of the MgO support. In addition, because Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts contained the alkaline earth metal Mg, they exhibited improved CO₂ adsorption properties compared to the Ni/Al₂O₃ catalyst. In particular, Ni/MgAl₂O₄, which had a high metal dispersion and large specific surface area, exhibited a higher CO₂ desorption amount (125 μmol/g) than the Ni/MgO-Al₂O₃ catalyst (110 μmol/g). Commercial catalyst A, which employed the acidic Al₂O₃ support showed poor CO₂ adsorption properties, due to the formation of bulk NiO. The commercial catalyst B presented CO₂ desorption peaks at 200 and 500 °C, and an increased CO₂ desorption amount of 81 μmol/g compared to the catalysts that employed Al₂O₃ as a support, due to the presence of the alkaline earth metal Ca.

2.2. Catalytic Test in DRM

The catalytic activity of the DRM reaction was evaluated in the temperature range of 600–750 °C, and the CH₄ and CO₂ conversions are presented in Figure 4. Since the DRM is an endothermic reaction, it exhibited high CH₄ and CO₂ conversions with increasing reaction temperature. The Ni/MgO catalyst, which formed a NiO-MgO solid solution, was reduced at temperatures above 1000 °C; therefore, the Ni/MgO catalyst did not show catalytic activity in the DRM reaction. Since the Ni/Al₂O₃ catalyst also formed a NiAl₂O₄ spinel, this resulted in low metal dispersion and low reducibility of the catalyst. For this reason, the Ni/Al₂O₃ catalyst showed a low CH₄ conversion (66%) at 750 °C, whereas, Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts exhibited high CH₄ conversions of 78 and 77%, respectively, compared with Ni/Al₂O₃. This is because of the high dispersion of the Ni metals due to the suppression of the formed NiAl₂O₄ and NiO-MgO solid solutions. On the other hand, commercial catalysts A and B showed low CH₄ conversions, due to the formation of bulk NiO, despite the presence of 15 wt% Ni metal. However, because the alkaline earth metal-containing commercial catalyst B formed a lower amount of NiAl₂O₄ compared with the commercial catalyst A, the catalyst B presented relatively high CH₄ and CO₂ conversions. This result indicates that the catalytic characteristics (such as metal dispersion and crystalline phase), which depend on the type of support employed, played an important role in determining the catalytic activity.

H₂ and CO yields and H₂/CO ratio are presented in Figure 4 and

Table 2. H₂/CO ratio of Ni catalysts with various supports (reaction conditions: Temp. = 750 °C, CH₄/CO₂/N₂ = 1/1/1, GHSV = 144,000 mL/g_cat∙h).

	Ni/Al2O3	Ni/MgO	Ni/MgO-Al2O3	Ni/MgAl2O4	Commercial A	Commercial B
H2/CO ratio	0.8	N/A 1	0.9	0.9	0.6	0.8

¹ N/A: Not available.

, respectively. The CO yield (56–86%) was higher than the H₂ yield (34–74%). In addition, the H₂/CO ratio of the produced syngas after the DRM reaction was 0.6–0.9. These results are attributed to the RWGS reaction (Equation (2)) as a side reaction, as mentioned above. Theoretically, the ratio of the H₂/CO produced in the DRM reaction (Equation (1)) is 1. However, the H₂/CO ratio was lower than 1, because the RWGS reaction occurs as a side reaction under the reaction conditions of the DRM process [29]. Among the tested catalysts, Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ presented ratios close to the equilibrium H₂/CO ratio of 0.9.

2.3. Coke Formation Study

To investigate the stability of the reaction and coke resistance, a stability test was conducted in the absence of the diluent (Figure 5). Coke formation reactions of the CH₄ decomposition reaction (Equation (3)) and CO disproportionation reaction (Equation (4)) occur simultaneously in the temperature range of 557–700 °C [10]. Thus, stability tests were performed at 600 °C and a TOS of 20 h. As the Ni/MgO catalyst did not exhibit catalytic activity in the catalytic test, in accordance with the reaction temperature, a stability test was conducted, except for the Ni/MgO catalyst. The Ni/Al₂O₃ catalyst tended toward constantly reduced CH₄ and CO₂ conversions over 20 h. The Ni/Al₂O₃ catalyst had 21% of the initial CH₄ conversion, the Ni/Al₂O₃ catalyst decreased to 16% of CH₄ conversion after 20 h. Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts, which added the Mg alkaline earth metal, had high CH₄ and CO₂ conversions compared to the Ni/Al₂O₃ catalyst, owing to the improved reducibility of the catalyst and high metal dispersion. In particular, the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts showed higher CH₄ and CO₂ conversions than the commercial catalyst A. On the other hand, the commercial catalyst B exhibited a CH₄ conversion of 20–22%, and had a higher CH₄ conversion than the catalyst that employed Al₂O₃ as a support.

To investigate the carbon formed on the surface of the catalyst, SEM and TGA analyses were performed on the recovered spent catalysts. The morphologies of the formed carbon were analyzed using SEM images, as shown in Figure 6. Sehested reported that three types of carbon were formed during reforming reactions: pyrolytic carbon, encapsulating carbon, and filamentous carbon [30]. Pyrolytic carbon is formed at temperatures higher than 600 °C, owing to thermal decomposition of hydrocarbon compounds. Since the encapsulating carbon covered particles of the catalyst, it caused immediate catalyst deactivation. Filamentous carbon is formed and grown between the Ni particles and the support, which causes catalyst pore plugging. For this reason, filamentous carbon results in catalyst deactivation, causing catalyst destruction and a pressure drop. The formation of filamentous carbon was also confirmed on the surface of the spent catalysts after the stability test, and SEM images of all the analyzed catalysts indicated the formation of a large amount of filamentous carbon on the catalyst surface.

Quantitative analysis of the amount of deposited carbon was performed through weight loss analysis using TGA up to 800 °C. The TGA results are shown in Figure 7 and

Table 3. Amount of coke over Ni catalysts from TG profiles (reaction conditions: Temp. = 600 °C, CH₄/CO₂/N₂ = 1/1/1, GHSV = 144,000 mL/g_cat∙h, TOS = 20 h).

Catalysts	Amount of Coke 1 [wt%]
Ni/MgO-Al2O3	21.4
Ni/MgAl2O4	29.3
Commercial B	47.1

¹ Calculated from the following equation; $\left(\mathrm{Amount} \mathrm{of} \mathrm{coke}\right)=\frac{\left(sample weight at 50 °\mathrm{C}\right)-\left(sample weight at 800 °\mathrm{C}\right)}{\left(sample weight at 50 °\mathrm{C}\right)}\times 100$.

. The weight loss of the spent catalyst was classified according to the temperature range; desorption of H₂O and CO₂ occurred in the temperature range of 250–300 °C, and the oxidation of carbon due to the release of CO and CO₂ occurred in the temperature range of 480–640 °C [31]. In addition, graphitic filamentous carbon is mainly oxidized at a high temperature of 530 °C [32]. Quantitative analysis of the formed carbon was performed using the recovered spent catalysts containing alkaline earth metals, which showed a relatively high catalytic activity during the stability test. Results from the TG profiles indicated that most of the weight loss of the spent catalysts occurred at temperatures above 500 °C. The amount of deposited carbon was calculated from the weight loss, in the temperature range of 50–800 °C (Table 3). The amount of deposited carbon on the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts was 21.4 and 29.3 wt%, respectively. In comparison, the commercial catalyst B contained twice the amount of deposited carbon (47.1 wt%). These results are related to the CO₂ adsorption characteristics of the catalysts [33]. Since CO₂ is an acidic gas, CO₂ adsorption was enhanced by the addition of basic materials, such as Mg and Ca. In addition, oxygen species produced by the dissociation of CO₂ molecules are used in the oxidation of deposited carbon on the catalyst surface. Therefore, increasing the basic properties of the DRM catalyst could improve catalyst durability. Therefore, commercial catalyst B, which showed relatively low CO₂ adsorption properties (81 μmol/g), contained a large amount of carbon compared to the Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts, which had high CO₂ adsorption properties, of 110 and 125 μmol/g, respectively.

3. Materials and Methods

3.1. Materials

Ni(NO₃)₂∙6H₂O (97%, Junsei) was used as the active metal precursor; γ-Al₂O₃ (PURALOX TH 100/150, SASOL), MgO (≥99%, Sigma-Aldrich), MgO-Al₂O₃ hydrotalcite (MgO:Al₂O₃ = 30:70, SASOL), and MgAl₂O₄ (MG30 pre-calcined at 800 °C, SASOL) were used as supports for the Ni-based catalyst. Ni-based commercial catalysts Ni/Al₂O₃ (FCR-4; Clariant) and Ni/CaAl₂O₄ (R-70; Clariant)) were used as reference catalysts.

3.2. Catalyst Preparation

Ni-based catalysts were prepared using the incipient wetness impregnation method. Nickel nitrate was dissolved in distilled water and impregnated with γ-Al₂O₃. The prepared Ni/Al₂O₃ catalyst was dried overnight at room temperature, followed by calcination at 800 °C for 6 h in air. Ni/MgO, Ni/MgO-Al₂O₃, and Ni/MgAl₂O₄ catalysts were also prepared by the same method, and 10 wt% Ni active metal was supported on the catalysts.

3.3. Characteristics

The active metal content supported on the catalyst was analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES, AVIO500, Perkin Elmer). The crystalline phases and composition of the catalysts were analyzed using an X-ray diffractometer (XRD, SmartLab High Temp, Rigaku, Tokyo, Japan). Diffraction peaks were obtained in the 2θ range of 10–90° with a step size of 0.02°. The specific surface area and pore size distribution of the catalyst were obtained using the Brunauer–Emmett–Teller and BarrettJoyner–Halenda methods (BET-BJH, BELSORP-MAX, MicrotracBEL Corp., Osaka, Japan). A sample (0.2 g) was pretreated at 300 °C for 3 h. N₂ adsorption/desorption isotherms and pore size distributions were analyzed at −196 °C using liquid nitrogen. H₂-chemisorption (BEL-METAL-3, MicrotracBEL Corp., Osaka, Japan) was performed to determine the metal dispersion of the catalyst. The sample (50 mg) was reduced at 700 °C for 2 h under H₂ flow, and H₂ gas was adsorbed at 50 °C using 20% H₂/Ar gas. H₂-temperature programmed reduction (H₂-TPR, BELCAT-B, MicrotracBEL Corp., Osaka, Japan) analysis was performed to investigate the reducibility of the catalyst and the interaction between the active metal and support. The analysis was performed by increasing the temperature up to 1000 °C (10 °C/min) under 10% H₂/Ar flow using a 50 mg sample. The CO₂ adsorption properties were analyzed through CO₂-temperature programmed desorption (CO₂-TPD, BELCAT-B, MicrotracBEL Corp., Osaka, Japan) analysis. The sample (50 mg) was reduced at 700 °C for 2 h under 10% H₂/Ar flow. The CO₂ was adsorbed at 50 °C using 10% CO₂/He gas and desorbed by increasing the temperature to 800 °C (10 °C/min). In addition, the morphology of the formed carbon in the spent catalysts was analyzed using scanning electron microscope (SEM, Regulus 8220, HITACHI, Tokyo, Japan) analysis. Quantitative analysis of carbon on the spent catalysts was performed using thermogravimetric analysis (TGA, TGA 92-18, Setaram, Caluire-et-Cuire, France). The weight loss was measured by loading 7 mg of the sample. The sample was oxidized by increasing the temperature to 800 °C at 10 °C/min in air.

3.4. Catalytic Tests

Catalytic activity was evaluated in a fixed bed reactor system. The 3/8′′ quartz reactor was filled with a catalyst (50 mg, 150–250 μm) and 1100 °C pre-calcined MgAl₂O₄ as a diluent (500 mg, 150–250 μm) in a ratio of 1:10. Before the catalytic test, the catalyst was reduced at 700 °C for 2 h with 10 vol.% H₂/N₂ gas. The evaluation of the catalytic activity was performed in a temperature range of 600–750 °C. The feeds of CH₄, CO₂, and N₂ were injected to equal ratio. The total flow rate was 120 mL/min, and GHSV corresponded to 144,000 mL/g_cat∙h. The reaction temperature was controlled using a K-type thermocouple (TC), the TC was located inside the catalyst layer. The produced gas was detected using a micro-gas chromatograph (3000A Micro GC, Agilent, CA, USA) equipped with a thermal conductivity detector (TCD) using MolSeive 5A and PlotU columns.

4. Conclusions

DRM reaction over Ni-based catalysts was studied as a function of the type of catalyst support used (Al₂O₃, MgO, MgO-Al₂O₃ (LDH), and MgAl₂O₄ (spinel)). The type of support significantly influenced the reducibility and metal dispersion of the Ni-based catalysts. The Ni/Al₂O₃ and Ni/MgO catalysts had low metal dispersion, due to the formation of NiAl₂O₄ and NiO-MgO solid solutions, respectively. In addition, these catalysts had a strong interaction between the Ni active metal and support, indicated by the high temperature (more than 900 °C) required for reduction. In contrast, Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts suppressed the formation of NiAl₂O₄ and NiO-MgO solid solutions, exhibited high metal dispersion, and were reduced at lower temperatures compared with the Ni/Al₂O₃ and Ni/MgO catalysts. Moreover, Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts containing Mg had enhanced CO₂ adsorption properties. In the DRM reaction, Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts exhibited high CH₄ conversion (>75%) at 750 °C compared to commercial catalysts. Furthermore, Ni/MgO-Al₂O₃ and Ni/MgAl₂O₄ catalysts showed enhanced coke resistance compared to commercial catalyst B. Therefore, the metal dispersion, basic properties, and metal-support interaction of the catalyst in accordance with the type of support were confirmed to significantly influence the catalyst performance in the DRM reaction.