CO₂ Reforming of CH₄ Using Coke Oven Gas over Ni/MgO-Al₂O₃ Catalysts: Effect of the MgO:Al₂O₃ Ratio

Abstract

Research is being actively conducted to improve the carbon deposition and sintering resistance of Ni-based catalysts. Among them, the Al₂O₃-supported Ni catalyst has been broadly studied for the dry reforming reaction due to its high CH₄ activity at the beginning of the reaction. However, there is a problem of deactivation due to carbon deposition of Ni/Al₂O₃ catalyst and sintering of Ni, which is a catalytically active material. Supplementing MgO in Ni/Al₂O₃ catalyst can result in an improved MgAl₂O₄ spinel structure and basicity, which can be helpful for the activation of methane and carbon dioxide molecules. In order to confirm the optimal supports’ ratio in Ni/MgO-Al₂O₃ catalysts, the catalysts were prepared by supporting Ni after controlling the MgO:Al₂O₃ ratio stepwise, and the prepared catalysts were used for CO₂ reforming of CH₄ (CDR) using coke oven gas (COG). The catalytic reaction was conducted at 800 °C and at a high gas hourly space velocity (GHSV = 1,500,000 h⁻¹) to screen the catalytic performance. The Ni/MgO-Al₂O₃ (MgO:Al₂O₃ = 3:7) catalyst showed the best catalytic performance between prepared catalysts. From this study, the ratio of MgO:Al₂O₃ was confirmed to affect not only the basicity of the catalyst but also the dispersion of the catalyst and the reducing property of the catalyst surface.

1. Introduction

Since the adoption of the Paris Agreement on Climate Change in 2015, it has been necessary to put in place measures to cut down greenhouse gas emissions to limit the increase in the global average temperature to 1.5 °C relative to preindustrial levels [1,2]. To achieve the CO₂ levels required by the Paris Agreement, we must reach zero emissions by 2050 [1]. However, zeroing greenhouse gases is still difficult due to the following issues. First, as the population increases, energy consumption continues to increase [3,4]. Among them, fossil fuels account for 81% of the total energy consumption [5]. Second, some carbon-based resources are essential for industries, such as steel making, where high-temperature heat is required [6,7,8]. Copious amounts of by-product gas are generated in each unit process in the steel industry, which is the core industry of our society [9]. Greenhouse gas emissions in the form of by-products accounted for approximately 9% of global emissions from 1900 to 2015 [10]. The by-product gas of the steel industry is largely divided into coke oven gas (COG), which is generated in the process of producing coke by oxidizing coal in a coke oven, and CO₂ generated in a blast furnace when iron oxide is reduced to iron [11]. Coke oven gas, a representative steel by-product gas, contains approximately 27% methane (CH₄), which causes greenhouse gas problems when emitted (coke oven gas composition: CH₄: 28.0%, CO₂: 2.6%, CO: 8.0%, H₂: 55.6%, N₂: 5.8%) [12]. A steam-reforming reaction (SRM: CH₄ + H₂O → CO + 3H₂) that can produce syngas from methane is being studied to utilize COG, but there are economic problems with the additional supply of steam [13,14].

The CO₂ reforming of CH₄ (CDR: CH₄ + CO₂ → 2CO + 2H₂) has the advantage of using CH₄ and CO₂ as reactants to produce syngas, which is an elemental raw material with high added value [15]. In addition, the syngas produced by the dry reforming reaction (H₂:CO = 1:1) shows a favorable H₂:CO ratio for oxo compound synthesis compared to the syngas produced through the steam reforming reaction (H₂:CO = 3:1) [16]. However, as the dry reforming reaction proceeds at high temperatures above 700 °C, favorable conditions are formed for sintering of active metals and supports to occur, and catalyst deactivation may occur due to carbon deposition by methane decomposition [17]. In addition, the dry reforming reaction is a strong endothermic reaction, partially lowering the catalyst bed temperature and promoting the occurrence of the Boudouard reaction (2CO → C + CO₂), causing carbon deposition on the catalyst surface, thereby deactivating the catalyst [18,19]. Therefore, to overcome these problems, it is necessary to develop a catalyst for dry reforming with high sintering and carbon deposition resistances.

Various studies have been conducted using noble metal (Ir, Rh, Ru, Pt, and Pd) catalysts, and strong sintering resistance and carbon deposition resistance have been demonstrated in the dry reforming reaction. However, there is a problem in entering the commercialization stage due to the low economic feasibility of the noble metal catalyst [20,21]. Therefore, although research on economical nickel (Ni)-based catalysts exhibiting excellent activity in dry reforming reactions is being actively conducted, the problem of catalyst deactivation due to the low sintering resistance and low carbon deposition resistance of Ni has become a major issue [22,23,24,25,26].

Research is being actively conducted to improve the catalyst deactivation problem of Ni-based catalysts. Out of them, the Al₂O₃-supported Ni catalyst has been broadly studied for the dry reforming reaction due to its high CH₄ activity at the beginning of the reaction [27,28,29].

However, there is a problem of deactivation due to carbon deposition of the Ni/Al₂O₃ catalyst and sintering of Ni, which is a catalytically active material [30].

Therefore, many studies have been conducted to improve the performance of Ni/Al₂O₃ catalysts. Alipour et al. reported that adding alkaline earth metal oxides (CaO or MgO), dopants can enhance the interaction between Ni and the Al₂O₃ support [31]. Sengupta et al. demonstrated that the surface basicity of the alkaline earth metal oxide can improve the activity of methane molecules, thereby alleviating the problem of carbon deposition in the catalyst during the dry reforming reaction [32]. Guo et al. found that MgAl₂O₄ can be used as a support for the dry reforming catalytic reaction of Ni because of its high melting point, good chemical stability, and low acidity [33].

Based on the studies outlined above, the supplementing of MgO in Ni/Al₂O₃ catalyst can result in an improved MgAl₂O₄ spinel structure and basicity, which can be helpful for the activation of CH₄ and CO₂ molecules.

In this study, Ni/MgO-Al₂O₃ catalysts prepared with different MgO:Al₂O₃ ratios were used with coke oven gas (COG) to compare their dry reforming performance (gas hourly space velocity (GHSV) = 1,500,000 h⁻¹) and to investigate how that performance relates to the characteristics of the catalysts. The correlations between the catalytic performance and the physicochemical properties were investigated.

2. Results and Discussion

MgO-Al₂O₃ was prepared by the one-step coprecipitation method with different MgO:Al₂O₃ ratios as a support. A Ni/MG catalyst was prepared by supporting a Ni precursor aqueous solution on the prepared MgO-Al₂O₃ support by the impregnation method, as shown in Scheme 1. Before chemisorption and CDR reaction, in situ reduction was performed for 3 h at 800 °C. Prior to XRD and TEM analyses, ex situ reduction was carried out for 3 h at 800 °C, and passivation was conducted for 3 h at 25 °C (Scheme 1).

2.1. Catalyst Characterization

The BET surface areas (S.A.) of catalysts are listed in

Table 1. Characteristics of Ni/MG catalysts with different MgO:Al₂O₃ ratios.

Catalyst	Support BET S.A. (m2/g) 1	Catalyst BET S.A. (m2/g) 1	Ni Dispersion (%) 2	Crystallite Size of Ni0 (nm) 3
Ni/MG10	202.6	139.2	1.46	13.6
Ni/MG30	200.9	120.8	3.41	9.4
Ni/MG50	164.2	104.2	3.32	9.7
Ni/MG70	161.5	104.0	3.18	9.9
Ni/MG90	102.0	76.6	3.24	11.9

¹ Predicted from N₂ adsorption–desorption at −196 °C. ² Predicted from H₂-chemisorption contemplating the Ni reduction degree. ³ Calculated from the Scherrer equation based on the XRD result.

. The BET S.A. of the prepared MgO-Al₂O₃ support decreased as the MgO content increased and further decreased as Ni was loaded onto the MgO-Al₂O₃ support. As a result, the BET S.A. of the prepared Ni/MG catalyst was shown to have the following order: Ni/MG10 (139.2 m²/g) > Ni/MG30 (120.8 m²/g) > Ni/MG50 (104.2 m²/g) > Ni/MG70 (104.0 m²/g) > Ni/MG90 (76.6 m²/g). When NiO is impregnated on a support, the BET S.A. of the catalyst has been shown to be reduced compared to that of the support because NiO fills the pores of the support [34,35,36]. Figure S1 shows the nitrogen isotherm adsorption and desorption results of the catalysts. The type IV hysteresis loop is identified in all catalysts, which indicates that the catalysts have mesoporous structures [16]. The mesoporous structure of Ni-based catalysts is known to provide accessible Ni active centers and to stabilize Ni particles by confinement effect [18]. The dispersion of Ni from the prepared catalysts was calculated using the results of the H₂-chemisorption analysis and is shown in Table 1. A relatively high level of Ni dispersion was confirmed in the Ni/MG30, Ni/MG50, Ni/MG70, and Ni/MG90 catalysts compared to the Ni/MG10 catalyst. The Ni/MG30 catalyst showed the highest Ni dispersion (3.41%) among the prepared catalysts.

Figure 1 demonstrates the XRD analysis results of the Ni/MG catalysts by diverse MgO:Al₂O₃ ratios. Peaks at 31.4, 37.0, 45.0, 59.7, and 65.5° were attributed to spinel NiAl₂O₄ (JCPDS #10-0339), and only the highest Al₂O₃-containing Ni/MG10 catalyst was identified. Peaks at 31.3, 36.9, 44.8, 59.4, and 65.2° were assigned to the spinel MgAl₂O₄ (JCPDS #21-1152) crystals, which can be identified with the increasing MgO content. The Ni/MG30 catalyst exhibited the strongest spinel MgAl₂O₄ peak intensity. The peaks at approximately 37, 43, 62, 75, and 79° were due to the cubic structure of unreduced NiO (JCPDS #04-0835) and MgO (JCPDS #65-0476). It was difficult to differentiate the positions of the NiO and MgO XRD diffraction peaks. As the MgO content increased, the intensity of the peak corresponding to the NiO-MgO solid solution produced by the interaction of NiO and MgO remaining after MgAl₂O₄ formation increased. All the reduced catalysts exhibited Ni⁰ peaks at 44.5, 51.8, and 76.4°, which is an active phase during dry reforming of COG [14]. The crystallite size of Ni⁰ was calculated from the peak (51.8°) assigned to the Ni⁰ (2 0 0) plane using the Scherrer equation due to the peak overlap at the Ni⁰ (1 1 1) plane peak, and the results are given in Table 1. In the prepared catalysts, the Ni/MG30 catalyst exhibited the smallest Ni crystal size (9.4 nm).

The oxidation/reduction properties of the catalysts were the primary factors influencing the catalyst activity in reforming reactions. TPR analysis was organized to understand the oxidation/reduction characteristics of the Ni/MG catalysts prepared with various MgO:Al₂O₃ ratios, and the results are shown in Figure 2. All prepared catalysts demonstrated large reduction peaks at 700–850 °C [27]. This was a peak owing to the reduction of the complex NiO species that interacted strongly with the support.

The composite NiO reduction peak of the Ni/MG catalyst at low temperature appeared with the following order: Ni/MG30 < Ni/MG10 < Ni/MG70 < Ni/MG50 < Ni/MG90. In the case of catalyst Ni/MG10, a high Al₂O₃ content was reported to be advantageous for forming a NiAl₂O₄ spinel structure; and when the NiAl₂O₄ spinel structure was formed, a reduced peak appeared at high temperature because of the strong interaction between NiO and Al₂O₃ [37,38,39]. On the other hand, Ni/MG30 was reduced at a lower temperature than the Ni/MG10 catalyst by the reduction of the NiO interacting with the MgAl₂O₄ spinel structure. However, with an increase in MgO, the reduction peak shifted to a higher temperature because of the strong interaction following the formation of the NiO-MgO solid solution [40,41].

Table 2. Reduction degree results of Ni/MG catalysts with different MgO/Al₂O₃ ratios.

Catalyst	Ni/MG10	Ni/MG30	Ni/MG50	Ni/MG70	Ni/MG90
Reduction degree (%) 1	84	96	88	67	51

¹ Calculated from (H₂ consumption amount (800 °C)/H₂ consumption amount (1000 °C)) × 100.

shows the results of the reduction degree analysis for Ni/MG catalysts under the reaction conditions. The reduction degree value is closely related to the number of active species in the reaction conditions. Therefore, the Ni/MG30 catalyst showing the highest value was expected to show the highest activity when applied to the dry reforming reaction.

CO₂-TPD analysis was used to examine the interaction of CO₂ with the synthesized Ni/MG catalyst. As shown in Figure 3, the Ni/MG catalyst exhibited a major desorption peak at 100–400 °C, which means that there was a strong basic site on the Ni/MG surface. According to the literature, the chemical properties of the support, especially the basicity, promoted the activation of CO₂ in the dry reforming reaction [42,43]. In addition, the conversion of both CH₄ and CO₂ was improved by increasing the removal rate of surface carbonaceous species, such as CH_x (x = 0–3), generated by the dehydrogenation of CH₄. The high CO₂ adsorption capacity also improved the coke resistance of the catalyst [42,43].

Table 3. CO₂-TPD results of MG supports and Ni/MG catalysts with different MgO:Al₂O₃ ratios.

Catalyst	Support Desorbed CO2 (cm3/gcat)	Total Desorbed CO2 (cm3/gcat)
Ni/MG10	18.0	6.1
Ni/MG30	58.8	9.7
Ni/MG50	71.2	8.2
Ni/MG70	154.3	8.0
Ni/MG90	39.8	7.7

shows the amount of CO₂ desorbed by the prepared MG supports and Ni/MG catalysts. In the MG supports, as the ratio of MgO increased, the basicity of the support increased stepwise and then decreased again at the ratio of MgO:Al₂O₃ = 9:1. The Ni/MG30 catalyst exhibited the highest CO₂ adsorption amount (9.7 cm³/gcat).

TEM and EDS mapping analysis was performed to confirm the supported state and composition of the prepared catalyst. Figure 4 shows the TEM and EDS mapping images of Ni/MG30 catalyst. The composition of the prepared Ni/MG catalyst followed (Ni: 13.28%, Mg: 28.01%, Al: 58.70%).

The elemental composition analysis of the Ni/MG catalyst prepared by inductively coupled plasma (ICP) was analyzed and is listed in Table S1. In ICP analysis results, for all the prepared catalysts, almost same catalyst composition was achieved, which is what we intended.

2.2. Reaction Results

Figure 5 shows the interrelationship between the physicochemical properties and the catalytic performance of the Ni/MG catalysts with various MgO:Al₂O₃ ratios. The BET S.A. of the prepared MgO-Al₂O₃ support decreased as the MgO content increased. In the case of the Ni/MG10 catalyst with the highest Al₂O₃ content in the support, the complex NiO reduction temperature increased, according to the formation of spinel NiAl₂O₄, and the catalyst showed low dispersion and a large crystallite size. As the MgO content increased, the complex NiO reduction temperature of complex NiO decreased according to the MgAl₂O₄ production. In the case of an excessive MgO content, a NiO-MgO solid solution, in which the excess MgO strongly interacted with NiO, was formed, the NiO reduction temperature increased to a high temperature, the dispersion decreased, and the crystallite size increased. The Ni/MG30 catalyst showed the highest CO₂ adsorption amount. As a result, the Ni/MG30 catalyst showed an appropriate correlation between complex NiO and the MgO-Al₂O_3, having the largest number of Ni⁰ active sites.

Figure 6 shows the CH₄ conversion of Ni/MG catalysts. To screen the prepared catalyst performance, the reaction was performed under high GHSV conditions (GHSV = 1,500,000 h⁻¹) at 800 °C. As a result of the reaction, the Ni/MG30 catalyst had the highest CH₄ conversion (84.2%), and the Ni/MG90 catalyst had the lowest CH₄ conversion (66.6%). Even at high GHSV, all the prepared catalysts showed stable activity for 12 h.

Figure 7 shows the CO₂ conversion of the Ni/MG catalysts. The CO₂ conversion was approximately 3% higher than the CH₄ conversion, and the H₂ yield was lower than the theoretical H₂ yield (H₂ yield (%): Ni/MG10 (77.6), Ni/MG30 (81.9), Ni/MG50 (79.8), Ni/MG70 (79.4), Ni/MG90 (53.9)). This was judged to be the result of the reverse water gas shift reaction (RWGS: H₂ + CO₂ → H₂O + CO) during the dry reforming reaction [44]. The detailed reaction results, including the feed gas and outlet gas compositions, are shown in

Table 4. CO₂ reforming of CH₄ using COG over Ni/MG30 catalyst (reaction results for 12 h).

Gas Composition (%) 1
Component	CH4	CO2	N2	H2	CO	Total
Reactant gas (COG) + CO2	21.39	25.67	4.46	42.39	6.10	100.00
Product gas	2.65	2.68	3.53	55.27	35.87	100.00
Yield (%)		Conversion (%)		H2/CO ratio
H2	CO	CH4	CO2	Reactant gas (COG) + CO2		Product gas
64.0	83.2	84.4	86.8	6.95		1.54

¹ Evaluated by micro-GC.

.

3. Materials and Methods

3.1. Preparation of Catalysts

MgO–Al₂O₃ was prepared using a one-step coprecipitation method with different MgO:Al₂O₃ ratios as a support. Mg(NO₃)₂·6H₂O (99%, Aldrich) and Al(NO₃)₂·6H₂O (98%, Aldrich) were used as the precursors. KOH (95%, Samchun) was utilized as the precipitation agent. To support preparation, the precursors were stoichiometrically quantified and then dissolved in distilled water and a constant temperature at 80 °C. In a precipitation step, 15 wt% KOH solution was added at 0.8 mL/min to reach pH 10.5. The prepared solution was aged for 3 days and was then washed several times with distilled water to remove impurities, including the remaining K⁺ and NO^3- ions. The precipitate was sufficiently dried at 100 °C to remove moisture. After that, it was calcined at 800 °C for 6 h. The Ni/MgO-Al₂O₃ catalyst was prepared through an impregnation method by supporting the Ni(NO₃)₂·6H₂O (97%, Junsei) on the prepared MgO-Al₂O₃ support. The Ni loading was 15 wt. %, and the catalyst was dried at 100 °C and then calcined at 800 °C for 6 h. The finished Ni/MgO-Al₂O₃ catalyst was named according to the MgO:Al₂O₃ ratio, as indicated in

Table 5. Ni/MgO-Al₂O₃ catalysts’ naming convention.

MgO:Al2O3	Name
1:9	Ni/MG10
3:7	Ni/MG30
5:5	Ni/MG50
7:3	Ni/MG70
9:1	Ni/MG90

.

3.2. Catalyst Characterization

Transmission electron microscopy (TEM) images of the samples were obtained using a JEM-F200 (JEOL, Japan) microscope. Samples were blended with ethanol by using ultra-sonication for 30 min. The prepared suspension was deposited on a grid. The elemental composition of prepared catalysts was analyzed by an inductively coupled, plasma-optical emission spectrometer (Varian, USA). The Brunauer–Emmett–Teller specific surface areas of the samples were investigated by the N₂ adsorption/desorption isotherms at −196 °C using an ASAP 2010 (Micromeritics, USA). X-ray diffraction (XRD) patterns were analyzed using a Rigaku Ultima IV diffractometer (RIGAKU, Japan). Detailed procedures of XRD and Ni⁰ crystallite size calculation method were reported in our previous work [14]. H₂-chemisorption data were obtained using an Autochem 2920 (Micromeritics, USA). The redox properties of the catalysts were evaluated via H₂ temperature-programmed reduction (H₂-TPR) using Autochem 2920. The reduction degree was estimated using the following equation, derived from the integration of the peak area below the TPR at 800 °C and 1000 °C:

$$\mathrm{Reduction} \mathrm{degree} \left(\%\right)=\frac{{\mathrm{H}}_2 \mathrm{consumption} \mathrm{amount} \left(800 °\mathrm{C}\right)}{{\mathrm{H}}_2 \mathrm{consumption} \mathrm{amount} \left(1000 °\mathrm{C}\right)}\times 100$$

H₂-chemisorption data were obtained using an Autochem 2920 (Micromeritics, USA). Detailed procedures of H₂-chemisorption were reported in our previous work [14]. CO₂ temperature-programmed desorption (CO₂-TPD) analyses were performed on an Autochem 2920 (Micromeritics, USA) instrument. In the pretreatment step, samples were fixed in a quartz U-tube and exposed with He at 300 °C for 10 min. The CO₂ adsorption step was performed in 10% CO₂/He for 30 min at 100 °C. Subsequently, physically adsorbed CO₂ was removed by purging with He for 1 h, and the treated samples were heated in a He flow from 100 to 1000 °C at a rate of 10 °C/min. The desorbed CO₂ was measured by a thermal conductivity detector (TCD).

3.3. Catalyst activity

The catalytic activity tests were performed under atmospheric pressure at 800 °C using a fixed-bed tubular quartz reactor with an inner diameter of 4 mm. The reactor temperature was monitored in real time and controlled by a temperature controller box. During the test procedure, 10 mg of the catalyst, 150 mg of the diluent, and a simulated coke oven gas (as the reactant gas: CH₄: 28.04%, CO₂: 2.56%, CO: 7.99%, H₂: 55.57%, N₂: 5.84%) were used, and CO₂ was supplied to increase the CO₂:CH₄ ratio to 1.2:1. The catalyst was loaded into the tubular quartz reactor, and a catalytic test was conducted at 800 °C. A high GHSV of 1.5 million h⁻¹ was applied for 12 h to screen the activity. Before each measurement, the sample was reduced in situ at 800 °C for 3 h under 5% H₂/N₂ conditions. Each gas was controlled using an independent mass flow controller (MFC). Effluent gases from the reactor were chilled and allowed to pass through a moisture trap to remove the remaining H₂O. Then, the product gases were analyzed online using a micro-gas chromatograph (Agilent 3000, USA). The CH₄, CO₂ conversions, H₂, CO yields, and H₂/CO ratio were defined as follows:

$$\mathrm{C}{\mathrm{H}}_4 \mathrm{conversion} \left(\%\right)=\frac{{\left[\mathrm{C}{\mathrm{H}}_4\right]}_{\mathrm{in}}-{\left[\mathrm{C}{\mathrm{H}}_4\right]}_{\mathrm{out}}}{{\left[\mathrm{C}{\mathrm{H}}_4\right]}_{\mathrm{in}}}\times 100$$

$$\mathrm{C}{\mathrm{O}}_2 \mathrm{conversion} \left(\%\right)=\frac{{\left[\mathrm{C}{\mathrm{O}}_2\right]}_{\mathrm{in}}-{\left[\mathrm{C}{\mathrm{O}}_2\right]}_{\mathrm{out}}}{{\left[\mathrm{C}{\mathrm{O}}_2\right]}_{\mathrm{in}}}\times 100$$

$${\mathrm{H}}_2 \mathrm{yield} \left(\%\right)=\frac{{\mathrm{H}}_2{}_{\mathrm{out}}-{\mathrm{H}}_2{}_{\mathrm{in}}}{2\times \mathrm{C}{{\mathrm{H}}_4}_{\mathrm{in}}}\times 100$$

$$\mathrm{CO} \mathrm{yield} \left(\%\right)=\frac{\mathrm{C}{\mathrm{O}}_{\mathrm{out}}-\mathrm{C}{\mathrm{O}}_{\mathrm{in}}}{\mathrm{C}{{\mathrm{H}}_4}_{\mathrm{in}}+\mathrm{C}{{\mathrm{O}}_2}_{\mathrm{in}}}\times 100$$

$${\mathrm{H}}_2/\mathrm{CO} \mathrm{ratio}=\frac{{\left[{\mathrm{H}}_2\right]}_{\mathrm{out}}}{{\left[\mathrm{CO}\right]}_{\mathrm{out}}}$$

where [X]_in and [X]_out are the inlet and outlet concentrations of X, respectively. In the same way, X_in and X_out are the inlet and outlet amounts of X, respectively.

4. Conclusions

To confirm the effect of the MgO:Al₂O₃ ratio in the Ni/MG catalyst for the dry reforming of COG, Ni/MG catalysts were prepared with varying MgO:Al₂O₃ ratios (1:9, 3:7, 5:5, 7:3, 9:1). The Ni/MG catalyst prepared with a MgO:Al₂O₃ ratio of 3:7 (Ni/MG30) showed the highest CH₄ and CO₂ conversion (84.2%, 86.9%) even at high GHSV (1.5 million h⁻¹) and was selected as the optimal production ratio.

In the case of the Ni/MG10 catalyst with the highest Al₂O₃ content in the support, the complex NiO reduction temperature was increased. In addition, it showed the formation of spinel NiAl₂O₄, with low dispersion and a large crystallite size. As the MgO content increased, the reduction temperature of complex NiO in Ni/MG30 catalyst decreased according to MgAl₂O₄ production. In the case of an excessive MgO content, a NiO-MgO solid solution, in which the excess MgO strongly interacted with NiO was formed, the NiO reduction temperature increased to a high temperature, the dispersion degree decreased, and the crystallite size increased. This resulted in the Ni/MG30 catalyst having the highest number of Ni⁰ active sites. As a result, the Ni/MG30 catalyst showed an appropriate correlation between complex NiO and the MgO-Al₂O₃ and showed the highest basicity. Also, due to the high reducibility and dispersibility of the catalyst surface, it has the largest number of Ni⁰ active sites. Consequently, the Ni/MG30 catalyst showed the highest catalytic activity between the prepared catalysts.