Efficient and Stable Ni/SBA-15 Catalyst for Dry Reforming of Methane: Effect of Citric Acid Concentration

Abstract

Citric acid, one of the representative chelate compounds, has been widely used as an additive to achieve the highly dispersed metal-supported catalysts. This study aimed to investigate the effect of citric acid concentration on the preparation of the highly dispersed Ni catalysts on mesoporous silica (SBA-15) for the dry reforming of methane. A series of Ni/SBA-15 catalysts with citric acid were prepared using the acid-assisted incipient wetness impregnation method, and the Ni/SBA-15 catalyst as a reference was synthesized via the impregnation method. First of all, the citric acid addition during the catalyst synthesis step regardless of its concentration resulted in highly dispersed Ni particles of ~4–7 nm in size in Ni/SBA-15 catalysts, which had a superior and stable catalytic performance in the dry reforming of methane (93% of CO₂ conversion and 86% of CH₄ conversion). In addition, the amount of coke formation was much lower in a series of Ni/SBA-15 catalysts with citric acid (~2–5 mg_coke g_cat⁻¹ h⁻¹) compared to pristine Ni/SBA-15 catalysts (~22 mg_coke g_cat⁻¹ h⁻¹). However, when the concentration of citric acid became higher, the more free NiO species that formed on the SBA-15 support, leading to large Ni particles after the stability test. The addition of citric acid is a very clear strategy for making highly dispersed catalysts, but its concentration needs to be carefully controlled.

1. Introduction

The rapid growth of the world’s economy and the increase in population has led to a sharp increase in energy consumption [1,2,3]. This has resulted in the accumulation of greenhouse gases such as carbon dioxide (CO₂) and methane (CH₄) in the atmosphere, which have become the main cause of climate change [1,2,3]. This causes extreme natural phenomena, such as severe flooding and drought, which have an amplified impact on the ecosystem, negatively influencing biodiversity [2,3]. To alleviate this global warming and climate change, therefore, scientists have suggested a method by which to utilize CO₂ and CH₄ as resources for syngas (CO and H₂) production, which can be potentially be converted into heavier weight or partially oxidized hydrocarbons [1,2].

The dry reforming of methane (DRM) is the one of the main methods that consumes the two major greenhouse gases (CO₂ and CH₄) in order to produce syngas (CO₂ + CH₄ → 2CO + 2H₂) [1,2,4]. DRM produces a ratio of H₂/CO that is equal to one, which can be easily utilized in industry [1,2]. For this process, catalysts are an essential entity, as they are responsible for altering the reaction rate, thus improving the syngas production without being consumed [4,5]. DRM reactions can be catalyzed by metals such as Pt, Pd, Ru, Co, and Ni, and they are usually mounted on Al₂O₃, SiO₂, CeO₂, ZrO₂, La₂O₃, and TiO₂ [1,4,5,6,7,8,9,10,11,12]. It is well known that precious metals are highly active in DRM, but those expensive noble metals have provoked the search for new catalysts made of low-cost and earth-abundant elements [1,4,5,9]. In addition to the active metals, the properties of the support have a great effect on the dispersion, reducibility, morphology, and oxidation state of the active metal, which influences the final performance of the catalyst [1,4,5,13]. Hence, it is necessary to carefully check the combination between the support and active metal.

According to the literature, controlling the metal particle size is an effective approach by which to design a high-performance catalyst and many efforts have been devoted to achieving a high metal dispersion [1,12,14,15,16]. Moreover, reducing the particle size could improve the metal support interaction, influencing the oxidation state of the active metal on the support and the catalytic efficiency [16]. The facile method employed to control the Ni particle size uses citric acid as an additive [16,17]. The addition of citric acid during the Ni impregnation step has been reported to be an effective strategy by which to enhance the metal dispersion [16,17]. When a chelate compound (citric acid in this study) is added to the impregnation solution, a large complex cluster is formed around the metal [17]. This cluster repelling the adjacent molecules inhibits the aggregation of metal ions due to the steric hindrance and this effect causes the metal particle to be dispersed better on the catalyst surface [16,17]. However, only a few recent studies have reported the activity of an Ni-supported SBA-15 catalyst in the DRM reaction by tuning the particle size and then the chemical state of the Ni nanoparticles depending on the citric acid concentration.

In this study, mesoporous silica (SBA-15) was selected as the support material, which could highly disperse the Ni particles induced by the addition of citric acid. In this study, the 7 wt.% Ni/SBA-15 catalysts were prepared using a citric acid-assisted incipient wetness impregnation method and a pristine Ni/SBA-15 catalyst was also prepared for reference. The mole ratio of citric acid to Ni was varied with the purpose of simultaneously tuning the Ni particle size and metal support interaction. The physicochemical properties of these catalysts were characterized via XRD, TEM, BET, H₂-TPR, XPS, and TGA, and correlated with the catalytic activity. Through this study, it is expected that the optimum ratio of citric acid for a SBA-15-supported catalyst during the Ni impregnation step will be determined.

2. Results and Discussion

2.1. Fresh Catalyst Characterization

2.1.1. Textural and Structural Properties

XRD analysis was carried out to understand the crystal structure of the as-calcined and as-reduced catalysts, as given in Figure 1. Figure 1a presents the XRD patterns of the as-calcined samples: CA0, CA0.35, CA0.7, and CA1.4. Regardless of the concentration of citric acid, basically, all the samples exhibited a broad peak at 22°, which is attributed to the SBA-15 amorphous silica support. For the CA0 catalyst, it showed the sharp and strong characteristic peaks of NiO (PDF #44-1159) at 37.2°, 43.2°, and 62.9°, while these NiO peaks became very weak in the catalysts, CA0.35, CA0.7 and CA1.4. This implied that the particle size of Ni was decreased by the addition of citric acid [17,18,19]. Figure 1b plots the XRD peaks of the as-reduced samples with a reduction process under a 4% H₂/N₂ atmosphere for 2 h at 800 °C. The diffraction patterns ascribed to the mesoporous SBA-15 silica were similar to those of the as-calcined samples. Different from the XRD peaks of the as-calcined catalysts, new characteristic peaks at 44.5°, 51.8°, and 76.3° were observed for all the reduced samples, attributed to metallic Ni [18,20]. This means that the NiO in mesoporous SBA-15 was successfully reduced to metallic Ni during the reduction process. Moreover, it was revealed that the Ni characteristic peaks of CA0.35, CA0.7, and CA1.4 were not highly developed compared to those of CA0 after reduction, indicating that nickel nitrate impregnated with citric acid would form small Ni nanoparticles on the SBA-15 support with high thermal stability [19,21].

Table 1. Textural properties of as-reduced Ni/SBA-15 and a series of Ni/SBA-15 with citric acid.

Samples	Average Particle Size of Ni a (nm)	Ni Dispersion a (%)	BET Surface Area b (m2 g−1)	Pore Volume b (cm3 g−1)	Ni contents c (wt.%)
SBA-15	-	-	824.03	0.9029
CA0	49.3	2.05	584.54	0.6449	7.28
CA0.35	8.2	12.23	517.81	0.5837	6.02
CA0.7	7.6	13.29	449.34	0.5545	7.7
CA1.4	7.3	13.83	455.26	0.5028	8.76

^a Estimated by H₂ pulse chemisorption. ^b Estimated by N₂ isotherm. ^c Estimated by ICP-MS.

summarizes the average particle size and dispersion of Ni, estimated by H₂ pulse chemisorption, and these results indicated that the addition of citric acid influenced the particle size of Ni, which corresponded well to the XRD analysis.

TEM analysis was conducted to visually observe the Ni particles on the as-reduced catalysts. Prior to this analysis, the samples were dispersed in ethanol using ultra-sonication and loaded homogeneously on the copper grid to obtain detailed images, which clearly show the mesopore structures of SBA-15 as well as the Ni particles. Figure 2a–d presents the TEM images of the as-reduced Ni/SBA-15 (CA0) and the series of Ni/SBA-15 with citric acid (CA0.35, CA0.7, and CA1.4) catalysts. The CA0 catalyst had bulk Ni particles placed also on the outer surface of the SBA-15 (Figure 2a), whereas the catalysts synthesized by the addition of citric acid (CA0.35, CA0.7, and CA1.4) show much smaller Ni particles located inside of the SBA-15 pore structure (Figure 2b–d). This result clearly indicates that the addition of citric acid during the metal impregnation process helps to form nanoparticles of metallic Ni, which are small enough to be located inside of the SBA-15 pore [21]. The size and distribution of the metallic Ni over SBA-15 were dependent on the citric acid addition and concentration, shown in Figure 2e–h. In the case of the CA0, the Ni particle size was widely varied from 10 to 30 nm (d_ave = 16.7 nm), whereas a very narrow size distribution of Ni particles was observed over CA0.35, CA0.7, and CA1.4 (d_ave = 4.45, 4.78, and 7.01 nm, respectively).

In these TEM images (Figure 2a–d), interestingly, the only thing changed by the citric acid was the particle size of Ni and there were no discernible changes in the support structure with the addition of citric acid. For a better understanding of the support structure, a nitrogen adsorption–desorption analysis was conducted over the as-reduced samples. Figure 3 exhibits the nitrogen isotherms and pore size distributions of the as-reduced samples. As given in Figure 3a, all the samples have the typical Type-IV isotherm, with obvious hysteresis in the range of 0.4–0.7 P/P₀ [19,21]. Figure 3b presents the pore-size distribution of each sample estimated using the BJH method. All the samples possessed mesopores of identical sizes, with diameters ranging from 2 to 8 nm, indicating that the pore structure and the size of the SBA-15 support were identical regardless of the addition of citric acid and its concentration, corresponding well to the TEM analysis. Table 1 presents the textural properties of the Ni/SBA-15 and the series of Ni/SBA-15 with citric acid, as estimated by the N₂ adsorption–desorption method. The BET surface area and the pore volume of CA0 were 584.54 m² g⁻¹ and 0.6449 cm³ g⁻¹, respectively. Those numbers were decreased with the addition of citric acid (BET surface area = 517.81 m² g⁻¹ and the pore volume = 0.5837 cm³ g⁻¹ for CA0.35), and furthermore, a higher citric acid concentration (CA0.7 and CA1.4) led a smaller BET surface area and pore volume (BET surface area = 449.34 m² g⁻¹ and 455.26 m² g⁻¹ and the pore volume = 0.5545 cm³ g⁻¹ and 0.5028 cm³ g⁻¹ for CA0.7 and CA1.4, respectively). This result would be due to the fact that Ni nanoparticles induced by the citric acid were placed on the inside of the support structure and caused the pore blockage, not because of support structure change.

2.1.2. Reduction Behavior and Oxidation State of Catalysts

H₂-TPR analysis was conducted to examine the reduction behavior of Ni/SBA-15 (CA0, CA0.35, CA0.7, and CA1.4) catalysts (Figure 4). According to the literature, the reduction peak at 250–400 °C is ascribed to the reduction of surface NiO particles, with a poor interaction with the support, and the reduction peak in the temperature range of 400–600 °C is attributed to the reduction of NiO particles in the pore with a strong interaction with the support [14,15,21]. The CA0 had very large peaks for NiO belonging to both the poor and strong interaction with support. In contrast to CA0, the reduction behavior was significantly changed in CA0.35, CA0.7, and CA1.4. The reduction peaks of the surface NiO were clearly decreased but those of the NiO having a strong interaction with the support were relatively developed, indicating that the NiO nanoparticles with a strong metal–support interaction had been formed due to the addition of citric acid [16,17]. In terms of the citric acid concentration, there was one more thing we needed to focus on. That is, the high concentration of citric acid in the impregnation solution that led to the bigger surface NiO having a poor metal–support interaction. According to the literature, it is reported that Ni catalysts with a strong metal–support interaction are essential for the stable catalytic performance and it suppressed the formation of carbon fibers [20]. Therefore, this H₂-TPR result presented that there were different kinds of nickel species induced by the citric acid and it is expected that the catalytic performance for DRM would be very different due to this reduction behavior change.

The XPS experiment investigated the oxidation state of Ni species on the catalyst surface of the as-reduced samples. Curve-fitting for the present analysis was performed after the Shirley-type background subtraction using a combination of Gaussian and Lorentzian functions. Figure 5 shows the Ni 2p_3/2 spectra for Ni/SBA-15 (CA0) and a series of Ni/SBA-15 with citric acid (CA0.35, CA0.7, and CA1.4). The XPS peaks of Ni 2p_3/2 at 851.8–853.0 eV, 855.0–856.6 eV, and 860.0–862.2 eV can be attributed to Ni⁰, Ni²⁺, and the satellite peak, respectively [18,22]. The peak position for Ni species tended to be moved to lower/higher binding energy by the addition of citric acid (CA0.35, CA0.7, and CA1.4), indicating that the citric acid addition influenced the oxidation state of Ni species [16,17].

Table 2. Ni 2p_3/2 position of as-reduced samples as estimated by XPS analysis.

Samples	Ni 2p3/2 Peak Position (eV)
	Sat.	Ni2+	Ni0
CA0	860.5	854.9	851.8
CA0.35	861.7	856.2	853.0
CA0.7	859.1	854.9	852.2
CA1.4	860.4	855.6	853.0

summarizes the peak position of Ni 2p_3/2 over as-reduced catalysts. The Ni⁰ and Ni²⁺ peak positions of CA0.35 were located at the high peak position compared to others. It indicated that nickel species with the strong metal–support interaction could be formed by the addition of a low concentration of citric acid during the Ni metal impregnation step [15,17,23]. According to the H₂-TPR and XPS analysis, the addition of citric acid definitely led to the formation of nickel species with a strong metal–support interaction compared to the catalyst without citric acid, but one more step further, its concentration needs to be carefully controlled.

2.2. Catalytic Activity

To evaluate the catalytic activities of Ni/SBA-15 catalysts (CA0, CA0.35, CA0.7, and CA1.4), DRM reaction tests were conducted at 96,000 mL·g⁻¹ h⁻¹ of GHSV with a CH₄/CO₂ ratio = 1, changing the reaction temperature from 800 to 600 °C. Figure 6 depicts the CO₂ and CH₄ conversions for CA0, CA0.35, CA0.7, and CA1.4 catalysts as a function of the time-on-stream with changing the reaction temperature. Basically, the conversion of CO₂ was slightly higher than that of CH₄ because the DRM reaction accompanied a reverse water–gas shift (H₂ + CO₂ → CO + H₂O) reaction as a side reaction [24]. As given in Figure 6, when the reaction temperature was decreased from 800 °C to 600 °C with an interval of 50 °C, the conversion of CO₂ and CH₄, for all catalysts, was accordingly decreased. This deterioration in the catalyst efficiency was due to the reduction in the reaction energy of the reactant on the catalyst surface with decreasing the reaction temperature. The catalytic performance at 800 °C was almost the same for all catalysts, but the conversion of CO₂ and CH₄ became segregated among the catalysts with a decreasing the reaction temperature. The catalytic performance of CA0 was drastically decreased with decreasing the reaction temperature, while that of CA0.35, CA0.7, and CA1.4 was not and their efficiency was much higher than that of CA0. Among the series of the Ni/SBA-15 catalyst with citric acid, furthermore, CA1.4 had the lowest catalytic performance, indicating that the catalyst impregnated with a high concentration of citric acid does not promise high catalytic efficiency.

A stability test was conducted under severe conditions (CO₂/CH₄ = 1, T = 700 °C, and GHSV = 96,000 mL·g⁻¹ h⁻¹) for 30 h. Figure 7 exhibits the CO₂ and CH₄ conversion over CA0, CA0.35, CA0.7, CA1.4 catalysts. The H₂/CO ratio for this stability test is presented in Figure S1. The difference in conversion between catalysts was clearly shown. CA0.35, CA0.7, and CA1.4 presented a high efficiency compared to CA0, and among a series of Ni/SBA-15 with citric acid, CA0.35 had the highest conversion of CO₂ and CH₄ for the 30 h DRM reaction. This indicated that the addition of citric acid in a low concentration during the Ni impregnation step led to a high catalytic performance. In the case of CA0, furthermore, the conversion of CO₂ and CH₄ was gradually decreased from 57% to 32% and 38% to 21%, respectively. However, all catalysts impregnated with citric acid showed very stable conversion for CO₂ and CH₄. This clearly indicated that the addition of citric acid in the Ni impregnation step led to stable catalytic efficiency. Table 1S presents the detailed comparison to other catalysts with different condition. It implies the superior catalytic efficiency induced by the citric acid.

2.3. Spent-Catalyst Characterization

XRD and TEM analysis were performed to investigate the catalyst structure and carbon formation after the 30 h stability test at 700 °C (CO₂/CH₄ = 1 and GHSV = 96,000 mL·g⁻¹ h⁻¹). Figure 8 plots the XRD patterns on the spent catalysts used in DRM. In this XRD analysis, it was difficult to observe the coke formation because the peak position attributed to carbon (PDF #26-1080) overlapped with the diffraction peak of silica, but the structure of nickel and silica was clearly investigated [15,25]. The diffraction patterns attributed to mesoporous SBA-15 silica were similar to those for the fresh samples, indicating the high thermal stability of SBA-15 [14,15,16]. However, the difference between fresh and spent catalysts was found in the intensity of characteristic peaks at 44.5°, 51.8°, and 76.3°, ascribed to metallic Ni [18,20]. The peaks of spent catalysts were relatively increased after the DRM reaction compared to those of fresh catalysts, indicating that the size of metallic Ni was grown due to the metal sintering. This was an expected result for all catalysts, but interestingly the metallic Ni peaks for CA0.35 were not significantly increased. This corresponded well to the characterization result, in which there is an optimum concentration of citric acid during the Ni impregnation step for the strong metal–support interaction.

Figure 9 shows the TEM images of the spent catalysts. First of all, it is found that there were relatively agglomerated Ni particles in the spent catalysts compared to those in fresh catalysts (Figure 9a–d). The particle size distribution of Ni was plotted on Figure 9e–h. The Ni particles in CA0, CA0.7, and CA1.4 were noticeably grown after the DRM reaction, as confirmed by the XRD, but this growth was small in CA0.35 due to the strong metal–support interaction. This clearly indicated that when the citric acid was added in a low concentration during the impregnation step, most nickel species would have a high thermal stability owing to the strong metal–support interaction, and did not lead to Ni particle agglomeration. In terms of carbon formation on the surface of the catalyst, it was also confirmed that the entangled and randomly grown carbon whiskers were deposited over CA0, CA0.35, CA0.7, and CA1.4 (Figure 9a–d). Some carbon fibers possessed the agglomerated Ni particles detached from the catalyst surface, with this phenomenon mostly observed in CA0 (Figure 9a).

TG analysis was carried out to measure the amount of carbon accumulation formed on the catalyst surface, given in

Table 3. The amount of coke formation on catalyst surface, confirmed by TG analysis.

Samples	Coke Formation (mgcoke gcat−1 h−1)
CA0	21.9
CA0.35	2.4
CA0.7	4.8
CA1.4	5.1

. The amount of coke accumulation over the reaction time is as follows: CA0 = 21.9 mg_coke·g_cat⁻¹ h⁻¹, CA0.35 = 2.4 mg_coke·g_cat⁻¹ h⁻¹, CA0.7 = 4.8 mg_coke·g_cat⁻¹ h⁻¹, and CA1.4 = 5.1 mg_coke·g_cat⁻¹ h⁻¹. This supports that adding the citric acid as the chelating agent during catalyst synthesis step would increase the resistance against coke formation and the excess concentration of citric acid rather leads to a negative effect on the catalytic performance.

3. Experimental Method

3.1. Materials

Nickel nitrate (Ni(NO₃)₂∙6H₂O, ≥98%, Sigma-Aldrich, St. Louis, MI, USA) and tetraethyl orthosilicate (Si(OC₂H₅)₄, ≥97%, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) were used as the sources of Ni and Si, respectively. A pluronic P123 triblock copolymer (EO₂₀PO₇₀EO₂₀, molecular weight = 5800 g mol⁻¹, Sigma-Aldrich) was employed as a structure-directing agent. All of the materials were used as received without any further purification.

3.2. Catalyst Preparation

The mesoporous silica SBA-15 was synthesized using a hydrothermal method. First of all, 10 g of P123 was fully dissolved in 300 mL of 2M hydrogen chloride for 2 h, and then 22 mL of tetraethyl orthosilicate (TEOS) was added to this solution. After vigorously stirring at 40 °C for 24 h, the mixture was transferred to the autoclave for the hydrothermal treatment at 100 °C for 24 h. Afterwards, the obtained precipitate was thoroughly washed with deionized water to remove impurities, dried at 70 °C for 24 h, and subsequently calcined at 650 °C for 6 h.

The nitrate form of nickel was added onto SBA-15 via incipient wetness impregnation (nominal amount of active metal fixed at 7 wt.%) using various concentrations of citric acid (The molar concentration of 0.35, 0.7, and 1.4), and then the synthesized sample was calcined at 400 °C for 4 h. The samples were marked as CA0.35, CA0.7, and CA1.4, respectively. A Ni/SBA-15 catalyst in the absence of citric acid was also prepared using the incipient wetness impregnation method as a reference sample, denoted as CA0.

3.3. Catalyst Characterization

X-ray diffraction (XRD) analysis was carried out using a SmartLab (Rigaku, Corp., Akishima, Japan) at a Cu Kα wavelength, 45 kV, and 200 mA. Transmission electron microscopy (TEM) images were obtained using Tecnai G2 F30 (FEI Co., Hillsboro, OR, USA) at 300 kV, with sample loading on a holey copper grid coated with carbon. The BET surface area of the catalysts was estimated by N₂ physisorption at −196 °C using a BELSORP-mini (BEL Japan. Inc., Osaka, Japan). The pore size distribution was analyzed in accordance with the Barrett–Joymer–Halenda (BJH) theory. Temperature-programmed reduction/desorption (TPR/TPD) analysis was performed in a BELCAT-II (BEL Japan. Inc.). For the H₂-TPR experiment, 100 mg of a sample was loaded onto a quartz tube and pretreated in a flow of 10% O₂ in He at 200 °C for 1 h. After pretreatment, TPR analysis was conducted in the temperature range of 50–1000 °C using 10% H₂ in Ar balance with a ramping rate of 5 °C min⁻¹. For the H₂-chemisorption experiment, 50 mg of calcined sample was pretreated in 5% H₂/Ar at 800 °C for 2 h and cooled down to 50 °C. Then, H₂ gas was injected in order to perform the chemisorption experiment, assuming that the adsorption stoichiometry of H/Ni = 1. The surface electron state of the reduced samples was investigated using a X-ray photoelectron spectrometer (K-Alpha+, Thermo Scientific, Waltham, MA, USA) equipped with a micro-focused monochromatic Al Kα X-ray source (1486.6 eV) at a constant energy of 50 eV. The XPS spectra were carefully calibrated and aligned using the C 1s transition at 284.6 eV. Thermogravimetric (TG) analysis was carried out using the Labsys TGA EVO (Setaram Instrumentation, Caluire-et-Cuire, France) at temperatures up to 1000 °C in air to estimate the amount of carbon formed on the catalyst surface.

3.4. Catalytic Reaction

The catalytic reaction was performed in a fixed-bed reactor (using a quartz tube of 4 mm in internal diameter). The reaction temperature was controlled using a thermocouple placed at the center of the catalyst bed. Prior to the reaction, a sample was sieved to achieve a uniform particle size. The 75 mg of powder was then in situ reduced at 800 °C for 2 h under a flow of 50 sccm (H₂/N₂ = 1/4) to complete the reduction of active metals. The flow was then switched to the reactant mixture (10 vol% of CO₂ and CH₄ in N₂ balance, CH₄/CO₂ = 1) with P = 1.0 atm. The total GHSV was at 96,000 mL·g⁻¹ h⁻¹. The stability test was conducted under the conditions of CH₄/CO₂ = 1, T = 700 °C, P = 1.0 atm, and GHSV = 96,000 mL·g⁻¹ h⁻¹. The reactants and products were quantified by on-line micro gas chromatography (GC) using a 3000 MicroGC (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a thermal conductivity detector. The CH₄ and CO₂ conversion was calculated using the following equation:

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

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

where [CH₄]_in, [CO₂]_in, [CH₄]_out, and [CO₂]_in are the inlet and outlet volume flows of CH₄ and CO₂ gas, respectively.

4. Conclusions

In order to investigate the effect that citric acid has as the chelating agent on loading Ni active sites, Ni/SBA-15 and a series of Ni/SBA-15 with citric acid were synthesized using the incipient wetness impregnation method for the DRM reaction and various characterization techniques were conducted to understand the changes in the physicochemical properties. By the addition of citric acid, the Ni dispersion was clearly improved and the size of the Ni particles was decreased, while the concentration of citric acid did not significantly change the Ni particle size. In terms of the metal–support interaction, however, the concentration of citric acid for the Ni impregnation step was highly important. The higher the concentration of citric acid added in the impregnation process, the more nickel particles with a weaker metal–support interaction were produced, confirmed by the H₂-TPR and XPS analysis. These properties were clearly shown in the DRM reaction test. The CA0.35 catalyst had a high catalytic efficiency for the 30 h stability test, and moreover, it generated the least amount of carbon on the catalyst surface and particle agglomeration was minimized compared to the others. In conclusion, using citric acid as the chelating agent has an influence on the formation of the Ni active site and the concentration is of great importance in the formation of a strong metal–support interaction.