Effect of SiO₂ Addition to Highly Loaded Ni on CeO₂ for CO₂ Methanation

Abstract

In this study, 60 wt% Ni on xSiO₂–(1 – x)CeO₂ catalysts were prepared by flame spray pyrolysis, and the effects of SiO₂ fraction (x = 0–0.2) on the material properties and catalytic activity of CO₂ methanation were investigated. After reducing the catalysts in 5%H₂−Ar at 500 °C, the catalytic activity was evaluated; the highest catalytic activity was exhibited at x = 0.05. Furthermore, SiO₂ addition was found to improve the catalytic activity after 110 hours of the reaction. Powder X-ray diffraction revealed that SiO₂ addition did not alter the crystallinity of the catalysts before and after the reduction. Conversely, the appropriate amount of SiO₂ doping increased the Ni surface area and pore volume of the reduced catalysts, as confirmed by H₂ chemisorption and N₂ adsorption–desorption measurements. H₂-TPR measurements showed an increasing trend in the reduction temperature of particulate NiO as the SiO₂ fraction increased. The shift of the reduction temperatures indicates the better contact between NiO and SiO₂, which hindered the sintering of Ni particles during the reduction at 500 °C. Therefore, an appropriate amount of SiO₂ addition prevented the growth of Ni particles, resulting in the increase of the Ni surface area. The increase of the Ni surface area contributed to the improvement of the catalytic activity.

Keywords:

• Flame spray pyrolysis
• Flame synthesis
• High loading
• Methanation
• Thermal stability

1. Introduction

Hydrogen has garnered increased interest as an energy carrier (Graetz 2009) because it can be produced using renewable energy (e.g., biomass and solar energy). However, the inherent flammability of H₂ presents a formidable challenge (Dadashzadeh et al. 2018). The reaction of hydrogen with CO₂ to form methane (Centi et al. 2013) is promising because CH₄ is less flammable than H₂ (Dadashzadeh et al. 2018). Furthermore, the utility of methane is as a fuel in daily activities, and thermal electric power plants are versatile. Consequently, the methane produced from the reaction of hydrogen with CO₂ can be integrated into existing infrastructure.

Numerous catalysts have been proposed for this reaction, with a particular focus on cost-effective Ni-based catalysts, such as Ni/CeO₂ (Tada et al. 2012; Fukuhara et al. 2017), Ni/CeO₂–Al₂O₃ (Wang and Lu 1998), Ni/ZrO₂ (Jia et al. 2019), and Ni/Y₂O₃ (Muroyama et al. 2016). Owing to its high reducibility (Tada et al. 2012) and ability to absorb CO₂ (Fukuhara et al. 2017), CeO₂ is a promising metal-oxide support. The aforementioned studies suggest that the active sites for CO₂ methanation are at/near the Ni–CeO₂ interfaces. One straightforward strategy for increasing the number of active sites is increasing the Ni loading while keeping the Ni size small, thereby effectively increasing the Ni–CeO₂ interfaces. However, Ni loading has an upper limit because excessive loading induces the formation of huge Ni particles (Zyryanova et al. 2014; Beierlein et al. 2019). Some studies have suggested that the optimal Ni loading for CO₂ methanation is approximately 15–30 wt% (Liu et al. 2013; Nie et al. 2017). Recently, highly loaded Ni catalysts (50–92.5 wt% of Ni content) such as Ni–Al hydrotalcite (He et al. 2014), Ni–La₂O₃ (Tang et al. 2019), Ni–CeO₂ (Fujiwara et al. 2021), and a sponge nickel (Tada et al. 2017), were demonstrated as active catalysts for CO₂ methanation. Despite their promising activity, their high Ni contents could induce the sintering of Ni particles, thereby degrading the activity for long-term use. To prevent the sintering of Ni particles in catalysts, the addition of SiO₂ to the catalysts has been examined frequently (e.g., Ni/ZrO₂–SiO₂; Charisiou et al. 2019). Therefore, SiO₂ addition to highly loaded Ni catalysts is expected to improve long-term performance.

Flame spray pyrolysis (FSP) was used to synthesize highly loaded NiO particles (60 wt% as Ni) on SiO₂–CeO₂. FSP facilitates the one-step production of multicomponent metal oxides (Teoh et al. 2010; Li et al. 2016) and scaling up of the production rate; kg h⁻¹ (Wegner et al. 2011). We previously demonstrated that FSP can deposit small particles at high metal loading on metal-oxide supports; for example, 50 wt% Ag on SiO₂ (Fujiwara et al. 2015) and 60 wt% Cu on ZrO₂ (Fujiwara et al. 2019; Tada et al. 2020). Moreover, particles containing SiO₂ (e.g. ZnO (Tani et al. 2002), CeZrO₂ (Schulz et al. 2003), WO₃ (Righettoni et al. 2010) and MoO₃ (Güntner et al. 2016)) prepared by FSP exhibited high thermal stability. Thus, FSP allows doping SiO₂ to various metal oxides. However, the effect of SiO₂ doping to highly loaded metal particles and its catalytic activity have not been examined. It is expected that overloading SiO₂ to Ni/CeO₂ degrades the catalytic activity for CO₂ methanation because Ni/SiO₂ is less active than Ni/CeO₂ (Liu et al. 2022). Therefore, in this study, the effects of the SiO₂ fraction in FSP-made 60 wt%Ni/CeO₂ were investigated and the optimal SiO₂ content for improving the catalytic activity and thermal stability was explored.

2. Experimental

2.1. Preparation of Ni/CeO₂-SiO₂ by Flame Spray Pyrolysis

As shown in Figure 1, 60 wt% Ni supported on xSiO₂–(1 – x)CeO₂ catalysts, namely Si–x, where x is the molar fraction of SiO₂ in the support, were prepared using an FSP reactor. The precursor solutions were prepared by dissolving nickel acetate tetrahydrate (Fujifilm Wako Pure Chemical, Osaka, Japan, purity >98.0%), cerium 2-ethylhexanoate in 2-ethylhexanoic acid (Alfa Aesar, Ward Hill, MA), and hexamethyldisiloxane (HMDSO) into a 1:1 mixture of methanol (Sigma-Aldrich, St. Louis, MO, Reagent Grade) and 2-ethylhexanoic acid (Sigma-Aldrich, St. Louis, MO, purity >99%). The SiO₂ fraction was controlled by the amount of HMDSO dissolved in the solution, and the total metal concentration (Ni + Ce + Si) was set to 0.2 mol L⁻¹.

Figure 1. Formation of Ni/xSiO₂–(1 – x)CeO₂ catalysts using an flame spray pyrolysis (FSP) reactor.

The prepared solution was fed to the FSP reactor at 3 mL min⁻¹ and dispersed to fine spray using 5 L_STP min⁻¹ of O₂ dispersant (technical grade). The spray was combusted using a plot flame (CH₄:O₂ = 1.5 L_STP min⁻¹:3.2 L_STP min⁻¹) to generate nanoparticles. The particles were collected using a vacuum pump (Busch Seco SV1040C, Maulburg, Germany) and filtered using a Φ257 mm glass fiber filter (Albet LabScience, GF6, Dassel, Germany) located 65 cm above the FSP reactor. If needed, the as-prepared catalysts were reduced at 500 °C in 5%H₂–Ar (100 mL min⁻¹) for 1 h. Subsequently, the reduced powder was cooled to room temperature in Ar (100 mL min⁻¹). The reduction condition was the same as that before the catalytic activity test.

2.2. Evaluation of Catalytic Performance for CO₂ Methanation

The catalytic performance was evaluated using a fixed-bed reactor. The catalyst (100 mg) and SiC powder (Fujifilm Wako Pure Chemical, Osaka, Japan, 400 mg) were thoroughly mixed in a mortar and poured into a quartz tube (inner/outer diameters of Φ6/Φ8 mm). The catalyst bed (sample + SiC) was 13–14 mm long for all the catalysts. The filled catalyst was reduced at 500 °C for 1 h under 5% H₂–Ar (100 mL_STP min⁻¹) and then cooled to room temperature (c.a. 25 °C). Afterward, a reactant gas (CO₂/H₂/N₂ = 1/4/1) was fed to the catalyst at 60 mL_STP min⁻¹. The temperature of the catalyst bed was measured using a K-type thermocouple (Φ1 mm) placed at the center of the catalyst bed. The temperature-ramping rates below and above 250 °C were 1 and 2 °C min⁻¹, respectively, and the temperature was maintained for at least 30 min before the products were analyzed. The composition of the product gas was measured using an online gas chromatograph (Shimadzu, GC-2014, Kyoto, Japan) equipped with a thermal conductivity detector. CO was not detected by the detector using He as a career gas in all conditions examined in this study. The conversion and reaction rate of CO₂ were evaluated using the following equations: (1) $${\text{CO}}_2\hspace{0.17em}\text{conversion (}\%\text{)}=\left(F_{\text{CO2}\_\text{in}}-F_{\text{CO2}\_\text{out}}\right)/F_{\text{CO2}\_\text{in}}\times 100$$(1) (2) $$\text{Reaction rate},\hspace{0.17em}{r}_{\text{CO2}}({\text{mol s}}^{-1}{g}_{\text{cat}}{}^{-1})=\left(F_{\text{CO2}\_\text{in}}-F_{\text{CO2}\_\text{out}}\right)/M_{\text{cat}}$$(2) where F_CO2__in and F_CO2__out were the flow rates of CO₂ (mol s⁻¹) at the inlet and outlet, respectively. M_cat was the mass of the catalyst in the reactor. The Arrhenius plot was obtained by plotting the natural log of the reaction rate against the inverse temperature. To obtain the reaction rate for the plots, the catalytic activity was evaluated using 50 mg of the catalysts (with 400 mg of SiC). From the Arrhenius plots, the apparent activation energy (E_a) was obtained using the Arrhenius equation (Eq. (3)(3) $$k=A\bullet \mathrm{\text{exp}}\left(-\frac{E_a}{R\bullet T}\right)$$(3) ). (3) $$k=A\bullet \mathrm{\text{exp}}\left(-\frac{E_a}{R\bullet T}\right)$$(3) where k, A, R, and T are the kinetic constant, frequency factor, gas constant (8.314 J mol⁻¹ K⁻¹), and temperature of the catalyst bed, respectively. In this study, we assumed that the kinetic constant is proportional to the reaction rate.

2.3. Particle Characterization

The powder X-ray diffraction (PXRD) patterns of catalysts were obtained using a diffractometer (Rigaku Ultima IV, Tokyo, Japan, Cu Kα, 40 kV, 40 mA). The crystallite sizes of CeO₂ (1 1 1), NiO (2 0 0), and Ni (1 1 1) were calculated from the peaks at 28°, 43°, and 44°, respectively, using Scherrer’s equation (Eq. (4)(4) $$\text{Crystallite size, }{d}_{\mathrm{\text{XRD}}} \mathrm{(}\text{nm}\mathrm{)}=\frac{\text{K }\mathrm{\lambda }}{\mathrm{\beta }\text{ cos}\mathrm{\theta }}$$(4) ). (4) $$\text{Crystallite size, }{d}_{\mathrm{\text{XRD}}} \mathrm{(}\text{nm}\mathrm{)}=\frac{\text{K }\mathrm{\lambda }}{\mathrm{\beta }\text{ cos}\mathrm{\theta }}$$(4) where K (=0.89) is the shape factor, λ (=0.154 nm) the X-ray wavelength, β the line broadening at half the maximum intensity in radians, and θ the Bragg angle.

The particle morphology was investigated by using a high angle annular dark field scanning transmission electron microscope (HAADF-STEM) equipped with a spherical aberration corrector for a STEM probe (JEOL JEM-ARM200F, Tokyo, Japan). The sample was dispersed in ethanol (Wako, Osaka, Japan, purity >99.5%), and the suspension was dropped onto a carbon-coated Cu grid (Ohken Shoji Co., Tokyo, Japan, NP-C15) to deposit the particles on the microgrid. The distributions of Ni, Ce and Si species in the particles were recorded using an energy dispersive X-ray (EDX) detector.

N₂ adsorption for the catalysts was performed using a BELSORP-Mini II (MicrotracBEL Corp., Osaka, Japan). Before the measurements, the samples were degassed at 150 °C under vacuum at <1 kPa for 1 h. The specific surface area (SSA) was determined from the amount of adsorbed N₂ on the particle surface at 77 K using the Brunauer–Emmett–Teller method.

The reducibility of the samples was evaluated by temperature-programmed reduction by H₂ (H₂-TPR) using BELCAT II (MicrotracBEL Corp., Osaka, Japan). The sample of c.a. 25 mg was filled into a quartz tube. Before the measurement, the sample was heated at 300 °C for 1 h and cooled down to 45 °C in an Ar flow. The sample was heated from 45 °C to 700 °C with 5 °C min⁻¹ of ramping rate in a 5% H₂–Ar flow (100 mL min⁻¹). The product gas was introduced to a molecular sieve trap (Shinwa Chemical Industries Ltd., 3A, Kyoto, Japan) and, subsequently, to the thermal conductivity detector. To calculate the H₂ consumption from the sample, a commercial CuO powder (Fujifilm Wako Pure Chemical, Osaka, Japan, purity >99.9%) was used as the reference material.

The surface area of Ni in the catalysts was investigated by H₂ pulse titration using BELCAT II (MicrotracBEL Corp., Osaka, Japan). About 50 mg of the sample was placed in a quartz tubular reactor and reduced by 5% H₂–Ar at 500 °C for 1 h. Then, the sample was purged with Ar at 500 °C for 15 min to remove all adsorbed H₂ on the surface and cooled to 40 °C. After stabilizing the temperature at 40 °C, 0.966 mL_STP of 10% H₂–Ar was injected into 50 mL min⁻¹ of Ar carrier every 2 min until the equilibrium of H₂ adsorption on the catalyst surface was reached. The surface area of metallic Ni was calculated using Eq. (5)(5) $$\text{SSA }\mathrm{(}{\mathrm{m}}_{\mathrm{\text{Ni}}}^2\mathrm{ }{\mathrm{g}}_{\mathrm{\text{cat}}}^{-1}\mathrm{)}=\frac{{2 \mathrm{\times } M}_{{\mathrm{H}}_2}\times {\mathrm{N}}_{\text{av}}}{{\mathrm{A}}_{\text{Ni}}}$$(5) , assuming that every Ni atom on the catalyst surface adsorbs one hydrogen atom. (5) $$\text{SSA }\mathrm{(}{\mathrm{m}}_{\mathrm{\text{Ni}}}^2\mathrm{ }{\mathrm{g}}_{\mathrm{\text{cat}}}^{-1}\mathrm{)}=\frac{{2 \mathrm{\times } M}_{{\mathrm{H}}_2}\times {\mathrm{N}}_{\text{av}}}{{\mathrm{A}}_{\text{Ni}}}$$(5) where M_H2, N_av, and A_Ni are the amount of H₂ molecules adsorbed on the catalyst (mol ${\mathrm{g}}_{\mathrm{\text{cat}}}^{-1}$), Avogadro’s number (6.02 × 10²³ [atom mol⁻¹]), and the number of Ni surface atoms per unit area (15.4 × 10¹⁸ [atom m⁻²]).

3. Results and Discussion

Figure 2(a) shows the CO₂ methanation activity of 60 wt% Ni on xSiO₂–(1 – x)CeO₂ catalysts (Si–x). For all the catalysts, the conversion of CO₂ was initiated at 175–200 °C and reached the equilibrium value above 300–350 °C. The concentration of CO in the product was below the detectable limit (<0.01%), implying selective CH₄ formation. When the SiO₂ fraction increased to x = 0.05, CO₂ conversion increased. Further increase in the SiO₂ fraction degraded the catalytic activity. Therefore, the optimal SiO₂ addition is beneficial for catalytic performance. The reaction rate of the optimal catalyst (Si–0.05) was 8.6 × 10⁻² mol h⁻¹ g_cat⁻¹ at 225 °C. Notably, the reaction rate of the optimal catalyst was higher than those of the Ru catalysts (e.g., 3 wt% Ru/CeO₂; 8.1 × 10⁻³ mol h⁻¹ g_cat⁻¹ at 225 °C (Wang et al. 2015) and 4 wt% Ru/TiO₂; 5.6 × 10⁻² mol h⁻¹ g_cat⁻¹ at 240 °C (Zhou et al. 2022). Therefore, FSP-made catalysts performed excellently.

Figure 2. (a) CO₂ methanation activity of Si–x. (b) Catalytic activity of Si–0 and Si–0.05 for 110 h.

Despite the high catalytic activity of the FSP-made catalysts, high Ni loading (60 wt%) could suffer from the sintering of Ni particles during the reaction. SiO₂ doping is expected to improve thermal stability by blocking the contact of Ni particles. Herein, the effect of SiO₂ on catalytic activity for 110 h was investigated. Figure 2(b) shows the CO₂ conversion of Si–0 and Si–0.05 within 110 h. During the test, the temperature initially increased to 250 °C and subsequently to 300 °C. The initial CO₂ conversions at 250 °C by the catalysts with x = 0 and 0.05 were 61 and 70%, respectively. The conversion at 300 °C by both catalysts did not change (88–91%) for 100 h. Afterward, the temperature was reduced to 250 °C and then maintained for 10 h. The conversions after temperature reduction were 55% and 64% for x = 0 and 0.05, respectively. Therefore, the catalytic activity slightly degraded after the reaction at 300 °C for 100 h, regardless of SiO₂ addition. Nevertheless, the conversion of the SiO₂-doped catalyst (x = 0.05) after the long-time reaction (at 110 h) was higher than the initial conversion of the non-doped catalyst (x = 0), indicating that the optimal SiO₂ addition improved the conversion after the long-time reaction.

As shown in Figure 2(a), the optimal amount of SiO₂ addition improved the catalytic activity. To elucidate the effect of SiO₂ on the FSP-made Ni/CeO₂, the material properties were investigated. First, the crystallinity of the catalysts was analyzed by PXRD. Figure 3(a) shows the PXRD patterns of Si–x. Despite the SiO₂ fraction, the peaks of bunsenite NiO (37° and 43°) and fluorite CeO₂ (28°, 33°, and 47°) were observed, which is consistent with other FSP-made NiO (Azurdia et al. 2008) and CeO₂ (Mädler et al. 2002). Si-related compounds were not detected by PXRD as they were present as amorphous SiO₂.

Figure 3. PXRD patterns of Si–x (a) before and (b) after reduction in 5%H₂–Ar at 500 °C for 1 h.

After reducing Si–x in 5%H₂−Ar at 500 °C for 1 h, the peaks of metallic Ni (44° and 52°) and fluorite CeO₂ (28°, 33°, and 47°) were observed, as shown in Figure 3(b). The peaks of NiO disappeared owing to the reduction of NiO into metallic Ni. In addition, the reduction increased the intensity of the CeO₂ peaks, indicating the particle growth of CeO₂. The increase in the peak intensity of Si–0.2 was lower than that of the others because the presence of SiO₂ prevents the growth of CeO₂ (Reddy et al. 2005).

To quantitatively evaluate the particle growth, the crystallite sizes of Si–x were obtained. Table 1 shows that the crystallite sizes of CeO₂ and NiO in Si–x before the reduction were 3–4 nm and 4–5 nm, respectively. After the reduction, the crystallite size of CeO₂ slightly increased to 4–5 nm, but the size did not vary according to the SiO₂ fraction. Similarly, SiO₂ addition did not alter the crystallite size of Ni. Therefore, SiO₂ doping did not affect the crystallinity of the FSP-made Ni/CeO₂.

Table 1. Crystallite sizes of Si–x before and after reduction in 5%H₂–Ar at 500 °C for 1 h.

SiO2 fraction, x	As-prepared Si–x		Si–x reduced in 5%H2–Ar at 500 °C for 1 h
	dXRD of CeO2	dXRD of NiO	dXRD of CeO2	dXRD of Ni
0	3	5	5	7
0.03	4	5	5	6
0.05	4	4	5	6
0.08	4	4	5	6
0.1	4	4	5	7
0.2	3	4	4	6

During the PXRD measurements, the reduced catalysts were exposed to the atmosphere. Consequently, the surface of Ni particles was subjected to oxidation, resulting in a small crystallite size compared to the actual Ni size. To assess the Ni size while mitigating the effects of surface oxidation, the Ni surface area was obtained by H₂ pulse titration. Figure 4 exhibits the Ni surface area of Si–x reduced in 5%H₂–Ar at 500 °C for different durations (1–15 h). As the SiO₂ fraction increased from 0 to 0.05, the Ni surface area of Si–x reduced for 1 h increased from 16 to 20 m_Ni² g_cat⁻¹. As x increased further, the surface area decreased. As a result, the optimal SiO₂ fraction for maximum Ni surface area was x = 0.05. Interestingly, at the optimal x, the catalytic activity reached the highest among the tested catalysts. The Ni surface area at x = 0.1 was comparable with that at x = 0, although the CO₂ conversion of the former was lower than that of the latter. This is due to the increase of inactive Ni/SiO₂ and the decrease of active Ni/CeO₂ (Liu et al. 2022). Therefore, the increase of the Ni surface area by the appropriate SiO₂ addition promoted the catalytic activity.

Figure 4. Ni surface area of Si–x reduced in 5%H₂–Ar at 500 °C for different durations (1–15 h).

The effect of the SiO₂ addition on the sintering of Ni particles was investigated by monitoring the evolution of the Ni surface area at the different reduction times (Figure 4). Over time, the surface area of Si–0 gradually decreased; the surface areas were 16, 15, and 14 m_Ni² g_cat⁻¹ at 1, 5, and 15 h of the reduction, respectively. Conversely, the surface area of Si–0.05 remained comparable (20, 19, and 19 m_Ni² g_cat⁻¹ at 1, 5, and 15 h). Similarly, the surface area of Si–0.1 did not change as reduction time increased. Therefore, the SiO₂ doping to FSP-made 60 wt% Ni/CeO₂ prevented the sintering of Ni particles.

The SiO₂ doping of the catalysts can hinder the sintering of Ni particles. Therefore, the porosities of Si–x are expected to change in the presence of SiO₂. Figure 5(a) shows the N₂ adsorption–desorption isotherms of Si–x reduced in 5%H₂-Ar at 500 °C for 1 h. For all the SiO₂ fractions, the isotherms were classified as type IV with a hysteresis above p/p_o = 0.85, indicating the presence of a porous structure. From the isotherms, the pore size distributions of the catalysts were obtained. Figure 5(b) shows that all the catalysts consisted of mesopores and macropores (10–100 nm). The pore distribution of x = 0.05 was broader than those of the others (x = 0 and 0.1). The broad distribution of the former was attributed to its large pore volume compared to that of the latter. FSP-made powders, including NiO and CeO₂ (Fujiwara et al. 2021), typically have large pore volumes because of their fractal structure (Pratsinis 1998). Therefore, the large pore volume of Si–0.05 contributed to its excellent catalytic activity.

Figure 5. (a) N₂ adsorption–desorption isotherms and (b) pore size distribution of Si–0, Si–0.05, and Si–0.1 reduced in 5%H₂–Ar at 500 °C for 1 h.

As proven by the H₂ adsorption measurements (Figure 4) and N₂ adsorption (Figure 5), an appropriate amount of SiO₂ doping prevents the Ni sintering. It is expected that SiO₂ particles were present between Ni and/or CeO₂ particles (Tani et al. 2002). Such SiO₂ particles hindered the sintering of Ni particles resulting in an increase in the Ni surface area (Charisiou et al. 2019). The reduction in the Ni surface area at x ≥ 0.1 may be due to excess SiO₂ doping. Overloaded SiO₂ particles were deposited on the Ni particles covering the Ni surface (Schulz et al. 2003). HAADF-STEM and EDX mapping measurements were performed to confirm the location of SiO₂ in the catalysts. According to HAADF-STEM images in Figure 6(a,e), the primary particle sizes of the reduced Si–0.05 and Si–0.2 were about 10–20 nm with high uniformity. For both catalysts, EDX mappings of Ni species (Figure 6(b,f)) exhibited 10–20 nm of special spots. In contrast, Si (c, g) and Ce (d, h) species were uniformly dispersed through the particles indicating that the sizes of CeO₂ and SiO₂ particles were smaller than those of Ni particles. In addition, the distributions of Si and Ce species were close to Ni particles. Therefore, CeO₂ and SiO₂ particles were located around Ni particles. At a high SiO₂ fraction (x = 0.2), the location of SiO₂ and CeO₂ was further investigated as illustrated in Figure 7. Because of the characteristic lattice fringes of CeO₂ (0.32 nm), CeO₂ particles were distinguished. The size of CeO₂ was below 5 nm, which was in agreement with the crystallite size. Also, amorphous compounds (dotted circles) that correspond to SiO₂ were found. Both CeO₂ and SiO₂ were present on relatively large particles (∼10 nm), which were Ni particles as confirmed by the EDX mappings (Figure 6(f)). Some of the SiO₂ particles on Ni particles hindered the contact between adjacent Ni particles, preventing the sintering of Ni particles. However, it is difficult to quantify the amount of SiO₂ located between Ni particles.

Figure 6. (a, e) HAADF-STEM and (b–d, f–h) EDX mapping images of Ni, Si and Ce species of Si–0.05 and Si–0.2 reduced under 5% H₂–Ar at 500 °C for 1 h.

Figure 7. High-resolution HAADF-STEM image of Si–0.2 reduced under 5% H₂–Ar at 500 °C for 1 h.

In this study, the activity test and material properties were investigated after reducing catalysts in 5%H₂–Ar at 500 °C. The reducibility of Si–x was explored to clarify the state of the Ni species in the catalysts after the reduction. Figure 8 shows the H₂-TPR profiles of Si–x catalysts. At x ≤ 0.2, two small peaks at approximately 180 and 240 °C were observed. In contrast, these peaks were not observed in Si–1 (60 wt% Ni/SiO₂). Large peaks above 250 °C were observed for all the catalysts. The peak locations shifted to higher temperatures at higher SiO₂ fractions. The peaks at relatively low temperatures (<250 °C) corresponded to the reduction of Ni–O–Ce sites at the Ni–CeO₂ interface and/or that of Ni²⁺ substituted into the CeO₂ lattice (Shan et al. 2003; Tada et al. 2021). Notably, the low-temperature peaks were not observed in Si–1, which did not contain CeO₂. The peaks above 250 °C corresponded to the reduction of NiO particles to metallic Ni, and the peak locations changed based on the interaction between NiO and the support (Shan et al. 2003). According to literature (He et al. 2009), interactions between NiO and SiO₂ particles form nickel silicate, for which the reduction temperature is higher than that of the particulate NiO. Therefore, the shift of the peaks (at 300–700 °C) to higher temperatures indicates better contact between NiO and SiO₂. This interaction prevents the sintering of Ni particles during the reduction at 500 °C, resulting in the increase of the Ni surface area, as shown in Figure 4.

Figure 8. H₂-TPR profiles of Si–x catalysts. As a reference, the H₂-TPR profile of Si–1 (60 wt% Ni/SiO₂) was obtained.

Using the profiles, the H₂ consumptions by Si–x were calculated, as shown in Table 2. For all the catalysts, the amounts of H₂ consumed by Si–x were comparable (8.5–8.8 mmol g_cat⁻¹). As discussed in Figure 8, the H₂ consumption predominantly originated from the reduction of nickel oxides. As confirmed by PXRD (Figure 3(a)), the major nickel species in Si–x was NiO. Therefore, assuming that all Ni species in Si–x were NiO, Si–x contains 8.8 mmol g_cat⁻¹ of NiO. The NiO content is consistent with the amounts of H₂ consumption (8.5–8.8 mmol g_cat⁻¹). Therefore, NiO in the catalysts was completely reduced to metallic Ni by the reduction at 500 °C because, at x ≤ 0.2, all reduction peaks appeared below 500 °C (Figure 8).

Table 2. H₂ consumption of Si–x calculated using the peak area of the H₂-TPR profiles.

Samples	H2 consumption, mmol gcat^–1
Si–0	8.8
Si–0.03	8.7
Si–0.05	8.7
Si–0.08	8.8
Si–0.1	8.5
Si–0.2	8.5
Si–1	8.8

Figure 9 shows the Arrhenius plots of Si–0 and Si–0.05 for the reaction rate of CO₂. The Arrhenius plots were obtained using the reaction conditions (200–230 °C), where the CO₂ conversion was 4–24%. The reaction rate of Si–0.05 was higher than that of Si–0. Conversely, for both Si–0 and Si–0.05, the slopes of the plots were similar. The apparent activation energy values obtained by the slopes were 100 and 105 kJ mol⁻¹ for Si–0 and Si–0.05, respectively. The activation energies were consistent with the values obtained by Ni/CeO₂ catalysts (95–112 kJ mol⁻¹) in the literature (Bian et al. 2020; Lin et al. 2021), indicating that SiO₂ addition did not modify the reactivity of the active sites in the catalysts. According to literature (Liu et al. 2022), Ni/SiO₂ shows lower activity than Ni/CeO₂ for CO₂ methanation. Thus, the Ni/SiO₂ compounds did not work as an active sites for the reaction, even though the optimal SiO₂ doping (x = 0.05) led to an increase in the Ni surface area. In the case of excess SiO₂ doping (x > 0.05), the Ni surface area decreased, and the fraction of inactive Ni/SiO₂ increased. Therefore, there is an optimal SiO₂ fraction that can be determined by maximizing the Ni surface area while minimizing the loss of active Ni/CeO₂ compounds.

Figure 9. Arrhenius plots of Si–0 and Si–0.05 for the reaction rate of CO₂.

4. Conclusions

In this study, 60 wt% Ni on xSiO₂–(1 – x)CeO₂ catalysts were prepared by FSP, and the effects of SiO₂ fraction (x = 0–0.2) on the material properties and CO₂ methanation activity were investigated. The SiO₂ addition did not alter the crystallinity of Ni/CeO₂ catalysts. Conversely, the appropriate amount of SiO₂ doping (x ≤ 0.05) increased the Ni surface area of the catalysts reduced in 5%H₂−Ar at 500 °C because SiO₂ particles located between Ni particles prevented the sintering of Ni particles as confirmed by HAADF-STEM and EDX mapping. The resistance of Ni sintering by SiO₂ was attributed to the increase of the pore volume at x = 0.05. However, excess SiO₂ addition (x ≥ 0.1) decreased both the Ni surface area and pore volume of the catalysts. At the optimal SiO₂ fraction (x = 0.05), where the Ni surface area was maximal, the highest catalytic activity for CO₂ methanation was exhibited. Furthermore, SiO₂ addition improved the long-term catalytic activity. Therefore, the optimal amount of SiO₂ addition to 60 wt% Ni/CeO₂ catalyst through FSP enhances the thermal stability and catalytic activity.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was supported by the tryout program (JPMJTM20BJ) of the Japan Science and Technology Agency and the research grant by the Information Center of Particle Technology, Japan. Electron microscopy measurements were supported by “ARIM Project of the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), Grant No. JPMXP1223OS0036” at the Research Center for Ultra-High Voltage Electron Microscopy (Nanotechnology Open Facilities) in Osaka University.