Experimental Study on CO₂ Methanation over Ni/Al₂O₃, Ru/Al₂O₃, and Ru-Ni/Al₂O₃ Catalysts

Abstract

CO₂ methanation is recognized as one of the best technologies for storing intermittent renewable energy in the form of CH₄. In this study, CO₂ methanation performance is investigated using Ni/Al₂O₃, Ru/Al₂O₃, and Ru-Ni/Al₂O₃ as the catalysts under conditions of atmospheric pressure, a molar ratio of H₂/CO₂ = 5, and a space velocity of 5835 h⁻¹. For reaction temperatures ranging from 250 to 550 °C, it was found that the optimum reaction temperature is 400 °C for all catalysts studied. At this temperature, the maximum values of CO₂ conversion, H₂ efficiency, and CH₄ yield and lowest CO yield can be obtained. With temperatures higher than 400 °C, reverse CO₂ methanation results in CO₂ conversion and CH₄ yield decreases with increased temperature, while CO is formed due to reverse water-gas shift reaction. The experimental results showed that CO₂ methanation performance at low temperatures can be enhanced greatly using the bimetallic Ru-Ni catalyst compared with the monometallic Ru or Ni catalyst. Under ascending-descending temperature changes between 250 °C and 550 °C, good thermal stability is obtained from Ru-Ni/Al₂O₃ catalyst. About a 3% decrease in CO₂ conversion is found after three continuous cycles (74 h) test.

1. Introduction

The efficient utilization of renewable energy resources from wind and sun is an ongoing challenge. Electrical power from wind and solar devices is intermittent and produced at locations where it is not directly consumed. Thus, solutions for long-range renewable energy transportation and storage are currently of great interest [1,2,3]. Power-to-Gas (PtG) technology might contribute to solving this issue [4,5]. PtG links the power grid with the gas grid by converting surplus power into a grid compatible gas via a two-step process: H₂ production by water electrolysis and H₂ conversion with captured CO₂ from combustion or other sources into CH₄ via methanation [6]. The resulting CH₄ is known as substitute natural gas (SNG). It can be either injected into the existing gas distribution grid, stored, or utilized in other well established natural gas facilities.

The stoichiometric CO₂ methanation reaction is:

$${\mathrm{CO}}_2+4{\mathrm{H}}_2\leftrightarrow {\mathrm{CH}}_4+2{\mathrm{H}}_2\mathrm{O}; \mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}=-165 \mathrm{kJ}/\mathrm{mole}$$

(1)

CO₂ methanation is composed of two successive reactions: reverse water-gas shift reaction (RWGS) and CO methanation:

$$\text{RWGS:}\hspace{1em}{\mathrm{CO}}_2+{\mathrm{H}}_2\leftrightarrow \mathrm{CO}+{\mathrm{H}}_2\mathrm{O}, \mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}=42 \mathrm{kJ}/\mathrm{mole}$$

(2)

$$\text{CO methanation:}\hspace{1em}\mathrm{CO}+3{\mathrm{H}}_2\leftrightarrow {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}, \mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}=-206 \mathrm{kJ}/\mathrm{mole}$$

(3)

Note also that CO₂ methanation is exothermic. Thus, lower reaction temperature is desirable for higher CH₄ selectivity. It is therefore important to enhance the RWGS reaction rate and CO methanation at low temperatures. Different mechanisms have been proposed for CO₂ methanation. Eckle et al. [7] and Fisher et al. [8] proposed a CO₂ methanation mechanism that involves direct dissociation of CO₂ to CO(_ads) and O(_ads) on the metal surface, with CO(_ads) subsequently hydrogenated into CH₄. CO₂ dissociation has been generally recognized as the rate-determining step of the reaction [9,10]. The CO₂ methanation reaction is also known as the Sabatier reaction and has been studied extensively using different types of metals and supports [11,12,13]. Ni-, Ru-, and Rh-based catalysts have been proven to have good catalytic activity in CO₂ methanation [14,15,16]. Among these metals, Ni-based catalysts are the most commonly used catalysts because of their high activity and CH₄ selectivity. It was also found that Ni-based catalysts could maintain very good activity over a long reaction time with high CH₄ selectivity. However, high CO₂ conversion is difficult to achieve at low temperatures on Ni catalysts because high activation energy is required [17]. Compared with Ni, Ru-based catalysts are more active in CO₂ methanation and produce CH₄ almost exclusively. Moreover, it has also been shown that Ru-based catalysts have good activity at low temperatures [18,19,20].

Some efforts have been made to develop effective Ni-based catalysts with satisfactory catalytic activity for CO₂ methanation at low temperatures. Rahmani et al. [21] pointed out that Ni dispersion can be enhanced by the addition of Ce. In the study by Lee et al. [22], a Ni-based catalyst prepared using dielectric barrier discharge (DBD) plasma produced higher metal dispersion and fewer defect sites than the impregnation method. Both studies pointed out that high Ni dispersion can enhance Ni catalyst activity at low temperatures. In a study by Xu et al. [23], a series of rare-earth metals (La, Ce, Sm, and Pr) were doped onto Ni-based catalysts and used in CO₂ methanation. It was found that the apparent activation energy could be decreased using rare-earth dopants and that low-temperature catalytic activity could be greatly intensified over these rare-earth element catalysts. Additionally, some studies showed that Ni support is one of the factors that affect Ni activity at low temperatures. The catalytic activity for CO₂ methanation of Ni with various metal oxides as supports was evaluated in a study by Muroyama et al. [24]. They found that the order of CH₄ yield at 250 °C was Ni/Y₂O₃ > Ni/Sm₂O₃ > Ni/ZrO₂ > Ni/CeO₂ > Ni/Al₂O₃ > Ni/La₂O₃. In a study by Abate et al. [25], CO₂ methanation performance was studied using Ni supported by γ-Al₂O₃-ZrO₂-TiO₂-CeO₂ composite oxide. Their experimental results revealed that the performance of composite oxide-supported Ni-based catalysts was superior to that of only γ-Al₂O₃-supported Ni-based catalysts.

Although Ru-based catalysts have been proven to present good activity in low-temperature CO₂ methanation reactions, their performance at high temperatures has not been widely reported in the literature. Conversely, Ni has low activity at low temperatures but good activity at high temperatures. Based on these observations, bimetallic Ru-Ni catalysts could combine the advantages of each catalyst for use in a wide range of reaction temperatures for CO₂ methanation. The Ru-Ni bimetallic catalyst has been widely adopted in many chemical reactions such as steam reforming of methane [26], dry reforming of methane [27], and CO cleanup in hydrogen-rich reformed gas for fuel cell applications [28,29]. Ru is the cheapest precious metal. Thus, Ru-Ni can keep the catalyst cost sufficiently low for industrial implementation. Polanski et al. [30] tested the catalytic performance of CO₂ methanation using Ni-supported Ru nanoparticles in silica. Their result indicated that the Ru-Ni catalyst is very productive and efficient for CO₂ methanation at low temperatures. In the study by Zhen et al. [31], bimetallic Ni-Ru catalysts supported on γ-Al₂O₃ were prepared using co-impregnation and sequential impregnation methods and used to investigate CO₂ methanation performance for temperatures in the 250–500 °C range under atmospheric pressure conditions. They found that the CO₂ methanation activities were highly dependent on the bimetallic catalyst preparation sequence. In a study by Navarro et al. [32], a structured Ru-Ni/MgAl₂O₄ catalyst was proposed for CO₂ methanation. Their results indicated that the catalyst exhibited high stability in a long-term test.

Although Ru-Ni catalysts have been successfully applied in many applications, their efficacy in CO₂ methanation involving higher CO₂ concentration needs to be clarified. For further insights, Ru/Al₂O₃, Ni/Al₂O₃, and Ru–Ni/Al₂O₃ catalysts with various Ru and Ni loadings were prepared and tested for CO₂ methanation in this study. The metal loading and reaction temperature effects on CO₂ conversion, H₂ efficiency, CH₄ yield, and CO yield were investigated in detail. The thermal stability of the prepared catalyst is also reported in this study.

2. Results and Discussion

2.1. Experimental Parameters

A catalyst with a weight of 0.5 g was used in this study, resulting in a reaction packed bed length of 4.2 cm. Accordingly, the catalyst bed length to catalyst particle size ratio and the ratio of the inside reactor diameter to particle size were in the ranges of 35~84 and 3~8, respectively. This ensured that the back mixing and channeling effects were minimized in the packed-bed reactor [33].

The activity test was performed from 250 to 550 °C in 50 °C increments and operated at atmospheric pressure. The reactant volume flow rate was fixed at 55 sccm, which was composed of 5 sccm CO₂, 25 sccm H₂, and 25 sccm N₂. This corresponded to a molar ratio of H₂/CO₂ = 5, while N₂ served as the balance inert gas and to avoid the thermal runaway since CO₂ methanation is an exothermic reaction [34,35,36]. Moreover, the N₂ flow rate was used as the reference flow to evaluate the flow rates of the gas species involved in the product. Based on the total reactant flow rate and catalyst bed volume, the corresponding gas hourly space velocity (GHSV) was 5835 h⁻¹. After removing water using a condenser, the dried product gas mixture was collected every 30 min for each temperature. To ensure stable reaction performance, the time on stream measurement for each temperature lasted at least 2 h.

2.2. Performance Indices

The CO₂ methanation performance was characterized using CO₂ conversion, H₂ efficiency, CH₄ yield, and CO yield, defined as follows,

$${\mathrm{C}\mathrm{O}}_2\text{ conversion: }{X}_{{\mathrm{CO}}_2}=\frac{{\dot{F}}_{{\mathrm{CO}}_2,\mathrm{in}}-{\dot{F}}_{{\mathrm{CO}}_2,\mathrm{out}}}{{\dot{F}}_{{\mathrm{CO}}_2,\mathrm{in}}}\times 100\%$$

(4)

$${\mathrm{H}}_2\text{ efficiency: }{\eta }_{{\mathrm{H}}_2}=\frac{({\dot{F}}_{{\mathrm{CO}}_2,\mathrm{in}}-{\dot{F}}_{{\mathrm{CO}}_2,\mathrm{out}})+({\dot{F}}_{\mathrm{CO},\mathrm{out}}-{\dot{F}}_{\mathrm{CO},\mathrm{in}})}{{\dot{F}}_{{\mathrm{H}}_2,\mathrm{in}}}\times 100\%$$

(5)

$${\mathrm{C}\mathrm{H}}_4\text{ yield: }{Y}_{{\mathrm{CH}}_4}=\frac{{\dot{F}}_{{\mathrm{CH}}_4,\mathrm{out}}}{{\dot{F}}_{{\mathrm{CO}}_2,\mathrm{in}}}\times 100\%$$

(6)

$$\text{CO yield: }{Y}_{\mathrm{CO}}=\frac{{\dot{F}}_{\mathrm{CO},\mathrm{out}}}{{\dot{F}}_{{\mathrm{CO}}_2,\mathrm{in}}}\times 100\%$$

(7)

In these equations, ${\dot{F}}_{{\mathrm{CO}}_2,\mathrm{in}}$ is the CO₂ molar flow rate at the reactor inlet, ${\dot{F}}_{{\mathrm{CO}}_2,\mathrm{out}}$, ${\dot{F}}_{{\mathrm{CH}}_4,\mathrm{out}}$, and ${\dot{F}}_{\mathrm{CO},\mathrm{out}}$ are the molar flow rates of CO₂, CH₄, and CO at the reactor outlet, respectively. In Equation (4), CO₂ conversion is defined based on CO₂ consumption. H₂ is expected to be the limiting reactant in large scale applications. Therefore, it is important to determine the amount of CO₂ conversion compared to the amount of H₂ available. This is defined as the H₂ efficiency, as shown in Equation (5). Note that CO formation from converted CO₂ is included in the H₂ efficiency. In the CO₂ methanation reaction, the CO₂ is regarded as the feedstock, while CH₄ and CO are the products from the reaction. Instead of selectivity, the inlet molar flow rate of CO₂ is used as the reference to define the yields of CH₄ and CO, as shown in Equations (6) and (7). The product yield for every mole of CO₂ supply can be identified. The CH₄ selectivity can be evaluated from CO₂ conversion and CH₄ yield, as defined in Equations (4) and (6), respectively.

2.3. Catalyst Characterization

FE-SEM-EDX (JSM-7401F, JEOL, (Tokyo, Japan) analysis was used to analyze the general morphology and the chemical composition of the prepared catalysts. A typical FE-SEM-EDX analysis for the 5 wt% Ru/Al₂O₃ catalyst is shown in Figure 1. It can be seen that Ru was well dispersed on alumina from FE-SEM micrograph because the particle brightness appeared to be quite homogeneous in the micrograph. The EDX analysis showed the presence of Ru, Al, and O. The Ru amount measured by EDX was close to the designed Ru loading of 5 wt% of Al₂O₃.

The catalyst crystal structure was characterized by X-ray diffractometer (XRD, Rigaku, Japan) using a Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. The 2θ angle was scanned at a rate of 1.5°/min within the 20° < 2θ < 90° range. XRD patterns of the calcined and reduced catalysts are shown in Figure 2. In Figure 2a for the calcined 10 wt% Ni/Al₂O₃, the peaks at 37.6°, 43.2°, and 63.0° were attributed to the (111), (200), and (220) planes of NiO species (JCPDS65-6920) [37], respectively. These peaks disappeared after reduction and the Ni crystalline corresponding to the diffraction peaks appeared at 44.5°, 51.7°, and 76.4°. As pointed out by Penkova et al. [38], the Ni loading is critical to the formation of nickel aluminate spinels (NiAl₂O₄). For the slow Ni loading cases in this study, the reduction conditions were adequate to reduce all the Ni in its metallic form [21,39]. Figure 2b shows the XRD patterns for the calcined and reduced 5 wt% Ru/Al₂O₃ catalyst. In the calcined Ru/Al₂O₃, RuO₂ peaks appeared at 28.1°, 35.1°, and 54.4°. After reduction, these peaks disappeared and the Ru peaks appeared at 42.2°, 44°, 58.3°, 69.4°, and 78.2°. Figure 2c shows the XRD patterns of the calcined and reduced 1 wt% Ru-15 wt% Ni/Al₂O₃ catalysts. It can be seen that peaks corresponding to Ni, Ru, NiO, and RuO₂ were observed in the reduced and calcined Ru-Ni/Al₂O₃ catalyst. The mean Ni or Ru crystallite size could be evaluated from XRD patterns shown in Figure 2a,b for 10 wt% Ni/Al₂O_3, and 5 wt% Ru/Al₂O₃, respectively. From the Scherrer equation, the estimated mean crystallite sizes of Ni and Ru were 22 nm and 10 nm, respectively. Note that bigger crystallite sizes resulting from high metal loading have been substantiated in the literature [40,41].

2.4. Ni Loading Effect

The time on stream measurement of CO₂ methanation performance from 250 to 550 °C under various Ni loadings characterized by CO₂ conversion, H₂ efficiency, CH₄ yield, and CO₂ yield is shown in Figure 3. Because the product gas was collected and analyzed every 30 min for each temperature within 2 h, there were four data for each temperature, as shown in Figure 3. From the result shown in Figure 3, a stable reaction could be obtained within 2 h for each temperature. Figure 3a shows the result for CO₂ conversion based on Equation (4). CO₂ conversion can be enhanced by increasing the Ni loading for low-temperature regimes (250 and 300 °C). However, higher CO₂ conversion was obtained for 15 wt% Ni/Al₂O₃ instead of 20 wt% /Al₂O₃ for temperatures in the 350 to 450 °C range. For temperatures higher than 500 °C, no noticeable difference was obtained in CO₂ conversion for both 15 and 20 wt% catalysts. However, the Ni loading effect on CO₂ conversion could still be found for temperatures higher than 500 °C. Based on Figure 3a, the Ni loading should be greater than 10 wt% to have effective CO₂ conversion.

Figure 3b shows the H₂ efficiency. In the stoichiometric CO₂ methanation reaction with a molar ratio of H₂/CO₂ = 4, only 1/2 of the H₂ is converted into CH₄ and the other 1/2 is converted into water. Additionally, 2 moles of H₂ are required to produce one mole of CH₄. Therefore, the maximum theoretical H₂ efficiency that could be achieved is only 25%. With a molar ratio of H₂/CO = 5 used in this study, the maximum H₂ efficiency would be 20%. Due to low catalytic activity, almost zero or low H₂ efficiency results from low temperature (250 and 300 °C) and low Ni loadings. Similar to CO₂ conversion, H₂ efficiency increases with the temperature, indicating that more H₂ is consumed in the reaction. For temperatures in the 350 to 450 °C range, higher H₂ efficiency was obtained from 15 wt% Ni/Al₂O₃ instead of 20 wt% Ni/Al₂O₃. The maximum H₂ efficiency was observed at 400 °C. For temperatures higher than 450 °C, H₂ efficiency decreased with increased temperature. In this high-temperature regime, higher H₂ efficiency was obtained from 15 wt% Ni/Al₂O₃.

In Figure 3c, the CH₄ yield as a function of the reaction temperature and time on stream is shown. Similar to CO₂ conversion, the CH₄ yield increased with increased temperature and Ni loading. Because of low catalytic activity, low CH₄ yield occurred when the temperature was low. The maximum CH₄ yield was obtained at 400 °C. With temperatures higher than 400 °C, the CH₄ yield decreased with increased temperature.

As shown in Figure 3d, CO formed in the reaction at high temperatures. As pointed out above, the CO₂ methanation reaction is composed of two successive reactions: RWGS and CO methanation. Therefore, CO is one of the intermediates in CO₂ methanation produced from RWGS. Because the RWGS reaction is endothermic and favored thermodynamically and kinetically at high temperatures, CO formation was found in the high-temperature regime, as shown in Figure 3d. Another reaction that contributes to CO formation is CO₂ dissociation:

$${\mathrm{C}\mathrm{O}}_2\text{ dissociation: }{\mathrm{CO}}_{2\mathrm{ads}}\to {\mathrm{CO}}_{\mathrm{ads}}+{\mathrm{O}}_{\mathrm{ads}}$$

(8)

As mentioned above [9,10], CO₂ dissociation into CO is considered a rate-determining step of CO₂ methanation.

To have a better understanding of the Ni loading and temperature effects on CO₂ methanation, the averaged values of CO₂ conversion, H₂ efficiency, CH₄ yield, and CO yield based on the results shown in Figure 3 are plotted in Figure 4. The equilibrium results calculated by taking CO₂ methanation and RWGS reactions into account under the same experimental operating conditions are also shown in Figure 4. As shown in Figure 4a, it is indicated that CO₂ conversion increased with increasing Ni loading. For the 20 wt% Ni/Al₂O₃ case, CO₂ conversion was lower than that obtained from 15 wt% Ni/Al₂O₃ for temperatures higher than 350 °C. The reason for decreased CO₂ conversion with high Ni loading may be due to decreased nickel dispersion resulting in bigger crystallite size [40,41]. Moreover, Figure 4a shows that the CO₂ conversion increases with temperature reached a maximum value at 400 °C, and then decreased with temperature. Since the CO₂ methanation is an exothermic reaction, the CO₂ conversion decrease in high temperature was due to the reverse CO₂ methanation reaction in which CO₂ was reproduced. As shown in Figure 4a, the maximum CO₂ conversion, i.e., 85%, was obtained using the 15 wt% Ni/Al₂O₃ catalyst at 400 °C. As the temperature exceeded 400 °C, the catalytic CO₂ conversion obtained from 15 wt% Ni/Al₂O₃ and 20 wt% Ni/Al₂O₃ was about the same as the thermodynamic equilibrium result.

As also shown in Figure 4a, the catalytic CO₂ conversion over Ni catalysts was lower than the equilibrium CO₂ conversion, which gradually decreased with the increase in temperature due to the thermodynamic nature of CO₂ methanation. From the stoichiometric reaction, the change in Gibbs free energy for CO₂ methanation is:

$$\mathsf{\Delta }{\mathrm{G}}_{298\mathrm{K}}=(-173+0.1983\mathrm{T}) \mathrm{kJ}/\mathrm{mole}$$

(9)

This is favorable for CH₄ production from the chemical equilibrium viewpoint for Gibbs free energy ΔG being less than zero. In such cases, the equilibrium constant is positive. However, with the increase in temperature, ΔG increases, and the reaction shifts towards the reactants. This accounts for the decrease in equilibrium CO₂ conversion at high temperature and is also called the reverse CO₂ methanation reaction. The reason for the poor catalytic activity at low temperature is that CO₂ methanation is an eight-electron process for fully reducing the CO₂ (+4) into CH₄ (−4), which usually has a great kinetic barrier to activate the stable CO₂ molecule, and requires an efficient catalyst to decrease the activation energy [42].

The H₂ efficiency corresponding to Figure 4a is shown in Figure 4b. Similar to CO₂, Figure 4b shows that the H₂ efficiency increases with the increase in Ni loading. H₂ efficiency increased with temperature, reached a maximum value at 400 °C, and then decreased with temperature. The maximum H₂ efficiency, i.e., 17%, was achieved with a 15 wt% Ni/Al₂O₃ catalyst.

Figure 4c shows the averaged CH₄ yield based on the results shown in Figure 3c. Based on the CO₂ conversion and H₂ efficiency results, the CH₄ yield can be enhanced by increasing the temperature and Ni loading. The maximum CH₄ yield was obtained at 400 °C with a value of 80% from 15 wt% Ni/Al₂O₃ catalyst. At temperatures higher than 400 °C, CH₄ yield decreased due to the reverse CO₂ methanation mentioned above. Additionally, CH₄ yield may also decrease due to methane-steam reforming (MSR) and CH₄ decomposition reactions:

$$\text{MSR: }{\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{CO}+3{\mathrm{H}}_2\text{, }\mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}={206}_{}^{} \mathrm{kJ}/\mathrm{mole}$$

(10)

$${\mathrm{C}\mathrm{H}}_4\text{ decomposition: }{\mathrm{CH}}_4\leftrightarrow 2{\mathrm{H}}_2+\mathrm{C}\text{, }\mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}={75.6}_{}^{} \mathrm{kJ}/\mathrm{mole}$$

(11)

Both reactions are endothermic and favor high temperatures. Also, CO was formed in the RWGS reaction causing less carbon to be formed into CH₄. Because of these reactions, it can be seen that catalytic CH₄ yield departs from the thermodynamic equilibrium result to a higher extent compared with the CO₂ conversion shown in Figure 4a. Note also that H₂ can be produced from MSR and CH₄ decomposition, while it is consumed in the RWGS reaction. The amount of net H₂ production from these reactions may affect the H₂ efficiency described in Figure 4b.

Figure 4d shows the CO yield as a function of the temperature and Ni loading. From Equation (10), CO may also be produced from MSR in addition to RWGS at high temperatures. Another possible reaction for CO formation is due to the Boudouard reaction:

$${\mathrm{CO}}_2+\mathrm{C}\leftrightarrow 2\mathrm{CO}\text{, }\mathsf{\Delta }{\mathrm{H}}_{298\mathrm{K}}={172.4}_{}^{} \mathrm{kJ}/\mathrm{mole}$$

(12)

In this reaction, CO₂ is reacted with C to form CO. Since this reaction is endothermic, it is favorable at high temperatures. Based on Figure 4d, less CO is produced when the Ni loading is high. When the Ni loading is low (5 wt% Ni/Al₂O₃ case), CO yield is high due to poor activity for converting CO into CH₄. Moreover, the temperature at which CO forms can be as low as 300 °C for 5 wt% Ni/Al₂O₃ catalyst. Note that 15 wt% Ni/Al₂O₃ produces the lowest amount of CO among the catalysts studied.

In Equation (11), carbon is formed from CH₄ decomposition and consumed in the Boudouard reaction, as shown in Equation (12). The net carbon production may be deposited onto the catalyst surface, leading to catalyst activity loss. Carbon balance can be performed based on the carbon supply in the reactant and carbon production in the product [43]. For CO₂ methanation, the carbon supply is the CO₂, while the carbon productions are in the forms of CO and CH₄. Using the data shown in Figure 4 as an example, for T = 400 °C and 15% wt% Ni/Al₂O₃ catalyst, CO₂ conversion was 88%, CH₄ yield was 80%, and CO yield was 1%. This corresponds to about 7% of carbon deposition onto the catalyst and reactor surfaces. Note that CO yield was primarily due to RWGS which was not active for low temperature and produced low CO yield. Also, there was no carbon formation found from thermodynamic equilibrium analysis for the reactant composition used in this study, which agreed with the study by Janke et al. [44]. The 7% difference between the carbon supply and carbon production may have been due to experimental uncertainty.

2.5. Ru Loading Effect

The CO₂ methanation performance using the Ru catalyst is shown in Figure 5. Similar to the Ni case, the CO₂ conversion, H₂ efficiency, CH₄ yield, and CO yield values shown in Figure 5 are based on the averaged values obtained from the time on stream measurement which is provided in Figure S1 in the supplementary information. From Figure 5a, the Ru activity increased with temperature and Ru loading. Compared with the Ni catalyst results shown in Figure 4a, Ru is more active at low-temperature regimes. As shown in Figure 5a, the CO₂ conversion was 38% with the 3 wt% Ru/Al₂O₃ catalyst, but only 10% CO₂ conversion was obtained with the 20 wt% Ni/Al₂O₃ catalyst shown in Figure 4a. Similar to the Ni case, the CO₂ conversion increased with temperature, reaching a maximum value at 400 °C, and then decreased as the temperature increased further. A maximum CO₂ conversion of 90% could be obtained from 3 wt% Ru/Al₂O₃ catalyst, which was also 5% higher than that obtained from 15 wt% Ni/Al₂O₃, as shown in Figure 4a. For temperatures higher than 400 °C, CO₂ conversion decreased dramatically with increased temperature. Compared with the result for Ni shown in Figure 4a, CO₂ conversion did not follow the variation trend as it did in the thermodynamic equilibrium case. This was because Ru is more active for RWGS reactions in the higher temperature regime, as will be discussed later.

Figure 5b shows the H₂ efficiency corresponding to the results shown in Figure 5a. Maximum H₂ efficiency of 18% was obtained from the 5 wt% Ru/Al₂O₃ catalyst at 400 °C. It is interesting to note that low H₂ efficiency was found for temperatures greater than 400 °C. At a temperature of 550 °C, very low H₂ efficiency was found for the 1 wt% Ru and 3 wt% Ru cases. That is, H₂ did not efficiently convert into CH₄ in this high-temperature regime.

Figure 5c shows that the CH₄ yield increased with increased Ru loading and temperature. At a temperature of 400 °C, a maximum CH₄ yield of 80% could be obtained in the 5 wt% Ru/Al₂O₃ case. With temperatures higher than 400 °C, the CH₄ yield decreased as the temperature increased. Note that low CH₄ yields resulted in temperatures of 500 and 550 °C. As mentioned above, the amount of H₂ converted into CH₄ was low. With a temperature of 550 °C, Sharma et al. [45] reported that low CH₄ yield resulted from using 5 wt% Ru/CeO₂ as the catalyst in their CO₂ methanation experiment. Similar to the Ni case, the CH₄ yield may also be reduced by reactions such as MSR and CH₄ decomposition.

Figure 5d shows the CO yield as functions of the Ru loading and temperature. For temperatures less than 400 °C, almost zero CO yield was obtained from all the Ru loadings studied. As the temperature increased beyond 400 °C the CO yield increased with increased temperature. In the high-temperature regime, CO₂ conversion was still high, although the CH₄ yield was low. From the measured CO results shown in Figure 5d, high CO₂ conversion occurred partially due to CO formation. In the 1 wt% Ru/Al₂O₃ case, the CO yield was as high as 40%. This indicated that Ru is highly active for endothermic RWGS for CO production but not for the subsequent CO methanation for methane production. It was also expected that the H₂ efficiency would be low since less H₂ is consumed in RWGS for producing water.

Based on the results shown in Figure 5, it can be concluded that Ru has higher activity at low temperatures compared with Ni for CO₂ methanation. However, Ru is not a suitable catalyst for CO₂ methanation at temperatures higher than 400 °C due to low CH₄ yield. In addition to the reaction temperature, Ru dispersion range on support is one of the factors that control the product from CO₂ methanation, as pointed out in the study by Kwak et al. [34]. With a low Ru loading, atomically dispersed Ru on Al₂O₃ produced CO exclusively in CO₂ methanation. With a higher Ru loading, 3D Ru clusters were formed that were highly active for CO₂ methanation. It was also suspected that bigger crystallite size resulting from increased temperature was one of the factors that reduced the CH₄ yield from the Ru catalyst.

2.6. Ru-Ni Loading Effect

CO₂ methanation performance using Ru-Ni catalysts is shown in Figure 6 and Figure 7. Similar to the Ni and Ru cases discussed above, CO₂ conversion, H₂ efficiency, CH₄ yield, and CO yield are based on the averaged time-on-stream measured data, which are provided in Figure S2 in the supplementary information. The CO₂ methanation performance using Ni, Ru, and Ru-Ni catalysts are plotted together. In Figure 6, CO₂ methanation performance using 1 wt% Ru-10 wt% Ni/Al₂O₃ catalyst is compared with those using 10 wt% Ni/Al₂O₃ and 1 wt% Ru/Al₂O₃ catalysts. From Figure 6a for a temperature of 250 °C, the CO₂ conversion increased from 7% with 10 wt% Ni/Al₂O₃ to 15% when 1 wt% Ru-10 wt% Ni/Al₂O₃ catalyst was used. As pointed out in studies by Zhen et al. [31] and Skriver and Rosengaard [46], due to the difference in surface free energy, the Ru segregation tendency on the Ni catalyst surface can be identified. This tendency leads to higher bimetallic Ru-Ni catalyst activity compared with monometallic Ni or Ru catalyst, as clearly shown in Figure 6a. The maximum CO₂ conversion was obtained at 400 °C, with a value of 88%, as shown in Figure 6a. In the high-temperature regime, CO₂ conversions obtained from Ru-Ni and Ni catalysts followed the same variation trend as that of the equilibrium result. It was also seen that 1 wt% Ru-10 wt% Ni/Al₂O₃ catalysts could improve the CO₂ methanation activity as compared with the monoatomic 1 wt% Ru catalyst in the high-temperature regime.

Figure 6b shows the H₂ efficiency corresponding to the results shown in Figure 6a. With the increased CO₂ conversion using the Ru-Ni catalyst, the H₂ efficiency was also enhanced using the Ru-Ni catalyst, as shown in Figure 6b. Similarly, the CH₄ yield was also improved, as shown in Figure 6c. Significant improvement could be obtained compared with the 1 wt% Ru catalyst case for the high-temperature regime. Using the Ru-Ni catalyst, the CO yield could be kept at the same level as that of Ni catalyst, as shown in Figure 6d.

In Figure 7, CO₂ methanation performance using 1 wt% Ru-15 wt% Ni/Al₂O₃ catalyst is compared with those using 15 wt% Ni/Al₂O₃ and 1 wt% Ru/Al₂O₃ catalysts. As discussed in Figure 4 CO₂ methanation performance could be enhanced by increasing the Ni loading; also, the 15 wt% Ni/Al₂O₃ catalyst produced maximum CO₂ conversion at 400 °C. With the addition of 1 wt% Ru, Figure 7 clearly shows that further enhancement of CO₂ methanation performance was obtained. As shown in Figure 7a, the CO₂ conversion improved from 14% using 15 wt% Ni/Al₂O₃ to 40% using 1 wt% Ru-15 wt% Ni/Al₂O₃ at a temperature of 250 °C. CO₂ conversion with a value of 80% could be obtained for temperatures in the 300 to 400 °C range. The corresponding H₂ efficiency is shown in Figure 7b indicating that higher H₂ efficiency could be obtained in the low-temperature regime. In Figure 7c, the CH₄ yield could be improved from 5% with 15 wt% Ni/Al₂O₃ catalyst to 38% at 250 °C when a 1 wt% Ru-15 wt% Ni/Al₂O₃ catalyst was used. With higher CH₄ yield, CO formation could be kept at the same level as the thermodynamic equilibrium result, as shown in Figure 7d.

2.7. Stability Test

The thermal stability of 1 wt% Ru-10 wt% Ni/Al₂O₃ catalyst was tested using three continuous ascending-descending temperature cycles. That is, the temperature was raised from 250 °C to 550 °C at 50 °C increments and then reduced from 550 °C to 250 °C also at 50 °C increments. The test procedure was the same as that described in Figure 3. For each temperature, the time-on-stream measurement lasted two hours. The detailed time-on-stream measurement is shown in Figure S3 in the supplementary information. To complete three cycles, the catalyst was under testing for a total of 74 h. The test result is shown in Figure 8. Note that the 1 wt% Ru–10 wt% Ni/Al₂O₃ catalyst used for the result shown in Figure 6 was reused for the stability test. As shown in Figure 8, it showed that good catalyst thermal stability could be obtained. After three cycles, the maximum CO₂ conversion changed from 82% to 78%, as shown in Figure 8a; the maximum H₂ efficiency changed from 15% to 14%, as shown in Figure 8b; the maximum CH₄ yield changed from 78% to 73%, as shown in Figure 8c; and the maximum CO yield changed from 18% to 16%, as shown in Figure 8d. The reason for the decrease in CO₂ methanation performance after a long period of operation may be due to the gradual re-oxidization of Ni to NiO and oxidization Ru to RuO₂ by CO₂, or to the water produced. Because Ru has better resistance to carbon deposition than Ni [47], the Ru contained in the Ru-Ni catalyst played an important role in enhancing the activity and stability of the catalysts and achieved high CO₂ conversion and CH₄ yield [48,49].

3. Experimental

3.1. Catalyst Preparation

Ni/Al₂O₃, Ru/Al₂O₃, and Ru-Ni/Al₂O₃ with various metal loadings were prepared in this study using the wet incipient impregnation method. Spherical Al₂O₃ particles with an average diameter of 0.5~1.2 mm and a specific surface area of 220 m²/g were used as the catalyst support (Aldrich, USA). Aqueous Ni(NO₃)₂·6H₂O and Ru(NO)(NO₃)₃ (Merck, USA) were used as the Ni and Ru precursors, respectively. The bimetallic catalyst Ru-Ni/Al₂O₃ was prepared by adding Ru to Ni/Al₂O₃ using sequential impregnation [50]. All of the prepared catalysts in this study are listed in

Table 1. Abbreviation and metal loading amount for Al₂O₃-supported Ni, Ru, and Ru-Ni catalysts.

Catalyst Type	Catalyst
Ni catalyst	5 wt% Ni/Al2O3
	10 wt% Ni/Al2O3
	15 wt% Ni/Al2O3
	20 wt% Ni/Al2O3
Ru catalyst	1 wt% Ru/Al2O3
	3 wt% Ru/Al2O3
	5 wt% Ru/Al 2O3
Bimetallic catalyst	1 wt% Ru-10 wt% Ni/Al2O3
	1 wt% Ru-15 wt% Ni/Al2O3

Note: 1. The weight percentage is based on the weight of the support. 2. The chemical composition was verified by XRF (X-MET5100, Oxford Instruments, UK) and the accuracy is within 5%.

. The chemical composition was verified using XRF (X-MET5100, Oxford Instruments, UK) and the accuracy was shown to be within 5%. Figure 9 shows the prepared Al₂O₃-supported Ni, Ru, and Ru-Ni catalysts. After impregnation, the catalysts were dried in an oven at a temperature of 110 °C for 24 h. The catalysts were then calcined in the air flow at 700 °C for 2 h.

3.2. Experimental Setup

A schematic diagram of the experimental setup is shown in Figure 10a. The CO₂ and N₂ were supplied from gas bottles while H₂ was supplied from a hydrogen generator (H₂PEM-165, USA). The flow rate of each gas species was controlled using a mass flow controller MFC 5850 E (Brooks, USA). The gases were mixed in a mixer before entering the reactor. The CO₂ methanation was carried out using a conventional catalytic packed-bed tubular reactor operated isothermally in a temperature-controllable oven. A quartz tube with an inner diameter of 4 mm was used as the reactor in which the catalyst was loaded in the central section with two ends fixed using quartz wool, as shown in Figure 10b. The catalyst was activated using in situ reduction at 700 °C with 5% H₂ in N₂ before each experimental run [21,39]. After the reduction, the temperature was lowered to 250 °C, which is the initial activity test temperature. The activity test was performed from 250 to 550 °C. A thermocouple inserted into the catalyst bed was used to monitor the reaction temperature. After removing the product water with a condenser, the product gas was collected and analyzed using gas chromatography (Agilent 6890, USA).

4. Conclusions

CO₂ methanation was studied experimentally using Ni-, Ru-, and Ru-Ni-supported by Al₂O₃ as the catalysts in a fixed-bed reactor at atmospheric pressure. The H₂/CO₂ molar ratio and space velocity were fixed at 5 and 5835 h⁻¹, respectively. Based on the measured results, the following conclusions can be made:

(1)

For the three types of catalyst studied, the optimum reaction temperature was found to be 400 °C. At this temperature, maximum CO₂ conversion, maximum H₂ efficiency, maximum CH₄ yield, and minimum CO yield were obtained for all metal loadings studied.

(2)

CO₂ methanation performance at low temperatures could be enhanced by increasing the catalyst loading for all the catalysts studied. Compared with Ni, Ru was more active in the low-temperature regime. At higher temperature regimes, CO₂ methanation performance followed a variation trend resulting from the thermodynamic equilibrium of the Ni catalyst. The measured results indicated that Ru was active for the reverse water-gas shift reaction in the high-temperature regime. This led to a low CH₄ yield and high CO yield.

(3)

The measured data indicated that the bimetallic Ru-Ni catalyst was more active compared with the monometallic Ni or Ru catalysts for CO₂ methanation in a low-temperature regime.

(4)

The Ru-Ni catalyst showed good thermal stability, i.e., about 3% and 5% decreases in CO₂ conversion and CH₄ yield resulted after 74 h testing, respectively.