**1. Introduction**

With the release of a large amount of CO2 into the atmosphere, the greenhouse effect has increased in recent years [1,2]. Facing this great challenge, there are three main ways to control CO2 emissions: 1. Reducing CO2 emissions, 2. capture and storage of CO2, and 3. chemical conversion and utilization of CO2 [3–5]. Converting CO2 into value-added chemicals is by far the most cost-effective method [6–8]. Clean and renewable energy resources (wind, solar, and tidal energy) produce discontinuous electricity, which is not capable of being connected to the grid. Whereas, hydrogen can be generated by electrolysis using this discontinuous electricity [9–11]. With this H2 supply, the CO2 methanation reaction is attracting more and more interest due to the use of both carbon dioxide and hydrogen obtained from renewable energy. Compared with hydrogen, methane has many advantages, which could be easily liquefied, stored, transported, and used by the natural gas infrastructure [12,13]. In 1902, Sabatier et al. came up with the CO2 methanation reaction (also called the Sabatier reaction) firstly, CO2 +4H2 → CH4 + 2 H2O, <sup>Δ</sup>*<sup>G</sup>* = −130.8 KJ·mol<sup>−</sup>1, <sup>Δ</sup>*<sup>H</sup>* = −165 KJ·mol−<sup>1</sup> [14]. It can be seen that the CO2 methanation reaction is highly exothermic, which is thermodynamically favorable but kinetically constrained [15]. Meanwhile, the heat released during the reaction will lead to the sintering of metal particles and deactivation of the catalysts [9,16]. Therefore, designing a high active and stable catalyst operating at low temperature is important [17].

Many studies have shown that VIII metals exhibit a catalytic performance for CO2 methanation, especially Ru [18], Rh [19], Pd [20], and Ni [21]. Although noble metal catalysts exhibit a higher catalytic activity and CH4 selectivity at low temperature, their

**Citation:** Li, L.; Wang, Y.; Zhao, Q.; Hu, C. The Effect of Si on CO2 Methanation over Ni-xSi/ZrO2 Catalysts at Low Temperature. *Catalysts* **2021**, *11*, 67. https://doi.org/10.3390/ catal11010067

Received: 19 November 2020 Accepted: 3 January 2021 Published: 5 January 2021

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large-scale industrial applications are limited by the high cost [21,22]. Therefore, Nickel as the most practical active metal has been widely investigated in the CO2 methanation reaction due to its abundance, low cost, and high activity [23–25]. Moreover, the properties of support play an important role in catalytic performance including the surface properties, the ability to disperse the active phase, and metal-support interaction. It is crucial to choose an appropriate support in preparing effective catalysts [26,27]. Many different metal oxides, such as CeO2 [28], MgO [29], Al2O3 [30], TiO2 [31,32], SiO2 [9,33], ZrO2 [34–38], and Y2O3 [39], have been used as support to promote CO2 methanation. ZrO2 becomes the most promising support and is getting increased attention since it has a higher concentration of oxygen defects [5]. Xu et al. [35] studied the CO2 methanation mechanism on the Ni/ZrO2 catalyst by in-situ FTIR and DFT methods, and found that c-ZrO2 could improve the electron mobility, the reducibility of Ni, and thus increase the catalytic activity of the catalysts. Martínez et al. [36] found that Ni/ZrO2 had a higher catalytic activity due to the strong interaction between Ni and ZrO2 by comparing the catalytic performance of Ni/ZrO2, Ni/SiO2, and Ni/MgAl2O4 catalysts. Hu et al. [37] also used ZrO2 as support, due to its good synergetic function, to explore the effect of La on the CO2 methanation reaction. Jia et al. [38] found that the structure of the catalyst had a significant influence on the catalytic performance. In addition, 71.9% CO2 conversion and 69.5% CH4 yield at 300 ◦C could be obtained on the Ni/ZrO2 catalyst prepared by the DBD plasma decomposition of nickel nitrate, while the CO2 conversion and CH4 yield were only 32.9% and 30.3% on the catalyst prepared thermally. Many researchers also reported that Ni/CeZrO2 catalysts exhibited an excellent low temperature catalytic performance for CO2 methanation due to the oxygen vacancies and high oxygen storage capacity of CeZrO2 [40–42].

In addition, promoters could further improve the catalytic activity. Therefore, the addition of a second element into nickel-based catalysts is considered as an effective method to improve the catalytic activity and stability at low temperature [14,21,43]. There are many types of promoters, including alkaline earth metals (Mg, Ca) [44,45], noble metals (Pt, Pd, Rh) [43], rare earth metals (La, Ce, Sm) [46], etc. The activation of CO2 can be enhanced by changing the surface basicity of the catalyst and the metal-support interaction [30,44,46]. Guilera et al. [30] studied the metal-oxide promoted Ni/Al2O3 catalyst, which exhibited a higher catalytic performance compared with the Ni/Al2O3 catalyst, due to the increase in basic sites and nickel dispersion. Xu et al. [46] found that the surface basicity and the intensity of CO2 chemisorption on Ni-based catalysts promoted by rare earth metals greatly increased, which could enhance the low-temperature catalytic activity of the catalysts. Wang et al. [47] found that the Ni-Si/ZrO2 catalyst (Si as promoter) had a better catalytic performance on the DRM reaction than the Ni-Zr/SiO2 catalyst (Zr as promoter). The Si on the Ni-Si/ZrO2 catalyst could promote the dispersion of Ni and improve the stability of metal Ni in the DRM reaction process. The highly dispersed Ni species on Ni-Si/ZrO2 increased the activation of CH4 and CO2, thus increasing the catalytic activity.

Since CO2 adsorption and activation were the key steps in both the DRM reaction and CO2 methanation, and CO2, CH4, and H2 co-existed in both the two systems. This study tried to apply the Ni-Si/ZrO2 catalyst to CO2 methanation to improve the activation of CO2 and promote CO2 methanation. Therefore, Ni-xSi/ZrO2 (Ni is 10 wt%, x = 0, 0.1, 0.5, 1 wt%) catalysts with different contents of Si were prepared by the co-impregnation method. The catalysts were characterized by different methods, including BET, H2-TPR, H2-TPD, H2-chemisorption, CO2-TPD, XRD, TEM, XPS, and TG-DSC, to explore the influence of Si.

## **2. Results and Discussion**
