Greenhouse gases are created by human activities, and have been known as the most influential factor in global climate issues since the middle of the 20th century. On the other hand, reserves of fossil fuels are scarce and expected to exist only for another 60 years. Therefore, as the main natural gas component, methane can be considered a sustainable, alternative source of energy without the involvement of fossil elements. This allows for regulation of supply and demand for a sustainable energy source, while also reducing the environmental concerns associated with the utilization of fossil fuels.
As an effective solution, the emission of carbon dioxide (CO2; being a common greenhouse gas) can be prevented, involving several strategies such as switching from conventional electricity sources to renewable carbon-free ones, employing biomass as feedstock for sustainable chemicals, capturing CO2 from emission sources, and obtaining CO2 from the atmosphere directly. Recently, well-developed power-to-gas processes have been proposed to both store electrical energy from renewable resources and convert it into storable carriers of chemical energy (e.g., methane (CH4)) [1]. There is a possibility of realizing the renewable energy storage by converting CO2 into CH4 using renewable H₂, according to Eq. (1):
CO2 + 4H2⇆CH4 + 2H2O ΔrH = − 165 kJ/mol (1)
As known, CH4 is a kind of synthetic or substitute natural gas, possessing an efficient energy carrier with the ability to be employed in various industrial processes as fuel. Moreover, extensive pipeline infrastructures are available for storing CH4 and transporting it to far distances. Due to the close similarity of CH4 to natural gas, one can consider it as a promising substitute for the currently existing natural gas processes. Depending on the catalyst used in an exothermic catalytic reaction, the methanation of CO2 can be performed at temperatures ranging between 200–550°C [2, 3, 4]. In fact, the aforementioned catalytic reaction can be expressed in a two-step reaction mechanism, whose first step occurs via a reverse water-gas shift reaction, converting carbon dioxide and hydrogen into carbon monoxide and water as given below:
CO2 + H2⇆CO + H2O ΔrH = 41 kJ/mol (2)
In the second step of the catalytic reaction, methane is formed from CO and H2 (i.e., CO methanation):
CO + 3H2⇆ CH4 + H2O ΔrH = − 206 kJ/mol (3)
Since the methanation process is thermodynamically limited by a reversible exothermic reaction [5], it is inevitable to design a tailor-made catalyst in order to achieve conversions near to equilibrium at the moderate condition of reaction temperature (less than 350°C) and high values of space velocity. In this regard, both supported noble metal and non-noble metal have been used as catalysts for the CO2 methanation [6]. Based on previous literature, different types of active metals such as Ni, Co, Fe, Cu, Ru, Rh, Ir, Pd and Pt have been utilized as a catalyst in the methanation reaction [7]. Among the active metals, Rh, Ru, Ni, and Co have shown superior performance for methanation of CO2 [8], owing to their similar outermost electronic structure and catalytic behavior. Thus, they can be used in various types of reactions, including the steam reforming of organics (e.g., ethanol) [9–11] and acetic acid [12], as well as the hydrogenation of different organics [13, 14]. While the catalysts synthesized based on Co have exhibited inefficient performance in the CO2 methanation compared to Ni-based catalysts, they possess high resistance against carbon deposition and tolerate harsh environments. On the other hand, Co is the most common metal used for Fischer-Tropsch synthesis (FTS) of catalysts at an industrial scale. This is because of the high water-gas shift activity, low methane selectivity and low synthesis cost of Co-based catalysts, making them suitable for industrial applications [15].
The catalyst support has a multifaceted effect on the morphology of the active phase, adsorption ability and catalytic feature. For this reason, different types of supports (including Al2O3, TiO2, ZrO2, CeO2[16–19], SiO2, MgO, C, MgAl2O4, HY, USY, MSN, MCM-41, and SBA-15) have so far been studied and investigated in previous reports [3, 20]. Notably, Al2O3 is the most common catalyst support for the CO2 methanation process, being widely used in recent years [21]. The catalyst support modification is an efficient approach to preparing a highly active catalyst with superior reducibility [22]. In this respect, various types of support modifiers such as ZrO2, SiO2, MgO, La2O3, CeO2, and TiO2 have been used in recent research [23, 24], providing catalysts with higher conversion efficiency, superior redox property, and greater thermal stability and resistance against sintering or high temperatures.
Essentially, mixed oxide composite supports are advantageous because of their inherent desirable properties, giving rise to the synergistic effect of all the individual components. Meanwhile, interactions taking place between the active metal and oxide support significantly influence the methanation reaction, thereby inducing higher chemisorption capability due to the presence of well-dispersed active metal species [24]. Silica-alumina (SiO2-Al2O3; SA) composite oxide possesses high thermal stability and surface area at high temperatures, proposing it as a good candidate for use as a catalyst support. For example, Zhang Han et al. prepared a series of NiO/MOx-Al2O3 (M = Mg, Si, and Zr) catalysts with high conversion efficiencies by using a modified grinding‐mixing method. The comparison between their results indicated that the activity of MgO‐modified NiO/Al2O3 catalyst outperformed that of the catalysts based on NiO/ZrO2‐Al2O3 and NiO/SA [25]. Also, Moghaddam et al. prepared mesoporous Ni/SA catalysts via a sol-gel method with different SiO2/Al2O3 molar ratios in order to use them in the CO2 methanation reaction, achieving CO2 conversion of 82.38% and CH4 selectivity of 98.19% at 350°C [26]. Nevertheless, no study has been conducted on the synthesis of Co-based catalysts supported on SA composite for CO2 methanation purposes, according to the best of our knowledge.
In this paper, a series of Co-based catalysts supported on mesoporous SA composites are synthesized using a two-step method (sol-gel followed by impregnation), and their structural, chemical and morphological properties are investigated. By changing Si/Al molar ratio and Co loading in the range of 0.1–10 and 10–25 wt.%, respectively, the catalytic activity of the resulting Co/SA nanocatalysts is evaluated and optimized in the production of substitute natural gas through CO2 methanation under reaction conditions of 200–500°C, H2/CO2 ratio of 3.5:1, atmospheric pressure, and different gas hourly space velocities (GHSVs). This study evidences highly efficient catalytic activity of Co-based SA catalysts for CO2 methanation purposes, being comparable to previous metal-based nanocatalysts.