## 1 Introduction  
A reduction of carbon emissions from industry and energy production by avoiding, capturing, or recycling carbon is urgently needed to limit the ever-increasing concentration of CO\({}_{2}\) in the atmosphere, the leading cause of global warming. The utilization of CO\({}_{2}\) as a C1 building block in organic synthesis to produce chemicals is limited to the production of urea, salicylic acid, and polycarbonates, and utilizes only a small percentage of the largely available feedstock [1]. Recycling CO\({}_{2}\) to produce fuels such as methane, methanol, dimethyl ether, or higher alcohols could be a convenient alternative that has not been thoroughly investigated yet. Research in this field may leads to reduced consumption of carbon-based fossil fuels, avoiding the introduction of new CO\({}_{2}\) to the atmosphere [2; 3]. The hydrogenation of CO\({}_{2}\) to produce methane and water, (CO\({}_{2}\) + 4H\({}_{2}\)\(\rightarrow\) CH\({}_{4}\) + 2H\({}_{2}\)O), known as Sabatier reaction or methanation, is exothermic (AH\({}^{\circ}\) = -164 kJ mol\({}^{-1}\)), and thermodynamically favored (\(\Delta\)G\({}^{0}\) = -131 kJ/mol), but with kinetic barriers that require the presence of a catalyst. CO\({}_{2}\) methanation is currently attracting much attention due to its potential application in the power to gas process (PtG), where the intermittent excess electricity, generated from renewable sources, can be used for the sustainable production of synthetic natural gas (SNG), which contributes to the reduction of greenhouse gas emissions by replacing natural fossil methane [4; 5]. In the presence of a catalyst, CO\({}_{2}\) methanation occurs above 200 \({}^{\circ}\)C, and is generally exploited below 400 \({}^{\circ}\)C, to limit the endothermic side reactions that occur at high temperatures: the reverse-water-gas shift (RWGS: CO\({}_{2}\) + H\({}_{2}\)\(\rightarrow\) CO + H\({}_{2}\)O, \(\Delta\)H\({}^{\circ}\) = 41 kJ mol\({}^{-1}\)), which produces CO instead of methane, and the reforming of methane with CO\({}_{2}\) (CH\({}_{4}\) + CO\({}_{2}\)\(\rightarrow\) 2 CO + 2H\({}_{2}\), \(\Delta\)H\({}^{\circ}\) = 247 kJ mol\({}^{-1}\)). Noble and transition metals of the VIII-X groups (Ru, Rh, Ir, Pd, Ni, Fe, Co), are active and selective for methanation. They have been supported on oxides (\(\gamma\)-Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), SiO\({}_{2}\), ZrO\({}_{2}\), CeO\({}_{2}\)), and extensively studied, as summarized in some recent reviews [2; 4; 6; 7; 8; 9; 10; 11]. The order of the metal elements for the specific activity or selectivity may differ in published papers, depending on the methanation conditions and on the metal/support interactions; anyhow, ruthenium is generally the most active. Nonetheless, Ni metal is the most commonly used for commercial applications, due to its high activity, high selectivity, and very low cost. However, Ni-based catalysts are more active at higher temperatures than noble metal, and are prone to deactivation by carbon deposition due to the Boudouard reaction and the reduction by H\({}_{2}\) of CO\({}_{2}\) and CO to carbon. The catalytic performances of Ni-based catalysts depend markedly on the properties of the support, which affect the dispersion of the catalytic phase, the reducibility of the metal, and the metal-support interactions. The mechanism of CO\({}_{2}\) methanation over nickel-based catalysts has been extensively studied using theoretical and experimental approaches, but many aspects of the mechanism remain controversial, and are still debated among researchers. Three main possible reaction mechanisms have been proposed [11; 12; 13]: the first pathway involves the associative adsorption of CO\({}_{2}\) with a surface H\({}^{\bullet}\), forming a formate intermediate (CHOO\({}^{\bullet}\)), which is subsequently hydrogenated to CH\({}^{\bullet}\), and then to CH\({}_{4}\) (formate pathway).