## 1 Introduction
The other mechanisms both start with the dissociative adsorption of CO\({}_{2}\) onto CO\({}^{\bullet}\) and O\({}^{\bullet}\), followed by the hydrogenation of CO\({}_{\rm ad}\) to form methane after direct cleavage of the C-O bond, or via an HCO* intermediate. However, in the last two routes, the intermediate is the adsorbed CO. The properties of the support and the operating conditions appear to influence the preferred reaction pathway and the rate-determining step. The methanation mechanisms on Ni supported on CeO\({}_{2}\) catalysts are still under investigation and can be deepened by analyzing the involved intermediates, using in operando DRIFT experiments [12; 14]  
CeO\({}_{2}\) is one of the most studied catalyst carriers, due to its redox properties, and to its ability to store and release oxygen (oxygen storage capacity, OSC) through the Ce\({}^{3+}\)/Ce\({}^{4+}\) couple on the surface, and due to its oxygen vacancies. CeO\({}_{2}\)-based catalysts showed excellent performances in the oxidation of CO [15] or alcohols [16], combustion of methane in the lean state [17], dry reforming [18; 19; 20], hydrogenation [21], and CO\({}_{2}\) methanation [14; 22]. The redox properties and the ability to highly disperse Ni have been suggested as responsible for the excellent catalytic activity and stability for CO\({}_{2}\) methanation [23; 24]. CeO\({}_{2}\) can be prepared in different nano-shapes, such as nano-rod, nano-cubic or nano-octahedra. The morphology affects the catalytic performances of Me/CeO\({}_{2}\) catalysts for various reactions: i.e., CO\({}_{2}\) methanation over Ni [3; 25], methane combustion over Pd [17], water gas shift reaction over Cu [26], dry reforming of methane or methanol over Ni [27; 28], hydrogenation of ethyl levulinate over Ru [21]. The nano-rod morphology appears the most active and stable for all these reactions, whereas the order of activity for nano-cubes and nano-octahedra may be reversed. The difference in activity of Me/CeO\({}_{2}\) catalysts with different nano-shapes is ascribed to differences in the specific exposed crystallographic facets that affect the metal/support interactions [3; 27; 29], the amount of oxygen vacancies [3; 17; 21; 25; 27; 28; 29], or acid-basic sites [28]. Experimental and theoretical studies on well-defined nanocrystals have established that the control of exposed facets in ceria is a promising way to direct the activity and selectivity of many reactions [29]. In light of these considerations and perspectives, it is interesting to further investigate the role of most active nanoshapes for the CO2 methanation, i.e., rods and cubes. Recently, we studied a series of Ni/nanorod-CeO2 catalysts with different Ni loading (2-13 wt%) [22] and a Pd (2 wt%)/nanorod-CeO2 [16], prepared by one-pot hydrothermal synthesis, founding that this preparation method allows one to obtain the high and homogeneous dispersion of supported metals, good stability of metal dispersion and support morphology after treatments at high temperatures. In this work, we investigated the influence of morphology on catalytic activity for the CO2 methanation of samples Ce0.95 Ni0.05O2-4, with nanorod or nanocube shape prepared by one-pot hydrothermal synthesis. This rapid and straightforward method ensures a high dispersion of nickel and a strong Ni-support interaction. The central core of this research is the study of the nature of reaction intermediates, and the dependence of their nature on the specific ceria nano-shape, through in operando DRIFT analysis.