This work reports on the fundamental properties of nanostructured catalysts active in the main carbon oxides’ conversion processes for sustainable energy supply: methanation and co-methanation of CO₂. Transition metals (e.g. Ni, Pd, Pt, Co, Ru, Rh) are active species in both reactions. Ni has been the most studied because of its cheapness. Monometallic and bi-metallic Ni and Ni₃Fe catalysts supported on Gadolinia-doped ceria (GDC) have been synthesized, characterized and tested in the temperature range 200–600[Formula: see text]C. In the methanation reaction, the monometallic catalyst showed higher performance with respect to the bi-metallic catalyst. At 400[Formula: see text]C, the CO₂ conversion overcomes 90% with CH₄ selectivity of 100%. In co-methanation, the highest CO₂, CO and H₂ conversion values over monometallic Ni/GDC catalyst were obtained at 300[Formula: see text]C; at higher temperatures, conversion decreases. The GDC support plays a pivotal role in both reactions, enhancing the basicity of the catalyst and improving the dissociation of carbon oxide species adsorbed on Ni sites.