CO2　hydrogenation　to　methane　is　gaining　increasing　interest　as　one　of　the　most　promising　ways　to　store　intermittent　renewable　energy　in　the　form　of　chemical　fuels.　Ni　particles　supported　on　CeO2　represent　a　highly　efficient,　stable　and　inexpensive　catalyst　for　this　reaction.　Herein,　Ni-doped　CeO2　nanoparticles　were　tested　for　CO2　methanation　showing　an　extremely　high　Ni　mass-specific　activity　and　CH4　selectivity.　Operando　characterization　reveals　that　this　performance　is　tightly　associated　with　ionic　Ni　and　Ce3+　surface　sites,　while　formation　of　metallic　Ni　does　not　seem　to　considerably　promote　the　reaction.　Theoretical　calculations　confirmed　the　stability　of　interstitial　ionic　Ni　sites　on　ceria　surfaces　and　highlighted　the　role　of　Ce-O　frustrated　Lewis　pair　(FLP),　Ni-O　classical　Lewis　pair　(CLP)　and　Ni-Ce　pair　sites　to　the　activation　of　H-2　and　CO2　molecules.　To　a　large　extent,　the　theoretical　predictions　were　validated　by　in　situ　spectroscopy　under　H-2　and　CO2　:　H-2　gaseous　environments.