We investigate the reactivity to H₂ of a chemisorbed CO₂ species at electron traps on oxide surfaces, taking the single electron F⁺ oxygen vacancy of the MgO(100) terrace as a model system. We find that multiple hydrogen addition steps form three interacting catalytic cycles, leading to the evolution of formaldehyde, methanol or methane. Our results have general implications for the reactivity of CO₂ on metal oxides.