The nanoscale description of the reaction pathways and of the role of the intermediate species involved in a chemical process is a crucial milestone for tailoring more active, stable, and cheaper catalysts, thus providing "reaction engineering" capabilities. This level of insight has not been achieved yet for the catalytic hydrogenation of CO2 on Ni catalysts, a reaction of enormous environmental relevance. We present a thorough atomic-scale description of the mechanisms of this reaction, studied under controlled conditions on a model Ni catalyst, thus clarifying the long-standing debate on the actual reaction path followed by the reactants. Remarkably, formate, which is always observed under standard conditions, is found to be just a "dead-end" spectator molecule, formed via a Langmuir-Hinshelwood process, whereas the reaction proceeds through parallel Eley-Rideal channels, where hydrogen-assisted C-O bond cleavage in CO2 yields CO already at liquid nitrogen temperature.