Mechanistic details of catalytic reactions are critical to the development of improved catalysts. Here, we perform high quality Born-Oppenheimer molecular dynamics simulations of the reaction mechanisms associated with hydrogen-assisted CO2 reduction on Ni(110). The simulation results show direct theoretical evidence for both associative and redox mechanisms in the reaction of atomic hydrogen with CO2. Because H-2 is dissociatively chemisorbed on Ni(110) with nearly unit probability, the mechanisms we find are also relevant to the reverse water-gas shift reaction (H-2 with adsorbed CO2). Furthermore, we provide the first real-time demonstration of both Eley-Rideal (ER) and hot atom (HA) mechanisms when H impinges on adsorbed CO2, and we show that both occur even for low kinetic energies. The trade-off between ER or HA mechanisms is found to be strongly dependent on CO2 coverage. The results are compared with recent gas/surface measurements.