Abstract

Understanding the origin and mechanism of the formation of methane from CO2 is important because its formation would be related also to the origin of life on Earth. Both processes seem indissociable. To form methane, CO2 is reduced to CO by hydrogen. The reduction of CO2 might also correspond to the first step allowing the transition from CO2 to organic molecules and then to the first block of life on Earth. In our experiments, we used a natural rock (chromitite) collected from an open mine. The major mineral phase naturally occurring in this sample is magnesiochromite (85-95%) with subordinate serpentine and chlorite (including Cr-bearing chlorite). For the first time, we provide an indisputable experimental proof of the (abiotic) formation of methane on a natural chromitite rock, without any previous pretreatment, in the presence of gaseous CO2 and H-2 under low temperature and atmospheric pressure, which are the expected atmospheric environmental conditions that existed on Earth's crust when methane was formed in Nature. Methane is formed by a heterogeneous catalytic hydrogenation process at low temperature and atmospheric pressure. These results suggest that this transformation also goes on in other natural rocks existing on the surface of Earth, probably with higher efficacity. This means that natural rocks on Earth may contain catalytic sites and play the role of catalysts. The catalytic activity can be assigned to the presence of crystallographic phases and their surface composition, which promote the surface adsorption and transformation of gaseous reactants. The results demonstrate that a natural rock can activate, via heterogeneous catalysis, a very stable molecule such as CO2. The literature demonstrates that N-2 can be also activated by the same process suggesting a coherent pathway to explain the formation of organic molecules and amino acids in Nature. In situ catalytic CO2 hydrogenation in natural cavities should be considered as a realistic alternative method for CO2 mitigation. It is supported that catalysis would play an important role in the formation of the first block of life on Earth.