Partial CO2 Reduction in Amorphous Cylindrical Silica Nanopores Studied with Reactive Molecular Dynamics Simulations

Abstract

It is known that pore confinement affects the structure and transport properties of fluids. It has also been shown that confinement can affect the equilibrium composition of a reactive system. Such effects could be related to the possible abiotic hydrocarbon synthesis in deep-sea hydrothermal vents, especially when the CO2 methanation reaction occurs within nanopores. In an attempt to identify possible rate-limiting steps of such a reaction, we report here molecular dynamics simulations conducted implementing the reactive force field. The reaction is considered within a cylindrical nanopore carved out of amorphous silica. Within the constraints of our simulations, which were conducted for 5 ns, no CH4 molecules were detected in the temperature range of 400-1000 K, suggesting that the silica pore hinders the complete CO2 reduction. This is consistent with the fact that silica is not an effective catalyst for CO2 methanation. Our simulations, in agreement with literature reports, suggest that the silica pore surface facilitates the partial reduction of CO2 to CO, which, within the conditions of our study, is found to be a stable product within the silica nanopores simulated. Analysis of the reaction products suggests that, although C-C bonds did not form, fragments reminiscent of carboxylic acids and formate were observed. Because these compounds are part of the biological Krebs cycle, our results suggest that confinement could provide prebiotic precursors of core metabolic pathways. Our results could be useful for further developing applications in which catalysts are designed to promote CO2 activation, for example, the one-step thermolysis of CO2