Ab initio kinetic models for the reactions of carbon dioxide with water under supercritical conditions

Understanding aqueous carbon reactions at elevated pressure and temperature conditions is of great importance to the carbon transport in the deep Earth, which also greatly influences the carbon budget in near-Earth reservoirs. Here, we constructed the Markov States Models (MSMs) based on ab initio molecular dynamics (AIMD) simulations to study the reaction kinetics of carbon dioxide (CO₂) in supercritical water in the bulk phase and under nanoconfinement. Our MSMs reveal that there are two distinct reaction paths for the formation of carbonate or bicarbonate ions. In the bulk solutions, CO₂ tends to directly react with H₂O or OH^-, whereas under nanoconfinement the pyrocarbonate ion (C₂O₅^2-) is an important reaction intermediate. Interestingly, the CO₂(aq) reactions are often mediated by rapid proton transfers along hydrogen bond wires, which are concerted in the bulk water, but stepwise under nanoconfinement. Our work provides a methodical and unbiased approach to study the reaction mechanism of CO₂ in supercritical water at the atomistic scale, which has great implications for the deep carbon cycle.