Nanoconfinement facilitates reactions of carbon dioxide in supercritical water

The reactions of CO₂ in water under extreme pressure-temperature conditions are of great importance to the carbon storage and transport below Earth's surface, which substantially affect the carbon budget in the atmosphere. Previous studies focus on the CO₂(aq) solutions in the bulk phase, but underground aqueous solutions are often confined to the nanoscale, and nanoconfinement and solid-liquid interfaces may substantially affect chemical speciation and reaction mechanisms, which are poorly known on the molecular scale. Here, we apply extensive ab initio molecular dynamics simulations to study aqueous carbon solutions nanoconfined by graphene and stishovite (SiO₂) at 10 GPa and 1000 ~ 1400 K. We find that CO₂(aq) reacts more in nanoconfinement than in bulk. The stishovite-water interface makes the solutions more acidic, which shifts the chemical equilibria, and the interface chemistry also significantly affects the reaction mechanisms. Our findings suggest that CO₂(aq) in deep Earth is more active than previously thought, and confining CO₂ and water in nanopores may enhance the efficiency of mineral carbonation.