ABSTRACT

The interaction of carbon-containing molecules with water is important in a range of fields, including biology, astronomy, atmospheric chemistry, and geology. In this work, the reactions of carbon and water at extreme pressure-temperature (P-T) conditions have been studied using molecular dynamics-based computational approaches. Ab initio molecular dynamics (AIMD) simulations show that while CO₂(aq) is stable at ambient conditions, it reacts readily at the high P-T conditions of Earth's upper mantle. As a result, carbonic acid plays an important role in aqueous geofluids. AIMD simulations also demonstrate that CO₂(aq) is destabilized in nanocon ned solutions compared to the bulk at high P-T conditions, due to extensive structuring of the fluid in confined settings. Chemical interactions between the uid and the confining interfaces further shift the fluid equilibria. To shed more light on the reactions of carbon in geofluids, the water-gas shift reaction at high P-T conditions was studied using a combination of AIMD simulations and free energy calculations. At industrial P{T conditions, the water-gas shift products, CO₂ and H₂, are thermodynamically favored over the reactants, CO and H₂O, although a heterogeneous catalyst is employed in industrial settings to overcome the reaction barrier. At the P-T conditions of Earth's upper mantle, no catalyst is necessary for CO to react with water, but the water-gas shift reaction does not proceed to completion. Instead, HCOOH forms. Finally, the freezing of supercooled water on carbon-bearing substrates was investigated using classical molecular dynamics combined with metadynamics simulations. The freezing of water on pristine graphene proceeds more readily than on graphene oxide because of the in uence of oxidized functional groups. This work sheds light on the reactions and properties of carbon in the deep carbon cycle, and elucidates the interplay between water and carbon compounds at extreme conditions that remain challenging to study in the laboratory.

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