The catalytic effect of H₂O on the hydrolysis of CO₃^2- in hydrated clusters and its implication in the humidity driven CO₂ air capture

The hydration of ions in nanoscale hydrated clusters is ubiquitous and essential in many physical and chemical processes. Here we show that the hydrolysis reaction is strongly affected by relative humidity. The hydrolysis of CO₃^2- with n = 1-8 water molecules is investigated using an ab initio method. For n = 1-5 water molecules, all the reactants follow a stepwise pathway to the transition state. For n = 6-8 water molecules, all the reactants undergo a direct proton transfer to the transition state with overall lower activation free energy. The activation free energy of the reaction is dramatically reduced from 10.4 to 2.4 kcal mol^-1 as the number of water molecules increases from 1 to 6. Meanwhile, the degree of hydrolysis of CO₃^2- is significantly increased compared to the bulk water solution scenario. Incomplete hydration shells facilitate the hydrolysis of CO₃^2- with few water molecules to be not only thermodynamically favorable but also kinetically favorable. We showed that the chemical kinetics is not likely to constrain the speed of CO₂ air capture driven by the humidity-swing. Instead, the pore-diffusion of ions is expected to be the time-limiting step in the humidity driven CO₂ air capture. The effect of humidity on the speed of CO₂ air capture was studied by conducting a CO₂ absorption experiment using IER with a high ratio of CO₃^2- to H₂O molecules. Our result is able to provide valuable insights into designing efficient CO₂ air-capture sorbents.