Toward Understanding the Kinetics of CO2 Capture on Sodium Carbonate

Sodium carbonate (Na₂CO₃) has been widely studied as a promising candidate for CO₂ capture from humid flue gas because of its low cost, high abundance, reusability, and moderate operating temperatures. However, the slow kinetics of CO₂ capture on unmodified Na₂CO₃ make it an unacceptable choice for practical applications. If the reaction kinetics could be dramatically improved, then Na₂CO₃ could be a viable material for large-scale carbon capture applications. The first step to systematic improvement of kinetics is to understand the rate-limiting steps in the uncatalyzed system. We have therefore investigated the structural, mechanistic, and energetic properties of CO₂ capture on the (001) and (-402) surfaces of Na₂CO₃ using density functional theory to identify the origin of the slow kinetics observed in experiments. We have identified reaction pathways for co-adsorbed CO₂ and H₂O that lead to bicarbonate formation on the (001) and (-402) surfaces having activation energies of 0.40 and 0.34 eV, respectively. We modeled surface carbonation reactions under conditions of high surface loading of water by performing ab initio molecular dynamics simulations at typical operating temperatures. Multiple reactions were observed on picosecond time scales. Our results indicate that the Na₂CO₃ carbonation reaction is not controlled by the kinetics of the reaction at the surface but is likely by diffusion limitations. We propose two possible scenarios that could result in diffusion control of the reaction rate.