Understanding the Mechanisms of CaO Carbonation: Role of Point Defects in CaCO3 by Atomic-Scale Simulations

In order to improve our understanding of the mechanisms underlying the carbonation of CaO by CO2, a practically important reaction, we present a detailed atomic-scale study, by means of ab initio density functional calculations, of the point defect properties in calcite CaCO3. We perform a thorough investigation of the chemical potentials relevant in experimental conditions. Focusing on C and O defects closely related to CO2 diffusion, we point out the absence of interstitial C except possibly in complex form with other defects, whereas interstitial O is found stable. Whereas O and C vacancies are not significant in neutral form at low temperature, their role is clearly revealed when charged states are included. The formation energies of O and C vacancies, together with the profiles of chemical potentials across the growing calcite layer, are consistent with joint C and O diffusion by separate vacancy mechanisms of roughly equal amplitudes. As regards the possible influence of complex point defects, our results suggest that interstitial diffusion of CO2 is unfavorable. Conversely, the strong binding of the CO2 vacancy may enhance CO2 diffusion.