Understanding and Improving the Kinetics of Bulk Carbonation on Sodium Carbonate

Slow kinetics of CO2 capture
on sodium carbonate (Na2CO3) restricts its application
in CO2 capture from flue gas despite advantages of low
cost, high abundance,
and economical retrofitting of extant power plants. Surface carbonation
on Na2CO3 is facile; hence, the observed slow
kinetics is likely due to diffusion limitations. We present an atomic-level
study of the kinetics of Na2CO3 bulk carbonation.
We proposal two possible pathways: the molecular interstitial diffusion
(MID) mechanism, where CO2 and H2O diffuse through
bulk-like sodium bicarbonate (NaHCO3) to react at the NaHCO3/Na2CO3 interface, and the ion counter
diffusion (ICD) mechanism, where H+ and HCO3– are generated
at the NaHCO3–gas interface, accompanied by counter-diffusion
of H+ and Na+. The MID mechanism is excluded
because of the high formation energy of CO2 in NaHCO3 (2.76 eV), computed from density functional theory (DFT).
We have identified and studied three main steps in the ICD pathway:
(1) H+ and HCO3– generation at the NaHCO3–gas interface; (2) generation of H+ vacancies
(VH–) and interstitial Na+ (INa+) in the
NaHCO3 layer along with Na+ vacancies (VNa–) and interstitial hydrogen (IH+) in the Na2CO3 layer; and (3) VH– and INa+ diffusion through
the NaHCO3 layer to the gas–solid interface. Our
DFT molecular dynamics simulations confirm facile generation of H+ and HCO3– on the NaHCO3 surface. We find that the kinetics of Na2CO3 bulk carbonation is controlled by the IH+/VNa– defect pair generation in Na2CO3; we predict that the kinetics can be enhanced by doping lithium
into Na2CO3, which decreases the defect formation
energy by 0.13 eV. This prediction was confirmed by our fixed-bed
experiments, which found a 125% increase in the initial CO2 absorption rate and a 29% increase in CO2 uptake after
36 min exposure in 0.7 wt % (1.0 at. %) Li-doped Na2CO3 compared with undoped Na2CO3.