Structure of Na Species in Promoted CaO-Based Sorbents and Their Effect on the Rate and Extent of the CO₂ Uptake

To advance CaO-based CO2 sorbents it is crucial to understand how their structural parameters control the cyclic CO2 uptake. Here, CaO-based sorbents with varying ratios of Na2CO3:CaCO3 are synthesized via mechanochemical activation of a mixture of Na2CO3 and CaCO3 to investigate the effect of sodium species on the structure, morphology, carbonation rate and cyclic CO2 uptake of the CO2 sorbents. The addition of Na2CO3 in the range of 0.1-0.2 mol% improves the CO2 uptake by up to 80% after 10 cycles when compared to ball-milled bare CaCO3, while for Na2CO3 loadings >0.3 mol% the cyclic CO2 uptake decreases by more than 40%. Energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy, X-ray absorption spectroscopy (XAS), and Na-23 MAS NMR, reveal that in sorbents with Na2CO3 contents <0.3 mol% Na exists in highly distributed, noncrystalline [Na2Ca(CO3)(2)] units. These species stabilize the surface area of the sorbent in pores of diameters >100 nm, and enhance the diffusion of CO2 through CaCO3. For Na2CO3 contents >0.3 mol%, the accelerated deactivation of the sorbents via sintering is related to the formation of crystalline Na2Ca(CO3)(2) and the high mobility of Na.