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Characteristics of NaNO3-Promoted CdO as a Midtemperature CO2 Absorbent.

In this study, we explored the reaction system CdO(s) + CO2(g) ⇄ CdCO3(s) as a model system for CO2 capture agent in the intermediate temperature range of 300-400 °C. While pure CdO does not react with CO2 at all up to 500 °C, CdO mixed with an appropriate amount of NaNO3 (optimal molar ratio NaNO3/CdO = 0.14) greatly enhances the conversion of CdO into CdCO3 up to ∼80% (5.68 mmol/g). These NaNO3-promoted CdO absorbents can undergo many cycles of absorption and desorption by temperature swing between 300 and 370 °C under a 100% CO2 condition. Details of how NaNO3 promotes the CO2 absorption of CdO have been delineated through various techniques using thermogravimetry, coupled with X-ray diffraction and electron microscopy. On the basis of the observed data, we propose a mechanism of CO2 absorption and desorption of NaNO3-promoted CdO. The absorption proceeds through a sequence of events of CO2 adsorption on the CdO surface covered by NaNO3, dissolution of so-formed CdCO3, and precipitation of CdCO3 particles in the NaNO3 medium. The desorption occurs through the decomposition of CdCO3 in the dissolved state in the NaNO3 medium where CdO nanoparticles are formed dispersed in the NaNO3 medium. The CdO nanoparticles are aggregated into micrometer-large particles with smooth surfaces and regular shapes.