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.