Investigation of the capacity decay of a CdO-NaI mixed sorbent for pre-combustion CO₂ capture

The mechanisms for the loss of both CO₂ working capacity and mass from a CdO-NaI composite were investigated to better assess the potential use of the material to facilitate the pre-combustion capture of CO₂ from syngas. Fresh activated material was used to analyse sorption and desorption using a CO₂-N₂ mixture. Mass spectrometric analysis of the exit gas revealed the loss of elemental iodine from the system over the period, attributed to the oxidation of iodide. Thermogravimetric analysis using iodine vapour suggested the iodide loss reaction to be a partially reversible equilibrium. X-ray photoelectron spectroscopy revealed the formation of a highly oxidised iodine species on the surface of the sorbent during initial calcination in both air and N₂, but this compound vanished after the use of the sorbent in 25 CO₂ sorption cycles. Elemental mapping showed that iodine was dislocated from the sodium, which it was considered to be originally associated to, supporting the theory of oxidation and evaporation (and possible re-deposition). Transmission electron microscopy revealed that the sorbent consisted of regular, spherical nanoparticles of approx. 250 nm diameter, which became more irregularly-shaped after CO₂ sorption cycles, considered to be due to void/crack formation caused by density changes upon calcination and carbonation. In situ powder X-ray diffraction revealed an increase in crystallinity of both CdO and NaI upon heating to CO₂ sorption temperature of 325 °C in N₂ atmosphere, compared to room temperature. If the oxidation of the iodide promoter can be inhibited, this is likely to improve the multicyclic CO₂ sorption stability of this material for future use.