Cadmium oxide/alkali metal halide mixtures-a potential high capacity sorbent for pre-combustion CO₂ capture

A series of cadmium oxide based materials were prepared by mixing cadmium carbonate with alkali metal halides. Subsequent heat treatment then transformed the cadmium carbonate into oxide to yield the active carbon dioxide sorbent. It was observed from thermogravimetric analysis that neat cadmium oxide does not sorb significant amounts of carbon dioxide, whereas doping the material with alkali halides facilitates conversion to cadmium carbonate. The cadmium oxide/sodium iodide mixture, in particular, was found to reversibly bind up to 24 wt% carbon dioxide in the temperature range of 250 to 300 °C, which is consistent with an almost stoichiometric conversion of the cadmium oxide to cadmium carbonate. The carbon dioxide could subsequently be released, in the same temperature range, when the gas supply was switched from carbon dioxide to an inert gas flow. The formation of the carbonate was separately verified by both infrared spectrometry and powder X-ray diffraction (XRD). In addition, XRD provided simultaneous detection of both the oxide and carbonate phases thus demonstrating their inter-dependency and is consistent with the absence of other cadmium phases. Le Bail refinement of the unit cell parameters did not reveal a significant change in the unit cell size of the cadmium oxide or carbonate due to mixing with alkali metal halides. Transmission electron microscopy on a 17.5% NaI sample indicated that the material consists of spherical particles of ∼250 nm diameter. Nitrogen physisorption experiments showed that the sodium iodide-enhanced material is non-porous and of a low surface area.