Thermal stability and high-temperature carbon dioxide sorption on hexa-lithium zirconate (Li6Zr2O7)

Lithium zirconates, Li2ZrO3 and Li6Zr2O7, were synthesized by solid-state reaction. The thermal analyses of Li6Zr2O7 showed a continuous decomposition process due to lithium sublimation. However, the thermal behavior of this compound changed slightly when different gas environments were used. If nitrogen was used, Li6Zr2O7 decomposed in a mixture of Li2ZrO3, ZrO2, and Li2O(g). Nevertheless, air environment produced a different and more complex decomposition mechanism at high temperatures. In this case, lithium reacted with the oxygen from the air to produce Li2O at the surface, producing a temporary increase of the total weight. Subsequently, Li2O and some oxygen, from the Li6Zr2O7 structure, sublimed to produce Li2ZrO3 and ZrO2. The CO2 absorption capacity of both zirconates was studied. The materials absorbed CO2 at around the same temperature, 450-650 degrees C. Still, Li6Zr2O7 absorbed 4 times more CO2 than Li2ZrO3. Furthermore, the CO2 sorption rate of LiZrO3 was much slower than that of Li6Zr2O7 at short times. Apparently, at the beginning of the absorption process, there was more lithium available to react with CO2 in the Li6Zr2O7 sample, as expected, although the sorption rates of both ceramics became similar after long times. A correlation is established between the lithium and CO2 diffusion through the Li2CO3 produced on the surface of the particles. The best temperature for the CO2 absorption on Li6Zr2O7 was 550 degrees C. Finally, XRD analyses, after the CO2 absorption, and cyclic thermogravimetric analyses showed that Li6Zr2O7 was not regenerated. In all cases, the final product was Li2ZrO3.