Thermogravimetric study of sequential carbonation and decarbonation processes over Na2ZrO3 at low temperatures (30–80 °C): relative humidity effect

Abstract

In the present work, Na₂ZrO₃ was synthetized via a solid-state reaction and characterized by powder X-ray diffraction and N₂ physisorption techniques, where desired structural and microstructural characteristics were confirmed. Then, the ceramic material was tested dynamically and isothermally in a low temperature range (30–80 °C) for carbonation and decarbonation processes using relative humidity (RH) values between 0 and 80%. Thermogravimetric results indicate that humidity has a positive influence over the carbonation process, thus the amount of CO₂ captured increases as a function of relative humidity. When high values of humidity (70 and 80%) were used, the increase in sample weight was higher than the theoretical amount of 57.2 wt% expected in wet conditions. This result was attributed to the formation of NaHCO₃ with a mesoporous microstructure. Then, the relative humidity effect was studied during the decarbonation stage as a sequential step after the carbonation process, using a N₂ flow. Infrared spectroscopy and thermogravimetric results showed that NaHCO₃ decomposition took place in this process. At low RH values, 0 and 20%, NaHCO₃ is decomposed in Na₂O and Na₂CO₃; whereas, when RH was increased between 40 and 80%, only the presence of Na₂CO₃ was observed. This result indicates that at high humidity conditions the Na₂O formation is avoided. Thermal curves show that Na₂CO₃ decomposition presented a maximum efficiency at 40% of RH, which seems to be the optimal condition for the decarbonation step. Finally, sequential carbonation–decarbonation tests were performed with sodium zirconate samples. Infrared and thermal analyses confirm that it is possible to accomplish successively at least eight cycles of carbonation and decarbonation steps and to obtain high NaHCO₃ and Na₂CO₃ regenerations.