Thermokinetic and microstructural analyses of the CO2 chemisorption on K2CO3–Na2ZrO3

Abstract

Sodium zirconate (Na2ZrO3) was synthesized via a solid-state reaction. Subsequently, a portion of the prepared Na2ZrO3 was mechanically mixed with 5wt.% potassium carbonate (K2CO3). The Na2ZrO3 and K-Na2ZrO3 samples were characterized, and the CO2 capture processes were evaluated. The potassium addition did not modify the structural or microstructural characteristics of the Na2ZrO3. However, during CO2 chemisorption, the material presented some important variations that depended on the potassium addition. The maximum CO2 capture of the Na2ZrO3 sample was observed at T≥550°C, while the CO2 capture of the K-Na2ZrO3 sample was significantly favored at 400°C. According to DSC analysis, the CO2 capture increase observed at lower temperatures was due to the formation of a K–Na carbonate eutectic phase. Additionally, for the samples demonstrating effective CO2 capture at relatively low temperatures, SEM microstructural analysis demonstrated the formation of a Na2CO3–ZrO2 mesoporous external shell. Moreover, some kinetic parameters were determined. The isothermal data were fitted to a double exponential model related to the direct CO2 chemisorption (k1) and diffusion processes (k2). The K-Na2ZrO3 sample presented higher k1 and k2 values than Na2ZrO3 at all temperatures investigated, confirming that the potassium addition improved the CO2 capture process. Further, the potassium in Na2ZrO3 significantly enhanced the CO2 capture at approximately 400°C, in comparison to the Na2ZrO3 sample. Finally, it was observed that potassium negatively compromised the chemisorption of CO2 after several cycles, due to potassium segregation and sublimation.