Unveiling the carbonation mechanism in molten salt-promoted MgO-Al₂O₃ sorbents

Abstract

Supports are commonly used to increase the stability and prevent the agglomeration of solid CO2 sorbents. However, the exact surface changes and carbonation mechanism on supported sorbents have not been directly observed. Knowing such insight would not only allow a better understanding of the surface phenomenon on supported sorbents but would also help to identify the exact role of the supports. In this study, real-time CO2 sorption observations on Al2O3-supported eutectic mixture (EM)-promoted MgO sorbents were obtained to determine the role of Al2O3 support in the sorption process. EM-MgO-Al2O3 exhibited stable sorption-regeneration cyclic performance and maintained a capacity of similar to 13 wt% after 12 cycles. In situ TEM was then utilized to directly observe the sorption and regeneration phenomenon on the sorbent. Results show that MgCO3 evolution proceeds with the formation of separated mushroom-like branches along the surface of the sorbent, contrary to the surface layer formation on unsupported sorbents. The same mushroom-like morphology was also observed during resorption, although smaller yet denser products formed due to MgO regeneration. Such morphology was observed due to the presence of Al2O3, which alters the sorbent surface, redistributes triple-phase boundaries, and directs the formation of MgCO3 towards the MgO active sites. This consequently avoids MgCO3 agglomeration and improves the cyclic stability of the sorbent. Hence, a better understanding of the carbonation mechanism and the role of support in solid sorbents was obtained.