Rethinking the existence of hexagonal sodium zirconate CO2 sorbent

Abstract

Sodium zirconate (sodium zirconium oxide; Na₂ZrO₃) is a widely investigated carbon dioxide (CO₂) sorbent. Since it was first discussed in the 1960s, Na₂ZrO₃ has been reported to adopt monoclinic, hexagonal, and cubic structures, and it is widely believed that the CO₂ capture performance of Na₂ZrO₃ is related to its crystal structure. Researchers have relied on the differences in the relative intensities of two peaks (2θ ∼16.2° and 38.7°) in the powder X-ray diffraction (PXRD) pattern to determine the phase of this compound. However, to date, a defined crystal structure of hexagonal Na₂ZrO₃ has remained elusive. Our findings show that the current literature discussion on the structure of Na₂ZrO₃ is misleading. With the use of 3D electron diffraction (3D ED), and PXRD, we prove that hexagonal Na₂ZrO₃ does not exist. The so-called hexagonal Na₂ZrO₃ is actually Na₂ZrO₃ with three different types of disorder. Furthermore, the two PXRD peaks (2θ ∼16.2° and 38.7°) cannot be used to distinguish the different phases of Na₂ZrO₃, as the change in the PXRD pattern is related to the extent of structure disorder. Finally, we also show that the CO₂ uptake properties of Na₂ZrO₃ are not related to the differences in crystal structures, but rather to the Na⁺ site occupancy differences in different Na₂ZrO₃ samples. In order to further develop applications of Na₂ZrO₃, as well as other mixed-metal oxides, their structures, and the existence of any disorder, need be understood using the methods shown in this study.