Deciphering the existence of hexagonal sodium zirconate CO2 sorbent

Abstract

Sodium zirconate (sodium zirconium oxide; Na2ZrO3) is amongst the most investigated carbon dioxide (CO2) sorbent. Na2ZrO3 is renowned for its high capture capacity and cyclic stability. It can effectively capture CO2 at temperatures that are found in industrial processes such as the manufacture of steel or cement. Na2ZrO3 is reported to adopt monoclinic, hexagonal, and cubic structures since it was first discussed in the 1960s. Researchers relied on the differences in the relative intensities between two peaks (2θ ~ 16.2 and 38.7 °) in the powder X-ray diffraction (PXRD) pattern to determine the phase of this compound. It is also widely believed that the CO2 capture performance of Na2ZrO3 is related to the crystal structure, yet the crystal structure of hexagonal Na2ZrO3 has remained elusive. With the use of 3D electron diffraction (3D ED), X-ray photoelectron spectroscopy (XPS), and PXRD, we show that the hexagonal Na2ZrO3 does not exist. The so-called hexagonal Na2ZrO3 is 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 changes in the PXRD pattern are related to the extent of structure disorder. Finally, we also show that the CO2 capture properties of Na2ZrO3 are related to the Na+ site occupancy between different Na2ZrO3 samples, and not differences in crystal structures. The findings from our work shows that the current literature discussion on the structure of Na2ZrO3 is misleading. In order to further develop Na2ZrO3 as well as other mixed-metal oxides for applications, their structures, as well as any disorder, needs be understood using the methods shown in this study.