Phase Formation and Degradation of Na 2 ZrO 3 under CO 2 Cycling Studied by Ex Situ and In Situ Diffraction.

We positively identified and quantified the solid-state phases involved in the carbonation/decarbonation cycle of Na 2 ZrO 3 when used for carbon capture. Previous work had only qualitatively inferred the phases present using diffraction pattern matching and thermogravimetric analysis. Here, we used the Rietveld-refinement method to analyze synchrotron X-ray and neutron powder diffraction data from samples treated ex situ . We then confirmed and extended our findings by in situ diffraction using a purpose-built gas-flow apparatus. This allowed us to resolve discrepancies in the earlier literature concerning which phases are present during the carbonation and regeneration processes. A key finding is the simultaneous presence of the monoclinic and tetragonal phases of ZrO 2 and that the "metastable" tetragonal phase is favored by smaller particles and can reincorporate into the bulk but the stable monoclinic phase does not. The result will help optimize the cycling of Na 2 ZrO 3 .