Encoding CO2 Adsorption in Sodium Zirconate by Neutron Diffraction

Recent research into sodium zirconate as a high-temperature CO₂ sorbent has been extensive, but detailed knowledge of the material's crystal structure during synthesis and carbon dioxide uptake remains limited. This study employs neutron diffraction (ND), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) to explore these aspects. An improved synthesis method, involving the pre-drying and ball milling of raw materials, produced pure samples with average crystal sizes of 37-48 nm in the monoclinic phase. However, using a slower heating rate (1 °C/min) decreased the purity. Despite this, the 1 °C/min rate resulted in the highest CO₂ uptake capacity (4.32 mmol CO₂/g Na₂ZrO₃) and CO₂ sorption rate (0.0017 mmol CO₂/g) after 5 min at 700 °C. This was attributed to a larger presence of microstructure defects that facilitate Na diffusion from the core to the shell of the particles. An ND analysis showed that the conversion of Na₂ZrO₃ was complete under the studied conditions and that CO₂ concentration significantly impacts the rate of CO₂ absorption. The TGA results indicated that the reaction rate during CO₂ sorption remained steady until full conversion due to the absorptive nature of the chemisorption process. During the sorbent reforming step, ND revealed the disappearance of Na₂O and ZrO₂ as the zirconate phase reformed. However, trace amounts of Na₂CO₃ and ZrO₂ remained after the cycles.