Tuning Na₂ZrO₃ for fast and stable CO₂ adsorption by solid state synthesis

This work assessed the possibility to tune the CO₂ capture performance of Na₂ZrO₃ with respect to CO₂ uptake and CO₂ sorption rate by varying the conditions used in the solid-state synthesis. The resulting Na₂ZrO₃ were characterized by XRD, SEM-EDS, XPS and TGA. A structural, chemical, microstructural and kinetic analysis of the Na₂ZrO₃–CO₂ system over one cycle was performed to identify the correlation with the sorbent performance. The heating rate, the molar ratio of the Na₂CO₃ and ZrO₂ used in the synthesis of Na₂ZrO₃, as well as additional powder processing steps of the reactants, all had a major impact on the sorbent's CO₂ capture performance. The best performing sorbent with the highest CO₂ uptake capacity (4.83 mmol CO₂/g) and absorption rate (30. 5 nmmol/s) at 700 °C was obtained when the Na₂CO₃ and ZrO₂ reactants were processed by ball milling varying the molar ratio of 1:1 and a synthesis heating rate of 1 °C/min. Under these conditions, the optimised Na₂ZrO₃ exhibited 86.5% conversion in 10 min with respect to the theoretical value. Na₂ZrO₃ synthesised using the optimised conditions as listed above were constructed with nanocrystals of ~ 20 nm in average diameter as observed using XRD (Sherrer's formula). The Na₂ZrO₃ synthesised in this study favoured the ionic solid-state diffusion of Na and O from the core to the surface of the material to readily react with CO₂. Moreover, an excellent cyclic stability of the sorbent over 70 sorption/desorption cycles was noted after an initial decay when the CO₂ cycles were shortened to 5 min.