XAS analysis of Na2CO3-modified CaO-based CO2 sorbents

https://doi.org/10.1039/D0CP04410E

“To determine in more detail the nature of the Na-rich phase in the sorbents during the different steps of the CO₂ capture process, Na K-edge XANES data were acquired. After the initial calcination step (800 °C, N₂), the Na K-edge spectra of the sorbents matched perfectly the reference spectra of Na₂CO₃ (Fig. S14, ESI†). This observation, combined with Ca K-edge XAS results (vide infra, Fig. S13, ESI†), confirmed that all of the Na₂CO₃-modified sorbents consisted of a mixture of CaO and Na₂CO₃ after the initial calcination step and no Ca–Na mixed phase was present. The Na K-edge XANES spectra of all of the sorbents collected after the 10th carbonation step had a similar profile with a very characteristic shape of the XANES spectrum that was different from the XANES spectrum of pure Na₂CO₃ (Fig. 3). Specifically, all spectra possessed a pre-edge peak at ca. 1073.2 eV (−1.5 eV with respect to the absorption edge, feature A, Fig. 3), while Na₂CO₃ possessed a pre-edge feature at ca. 1073.5 eV (−1.2 eV with respect to the absorption edge). Additionally, contrary to the spectrum of Na₂CO₃ with a white line maximum at 1078.5 eV, the white line region of the Na₂CO₃-modified sorbents consisted of three defined features, i.e. B (ca. 1076.3 eV), C (ca. 1078 eV) and D (ca. 1080.6 eV, maximum intensity of the spectra) (Fig. 3). Notably, the obtained Na K-edge spectra of the Na₂CO₃-modified sorbents did not correspond to any previously reported XANES spectrum of Na-based materials. On the other hand, the spectra matched perfectly the Na₂Ca(CO₃)₂ reference (P2₁ca space group,³⁰ see XRD of the synthesized reference in Fig. S15, ESI†). The Na K-edge XAS spectrum of Na₂Ca(CO₃)₂ is shown in Fig. 3 and, to the best of our knowledge, is reported here for the first time. A perfect match between the spectrum of Na₂Ca(CO₃)₂ and the spectra of the cycled Na₂CO₃-modified sorbents (collected after the carbonation step) revealed the formation of the Ca–Na double carbonate (Na₂Ca(CO₃)₂) under carbonation conditions (Fig. 3). Hence, the Na-rich phase that was observed by cross-sectional FIB-SEM (Fig. 2f) to cover the surface of CaO/CaCO₃ particles was Na₂Ca(CO₃)₂. Fig. 3 demonstrates that Na K-edge XANES can be used as a fingerprint to distinguish between Na in Na₂CO₃ or Na₂Ca(CO₃)₂ environments. Being an element selective technique, XANES allowed us to study the Na environment in the materials, even when using relatively low Na contents (i.e. < 10 wt% Na₂CO₃); a composition for which XRD failed to provide sufficient information on the Na containing phases.”

“Fig. 3 Na K-edge XANES spectra of Ca/1Na, Ca/10Na and Ca/20Na after 10th carbonation, as well as the references Na₂CO₃ and Na₂Ca(CO₃)₂.”