XAS analysis of Na2CO3-modified CaO-based CO2 sorbents

“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)2Fig. 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.”

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