CaO-based sorbents are cost-efficient materials for high-temperature CO2 capture from point sources. In the calcium looping process, CaO is carbonated using the CO2 in flue gases to form CaCO3 and re-generated back to CaO via calcination. This cycle is repeated multiple times. However, CaO-based sorbents are prone to rapid deactivation over repeated carbonation-calcination cycles due to sintering, resulting in a substantial loss of their cyclic CO2 uptake. Different approaches to mitigate deactivation are being investigated currently, including the addition of alkali metal salt promoters (such as Na2CO3). Yet, the effect of the addition of Na-containing species to CaO-based sorbents remains poorly understood. Hence, this work is set to investigate the promotional effect of Na on CaO-based sorbents. To this end, we use partially amorphous Na-modified CaO, synthesized via high-energy ball-milling of CaCO3 and Na2CO3 (0.1 - 20 mol. %), with a homogeneous distribution of Na throughout the material. Using kinetic modelling and advanced characterization techniques, i.e. in situ X-ray powder diffraction, pair distribution function analysis, 23Na-nuclear magnetic resonance spectroscopy, X-ray absorption at the Na K-edge and transmission electron microscopy, we demonstrate that Na2CO3 contents in a narrow range of 0.1 – 0.3 mol. % promote the carbonation kinetics of CaO via the formation of atomically-dispersed [Na2Ca(CO3)2] clusters without substantially enhancing the sintering process. For Na2CO3 contents &gt; 0.5 mol. %, the materials rapidly deactivate via enhanced sintering and the formation of micrometer-sized Na2Ca(CO3)2 crystallites at the surface of CaO. Our work provides novel insight into the atomic-scale promotion and the deactivation mechanisms of Na-modified CaO, which are relevant for the design of more effective CaO-based CO2 sorbents.

Elucidating the atomic-scale promotion and deactivation mechanisms of Na-modified CaO-based sorbents for high-temperature CO2 capture

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