We can report the following achievements in the period 01.06.2019-31.05.2024
A) Development of (in situ/operando) characterization techniques
We have developed and applied successfully pair distribution function (PDF) analysis of in situ total scattering data (synchrotron based) under CO2 capture relevant conditions. We have also introduced for the first time Na K edge XAS to the research of alkali earth metal oxide based CO2 sorbents. We have designed and developed a reaction cell to allow for X-ray reflectivity (XRR) and X-ray grazing incident diffraction (GIXRD) under CO2 capture conditions. In situ electron microscopy techniques proved to be very challenging owing to electron beam induced damage of the material and phase transformations. Hence, most electron microscopy work has been done by ex situ experiments whereby samples are taken in "frozen states" at given times of the reaction.
B) Fabrication of model structures/materials
Using laser ablation, we have been able to manufacture grooves of controlled shape and depth (micrometer scale) into MgO single crystals. In addition, we have been able to synthesize oxygen isotope labelled (18O and 17O) MgO and NaNO3 with enrichment levels of up to 95% and 50% respectively for in situ Raman spectroscopy. These unique materials were used to probe the role of molten promoters in MgO based CO2 sorbents. In collaboration with colleagues, we have also been able to make thin films of controlled thickness via sputtering, but sample transfer is very challenging, in particular for CaO due to its hygroscopic nature. In addition, we were able to synthesize CaO-based nano particles and structures with controlled porosity. Using atomic layer deposition such structures were covered by metal oxide stabilizers of well controlled thickness to probe the cyclic dynamics of the stabilizer. These model materials synthesized proved to be highly advantageous for the objective to elucidate the carbonation mechanism and deactivation pathways of CaO- and MgO-based CO2 sorbents.
Using the newly developed experimental techniques and model structures we were able to progress towards all objectives of the proposal.
For example, concerning the role of the molten alkali metal salt promoters we could demonstrate that the presence of a promoter already effects strongly the decomposition pathway of a relevant MgO precursor, i.e. hydromagnesite (Rekhtina et al., Nanoscale, 2020). In addition, using grooves of variable depths in a single crystal MgO(100), we revealed that MgCO3 formation occurs at the interphase promoter-MgO with a crystal growth in a preferential direction with respect to the MgO(100) crystal. Furthermore, we revealed that the molten salt promoter accelerates the dissolution of MgO leading to pyramidal-shaped pitches and a high concentration of ionic [Mg2+ --- O2-] pairs, the formation of which was found to be the rate limiting step in the carbonation process (Bork et al., PNAS, 2021; Landuyt et al., JACS Au, 2022). Further, it was discovered that the addition or in situ formation of nucleation seeds greatly accelerates the carbonation kinetics.
We progressed also appreciably in addressing the controversy whether the addition of alkali metals (notably Na and K) to CaO can improve its CO2 uptake. We demonstrate that the addition of high amounts of Na2CO3 (> 1 wt%) to CaO reduces severely its CO2 uptake, while very small quantities of Na2CO3 improved the CO2 uptake appreciably. The performance degradation is due to the formation of the double salt Na2Ca(CO3)2 that leads to strong sintering. During carbonation the formation of a dense layer of Na2Ca(CO3)2 that covers unreacted CaO, prevents its full carbonation to CaCO3. (Kurlov et al., PCCP, 2020). In addition, using Na K-edge XAS, 23Na magic angle spinning nuclear magnetic resonance spectroscopy (NMR) and transmission electron microscopy (TEM), it was revealed that in sorbents with promotional Na2CO3 loadings (i.e. 0.1-0.2 mol%), Na existed in highly distributed, non-crystalline [Na2Ca(CO3)]2 units that stabilize the pore network of the sorbent and enhanced the diffusion of CO2 through CaCO3 to unreacted CaO (Krödel et al., Adv. Funct. Mater., 2023). In a separate work we discovered that glassy stabilizers can delay appreciably the deactivation of CaO-based CO2 sorbents (Krödel et al., JACS Au, 2023).
