https://doi.org/10.1016/j.ccst.2021.100001
” The formation of calcium carbonate (CaCO3) can be clearly observed from the in-situ DRIFTS after 30 mins CO2 adsorption and 8 mins purge. The peaks at about 2971, 2867, 2516, 1789 cm-1 and etc. (as marked in Fig.S4a) could be assigned as the characteristic peaks of CaCO3. The peaks at around 3600-3750 cm-1 and 2300-2400 cm-1 are contributed by gas-phase CO2 or the weakly adsorbed CO2. Helium purging can effectively remove the residual and weak adsorbed CO2 in a short time(~4 mins).
The in situ H2 regeneration of CaO was then carried by switching gas from He to H2. We observed a distinct CO2 release when introducing H2, which can be clearly identified from the peaks at around 2300-2400 cm-1 (Fig.S4b)[19] and mass spectrum at about 3140-3240 s (Fig.S4c). The small peaks at around 2142 cm-1 might be contributed by the gas phase CO[20-22]. It can be confirmed from the mass spectrum (Fig.S4c) that there is no CH4 generated in the whole process. Therefore, H2O can be used as a probe MS signal to monitor the progress of the reaction (Eq.S7). The reduction of surface carbonates is very fast, which can be confirmed by mass spectrum at 3140-3240 s (Fig.S4c) and the miscellaneous peaks at around 3500-4000 cm-1 and 1200-2000 cm-1 (Fig.S4b), which are contributed from H2O. With the progress of the reaction, the release of gas-phase CO2 has been minimal, while the production of H2O can still be maintained at a certain level, which proves that the continuous reaction between H2 and surface or subsurface carbonates. We observed the turning point in both mass spectrometry and in-situ DRIFTS at about 16mins of H2 regeneration. It is speculated that the surface or subsurface carbonates have been completely regenerated in this period (as illustrated in the surface reaction in Fig.S5).
Characteristic peaks (at about 3600, 2970, 2870, 2600 cm-1 and etc.) are related to the surface hydroxyl groups combined with Ca or CaO; this is due to the reaction between the H2O and the surface CaO. A significant increase of the peak at around 3600 cm-1 corresponding to H2O was observed in the initial 16 mins H2 regeneration. The peak decreased quickly after 16 mins; this might be due to the consumption of active carbonates formed on the surface of CaO (Fig.S4d). The intensity of peak at around 3600 cm-1 maintained for about 15 mins after the fast decrease (Fig.S4d), demonstrating the continuous generation of hydroxyl. The other increasing peaks also confirm the ongoing reduction of carbonates.
Therefore, the carbonation forms carbonates which are converted into CO in the atmosphere of H2. The H2 regeneration of CaCO3 could have two stages. One is the fast reduction of surface or subsurface carbonates with abundant H2O generation (as shown in Fig.S4c); another is bulk carbonates reduction with relatively slow kinetics , but possibly ongoing in a prolonged period (the reaction path is illustrated in Fig.S5). ”
Fig. S4. In situ DRIFTS spectra of ICCU over CaO. (a) CO2 adsorption (15% CO2/N2 for 30 mins) and sample purge (100% He for 8 min) at 650 ˚C, (b) H2 regeneration of CaO (100% H2 for 50 mins) at 650 ˚C, (c) MS signal of ICCU over CaO (CO signal was flattened by the fragment peak of CO2, therefore, we applied H2O as reaction probe product of ICCU) and (d) relative intensities of surface species as a function of time of H2 regeneration(b).
Fig. S5. Schematic ICCU mechanism over CaO and the two stages of H2 regeneration of CaO.