Mechanisms of using CaO-alone for ICCU-RWGS

https://doi.org/10.1016/j.ccst.2021.100001

“By investigating the reaction mechanism using in-situ diffused reflectance infrared Fourier transform spectroscopy coupled with mass spectrum (DRIFTS-MS), we confirm the formation of calcium carbonate (CaCO₃) during the CO₂ adsorption (character peaks assigned in Fig. S4a). And we found the direct reaction between H₂ and carbonates by the dissociation of carbonates into surface hydroxyl and CO (fig. S4b). It is noted that the conversion of CO₂ in the ICCU process using CaO-alone is still higher than most of the reported CO₂ conversion from conventional reverse-water-gas shift reaction (RWGS) where transition-metal-based catalysts were used (fig. S8) (Daza and Kuhn, 2016). We further replaced H₂ with N₂ to confirm the exceptional performance of CaO in ICCU, and there is no distinct CO production in the presence of N₂. It is interesting to observe that the yield of CO₂ is similar in both N₂ and H₂ atmosphere during the utilization stage of ICCU. For example, the yield of CO₂ is 1.34 and 1.53 mmol/g_CaO, and the yield of CO is 4.15 and 0.01 mmol/g_CaO at 700 °C in H₂ and N₂, respectively. Therefore, it is proposed that CaO acts as CO₂ adsorbent and catalyses CO₂ for producing CO by directly reducing carbonates with the assistance of H₂, which is also confirmed by in situ DRIFTS-MS (fig. S4b and S4c). The ICCU mechanism of CO₂ adsorption over CaO and the H₂ regeneration of CaCO₃ were proposed in fig S5 and discussed in Supplementary Text. Typically, the extensive hydroxyl groups in the initial hydrogenation stage indicate the effective H₂ dissociation on the surface of CaCO₃, and the CO can be dissociated from the hydrogenated carbonate and released out quickly. The formates species or the hydrogenated carbonate (as shown in Fig. S5) might be the critical intermediates. ”

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.

Fig. S8. Catalytic performance of recent studies on RWGS (reverse-water-gas shift reaction)[23-64] and commercial catalysts price comparsion[65] (From https://www.kemcore.com/; https://www.alfa.com/en/; https://www.energy.gov/).