Mechanisms of using CaO-alone for ICCU-RWGS

“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/gCaO, and the yield of CO is 4.15 and 0.01 mmol/gCaO 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;;

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