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Mechanism of Ru/CeO2-MgO DFMs for ICCU-methanation

The following content is copied from this reference (https://doi.org/10.1016/j.fuel.2022.123420) regarding the reaction mechanism of Ru/CeO2-MgO DFMs.

“The mechanism of ICCM over Ru/CeO2-MgO was studied by in situ DRIFTS characterization using Ru/rod-CeO2-MgO catalyst with Ru/rod-CeO2 as the benchmark, as shown in Fig. 9. During the CO2 adsorption of Ru/CeO2-MgO, a set of peaks appeared and gradually increased, including the peaks at 1614 cm−1 (bidentate carbonates [58]) and 1815 cm−1 (carbonyl vibration of carbonates), which represented the CO2 adsorption on MgO by forming carbonates. The peaks belonging to CO2 adsorption can be saturated within 15 mins over Ru/CeO2-MgO combined materials. As for the key intermediates, we observed CO2 dissociation at the initial time of CO2 adsorption, including the peaks at 2037 cm−1 and 1993 cm−1 (linear or bridged Ru-CO [14][59][60][61]). The dissociation of CO2 on Ru was fast and stable under Ar purge and vanished quickly under H2. With the introduction of H2, there is also a significant peak shift from 1614 cm−1 to 1582 cm−1(conjugated C-O/C = O of formats [14][61]) and OCO peak at 1367 cm-1 [62], representing the possible formates reaction pathway on the surface of MgCO3. Ru/rod-CeO2 can also adsorb CO2 to form carbonates [63], which may be related to the oxygen vacancies on the surface. In short, there are mainly two reaction pathways in ICCM over Ru/CeO2-MgO, including CO2 dissociation on Ru and formates on MgCO3. The gradually increased CO2 conversion during cyclic ICCM over Ru/rod-CeO2-MgO might be attributed to the promotion of the formates pathway.”

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“Fig. 9. In situ DRIFTS of ICCM over Ru/rod-CeO2-MgO and Ru/rod-CeO2.”

“Therefore, as illustrated in Fig. 10, it is suggested that (1) in the first step of CO2 adsorption, MgO acts as the main adsorbent to form MgCO3 while CeO2 with oxygen vacancies can promote extra CO2 adsorption by forming bicarbonates. (2) two reaction routes for the release and transformation of the adsorbed CO2 under 5% H2/N2 atmosphere are proposed. Route 1 is the decomposition of MgCO3 to release gas-phase CO2 and then generate CH4 with the assistance of H2 and active sites by the CO2 dissociation pathway. Route 2 is the reduction of carbonates (Eq. (6)) by the formates pathway. Route 2 does not rely on the gas phase CO2 spillover and can achieve ideal ICCM performance, ∼100% CO2 conversion.”

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“Fig. 10. Schematic diagram of ICCM over Ru/rod-CeO2-MgO.”

 

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