Reaction mechanisms of Ru-based DFMs for ICCU-methanation

The following sentences are copied from this reference (https://doi.org/10.1016/j.ccst.2022.100052) regarding the reaction mechanism of Ru-based DFMs.

“For the mechanism of the CO₂ capture and methanation on Ru-based DFMs, Farrauto et al. (Duyar et al., 2015; Zheng et al., 2016) proposed that CO₂ is mainly chemisorbed on CaO sites and spillover to the Ru sites upon H₂ introduction. Due to the methanation activity of Ru, CO₂ can then be reduced by H₂ for the generation of CH₄ on the Ru surface as shown in Fig. 15. An in-situ DRIFTS study was conducted to elucidate the possible mechanism of 5%Ru-6.1%Na₂O/Al₂O₃ DFM (Proaño et al., 2019). During the capture stage on the 5%Ru/Al₂O₃ surface, CO₂ was absorbed on the Al₂O₃ OH groups and the Ru sites. After the addition of Na₂O, the CO₂ was mainly absorbed on Al-O⁻-Na⁺ sites with the formation of bidentate carbonates. The adsorbed bicarbonates and bidentate carbonates spillover to the Ru-support interface for methanation with the formate species as intermediates, as shown in Fig. 16. Nonetheless, it was assumed that the carbonyl was the intermediate on 1%Ru/Al₂O₃, while on the 5%NaNO₃/1%Ru/Al₂O₃, both carbonyl, formate, carbonate, and biocarbonate species were likely to be the reaction intermediates for methanation (Park et al., 2021). On Li-Ru/Al₂O₃ DFM, in-situ DRIFTS and TG-MS indicated that the formed Li-Al phases could facilitate the prompt sorption of CO₂ in the form of bidentate carbonates, with further adsorbed CO₂ being absorbed as weakly bonded bicarbonates at a slower rate (Cimino et al., 2022). The spillover mechanism of CO₂ from Li-Al oxide to Ru sites for methanation was also confirmed by in-situ DRIFTS in this study.”

Fig. 15. Schematic mechanism of the CO₂ capture and methanation on Ru-CaO/Al₂O₃ DFM (Zheng et al., 2016).

Fig. 16. The proposed CO₂ methanation mechanism on 5%Ru/Al₂O₃ (right side) and 5%Ru-6.1%Na₂O/Al₂O₃ (left and right side) (Proaño et al., 2019).