The presence of O2 in ICCU-methanation and the stability of LaNiO3/CeO2-derived DFM

“The developed DFM should also selectively capture CO2 from O2-containing flue gas at relatively high temperatures and then, hydrogenate the adsorbed species to methane with H2. With the aim of evaluating the influence of the presence of O2 during storage period a 10% of O2 is jointly fed with a 10% of CO2 during adsorption cycles. Fig. 10 plots the evolution of CH4 and CO productions with the number of CO2 adsorption/hydrogenation cycle at 400 °C for 30% LaNiO3/CeO2-derived sample. Note that the cycles 1–2 and 8–9 were carried out in the absence of O2 in the feed stream, whereas this compound was fed in cycles 3–7.”


Fig. 10. CH4 and CO productions during CO2 adsorption and hydrogenation to CH4 at 400 °C without (cycles 1–2 and 8–9) and with (cycles 3–7) O2 during the adsorption step for the 30% LaNiO3/CeO2 DFM. The feed was 10% CO2/Ar or (10% CO2 + 10% O2)/Ar (1 min) in the adsorption step and 10% H2/Ar (2 min) in the hydrogenation step.”

“Comparing cycles 1–2 with cycles 3–7, a negative effect on CH4 production can be detected for the experiments in the presence of O2 during the adsorption period. Indeed, the CH4 yield immediately decreases from 99 to 47 μmol g−1 from the 2nd to 3rd cycle, whereas no significant changes are observed for cycles 4–7. Zheng et al. [54] justified the loss of activity for O2-containing experiments by the oxidation of the active metallic phase during the adsorption step. In agreement with their results, a small CO2 signal and a significant decrease in methane production is observed at the beginning of the hydrogenation period for the oxygen-containing experiment with respect to oxygen-free experiment (Figure S5). This fact reveals that some carbonates, adsorbed during the storage period, are released without being hydrogenated due to the absence of enough Ni0 active sites to reduce them towards CH4. In any case, the decrease in methane production for O2-containing experiments is significantly lower to that observed for 10% Ni-6.1% NaO/Al2O3, where no methane formation was observed when the sample was exposed to O2 and H2O during the CO2 capture step [19]. On the other hand, it is worth to mention that, in contrast to that observed in our previous work for conventional 10% Ni–10% Na2CO3/Al2O3 [11], CO production remains invariable after inclusion of O2 in the feed stream. This trend discards the promotion of RWGS reaction (Eq. (14)) due to a partial oxidation of Ni0 to NiO.

During the next cycles (i.e. from the 8th to the 9th), CO2 adsorption/hydrogenation cycles were again carried in an oxygen-free environment. Remarkably, the CH4 production is recovered immediately after the oxygen is removed from the feed stream. Note that the 8th and 9th cycles show similar CH4 and CO productions than 1st and 2nd cycles. Thus, these results reveal that the here discovered DFM has a high ability to restore activity once O2 is not fed during CO2 adsorption, which it is one of the main limitations of the conventional Ni-based formulations [19][55][56]. In agreement with the H2-TPR results, the high reducibility of different Ni species implies that Ni can be easily reduced back during the hydrogenation step at low temperature. Therefore, 30% LaNiO3/CeO2-derived DFM can be considered a superior candidate for real conditions process at intermediate-high temperatures.”

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