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In this study, a systematic investigation on Dual Functioning Materials (DFMs) for the capture and methanation of CO2 is carried out. The attention is focused on the nature of the CO2 adsorbent component (storage material, SM) varying between alkaline (Li, Na, K) and alkaline-earth (Mg, Ca, Ba) metal oxides in combination with Ru, both supported on an Al2O3 support. Combining gas phase reactivity analysis and FT-IR characterization, the samples are characterized in terms of CO2 storage capacity. It is found that all the SM-containing samples adsorb significant amounts of CO2 as carbonate species, with the higher amounts being adsorbed when the more thermally stable species are formed, i.e., when Ca, Ba, or K are employed as SMs. In all cases, the hydrogenation of the adsorbed carbonates to CH4 occurs at lower temperature, if compared to their thermal desorption. However, in the case of Ca- and Ba-based DFMs, resilient carbonates are present on the material surface. It was found that the SMs able to form the more thermally stable carbonates upon CO2 adsorption also showed the best performances in capture/methanation cycles at 350 °C, even if some residual carbonates were left on the DFM after the hydrogenation step. In particular, the following order of reactivity has in fact been observed in terms of CH4 production: Ru-K ≥ Ru-Ba Ru-Ca Ru-Na ≫ Ru-Mg = Ru-Li = Ru. The presence of steam and O2 during the capture step has a detrimental effect on the CO2 adsorption for all samples and, as a result, on CH4 production due to the competition of CO2 and water for the same adsorption sites. Thus, only SMs able to form strongly bound carbonates species upon CO2 exposure can retain significant CO2 storage capacity also in the presence of water in the adsorption feed.
Porta et al. (Wed,) studied this question.