Storage Material Effects on the Performance of Ru-Based CO2 Capture and Methanation Dual Functioning Materials

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