Spatial arrangements of dual functional materials for CO₂ capture and in-situ methanation at low temperature

Integrated CO₂ capture and methanation (ICCM) is a negative-emission technology with simplified processes for CO₂ capture from industrial sources and in-situ conversion into synthetic natural gas. Advancing ICCM system towards low-temperature operation has additional advantages, including reducing energy consumption and improving CH₄ selectivity, however, it requires further development of Dual Functional Materials (DFMs) capable of in-situ methanation at lower temperature. In this study, Na-based DFMs with varying spatial arrangements and proximities between catalysts and adsorbents, ranging from the milli scale to the micro scale, were synthesized. The catalytic performance of Ni and Ru components was also compared. The closest catalyst-adsorbent proximity between Na₂CO₃ adsorbents and Ru sites in co-extruded DFM attributed to high CH₄ production of 306 μmol_CH4·g^-1_DFM and > 99 % CH₄ selectivity at temperature as low as 220 °C. Mechanistic studies revealed that the low-temperature in-situ methanation proceeded via a direct hydrogenation route that facilitated by hydrogen spillover. Dissociated hydrogen (H*) species migrated from the metallic sites to the adsorbent sites via γ-Al₂O₃ support, enabling stepwise hydrogenation of captured CO₂. While both Ni- and Ru-catalyzed DFMs exhibit strong H₂ dissociation activities, the enhanced catalyst-adsorbent proximity and Ru/γ-Al₂O₃ interface significantly accelerated hydrogen spillover. During the stepwise hydrogenation of captured CO₂, the accelerated spillover improved the activation of carbonate into formate (HCOO*) species, facilitating the conversion of adsorbent component. Furthermore, the enhanced H* spillover also improved CH₄ selectivity by promoting hydrogenation of CO*_(attached) species, instead of desorption into CO_(g).