Development of K2CO3-based catalytic adsorbents for direct air capture of CO2 and its methanation

Abstract

Direct air capture (DAC) using alkali metal carbonates has gained increasing attention due to their low cost, thermal stability, and strong affinity for CO2. However, their slow carbonation kinetics under atmospheric conditions remain a major limitation. To address this, K2CO3 was supported on various porous materials and systematically evaluated for DAC performance under real air conditions. Among the investigated supports, γ-Al2O3 exhibited the highest CO2 uptake, which was attributed to the formation of the hydroxyl-carbonate phase KAl(CO3)(OH)2 generated through strong interactions between K2CO3 and the alumina surface. Characterization studies further confirmed that the Brønsted acid sites of γ-Al2O3 play a central role in promoting this phase, as demonstrated by NH3-TPD analysis and comparison with samples enriched in either Brønsted or Lewis acidity. To optimize the formation of KAl(CO3)(OH)2, various synthesis parameters including K2CO3 loading, calcination temperature, water-assisted preparation, and ball milling were examined. The optimized adsorbent, 30K/Al synthesized with an appropriate amount of added water, exhibited enhanced CO2 capture even at low humidity levels. Although this hydroxyl-carbonate phase requires elevated temperatures for CO2 desorption, its high-temperature stability was effectively leveraged in an integrated DAC-methanation system. By incorporating Ni as a methanation catalyst, the dual-functional 10Ni/30K/Al (H2O/K = 10) material achieved both high CO2 uptake and improved CH4 selectivity, demonstrating that KAl(CO3)(OH)2 enables stable CO2 retention up to the methanation reaction temperature. Overall, this work highlights the decisive role of support acidity in governing the interaction between K2CO3 and γ-Al2O3, and establishes acidity control as a practical design strategy for developing high-performance K2CO3-based DAC sorbents. The insights gained here offer guidance for sorbent development not only for DAC applications but also for their direct integration with catalytic CO2 utilization processes.