Adsorption site and active species interactions in direct air capture and subsequent CO2 utilization

Abstract

This research investigates material design strategies for direct air capture （DAC） and integrated CO2 utilization under realistic atmospheric conditions, with a particular emphasis on adsorption site engineering and interfacial chemistry. The primary objective is to elucidate how framework structure, cation characteristics, and metal–sorbent interactions govern CO2 capture behavior and subsequent conversion pathways, thereby enabling the rational design of high- performance DAC systems. A series of studies were conducted under practical conditions, including low CO2 concentrations （~400–500 ppm）, varying humidity, and the presence of competing gases, to systematically evaluate and optimize material performance. Four distinct material platforms, ranging from zeolite adsorbents to alkali-based sorbents and dual- functional catalytic systems, were explored to identify key design principles for efficient DAC and CO2 utilization. The first study focused on the CO2 adsorption–desorption performance of representative zeolites （ZSM-5, Beta, Mordenite and Y） in both H- and Na-forms. Across all framework types, Na+ exchange significantly enhanced CO2 adsorption capacity compared to their H-form counterparts, highlighting the importance of cation-mediated electrostatic interactions. Among them, Na-ZSM-5 exhibited superior DAC performance due to its optimal pore dimensions and enhanced acid–base properties. Notably, low Si/Al ratio Na-ZSM-5 demonstrated the highest effective CO2 uptake at low concentrations and maintained stable cyclic adsorption–desorption performance, confirming its durability. Furthermore, under real atmospheric conditions containing O2 and moisture, this material retained strong CO2 capture capability, underscoring its practical applicability as a scalable DAC adsorbent. The second study systematically investigated the role of cation charge density in ZSM-5 zeolites exchanged with alkali （Na+ and K+） and alkaline earth metals （Mg2+, Ca2+ and Ba2+）. The results revealed that cation charge density is a critical descriptor governing DAC performance. Low charge density cations （e.g., K+） exhibited insufficient electrostatic interaction with quadrupolar CO2, leading to poor capture efficiency, whereas excessively high charge density cations （e.g., Mg2+ and Ca2+） induced site blocking due to strong adsorption of H2O and CO2. In contrast, Ba2+, with an intermediate charge density, provided an optimal balance, resulting in the highest DAC capacity （0.4 mmol g‒1 at 500 ppm CO2, RH 13%） along with fast kinetics and excellent cyclic stability. In-situ DRIFTS analysis further revealed a moisture swing mechanism, where adsorbed CO2 is reversibly desorbed by water, highlighting the dynamic role of humidity in DAC operation. Subsequent studies explored K2CO3-based sorbents, focusing on the critical role of support-dependent interfacial chemistry. Through systematic support screening, γ-Al2O3 was identified as a unique support capable of inducing the formation of a K-dawsonite-related K– O–Al interfacial phase, which provides additional CO2-reactive sites beyond bulk K2CO3. This phase forms upon high-temperature calcination （≥ 550 °C） and is further promoted by controlled water addition. The optimized K2CO3/γ-Al2O3 sorbent achieved a CO2 uptake of 1.58 mmol g‒1under real-air DAC conditions. Mechanistic studies revealed a reversible transformation: pretreatment decomposes K-dawsonite into K2CO3 and interfacial K–O–Al species, while humid DAC conditions regenerate K-dawsonite alongside KHCO3 formation. Upon Ni incorporation for CO2 utilization, this dynamic interfacial environment further enhanced CO2 uptake （up to 2.62 mmol g‒1） and improved CH4 selectivity, demonstrating the effectiveness of synthesis-controlled interfacial phase engineering. Finally, an integrated DAC–methanation system was developed using a Ni/K2CO3/γ- Al2O3 dual-functional material. In this system, Ni serves as the active phase for CO2 hydrogenation, while K2CO3 functions as a CO2 sorbent. The combined system exhibited a significantly enhanced CO2 capture capacity （2.35 mmol g‒1）, approximately three times higher than K2CO3/γ-Al2O3 alone, indicating strong synergistic interactions between Ni and the sorbent phase. This enhancement is attributed to the formation of KO⁻ superbase species via reductive decomposition of K2CO3 during H2 pretreatment, which creates additional active sites for CO2 adsorption. Simultaneously, K2CO3 suppresses the formation of inactive NiAl2O4 by weakening strong metal–support interactions, thereby improving Ni reducibility and dispersion. The effect of moisture was also systematically evaluated, revealing that moderate humidity （30–50% RH） maximizes CO2 uptake, while higher humidity enhances CO2 utilization efficiency （up to 35%） by promoting *HCO3 intermediate formation, as confirmed by in-situ DRIFTS. These intermediates play a crucial role in facilitating the methanation pathway and enhancing CH4 productivity. In conclusion, this work demonstrates that precise control of adsorption sites, cation properties, and interfacial phases is essential for achieving high-performance DAC and integrated CO2 conversion systems. By systematically tailoring framework composition, cation charge density, and metal–sorbent interactions, the developed materials exhibit enhanced CO2 capture capacity, stability, and conversion efficiency under realistic conditions. The insights obtained from this study provide fundamental design principles for next-generation DAC materials and highlight the critical role of site-specific engineering in advancing scalable and sustainable carbon capture and utilization technologies.