Design of alkali metal oxide adsorbent for direct air capture: Identification of physicochemical adsorption and analysis of regeneration mechanism

• Dual loop of the CO 2 capture and regeneration mechanism of CaO/HcATP are summarized. • The ratio of physisorption and chemisorption of CaO/HcATP was 33.2 % to 66.8 %. • The adsorption capacity of adsorbent is restored to 90.7 % upon regeneration of silanol groups. • The maximum adsorption capacity of CaO/HcATP was 667.1 ± 44.6 µmol/g. Direct air capture (DAC) represents an advanced negative carbon emission technology, with the key being high-performance CO 2 adsorbents. First, this work carefully identifies CO 2 physisorption and chemisorption by CaO/HcATP (CaO loaded on acid-modified attapulgite) as DAC adsorbent. The chemisorption of amorphous "CaO" plays a crucial role in both the adsorption capacity and rate, with contributions of 66.8 % and 50.85 %, respectively. The adsorption capacity of CaO/HcATP is only 212.4 ± 25.7 µmol/g via the simple CO 2 physisorption and improved by 426.7 µmol/g owning to the chemisorption of amorphous CaO. Second, the concentration of silanol groups on CaO/HcATP plays a pivotal role in the adsorption process. The concentration of silanol groups decreases to 3.85 OH/nm 2 after undergoing 30 cycles of adsorption-desorption. Then it increases to 9.54 OH/nm 2 by adsorbing the moisture in the air, resulting in a recovered adsorption capacity of 90.7 %. Furthermore, the pseudo-first-order adsorption kinetics model effectively predicted the experimental results. Finally, the dual loop of CO 2 capture and regeneration is summarized using the CaO/HcATP as DAC adsorbent. The amorphous "CaO" interacts with the surface silanol of HcATP, synergistically capturing CO 2 in the form of "CaO···CO 2 ", which reduces desorption energy consumption. The wetting property of HcATP contributes to the regeneration of CaO/HcATP. This work contributes to establishing fundamental principles for designing cost-effective DAC adsorbents.