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

Abstract

Direct air capture (DAC) represents an advanced negative carbon emission technology, with the key being high-performance CO2 adsorbents. First, this work carefully identifies CO2 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 CO2 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/nm2 after undergoing 30 cycles of adsorption-desorption. Then it increases to 9.54 OH/nm2 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 CO2 capture and regeneration is summarized using the CaO/HcATP as DAC adsorbent. The amorphous ''CaO'' interacts with the surface silanol of HcATP, synergistically capturing CO2 in the form of ''CaO···CO2'', 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.

Keywords

• Direct air capture
• Adsorbent
• Physisorption
• Chemisorption