A Mechanistic Framework for CO2 Adsorption over PEI-Functionalized LDO Adsorbents under Coupled Humidity and Temperature Conditions

Abstract

Adsorption-based direct air capture (DAC) using supported amine adsorbents is a promising strategy for reducing atmospheric CO2. However, understanding of the adsorption–desorption mechanism, especially under humid conditions, remains inconsistent due to experimental conditions in prior studies, which often overlook synergistic interactions among multiple parameters. This work systematically investigates the adsorption pathways of a benchmark adsorbent, PEI-functionalized layered double oxide (LDO), under varying humidity, temperature, CO2 partial pressure, and amine loading using an in situ diffuse reflectance infrared Fourier transform spectroscopy platform. A bicarbonate species, commonly reported in the literature, is observed under humid conditions, but its weak spectral features and delayed emergence indicate it is not the predominant adsorption product. In addition, kinetic analysis based on the linear driving force model reveals that while H2O facilitates proton transfer and accelerates the formation of adsorbed species, it ultimately suppresses kinetics through competitive adsorption, diffusion barriers, and strong hydrogen bonding effects. Optimal adsorption kinetics occur at 60% relative humidity. This study establishes a unified mechanistic framework for the PEI-LDO system by coupling multiple environmental parameters, clarifying the distinct roles of primary/secondary amines, bicarbonate species, and hydrogen bond networks across diverse conditions, thereby helping to reconcile long-standing discrepancies in the literature, particularly regarding the role of water. These insights provide actionable guidance for the targeted design of PEI-LDO adsorbents and the informed optimization of DAC operating conditions.