Water-promoted CO₂ adsorption on the ZrO₂(001) surface: A first-principles mechanistic study

Direct air capture (DAC) is an important route toward negative carbon emissions. Because atmospheric CO₂ is present at an ultralow concentration and always coexists with water vapor, the interfacial interaction between CO₂ and H₂O plays a key role in capture performance. As a common support for alkali-metal-based composite adsorbents, ZrO₂ has good thermal stability and abundant surface acid-base sites, yet its interfacial response to CO₂ under humid conditions remains unclear. In this work, first-principles calculations were employed to systematically investigate the adsorption behavior and interfacial mechanisms of single CO₂, single H₂O, and CO₂+nH₂O (n = 1∼4) on the ZrO₂(001) surface. The results show that isolated H₂O preferentially adsorbs at the 5-coordinated Zr site through stable chemisorption, whereas isolated CO₂ exhibits a weak adsorption energy of only 0.45 eV at its optimal site, indicating the limited intrinsic CO₂ capture ability of pristine ZrO₂(001). After introducing H₂O, both the co-adsorption energy and interaction energy become negative, demonstrating a synergistic effect, which becomes much more pronounced when n > 2. Under low-water conditions, H₂O mainly stabilizes CO₂ indirectly through local electrostatic regulation, hydrogen bonding, and weak intermolecular interactions. When n > 2, the multi-water network further improves orbital matching between the O atoms of CO₂ and surface Zr sites and strengthens direct bonding. Excited-state spectra and electron-hole analyses further show that multiple water molecules not only modify interfacial interactions but also alter the dominant electronic transition characteristics. This work reveals the intrinsic mechanism of cooperative CO₂/H₂O adsorption on ZrO₂ at the atomic and electronic scales, providing a theoretical basis for understanding the adsorption behavior at interfaces of DAC-related oxides.