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.