Decoupling charge transfer and spin in CO₂ activation: spin-selective frontier orbital control in doped Al₁₂X^- superatomic clusters

Understanding the electronic origin of CO₂ activation is essential for the rational design of efficient catalysts, yet conventional descriptors such as ionization potential, charge transfer, and the d-band center provide only a partial picture. Here, using density functional theory, we investigate CO₂ activation on endohedrally and exohedrally doped Al₁₂X^- superatomic clusters. Endohedral doping produces activated chemisorption without direct metal-adsorbate contact, whereas exohedral doping exposes the dopant as a directly accessible active site. Despite their distinct adsorption pathways, both systems reveal a common mechanism: CO₂ activation is governed by the ability of the clusters spin-polarized frontier electronic structure to reorganize in response to adsorption. Charge- and spin-density analyses further show that substantial charge transfer to CO₂ can occur without concomitant spin transfer. The Al₁₂ cage acts primarily as the electron reservoir, while the dopant mediates adsorption through spin-selective orbital reorganization. This mechanism accounts for the observed adsorption trends, including the approximate volcano dependence on the β-spin d states in endohedral transition-metal dopants and the transition from majority-spin frontier-orbital control to minority-spin reorganization across the exohedral series. These findings establish spin-selective electronic flexibility, rather than charge donation alone, as a key principle governing CO₂ activation in doped superatomic clusters.