Direct Infrared Discrimination of End-On and Bridged CO₂ Adsorption via ¹⁶OC¹⁸O Linkage Isomerism

Identification of the adsorption geometry of molecular CO₂ on solid surfaces is essential for the rational design of effective adsorbents and catalysts for CO₂ capture and utilization, yet remains a persistent experimental challenge. In this work, we show that mixed-oxygen CO₂ isotopologues containing ¹⁶O and ¹⁸O, optionally combined with ¹³C substitution, provide a direct infrared spectroscopic criterion for determining the adsorption geometry of molecular, near-linear CO₂ at low coverage. The approach exploits isotopic linkage isomerism (ILI) of the intrinsically asymmetric ¹⁶OC¹⁸O isotopologue. End-on (η¹-O) adsorption necessarily generates two linkage isomers (coordination through either ¹⁸O or ¹⁶O) producing two ν₃ bands of equal intensity. The magnitude of the splitting correlates with adsorption strength. In contrast, bridged (μ-O,O') adsorption involving two equivalent bonds suppresses ILI and yields a single ν₃ band. The general applicability of this diagnostic criterion is demonstrated for CO₂ adsorbed on alkali- and alkaline-earth-exchanged zeolites, Brønsted- and Lewis-acidic zeolites, metal-organic frameworks containing open metal sites or hydroxyl groups, and nonporous metal oxides. Pure end-on adsorption is observed on LiX and CaX, on acid sites in H-ZSM-5, on μ₃-OH groups in UiO-66, and on TiO₂, whereas Na-LTA and MIL-53 exhibit predominantly bridged adsorption. Mixed adsorption regimes are identified in NaX, NaY, and Ni-MOF-74. Isotopic shift factors between different CO₂ isotopologues further aid band assignment in complex spectra. This experimentally simple approach provides a robust and broadly applicable spectroscopic tool for resolving CO₂ binding geometries across a wide range of materials.