Direct Infrared Discrimination of End-On and Bridged CO 2 Adsorption via 16 O═C═ 18 O 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.