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

High Resolution Image Download MS PowerPoint Slide Identification of the adsorption geometry of molecular CO 2 on solid surfaces is essential for the rational design of effective adsorbents and catalysts for CO 2 capture and utilization, yet remains a persistent experimental challenge. In this work, we show that mixed-oxygen CO 2 isotopologues containing 16 O and 18 O, optionally combined with 13 C substitution, provide a direct infrared spectroscopic criterion for determining the adsorption geometry of molecular, near-linear CO 2 at low coverage. The approach exploits isotopic linkage isomerism (ILI) of the intrinsically asymmetric 16 O═C═ 18 O isotopologue. End-on (η 1 -O) adsorption necessarily generates two linkage isomers (coordination through either 18 O or 16 O) producing two ν 3 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 ν 3 band. The general applicability of this diagnostic criterion is demonstrated for CO 2 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 μ 3 –OH groups in UiO-66, and on TiO 2, 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 CO 2 isotopologues further aid band assignment in complex spectra. This experimentally simple approach provides a robust and broadly applicable spectroscopic tool for resolving CO 2 binding geometries across a wide range of materials.