CO-Adsorption on MgO Revisited: Polarization-Resolved Identification of Step Edges as the Dominant Defects on MgO(100) and MgO(110).

Magnesium oxide is a paradigmatic ionic oxide, yet its experimentally observed chemical activity remains difficult to reconcile with its wide band gap and non-reducible character. Here, we combine polarization-dependent infrared reflection-absorption spectroscopy with hybrid density functional theory to investigate CO adsorption on well-defined MgO single-crystal surfaces, explicitly disentangling the roles of surface orientation, point defects, and low-coordination step-edge sites. Using CO as a site-sensitive vibrational probe, we demonstrate that the reactivity of MgO(100) originates from fourfold-coordinated Mg4c 2+ step-edge sites rather than terrace Schottky defects. Distinct vibrational signatures, adsorption geometries, and binding energies are observed in quantitative agreement between experiment and theory. On the more open MgO(110) surface, where Mg4c 2+ sites are intrinsic to the terrace, a coordination-controlled trend emerges: Mg4c 2+ sites bind CO more strongly and exhibit higher-frequency bands than Mg5c 2+ sites. Analysis of transition dipole moments (TDM) further reveals a substantial enhancement of infrared intensities at low-coordinated sites, underscoring the need to account for site-specific dipole strengths in spectral interpretation. These results establish extended low-coordination motifs as the dominant defect sites on MgO surfaces, with direct implications for metal anchoring and the design of defect-engineered oxide catalysts.