Adsorption of CO on α-Al₂O₃(0001): A Combined Experimental and Computational Study

α-Al₂O₃ is a widely utilized material with diverse technological applications, particularly as a catalyst support in heterogeneous catalysis. Here, a systematic investigation of the interaction between CO and the α-Al₂O₃(0001) single-crystal surface is presented by combining polarization-resolved infrared reflection absorption spectroscopy with theoretical calculations. The latter includes a comprehensive analysis of multiple coverage scenarios using periodic density functional theory calculations, as well as various embedded quantum cluster models to evaluate the performance of hybrid functionals and wavefunction methods such as MP2. The combined results reveal that the Al-terminated α-Al₂O₃(0001) surface exhibits high reactivity and is stabilized by partial hydroxylation even under ultrahigh vacuum conditions. This is evidenced by two characteristic CO bands (2172-2195 cm^-1 for CO-Al³⁺ and 2163 cm^-1 for OH^…CO) with distinct binding energies, which are consistent with theoretical predictions.