Binding Strengths and Orientations in CO 2 Adsorption on Cationic Scandium Oxides: Governing Factor Revealed by a Combined Infrared Spectroscopy and Theoretical Study.

Carbon dioxide (CO 2 ) adsorption is a critical step to curbing carbon emissions from fossil fuel combustion. Among various options, transition metal oxides have received extensive attention as promising CO 2 adsorbents due to their affordability and sustainability for large-scale use. Here, the nature of binding interactions between CO 2 molecules and cationic scandium oxides of different sizes, i.e., ScO + , Sc 2 O 2 + , and Sc 3 O 4 + , is investigated by mass-selective infrared photodissociation spectroscopy combined with quantum chemical calculations. The well-accepted electrostatic considerations failed to provide explanations for the trend in the binding strengths and variations in the binding orientations between CO 2 and metal sites of cationic scandium oxides. The importance of orbital interactions in the driving forces for CO 2 adsorption on cationic scandium oxides was revealed by energy decomposition analyses. A molecular surface property, known as the local electron attachment energy, is introduced to elucidate the binding affinity and orientation-specific reactivity of cationic scandium oxides upon the CO 2 attachment. This study not only reveals the governing factor in the binding behaviors of CO 2 adsorption on cationic scandium oxides but also serves as an archetype for predicting and rationalizing favorable binding sites and orientations in extended surface-adsorbate systems.