Carbon dioxide (CO2) 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 CO2 adsorbents due to their affordability
and sustainability for large-scale use. Here, the nature of binding
interactions between CO2 molecules and cationic scandium
oxides of different sizes, i.e., ScO+, Sc2O2
+, and Sc3O4
+,
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
CO2 and metal sites of cationic scandium oxides. The importance
of orbital interactions in the driving forces for CO2 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 CO2 attachment.
This study not only reveals the governing factor in the binding behaviors
of CO2 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.