Stability Trends of Surface and Subsurface Interstitial Transition Metal Species on CeO2(111)

Single-atom catalysts (SACs) on reducible oxides are generally assumed to consist of isolated metal atoms located at the surface and directly accessible to reactants. Recent studies on Pt/CeO$_2$(111), however, revealed that Pt preferentially occupies subsurface interstitial sites. Here, we investigate whether this behavior is unique to Pt or represents a more general phenomenon. Using density functional theory, we systematically compare the stability of surface and subsurface configurations for transition-metal atoms from Fe to Au on stoichiometric CeO$_2$(111). Pronounced periodic trends emerge: the tendency toward subsurface incorporation increases from the 3d to the 5d series and from the Fe group toward the Ni group, with Pt and Pd exhibiting the strongest preference. Several metals also induce oxygen migration and surface reconstruction, generating additional stable configurations. Calculated CO vibrational frequencies provide spectroscopic fingerprints for experimental identification. These results demonstrate that subsurface incorporation is a general structural motif in SACs on reducible oxides, challenging the conventional view of surface-bound single-atom catalysts.