Subsurface Stabilization of Interstitial Pt Atoms on CeO ₂ (111): Rethinking Single‐Atom Catalyst Architectures

at low coverages, indicating that the surface-bound Pt atoms are not present in detectable amounts. DFT calculations demonstrate that this unexpected observation is consistent with Pt atoms occupying buried interstitial sites. Such subsurface single-atom sites, not considered in previous studies, are thermodynamically favored at low coverages, adopt an unusual oxidation state, and are inaccessible to direct CO binding. Our findings challenge the prevailing assumption that single atoms remain surface-bound and highlight the critical role of subsurface interstitial species, prompting a rethinking of how active sites in single-atom catalysts are stabilized on reducible oxides.