Subsurface Stabilization of Interstitial Pt Atoms on CeO₂(111): Rethinking Single-Atom Catalyst Architectures

Single-atom catalysts (SACs) offer maximal efficiency by stabilizing isolated metal atoms on oxidic supports. Platinum on cerium oxide (CeO₂) is a key SAC system, where adsorbed CO typically exhibits red-shifted vibrational modes due to Pt back-donation. Using polarization-resolved IR spectroscopy on Pt-deposited CeO₂(111) single-crystal surfaces, we do not observe CO vibrational bands below 2140 cm^-1 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.