respectively. These single-atom model catalysts differ from Pd or Pt monomers in PdAg/Pd or PtAg/Pt monolayer surface alloys by the different environment of the active Pd or Pt surface atoms, with Ag neighbors in all direction for the single-atom systems, while in the other case surface Pd or Pt atoms bind to an underlying Pd or Pt substrate. Muir and Trenary observed by IR spectroscopy that upon room temperature deposition of small amounts of Pd, up to approximately 0.002 monolayers, Pd/Ag(111) forms a single-atom alloy, which was demonstrated by CO titration .