Ethylene and oxygen species adsorbed on a defect oxidized surface Ag(1 1 1): Theoretical analysis by DFT method

We suggest a cluster model ASv→Ag12–3O of the oxidized surface Ag(1 1 1) with a defect. The defect is simulated by cationic vacancy V. Density functional theory (B3LYP/LANL1MB approximation) is used to calculate ethylene and oxygen adsorption on the regular (ASr) and defect (ASd) sites on the Ag(1 1 1). Oxygen interaction with site ASr produces atomic oxygen species (ASr–O). Oxygen adsorption on site ASd is accompanied by its association with subsurface oxygen atoms to form a quasimolecular structure of metal ozonide type –Ag–O–Oep–O–Ag–, containing electrophilic oxygen Oep. Energies of atomic oxygen binding to the regular and defect surfaces are found to be approximately equal. On the regular surface, ethylene forms a π-complex with binding energy Eπ(Ag–C2H4)=14.2 kcal/mol. On the defect surface, ethylene produces a metal–ethylene-peroxide cycle such as Ag–O–O–C2H4–Ag. Determined are the frequencies of normal vibration for ethylene and oxygen species, adsorbed on the regular and defect surfaces. In the case of associative oxygen species and complete isotope replacement 16O→18O, the main frequency at 1000 cm−1 shifts by Δν=57–61 cm−1, but this shift decreases to Δν=25–30 cm−1 for isotope mixtures 16O/18O. For the adsorbed species of ethylene–oxygen mixtures, IR spectra show the frequencies within which 170–180 cm−1 are associated with stretching of bond Ag–C. Frequencies at 300–490 cm−1 are assigned to mode ν(Ag–O) of the functional group Ag–O–Oep–O–Ag. The most intensive modes at 950 and 600 cm−1 are likely to stretching and bending of the functional groups containing the O–O–O and O–O–C bonds.