Ethylene and oxygen species adsorbed on a defect oxidized surface Ag(111) - Theoretical analysis by DFT method

We suggest a cluster model AS(v) --> Ag12-3O of the oxidized surface Ag(111 ) with a defect. The defect is simulated by cationic vacancy V. Density fun ctional theory (B3LYP/LANL1MB approximation) is used to calculate ethylene and oxygen adsorption on the regular (AS(r)) and defect (AS(d)) sites on th e Ag(1 1 1). Oxygen interaction with site AS, produces atomic oxygen specie s (AS(r)-O). Oxygen adsorption on site AS(d) is accompanied by its associat ion with subsurface oxygen atoms to form a quasimolecular structure of meta l ozonide type -Ag-O-O-ep-O-Ag-, containing electrophilic oxygen O-ep. Ener gies of atomic oxygen binding to the regular and defect surfaces are found to be approximately equal. On the regular surface, ethylene forms a pi -com plex with binding energy E-pi(Ag-C2H4) = 14.2 kcal/mol. On the defect surfa ce, ethylene produces a metal-ethylene-peroxide cycle such as Ag-O-O-C2H4-A g. Determined are the frequencies of normal vibration for ethylene and oxyg en species, adsorbed on the regular and defect surfaces. In the case of ass ociative oxygen species and complete isotope replacement O-16 --> O-18, the main frequency at 1000 cm(-1) shifts by Deltav = 57-61 cm(-1), but this sh ift decreases to Deltav = 25-30 cm(-1) for isotope mixtures O-16/O-18. For the adsorbed species of ethylene-oxygen mixtures, IR spectra show the frequ encies within which 170-180 cm(-1) are associated with stretching of bond A g-C. Frequencies at 300-490 cm(-1) are assigned to mode v(Ag-O) of the func tional group Ag-O-O-eP-O-Ag. The most intensive modes at 950 and 600 cm(-1) are likely to stretching and bending of the functional groups containing t he O-O-O and O-O-C bonds. (C) 2001 Elsevier Science B.V. All rights reserve d.