AVS 47th International Symposium
    Surface Science Wednesday Sessions
       Session SS2+VT-WeA

Paper SS2+VT-WeA9
Theory of Oxygen Adsorption on Ag(111): A DFT-GGA Investigation

Wednesday, October 4, 2000, 4:40 pm, Room 209

Session: Adsorption and Desorption Phenomena I
Presenter: C. Stampfl, Northwestern University
Authors: W. Li, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
C. Stampfl, Northwestern University
M. Scheffler, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Correspondent: Click to Email
O/Ag(111) is an important system with unique catalytic behavior for several large-scale industrial processes, e.g., ethylene epoxidation and partial oxidation of methanol to formaldehyde. In spite of its importance, very little is known about the behavior of O at Ag(111) (which depends sensitively on pressure and temperature) on the microscopic level, and the precise atomic location and chemical nature of the various O species that form, e.g. on-surface, subsurface, in-surface. Using density-functional theory within the generalized gradient approximation, we investigate the interaction between oxygen and Ag(111). We found that the interaction between silver and oxygen is very weak. Adsorption becomes unfavorable for coverages between 0.33 and 0.50, with respect to the gas molecule O@sub 2@. Compared to hcp site, the fcc site is preferable, and the difference in adsorption energy differs by less than 0.17 eV/atom for the whole range of coverage considered. Strong charge transfer from silver to oxygen has been found due to the large difference in electronegativity between oxygen and silver, and results in a significant change in work function, which increases monotonically with oxygen coverage. Experimental studies report that at elevated temperatures two ordered phases form: a (4x4) structure (T@>=@400K),@footnote 1@ and a high-temperature (@sr@3 x @sr@3)R30° (T@>=@800K)@footnote 2@ structure. With respect to the latter, a surface substitutional site has been proposed. Our results show that this structure is unfavorable. For subsurface adsorption we find that oxygen prefers the octahedral site, but at coverage 0.33 it is also endothermic with respect to free O@sub 2@. We discuss alternative geometries for this phase. @FootnoteText@ @footnote 1@ G. Rovida et al., Surf. Sci. 43, 230 (1974); C. I. Carlisle et al., Phys. Rev. Lett. 84, 3899 (2000). @footnote 2@ B. Pettinger et al., Phys. Rev. Lett. 72, 1561 (1994); X. Bao et al. Phys. Rev. B 54, 2240 (1996).