Surface Science Letters Stranski–Krastanov like oxide growth on Ag(1 1 1) at atmospheric oxygen pressures A. Reicho, A. Stierle * , I. Costina, H. Dosch Max-Planck-Institut fu ¨ r Metallforschung, Heisenbergstr. 3, D-70569 Stuttgart, Germany Received 22 September 2006; accepted for publication 1 December 2006 Available online 3 January 2007 Abstract The oxidation behavior of Ag(1 1 1) was studied by means of in situ surface X-ray diffraction at atmospheric oxygen pressure. Expo- sure to 1 bar oxygen at 773 K reveals a competing growth of three different oxygen-induced structures on Ag(1 1 1), namely the well- known p(4 · 4) reconstruction, a surface oxide in a p(7 · 7) coincidence structure and the bulk oxide Ag 2 O in ð  1  1  1Þ orientation. The latter two exhibit the same honeycomb on hexagon arrangement of the Ag sublattice with respect to the Ag(1 1 1) surface. An inverted stacking of Ag planes in the bulk oxide islands is observed as compared to the Ag(1 1 1) substrate, which sheds new light on the Ag 2 O formation process. Finally, we present a structural model of the p(7 · 7) reconstruction, based on a three-layer O–Ag–O slab of Ag 2 O(1 1 1). Ó 2006 Elsevier B.V. All rights reserved. Keywords: Silver; Oxidation; X-ray diffraction 1. Introduction The high temperature and high pressure oxidation of the transition metals Ru, Rh, Pd and Ag is a matter of partic- ular interest, because these metals are widely used as oxida- tion catalysts [1]. A future vision is to get a direct microscopic control of the emerging surface structures and ultimately of the real-time oxidation/reduction dynam- ics allowing one to tailor such catalytic reactions to better performance. A necessary prerequisite to the microscopic control is the full atomistic understanding of the surface structures which form at high temperature and at high oxy- gen pressures. Supported Ag catalysts are used for the selective oxida- tion (‘‘epoxidation’’) of ethylene (2C 2 H 4 +O 2 ! 2C 2 H 4 O) and for the partial oxidation of methanol to formaldehyde (2CH 3 OH + O 2 ! 2CH 2 O + 2H 2 O). Ethylene oxide and its derivates are basic chemicals for industry, used in a many technologies with a world-wide production of more than 10 million tons as in medicine for disinfection, steril- ization, or fumigation, or in transport and energy technol- ogies for engine antifreeze and heat transfer. Because of its ability to kill most bacteria, formaldehyde is extensively used as disinfectant and as preservative in vaccinations. Therefore, the optimization of these two Ag-supported cat- alytic reactions is of paramount importance. Current strat- egies employed in the industrial process to enhance selectivity include the empirical use of inhibitors (Cl) [2] and promoters (Cs) [3], however, on the way to a knowl- edge-based control of these reactions one has first to under- stand the surface structure of oxidized silver under relevant conditions in full detail. The formation of extended Ag(1 1 1) facets is observed on polycrystalline silver during the above industrial cata- lytic oxidation reactions [4,5], in turn fundamental research (experiment and theory) has been devoted to the detailed understanding of oxidation of this surface [4–12]. It has been reported in the early 70s, that the oxygen exposure of Ag(1 1 1) surfaces leads to a p(4 · 4) reconstruction. Fol- lowing geometrical arguments (low misfit), a model for the p(4 · 4) reconstruction was constructed, based on a 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.12.008 * Corresponding author. Tel.: +49 711 689 1842; fax: +49 711 689 1902. E-mail address: stierle@mf.mpg.de (A. Stierle). www.elsevier.com/locate/susc Surface Science 601 (2007) L19–L23