Direct Observation of C 2 Hydrocarbon-Oxygen Complexes on Ag(110) with a Variable-Low-Temperature Scanning Tunneling Microscope J. R. Hahn* and W. Ho Department of Physics and Astronomy and Department of Chemistry, UniVersity of California, IrVine, California 92697-4575 ReceiVed: March 18, 2005; In Final Form: August 3, 2005 A variable-low-temperature scanning tunneling microscope (STM) was used to observe oxygen (O 2 ), ethylene (C 2 H 4 ), and acetylene (C 2 H 2 ) molecules on a Ag(110) surface and the various complexes that were formed between these two hydrocarbons and oxygen at 13 K. Ethylene molecule(s) were moved to the vicinity of O 2 either by STM tunneling electrons at 13 K or thermally at 45 K to form (C 2 H 4 ) x -O 2 (x ) 1-4) complexes stabilized by C-H‚‚‚O hydrogen bonding. Acetylene-oxygen complexes involving one or two acetylene molecules were observed. The heterogeneous selective oxidation of hydrocarbons is a topic of enormous industrial importance and has therefore been extensively studied. Most thermodynamically favorable hydro- carbon oxidation reactions give rise to undesirable products such as CO 2 and H 2 O; 1-3 hence, achieving acceptable selectivity in hydrocarbon oxidation is particularly important in industrial processes. Selectivity with respect to the formation of useful partial oxide (intermediates) can only be accomplished via kinetic control; however, such control requires a good under- standing of the surface reaction mechanisms. 1,4 Most previous studies of hydrocarbon oxidation have been performed at silver surfaces because silver-catalyzed partial oxidation of hydrocar- bons, in particular, epoxidation (ethylene to ethylene oxide), is the most widely exploited catalytic oxidation reaction on an industrial scale. 5 Achieving a molecular level understanding of the epoxidation mechanism is arguably the most important challenge in this field. 6-10 Thus, to enhance the basic knowledge on hydrocarbon catalytic oxidation reactions, we carried out molecular scale imaging of the complexes formed between an oxygen molecule and C 2 hydrocarbons (ethylene or acetylene) on the silver surface using a scanning tunneling microscope. An interesting aspect of hydrocarbon oxidation is its unique- ness: not only is silver the only metal that catalyzes the epoxidation reaction but also ethylene is the only hydrocarbon that is epoxidized with high selectivity (up to 80%). 11,12 Olefins such as propylene, 11,13 styrene, 14 3,3-dimethylbutene, 15 nor- borene, 16 and butadiene are also epoxidized on a silver surface but with very low selectivities (below 5%). 11,13 Other molecules such as butenes and penetenes are mainly combusted into CO 2 and H 2 O. 17 Alkynes (acetylene and propyne) also undergo complete oxidation; no epoxides have been observed to form on a silver surface. 18,19 In addition, the identity of the adsorbed oxygen species responsible for the ethylene epoxidation and combustion has been the subject of a long-standing controversy. Three mechanisms have been put forward for epoxidation at a silver surface, involving surface molecular oxygen, 20-23 atomic oxygen, 24-28 and subsurface oxygen 26 as the active species. Thus, three main factors appear to affect the mechanism by which hydrocarbons are oxidized at metal surfaces: the metal used, the structure of the hydrocarbon, and the identity of oxygen species. Despite numerous reports on the epoxidation on silver, an understanding of the complete reaction pathway is still required. In this paper, we present scanning tunneling microscopy (STM) images of hydrocarbon-O 2 complexes formed on Ag- (110) after one or more ethylene or acetylene molecules had been manipulated into the vicinity of an oxygen molecule on the silver surface either with tunneling electrons (at 13 K) or thermally (at 45 K). Elucidation of the characteristics of these complexes may provide insights into the fundamental mecha- nisms of hydrocarbon chemistry. Experiments were performed using a homemade, variable- temperature STM, 29 housed inside an ultrahigh vacuum chamber with a base pressure of 2 × 10 -11 Torr (2.7 × 10 -9 Pa). The Ag(110) sample was prepared by 500 eV neon ion sputtering followed by annealing at 693 K. Polycrystalline tungsten tips were prepared in situ by self-sputtering and annealing. Adsor- bates were introduced into the chamber via a capillary array doser attached to a variable leak valve. The O 2 molecules were adsorbed on the sample at 45 K to ensure molecular chemi- sorption. The O 2 coverage was kept below 0.01 monolayer (ML) to permit investigation of individual well-isolated molecules. Coadsorption of ethylene and acetylene molecules onto the Ag- (110) surface at coverages of less than 0.01 ML was performed at either 13 or 45 K. Atomically resolved imaging was achieved by transferring an ethylene molecule to the tip at 13 K. 30 Figure 1a shows a constant-current STM image taken with a bare metallic tip of one physisorbed ethylene and two chemi- sorbed oxygen molecules. The ethylene molecule appears as an oval-shaped protrusion elongated along the [001] direction on Ag(110) under typical imaging conditions, while the oxygen molecules appear as an oval-shaped depression elongated along the [11 h0] direction. Atomically resolved imaging the same area using an ethylene-terminated tip (Figure 1b) established that ethylene binds at the atop site, in agreement with previous theoretical calculations. 31 Previous studies have examined the structures of ethylene adsorbed on Ag(110) surfaces. 32-37 Using high-resolution electron energy spectroscopy (HREELS), Backx et al. 32-34 revealed that (i) ethylene adsorbs onto Ag(110) * To whom correspondence should be addressed. Department of Chemistry, Chonbuk National University, Jeonju 561-756, Korea. E-mail: jrhahn@chonbuk.ac.kr. 20350 J. Phys. Chem. B 2005, 109, 20350-20354 10.1021/jp051431c CCC: $30.25 © 2005 American Chemical Society Published on Web 10/07/2005