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Derouin, Jonathan; Farber, Rachael G.; Turano, Marie E.; Iski, Erin V.; and Killelea, Daniel. Thermally Selective Formation of Subsurface Oxygen in Ag(111) Just Accepted "Just Accepted" manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides "Just Accepted" as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. "Just Accepted" manuscripts appear in full in PDF format accompanied by an HTML abstract. "Just Accepted" manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). "Just Accepted" is an optional service offered to authors. Therefore, the "Just Accepted" Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the "Just Accepted" Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these "Just Accepted" manuscripts. Thermally Selective Formation of Subsurface Oxygen inAg (111) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 AbstractA long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what oxygen species are of greatest chemical importance.This is due to the coexistence of several different surface phases on oxidized silver surfaces. A further complication is subsurface oxygen (O sub ). O sub are O atoms absorbed into the near surface of a metal, and are expected to alter the surface in terms of chemistry and structure, but these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms to determine how O sub is formed and how its presence alters the resultant surface structure. Using a combination of surface science techniques to quantify O sub formation and the resultant surface structure, we observed that once 0.1 ML of O sub has formed, the surface dramatically, and uniformly, reconstructed to a striped phase at the expense of all other surface phases. Furthermore, O sub formation was hindered at temperatures above 500 K.The thermal dependence for O sub formation suggests that at industrial catalytic conditions of 475 -500 K for the epoxidation of ethylene-to-ethylene oxide, O sub would be present and is a factor in the subsequent reactivit...

Oxygen chemisorption on rhodium surfaces gives rise to several surface structures depending on the total oxygen coverage. In this study, Rh(111) was exposed to O2 or O + O2, and the oxygen surface structures formed at coverages greater than or equal to 0.5 ML were imaged using scanning tunneling microscopy (STM). The STM images showed that the (2 × 1)-O adlayer domains are predominant on the Rh(111) surface. Exposure of Rh(111) to O atoms yielded O coverages greater than 0.5 ML; (1 × 1)-O domains were observed to form along terrace step edges, and their areal density increased with exposure. However, (2 × 1)-O adlayers were still present on the surface. The STM images reveal that the surface coverage was appreciably less than the total amount of oxygen, suggesting that O uptake resulted in significant absorption into the selvedge, even at modest surface O coverages and temperatures. We compare these observations to previous surface scattering experiments and calculations and demonstrate that our findings resolve several inconsistencies and clearly demonstrate that despite the apparent simplicity of the O/Rh system subtle details remain important, and multiple O structures were present at any O coverage from 0.25 to >1.0 ML. This indicates the rich complexity of O-transition metal interactions and suggests that accurate models of oxygen on rhodium surfaces must include several coexisting surface structures.

Recent studies have shown the importance of oxide surfaces in heterogeneously catalyzed reactions. Because of the difficulties in reproducibly preparing oxidized metal surfaces, it is often unclear what species are thermodynamically stable and what factors effect the oxide formation process. In this work, we show that the thermodynamically stable phases on Rh(111) after exposure to atomic oxygen are the (2×1)-O adlayer and the trilayer surface oxide, RhO2. Formation of RhO2 was facilitated by surface defects and elevated concentrations of dissolved O atoms in the subsurface region. As the concentration of subsurface O atoms decreased, the coverage of RhO2 decreased so that only the (2×1)-O adlayer was present on the surface. The importance of subsurface oxygen species in RhO2 formation and stability indicates a complex relationship between surface structure and subsurface oxygen concentration.

The uptake and chemical speciation of oxygen in and on Ag(111) surface is described. An Ag(111) surface was exposed to gas-phase oxygen atoms under ultrahigh vacuum compatible conditions at various surface temperatures. The O uptake was quantified using temperature-programmed desorption measurements and showed that oxygen exposures at temperatures above 500 K yielded only surface-adsorbed oxygen in a single surface reconstruction. At temperatures below 500 K, O uptake continued past O surface saturation, and a maximum in the uptake with respect to exposure temperature was observed at 450 K. A model where O atoms must diffuse out of subsurface absorption sites to free room for further O describes this observation. The chemical speciation of the oxygenaceous species formed under these conditions was achieved using X-ray photoelectron spectroscopy. These data show that a single O species initially formed on the surface, but at higher coverages, a new, three-dimensional oxygenaceous phase developed. Because of the importance of silver in heterogeneously catalyzed partial oxidation reactions, these results show that oxygen species embedded below the surface plane must be incorporated into accurate models of Ag-surface catalyzed reactions.

We report on a combined TPD and STM study of O2 adsorption and dissociation on various Pt surfaces with varying (111) terrace widths and either (110) or (100) step geometries. Our quantitative TPD results show that (110) stepped surfaces adsorb considerably more oxygen at 100 K, regardless of terrace width, than either (100) stepped surfaces or planar Pt(111). These results suggest that O2 dissociates on the (110) stepped surfaces at 100 K, well lower than required for temperature-induced dissociation on (111) planes. The amount of oxygen desorbing from recombinative desorption of adsorbed oxygen atoms is also greater on (110) stepped surfaces. In addition, the partitioning of adsorbed oxygen between molecular and dissociative states depends on the step geometry; (110) stepped surfaces show an uptake plateau indicative of a threshold surface concentration for low-temperature dissociation, whereas (100) stepped surfaces do not. Scanning tunneling microscope (STM) images for various O coverages and surface deposition temperatures confirm low-temperature dissociation on a (110) stepped surface. The STM images also show that terrace width is not a factor in the lowered dissociation barriers for O2 on (110) stepped surfaces.

Oxygen atoms on transition metal surfaces are highly mobile under the demanding pressures and temperatures typically employed for heterogeneously catalyzed oxidation reactions. This mobility allows for rapid surface diffusion of oxygen atoms, as well as absorption into the subsurface and reemergence to the surface, resulting in variable reactivity. Subsurface oxygen atoms play a unique role in the chemistry of oxidized metal catalysts, yet little is known about how subsurface oxygen is formed or returns to the surface. Furthermore, if oxygen diffusion between the surface and subsurface is mediated by defects, there will be localized changes in the surface chemistry due to the elevated oxygen concentration near the emergence sites. We observed that oxygen atoms emerge preferentially along the boundary between surface phases and that subsurface oxygen is depleted before the surface oxide decomposes.