Abstract No

No. Adib0568 Dissociative Adsorption of CCl4 on the Fe3O4(111)-(2×2) Selvedge of α-Fe2O3(0001) K. Adib (Columbia University, BNL), D. Mullins (ORNL), G. Totir (Columbia University), N. Camillone (Columbia University, BNL), K. Rim, G. Flynn (Columbia University), R.M. Osgood (Columbia University, BNL) Beamline: U12A Introduction: Recently the fundamental chemistry of environmentally important adsorbed molecules has been studied via the use of molecular specific surface probes, in conjunction with model mineralogical surfaces. These studies start with well-characterized single-crystal surfaces with the aim of developing a molecular-level understanding of the reactive chemistry. The surface and the surface chemistry is then progressively made more complex and, hence, realistic. This approach has the advantage of allowing probes such as synchrotron photoemission spectroscopy, low energy electron diffraction, and scanning tunneling microscopy to characterize the surface and its adsorbate chemistry. Methods and Materials: We use ultrahigh vacuum for detailed studies of the chemistry of wellcharacterized surfaces formed on α-Fe2O3 (hematite) single crystals. Natural single crystal samples of hematite were cut and polished along the (0001) orientation. They were processed in ultrahigh vacuum to produce a surface selvedge of Fe3O4(111)-(2×2) and exposed at ~100 K to CCl4. The surface reactions were studied using Temperature-programmed desorption [1], Auger electron spectroscopy [2], synchrotron based X-ray photoelectron spectroscopy [3] and scanning tunneling microscopy [4]. Results: CCl4 was found to dissociate on the Fe3O4 surface at 100 K producing chemisorbed Cl and adsorbed CCl2. TPD shows that the large majority of the dissociatively adsorbed CCl2 fragments extract lattice oxygen and desorb as phosgene at >275 K. However, the XPS spectra show no evidence for the formation of surface-bound phosgene, at 100 K, indicating that its formation involves two steps. The first step, dissociation, is spontaneous at 100 K, whereas the second, oxygen atom abstraction to form phosgene, requires thermal excitation. Cl chemisorption yielded two separate species, the monoand dichloride terminations of surface iron sites. The identification of these two surface terminations is based on the coverage dependence and the surface temperature history of their Cl 2p3/2 peak intensity. For example, heating to >450 K allows the monochloride to transform into iron dichloride, indicating Cl adatom mobility at these temperatures. Acknowledgments: Support was provided by the National Science Foundation under Grant No. 9810367 to the Environmental Molecular Sciences Institute (EMSI) at Columbia University. Support was also provided by the U.S Department of Energy under contract No. DE-AC0298CH10886 with the National Synchrotron Light Source, Brookhaven National Laboratory, No. DE-AC05-00OR22725 with Oak Ridge National Laboratory. References: [1] K. Adib, N. Camillone III, J.P. Fitts, K.T. Rim, G.W. Flynn, S.A. Joyce and R.M. Osgood Jr., “CCl4 Chemistry on the Magnetite Selvedge of Single-Crystal Hematite: Competitive Surface Reactions,” Surface Science 497 127-138. [2] N. Camillone III, K. Adib, J.P. Fitts, K.T. Rim, G.W. Flynn, S.A. Joyce and R.M. Osgood Jr, “Surface Termination Dependence of the Reacitivity of Single Crystal Hematite with CCl4,” Surface Science 511 267. [3] K. Adib, D.R. Mullins, N. Camillone III, J.P. Fitts, K.T. Rim, G.W. Flynn, and R.M. Osgood Jr., “Dissociative Adsorption of CCl4 on the Fe3O4(111)-(2×2) Selvedge of α-Fe2O3(0001),” Accepted for publication in Surf. Sci. [4] K.T. Rim, J.P. Fitts, T. Mueller, K. Adib, N. Camillone III, R.M. Osgood, Jr., S.A. Joyce, and G.W. Flynn, “Scanning Tunneling Microscopy Study of Natural α-Fe2O3(0001) Surface: Termination and Reactivity,” in preparation.