Papers

AVS2001 Session SS2-MoA: Molecular Interactions with Oxide Surfaces

Monday, October 29, 2001 2:00 PM in Room 121

Monday Afternoon

Start	Invited?	Item
2:00 PM		SS2-MoA-1 Probe Molecules on Hydroxylated γ-Al₂O₃/NiAl(100): Characterization of Surface OH Acidity K.A. Layman, J.C. Hemminger (University of California, Irvine) Hydroxylated thin films of γ-Al₂O₃ are grown by exposing a NiAl(100) single crystal to 100 L H₂O at 1000 K. In HREELS analysis, these films exhibit the phonon modes in agreement with γ-Al₂O₃ films grown on the NiA l(100) substrate using 2400 L O₂ at 1000 K. In addition to the observed phonon modes, a single OH stretch is observed at 3270 cm^-1 (FWHM approximately 100 cm^-1). This frequency is indicative of non-interacting OH groups bonded to 2 or 3 Al atoms. To characterize the acidity of the surface OH groups, we have studied the adsorption of probe molecules, such as pyridine, benzene, and toluene, on the hydroxylated γ-Al₂O₃ surface at 150 K. The probe molecules, acting as bases, form acid-base complexes with the surface OH groups. HREELS spectra were recorded as a function of probe molecule coverage. Complex formation causes the OH bond strength to decrease and the OH stretch to shift to lower frequency. The adsorption and complex formation is observed to be reversible. The OH frequency shift is dependent on the probe molecule basicity and the OH acidity. Our results indicate that two types of isolated OH with different acidity exist on the surface. The correlation between OH frequency shift and probe molecule basicity allows us to determine the acid strength of the surface OH groups quantitatively.
2:20 PM		SS2-MoA-2 Evidence for a Radical-Radical Reaction between O₂ and OH on TiO₂(110) M.A. Henderson, C.L. Perkins (Pacific Northwest National Laboratory) Water readily dissociates at oxygen vacancy sites on TiO₂(110) to form bridging OH groups.^1-3 These OH groups exhibit properties that are more consistent with what one would expect from OH radicals rather than from OH^- ions. A surface consisting solely of these OH groups is easily prepared by adsorption of a multilayer water exposure at 130 K followed by heating to 300 K to desorb all molecular water. Recombination of the bridging OH groups occurs in a TPD peak at about 500 K. ELS spectra of the 10% vacancy-covered surface before and after exposure to water both possess the 0.8 eV loss feature attributable to Ti³⁺ cations. The inability of water/OH to oxidize Ti³⁺ cations at vacancy sites is consistent with earlier photoemission studies.⁴ In contrast, this ELS feature is absent after O₂ exposure at RT due to oxidation of the vacancies. Surprisingly, exposure of O₂ to bridging OH groups results in replacement of the 500 K recombinative desorption state of water with a sharp H₂O TPD state at 300 K. Concurrent with this change, the Ti³⁺ cations are oxidized and O adatoms are deposited on the surface. We speculate that the most likely explanation for this behavior is a radical-radical reaction between O₂ and OH. Such a reaction does not occur between OH groups and diamagnetic molecules like CO₂.⁵ These findings suggest that bridging OH groups on TiO₂(110) formed from the dissociative adsorption of H₂O at vacancy sites might be useful in exploring the thermal chemistry OH radicals believed to be formed on TiO₂ photocatalysts. ¹. M.B. Hugenschmidt, et al., Surf. Sci. 302 (1994) 329. ². M.A. Henderson, Langmuir 12 (1996) 5093. ³. W.S. Epling, et al., Surf. Sci. 412/413 (1998) 333. ⁴. R.L. Kurtz, et al., Surf. Sci. 218 (1989) 178. ⁵. M.A. Henderson, Surf. Sci. 400 (1998) 203. R.
2:40 PM		SS2-MoA-3 Sulfur Adsorption and Reaction with a TiO₂(110) Surface: O - S Exchange and Sulfide Formation J. Hrbek, J.A. Rodriguez, J. Dvorak, T. Jirsak (Brookhaven National Laboratory) Upon sulfur adsorption on TiO₂(110) at 600 K all surface oxygen is replaced by S. High-resolution photoemission data show a complete loss of oxygen from the surface layer, a large binding energy shift and attenuation of Ti core levels, and the presence of three different S species. The bonding of sulfur is examined using first-principles density-functional calculations and the periodic supercell approach. At saturation the top layer of the oxide surface is converted to sulfide, with the majority of sulfur buckled above the Ti lattice plane and the remaining sulfur bonded in bridging sites. A mechanism for this self-limiting thermodynamically unlikely surface reaction is proposed. This research was carried out at Brookhaven National Laboratory under contract DE-AC02-98CH10086 with the US Department of Energy (Division of Chemical Sciences).
3:00 PM		SS2-MoA-4 Molecular Adsorbate Geometries and Bondlengths on NiO(100); A Failure of Current DFT Theories M. Kittel, J.-T. Hoeft, M. Polcik (Fritz-Haber-Institut der MPG, Germany); R.L. Toomes, J.-H. Kang (University of Warwick, UK); M. Pascal, C.L.A. Lamont (University of Huddersfield, UK); D.P. Woodruff (University of Warwick, UK) Current density-functional theory has been shown to be extremely successful in reproducing the local geometries and bondlengths of adsorbates on metal surfaces, for which there is a significant body of experimental data. In the case of oxide surfaces, however, there is a dearth of experimental structural data to compare with the results of such theory. Here we present the results of quantitative structure determinations for new studies of CO and NH₃ on NiO(100) epitaxial films using photoelectron diffraction in the scanned-energy mode (PhD) combined with full multiple scattering simulations. Both species, like NO which we studied previously, occupy sites atop surface Ni atoms, with bondlengths which are slightly longer than we have found for the same species on metallic Ni surfaces, but much shorter (by up to 0.79 Å) than predicted by theory. The results highlight a major failure of current theory to provide an adequate description of molecule/oxide bonding, which was previously only evident through a comparison of experimental thermal desorption data with theoretical bonding energies. A comparison of the molecule-Ni nearest neighbour bondlengths for these singly-coordinated sites on NiO(100) with those found on metallic Ni(111) shows that while the bondlengths on the oxide surface are systematically longer than on the metal surface, these effects are far more subtle than those reflected by current theoretical treatments.
3:20 PM		SS2-MoA-5 The Stability of Polar Oxide Surfaces and Their Interaction with Deposits C. Noguera (CNRS, France); J. Goniakowski (CRMC2, France); F. Finocchi (Laboratoire de Physique des Solides, France) It has long been recognized that the most interesting or active surfaces for the applications are not necessarily the most perfect ones. In insulating oxides, geometric defects, nanofacets, or stoichiometric defects are usually associated to specific electronic states located in the forbidden gap region, which confer a higher reactivity to the surfaces. From this view point, polar oxide surfaces are of prominent interest: their intrinsic instability, due to the existence of a non-zero dipole moment in the repeat unit perpendicular to the surface, may be overcome by a deep modification of the surface electronic structure --- total or partial filling of surface states, sometimes leading to surface metallization --- or by strong changes in the surface stoichiometry --- spontaneous desorption of atoms, faceting, large-cell reconstructions, etc. We will examine several of these issues, through the results of first principles simulations of the MgO(111) surface, based on the density functional method. We will discuss the relative stability of various surface reconstructions, in relation with recent grazing incidence X Ray diffraction and atomic force microscopy experiments. The mechanism by which is achieved the charge compensation necessary to heal the polarity when transition metal atoms are deposited on the surface will be described and the trends in the adhesion energy along the transition series will be discussed. Finally we will present recent results on the dissociation of water on MgO(111).
4:00 PM		SS2-MoA-7 Small Molecule Adsorption on SrTiO₃ and CaTiO₃ Surfaces K.F. Ferris, L.-Q. Wang (Pacific Northwest National Laboratory) Interactions of water and formate with stoichiometric and defective on (001) MTiO3 surfaces (M=Ca, Sr) have been studied using semiempirical and first principles electronic structure calculations. Preliminary results for water interaction with (001) CaTiO 3 indicate weaker adsorption than TiO2 surfaces, consistent with experimental results. Though energetically similar, water adsorption on TiO2- and CaO-terminated CaTiO3 surfaces result in a distinctly different geometric arrangements due to the surface ox ygen atoms and the charge distribution of these acceptor sites. Stepped (001) CaTiO3 surfaces have oxygen sites with greater accessibility and lower coordination, and are predicted to have increased reactivity for H2O and HCOOH. Preliminary results for fo rmate interaction with (100) SrTiO3 indicate strong adsorption consistent with experimental results, but in a distinctly different geometric arrangement from the TiO2 surfaces due to absence of bridging oxygen sites. Further results will be discussed in terms of potential reaction mechanisms and correlations with ongoing experimental studies. This work was supported by the U.S. Department of Energy, Office of Science, Material Sciences Division, under contract DE-AC06-76RLO 1830.
4:20 PM		SS2-MoA-8 Dramatic Cooperative Effects in Chemisorption of NO_x on Oxide Surfaces W.F. Schneider (Ford Research Laboratory); M. Miletic (University of Michigan); K.C. Hass (Ford Research Laboratory); J.L. Gland (University of Michigan) NO_x adsorption by base metal oxide sorbents is central to numerous NO_x aftertreatment technologies, including those essential to enabling fuel-efficient "lean burn" combustion for transportation applications. While the ability of base metal oxide surfaces to form surface nitrites and nitrates is well established by TPD, XPS, and IR experiments, the identity and mechanisms of formation of the adsorbates are not well understood, inhibiting attempts to design oxides with more useful adsorption properties. In this work, first-principles DFT calculations are used to study NO_x adsorbates on alkaline earth oxide surfaces. We show that isolated NO or NO₂ are weakly physisorbed to these surfaces, but that adsorption is strongly enhanced by the formation of adsorbate pairs. This remarkable and unprecedented cooperative chemisorption phenomenon is a consequence of the amphoteric character of NO and NO₂ and the acidity and basicity enhancements possible when charge is transferred between two adsorbates. These results provide a novel and consistent picture of oxide surface nitrition and nitration and are a key step along the path of rationally designed NO_x adsorbants.
4:40 PM		SS2-MoA-9 CO₂ and BF₃ Adsorption on Cr₂O₃ (1012): Probing Surface Basicity and Oxygen Anions M.W. Abee, D.F. Cox (Virginia Polytechnic Institute & State University) The basic properties of oxide surfaces are often associated with surface lattice oxygen anions. CO₂ is the standard probe molecule for investigating surface basicity, but little information is available in the surface science literature concerning its interaction with well-defined (single crystal) oxide surfaces. On Cr₂O₃ (1012), CO₂ interacts primarily with cation/anion site pairs to form bidentate carbonates that are stable at room temperature. These sites are associated with five-coordinate Cr³⁺ cations on the stoichiometric, non-polar (1012) surface. Terminating the surface cations with chromyl oxygen (Cr=O) via dissociative O₂ chemisorption breaks the interaction and gives rise to a weakly-bound CO₂ moiety. BF₃, a strong Lewis acid, interacts directly with anions on the stoichiometric surface, giving a surface/adsorbate complex stable to above room temperature. However, unlike CO₂, BF₃ forms a more stable complex with chromyl oxygen on the oxide-terminated surface. The results demonstrate that thermal desorption experiments with BF₃ provide a means for probing the Lewis basicity of surface oxygen anions of different coordination.
5:00 PM		SS2-MoA-10 Synchrotron X-ray Photoelectron Studies of the CCl₄ Chemistry on Fe₃O₄ (111)- 2x2 Surfaces K. Adib, N. Camillone III, J.P. Fitts, Z. Zhu, R.M. Osgood, Jr. (Columbia University); S.A. Joyce (Pacific Northwest National Laboratory); D.R. Mullins (Oak Ridge National Laboratory) We have used synchrotron x-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) to investigate the reactions of CCl₄ with Fe₃O₄ (111) surfaces. Natural single crystals of α-Fe₂O₃ were cut and polished in the (0001) orientation. They were processed in ultrahigh vacuum to produce a surface selvedge of Fe₃O₄ (111)- 2x2 and exposed at ~100 K to CCl₄. TPD results indicate a highly reactive surface in which the dissociation of CCl₄ into Cl and CCl₂ plays an important role. XPS results confirm the presence of three Cl-containing species immediately upon dosing. The data is consistent with presence of unreacted CCl₄, and chemisorbed CCl₂ and Cl on the surface. We propose that upon subsequent heating of the surface, the chemisorbed species can (1) abstract an oxygen and desorb as OCCl₂, (2) associatively desorb as C₂Cl₄ or (3) recombinatively desorb as CCl₄. We have observed differences in the TPD spectra following the initial dosing of the surface as compared to those following subsequent dosings. These marked changes in the surface are explained in terms of incomplete desorption of iron chloride and carbon species upon heating as verified by XPS. At sufficiently high dosages of CCl₄, the iron chlorides formed on the surface consisted of multiple species, possibly FeCl₂ and FeCl₃.