Papers

AVS2004 Session SS1-ThA: Metal Oxides and Clusters IV: Oxide Surface Chemistry

Thursday, November 18, 2004 2:00 PM in Room 210B

Thursday Afternoon

Start	Invited?	Item
2:00 PM		SS1-ThA-1 Partial Dissociation of Water on the Surface of ZnO B. Meyer, D. Marx (Ruhr-Universität Bochum, Germany); O. Dulub, U. Diebold (Tulane University); M. Kunat, D. Langenberg, C. Wöll (Ruhr-Universität Bochum, Germany) Due to the interplay between chemical bonding, van der Waals forces, and hydrogen bonding, the interaction of water with solid substrates gives rise to complex phenomena such as complete dissociation, partial dissociation at defects, multilayer formation, and wetting. Recently, an intriguing, yet controversial, intermediate scenario was advanced, where the interaction between water molecules results in a partial dissociation of water on perfect surfaces, leading to superlattices with long-range order. Applying a broad array of methods, including diffraction (He-atom scattering, LEED), scanning tunneling microscopy, and thermodynamic measurements supplemented by density-functional total-energy, Car-Parrinello molecular dynamics, and STM computations, conclusive evidence is given that such a phenomenon is encountered for H₂O on the perfect ZnO(10-10) surface. At monolayer coverage, every second water molecule is found to auto-dissociate, subject to a low activation barrier, upon a favorable hydrogen-bonding interaction with a neighboring water molecule, i.e. without the need to invoke defects or impurities. This process leads to a (2x1) superlattice with long-range order which is stable from well below room temperature up to temperatures close to the boiling point of liquid water.
2:20 PM		SS1-ThA-2 Photoemission of Adsorbed Xenon Studies on the Characterization of Reaction Sites on Oxygen-Modified Ni(110) Surfaces H. Guo, F. Zaera (University of California at Riverside) Considerable attention has been paid in our laboratory to the study of the adsorption and reactions of surface intermediates of relevance to catalysis on clean, hydrogen- and oxygen-modified Ni(110) to understand the properties of these surfaces in hydrocarbon conversion, and in trying to identify specific sites selective for the promotion of desirable reactions. STM studies have evidenced that surface oxygen in lower coverage regimes not only induces reconstruction of Ni(110) in extending domains, but also modifies the electronic structure at local sites. Such surfaces have been found particularly effective in the production of heavier hydrocarbons. However, the relationship between this distinctive catalysis and the surface structures responsible for it cannot be established without a better knowledge of the local surface properties at the atomic scale. With this in mind, we have carried out experiments using photoemission of adsorbed xenon (PAX) to characterize specific adsorption sites in heterogeneous surfaces produced by oxygen adsorption on Ni(110) single crystals. This technique, which provides both energetic and local electronic information on small surface atom ensembles, has been used in combination with chemical titrations using probe molecules such as carbon monoxide and ammonia to determine correlations between electronic structures and reactivity. It was determined that ammonia prefers a direct interaction with the terminating atoms of the -Ni-O- added rows that form on Ni(110) upon oxygen treatments. Those sites appear to be key for the selective conversion of hydrocarbons.
2:40 PM		SS1-ThA-3 Oxide and Carbonate Surfaces as Environmental Interfaces: The Importance of Water in Surface Composition and Surface Reactivity V.H. Grassian (University of Iowa) Environmental molecular surface science is an important and expanding area of current research. This presentation focuses on advances in the molecular level understanding of the chemistry that occurs on oxide and carbonate surfaces in the atmosphere. In particular, the importance of water in the surface composition and surface reactivity of two representative oxide and carbonate surfaces, MgO(100) and CaCO₃(104) will be discussed. Reactions of trace atmospheric gases, including HNO₃, with MgO(100) and CaCO₃(104) as a function of relative humidity highlight the role of surface hydroxyl groups and molecularly adsorbed water in these reactions.
3:20 PM		SS1-ThA-5 Interactions of S-containing Molecules and Water Vapor with Polycrystalline UO₂ B.V. Yakshinskiy, T.W. Schlereth, M.N. Hedhili, T.E. Madey (Rutgers, The State University of New Jersey) The interaction of sulfur dioxide (SO₂), thiophene (C₄H₄S) and water vapor (D₂O) with a polycrystalline stoichiometric UO₂ and oxygen-deficient UO₂ surfaces has been studied under UHV conditions over the temperature range 100 K to 600 K, using XPS (X-ray photoelectron spectroscopy) and TPD (temperature programmed desorption). This work is motivated by potential catalytic applications of stockpiles of depleted uranium. All three molecules are relatively unreactive on the stoichiometric UO₂ surface, they adsorb and desorb in molecular form. The creation of oxygen vacancies by 1.5 keV Ar ion sputtering is found to enhance the UO₂ surface reactivity towards the desulfurization of C₄H₄S and SO₂ and the dissociation of water with formation of OD species. Heating of the oxygen-deficient surface with a preadsorbed water monolayer causes desorption of molecular D₂. The oxygen remains and restores the sample surface to its initial stoichiometric state. The healing of sub-surface defects occurs through thermal diffusion of atoms from the sample bulk at ~ 500 K.
3:40 PM		SS1-ThA-6 Reactions of Substituted Hydrocarbons with Cerium Oxide Thin Filmsfootnote1 D.R. Mullins, M.D. Robbins, T.S. McDonald (Oak Ridge National Laboratory) Fully oxidized ceria surfaces are largely inactive with respect to the adsorption and reaction of most adsorbates under UHV conditions. Reduced surfaces, however, provide active sites where some adsorbates such as H₂O, NO and SO₂ can adsorb and react. Other molecules, such as CO, H₂ and C₂H₄ do not interact strongly with either an oxidized or a reduced surface. Based on the ethylene behavior, it appears that hydrocarbons may not interact strongly with ceria whereas on metals they frequently decompose. Substituted hydrocarbons may therefore bind to the surface through the heteroatom at the oxygen vacancy while the hydrocarbon part of the molecule may not interact strongly with the surface. We have recently completed a study of methanol and methanethiol adsorption on ceria as a function of temperature, exposure and Ce oxidation state. CH₃OH reacts at low temperatures with oxidized CeO₂ to produce H₂O at 200 K, and CH₂O and CH₃OH near 600 K. This leads to the reduction of the ceria. This is the first molecule we have examined that is capable of reducing a ceria film in UHV. Surprisingly, CH₃SH does not interact strongly with the CeO₂. It desorbs molecularly by 300 K and does not reduce the oxidized surface. On reduced ceria, the oxygen vacancies result in more methanol adsorption which undergoes more extensive decomposition producing CO and H₂ near 600 K. As the degree of ceria reduction increases, more H₂ and less H₂O are produced. Methanethiol does adsorb on the reduced surface producing CH₃S and OH. The C-S bond cleaves near 600 K and methyl reacts with the hydroxyls to produce CH₄. ¹ Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.
4:00 PM		SS1-ThA-7 Adsorption and Reaction of Acetaldehyde and Methanol on Stoichiometric and Defective Mixed-Metal Oxide Surfaces L.Q. Wang, S.A. Azad, K.F. Ferris, C.H.F. Peden, M.H. Engelhard (Pacific Northwest National Laboratory) The adsorption and reaction of oxygenated hydrocarbons on metal oxide surfaces are of much interest from both fundamental and practical perspectives. The reactivity of these catalytic processes largely depends on the characteristics of the oxide catalysts defined by their surface structures, acid-base properties and surface defects. Oxygenated hydrocarbons are often used as fuels and fuel additives , and they may be formed as a result of incomplete combustion of fuel in the engine. To efficiently reduce these toxic exhaust products, it is especially helpful to have a fundamental understanding of the adsorption and reaction of oxygenated hydrocarbons on metal oxide surfaces. In this presentation, we examined the interactions of acetaldehyde and methanol with stoichiometric and defective SrTiO₃(100) surfaces using x-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and first-principles density-functional calculations. The results obtained from methanol and acetaldehyde on SrTiO₃(100) surfaces are compared with our results on Ce_0.8Zr_0.2O₂(111) surfaces and with the previous results on single crystal TiO₂ surfaces. Both acetaldehyde and methanol adsorb mostly non-dissociatively on the stoichiometric SrTiO₃(100) surface that contains predominately Ti⁴⁺ cations. Theoretical calculations predict weak adsorption of acetaldehyde and methanol on TiO₂-terminated SrTiO₃(100) surfaces, in agreement with the experimental results. The stronger binding of acetaldehyde and methanol on TiO₂ surfaces than on SrTiO₃(100) surfaces is attributed to the more covalent nature of the Ti⁴⁺ cation sites in the mixed-metal oxides and the unique surface structure due to the absence of the bridging oxygen atoms on the TiO₂-terminated SrTiO₃(100). The Ti⁴⁺ sites on the stoichiometric SrTiO₃(100) surface are not sufficiently active for surface reactions such as aldol condensation, as opposed to the Ti⁴⁺ ions on the TiO₂ (001) surface. However, decomposition and redox reactions for both methanol and acetaldehyde occur in the presence of surface defects created by Ar⁺ sputtering. The decomposition products following reactions of acetaldehyde on the defective surface include H₂, C₂H₄, CO, C₄H₆ and C₄H₈. Reductive coupling to produce C₂H₄ and C₄H₈ is the main reaction pathway for decomposition of acetaldehyde on the sputter reduced SrTiO₃(100) surface. Adsorption of CH₃OH on the reduced SrTiO₃(100) surface produces the decomposition products of H₂, CO, and CH₄. As compared with SrTiO₃(100) surfaces, Ce_0.8Zr_0.2O₂(111) surfaces exhibit enhanced adsorption and reactivity for methanol and acetaldehyde. Both acetaldehyde and methanol mostly adsorb dissociatively on the oxidized Ce_0.8Zr_0.2O₂(111) surfaces. The formation of furan was surprisingly observed on reduced Ce_0.8Zr_0.2O₂(111) surfaces following the adsorption of acetaldehyde.
4:20 PM		SS1-ThA-8 The Adsorption of Bromobenzene on Periodically-Stepped and Flat NiO(100) Surfaces S.C. Petitto, E.M. Malone, M.A. Langell (University of Nebraska-Lincoln) Bromobenzene was adsorbed onto both stepped and flat NiO(100) surfaces to model surface defects relevant to heterogeneous chemical processes. Both surfaces were characterized using Auger electron (AES) and X-ray photoelectron (XPS) spectroscopies, low energy electron diffraction (LEED), and thermal desorption mass spectrometry (TDS). The stepped NiO(100) substrate was cut and polished at an angle vicinal to the (100) surface, resulting in monoatomic steps with 7-atom terraces. The LEED diffraction patterns show sharp diffraction features for both surfaces, and diffraction spot splitting correlating to appropriate terrace and step height dimensions for the stepped surface. Both substrates interact with bromobenzene at 120 K to produce first a molecularly adsorbed monolayer species and then a multilayer adsorbate state as the exposure is increased. The stepped NiO(100) surface has an additional TDS peak not observed for flat NiO(100) and which results in dissociation adsorption initiated by cleavage of the Br-C₆H₅ bond. Bromine that remained on the surface appeared as nickel bromide.
4:40 PM		SS1-ThA-9 Synchrotron X-Ray Photoelectron Spectroscopy Studies of the Thermal Chemistry of (trimethyl) Methylcyclopentadienyl Platinum on Tio₂ (110) K. Adib (Brookhaven National Laboratory); M.A. Barteau (University of Delaware); J. Hrbek (Brookhaven National Laboratory); J.M. White (University of Texas at Austin) Pt/TiO₂ is one of the most important systems used in the photocatalytic decomposition of water to hydrogen as well as environmental purification of organic waste. In this regard, the use of organometallic precursors as sources of Pt metal islands on TiO₂ surfaces offers an attractive alternative to metal-vapor deposition techniques. We have used synchrotron X-ray photoelectron spectroscopy (XPS) to investigate the thermal chemistry of (trimethyl) methylcyclopentadienyl platinum (MeCpPtMe₃) on the stoichiometric rutile (110) surfaces. Our results indicate that the submonolayer adsorption of MeCpPtMe₃ on nominally stoichiometric TiO₂ (110) at 300 K does not result in substantial decomposition of the adsorbate. While subsequent annealing of the surface to 450 K enhances the decomposition of the MeCpPtMe₃, as evidenced by the appearance of additional Pt 4f peaks, there is no evidence of the desorption of the resulting carbon fragments even after extended periods of annealing. Predosing of nominally stoichiometric TiO₂ (110) surfaces with molecular oxygen at 300 K substantially enhances the decomposition of subsequently deposited MeCpPtMe₃. This decomposition is accompanied by the formation surface-bound CO_x species, possibly carboxylate groups, suggesting strong interactions between the adsorbate and substrate. Heating to 850 K can result in the removal of more than 98% of the surface bound carbon species, including the CO_x, but does not result in the formation of a carbon-free Pt/TiO₂ surface.
5:00 PM		SS1-ThA-10 The Reaction of DL-Proline on TiO₂(110) Single Crystal Surfaces. K. Adib (Brookhaven National Laboratory); G. Fleming (University of Auckland, New Zealand); J.A. Rodriguez (Brookhaven National Laboratory); H. Idriss (University of Auckland, New Zealand); M.A. Barteau (University of Delaware) Titanium metal is widely used as a medical implant in the aiding of healing fractures in teeth and bone. The choice of titanium as an implant material is based on both its mechanical properties and on its relative chemical inertness. Once placed in the body's aqueous environment, the implant undergoes an oxidation process where the formation of a thin oxide layer in the range of 10 to 100 nm thick occurs. This layer is crucial since it prevents the Ti metal from further reacting with the biological molecules. However, the nature of interaction of the bio-molecule with this thin TiO₂ surface will ultimately determines its conformation. If the conformation of the bio-molecule is altered from its naturally occurring state, it may cause the body to undergo an auto immune response and reject the implant. Surface science studies can address the nature of interaction of prototype amino acids with TiO₂. In this work we study the reaction of DL-Proline on the surface of a model TiO₂ surface, the rutile (110) surface. Proline was chosen as it is a constituent of collagen I, a major high tensile structural protein found in teeth, bone and cartilage. The reaction of DL-Proline on stoichiometric and O-defected surfaces has been investigated by temperature programmed desorption while the surface species at different reaction temperatures were monitored by X-ray core level shifts and by their valence band. Proline binds to the surface via its COO group. The presence of two N(1s) lines upon adsorption at 300 K indicates the presence of two distinct species that are tentatively assigned to -NH₂⁺- and -NH- attributed to the zwitterionic and non-zwitterionic forms of the amino acid, respectively. Complex reactions are seen for Proline upon heating the surface, and products such as CH₂=C=O and HCN are seen in the gas phase. In addition, large amounts of organic species containing O and N are still present on the surface even after heating to 600 K.