AVS1997 Session SS1-MoM: Metal Oxide Surface Science I

Monday, October 20, 1997 8:20 AM in Room A3/4

Monday Morning

Start	Invited?	Item
8:20 AM	Invited	SS1-MoM-1 Clean and Modified Oxide Surfaces: Structure and Reactivity H.-J. Freund (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) We have studied the surfaces of a variety of oxide surfaces including non-polar and polar surfaces. The structure and reactivity of the clean surfaces, which were prepared as thin films, have been investigated applying a variety of methods such as electron spectroscopy, infrared spectroscopy, thermal desorption spectroscopy as well as low energy electron diffraction, scanning tunnelling microscopy, and transmission electron microscopy. Specifically adsorption of CO, NO, O₂, H₂O and simple hydrocarbons such as C₂H₄ and C₆H₆ on NiO and Cr₂O₃ surfaces have been studied. In order to prepare model systems for supported metal catalysts we have deposited metals such on Pt, Pd, Rh, Ir and Ag on Al₂O₃ surfaces. The electronic and geometric structures of the metal aggregates have been studied by the above mentioned methods. It is possible to stimulate luminescence in deposited metal particles. The measured spectra in the VIS-UV regime reflect the electronic structure of the particles. The latter depends on the presence of adsorbates on the aggregates. The mutual influence will be discussed. Finally, we shall report on size dependent reaction (i.e. dissociation) probabilities of CO observed upon interaction with the metal deposits.
9:00 AM		SS1-MoM-3 Metal Adsorption Calorimetry and Adhesion Energies on Clean Single-Crystal Oxide Surfaces D.J. Bald, J.T. Stuckless, D.E. Starr, C.T. Campbell (University of Washington) The heats of adsorption of metals on clean single crystalline surfaces have been measured calorimetrically for the first time. A pulse of metal vapor from a chopped atomic beam adsorbs onto an ultrathin single crystal's surface in ultrahigh vacuum, causing a transient temperature rise. The heat input is detected by a pyroelectric polymer ribbon, which is mechanically driven to make a gentle mechanical / thermal contact to the back of the single crystal sample during calorimetry measurements. The differential heat of adsorption is thus measured as a detailed function of coverage up through multilayer deposition. The sticking probability of the metal is also measured by a line-of-site modification of the King and Wells method. The integral heat of adsorption also provides the adhesion energy of the metal film, if the surface free energy of the clean, bulk metal is known. Adsorption and adhesion energies are reported for Pb adsorption onto: Mo(100), single-crystalline Mo oxide thin films, and ultrathin MgO(100) films that grown on Mo(100); and for Cu on a disordered W(100)-oxide thin film. Both metals have initial adsorption energies on all oxides studied which are substantially less than their bulk sublimation energies, and therefore ultimately cluster into 3D islands as growth proceeds, or upon annealing, as revealed by AES.
9:20 AM		SS1-MoM-4 Oxygen Adsorption and Oxide Formation on Ni₃Al(111) C. Becker, J. Kandler, R. Linke, T. Pelster, M. Tanemura, K. Wandelt (Universität Bonn, Germany) The interaction of oxygen with the ordered Ni₃Al(111) surface has been investigated in the temperature range from 300K to 1000K using HREELS, UPS, LEED, and PAX. The (2x2) LEED pattern of the clean Ni₃Al(111) surface indicates a bulk-like termination. After oxygen adsorption at 300K the LEED pattern is diffuse suggesting the formation of an amorphous overlayer. The UPS and HREELS spectra show evidence for oxygen interaction with aluminum and, to a lesser extend, with nickel atoms. At 600K adsorption temperature the geometrical surface order is restored, however, the observed (1x1) LEED pattern indicates of loss of chemical order. Again the UPS and HREELS spectra suggest interaction of oxygen with aluminum and nickel. For an adsorption temperature of 800K a new LEED pattern is observed corresponding to a (15/13x15/13) superstructure in addition to the (1x1) substrate spots. The HREELS spectra show an intense loss at 82meV, which is also known from oxygen in threefold hollow sites on Al(111). Hence, we conclude the build-up of an oxygen covered aluminum overlayer. This is in line with the UPS results, which show a narrow O(2p) emission suggesting a highly ordered surface oxygen state. Finally, during oxygen exposure at 1000K we observe the growth of a γ-Al₂O₃ structure on an, now, ordered Ni₃Al(111) substrate surface. This structure has been identified by means of the characteristic losses HREELS at 55meV, 80meV, and 113meV together with the emergence of overlayer spots in LEED exhibiting the lattice spacing of γ-Al₂O₃ (3.02Å). For oxygen exposures above 800K an island growth of the overlayers has been found with PAX.
9:40 AM		SS1-MoM-5 The Growth and Characterization of Ag and Au Clusters on Al₂O₃/Re(0001) Q. Guo, D.W. Goodman (Texas A&M University) The deposition of silver and gold onto ordered alumina films on a Re(0001) surface has been studied by Auger electron spectroscopy (AES), low energy electron spectroscopy (LEED) and high resolution electron energy loss spectroscopy (HREELS). The results show that Ag and Au initially grow as clusters at room temperature. The energy shifts of the Ag and Au surface plasmons as a function of coverage are dramatic and have been used to estimate cluster "size". At a coverage <0.1 monolayer (ML), Ag clusters of uniform size have been found. From 0.1 to 3.8 ML, the Ag cluster size gradually iincreases with Ag coverage. However, at a coverage ≥ 3.8 ML (CA. 3 nm) a surface plasmon characteristic of metallic Ag is found. For Au, a surface plasmon characteristic of metallic Au appears at a coverage >0.6 ML.
10:00 AM		SS1-MoM-6 Laser Desorption Mass Spectrometry and Vibrational Spectroscopy of Hydrocarbon Chemistry on Hydroxylated and Non-Hydroxylated Al₂O₃ Surfaces M.M. Ivey, A. Avoyan, H.C. Allen, J.C. Hemminger (University of California, Irvine) γ-Al₂O₃ powders have important uses as catalysts. Aluminum oxide is also a large component of soil particles in the atmosphere, which could provide a surface for reactions of airborne toxins. To better understand these systems, we are studying the chemistry of hydrocarbons on thin films (10-15Å) of Al₂O₃ in ultrahigh vacuum. Thin films of γ-Al₂O₃ are grown on NiAl (100) substrates by exposure to O₂ at elevated temperatures. A hydroxylated γ-Al₂O₃ film is generated by similar oxidation utilizing water exposure. These films are sufficiently thin to allow characterization by electron spectroscopies. We have characterized both the hydroxylated and the non-hydroxylated surfaces using vibrational spectroscopy (HREELS), LEED and Auger. The adsorption chemistry is studied using these techniques as well as laser induced desorption coupled with Fourier transform mass spectrometry. Our data indicate that 1,3-butadiene is weakly adsorbed on the non-hydroxylated surface at low temperatures, but is reactive on the hydroxylated-Al₂O₃ surface.
10:20 AM		SS1-MoM-7 Thermal and Electron-Stimulated Desorption of Na from Amorphous SiO₂ Films Grown on a Re(0001) Surface. B.V. Yakshinskiy, T.E. Madey (Rutgers, The State University of New Jersey) In an effort to probe mechanisms for the origin of Na in the atmospheres of the planet Mercury and the Moon, we are studying the adsorption and desorption of Na deposited on model mineral surfaces, amorphous SiO₂ thin films. Amorphous stoichiometric silica thin films (up to 100 Å thick) have been grown on a Re(0001) surface at room temperature by silicon evaporation in an oxygen ambient (10^-5 Torr ) followed by annealing to 1000K.The adsorption-desorption of Na deposited onto these films have been investigated using x-ray photoelectron spectroscopy, low energy ion scattering and thermal desorption spectroscopy (XPS, LEIS, TDS) as well as electron-stimulated desorption (ESD). The core level binding energies of the stoichiometric SiO₂ films show thickness-dependent and temperature-dependent shifts. Sodium deposition onto SiO₂ causes the XPS peaks to shift to higher binding energy, reflecting a downward bending of the bands of silica. Based on LEIS and XPS measurements, Na is found to grow on amorphous SiO₂ as a monolayer followed by island formation (Stranski-Krastanov growth mode); TDS and angle resolved XPS indicate there is little diffusion of Na to the sample bulk at room temperature. Thermal desorption spectra show monolayer Na desorption in the range 500-1000K, with the multilayer peak at 350K. Both TDS data and growth modes are different for Na on the clean Re (0001) surface, although monolayer coverages are the same as for SiO₂. ESD of Na/SiO₂ causes desorption of Na⁺ ions, which have an appearance threshold at 25eV that is identified with excitation of the O2s core level.
10:40 AM		SS1-MoM-8 CO Oxidation on Gold-Covered Pt(335), A Potential Sensor Electrode D.C. Skelton (Michigan State University/General Motors R&D Center); R.G. Tobin (Tufts University); C.L. DiMaggio, D.K. Lambert, G.B. Fisher (General Motors R&D Center) As part of a study of sensor electrode materials of differing activities for catalyzing CO oxidation, we have studied the adsorption of CO, oxygen, and their reaction on a Pt(335) surface as a function of gold coverage, especially at two approximate gold coverages: just over one monolayer of gold and three fourths of a monolayer of gold. The interest in Au/Pt surfaces arises because, when used as a working electrode in a zirconia-based sensor, these surfaces have been found to make the sensor signal selective to the concentration of CO and other oxidizable gases [1]. The Au/Pt(335) surface was studied with three major techniques: temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and steady state reaction (SSR) measurements. The Pt(335) surface covered with a full monolayer of gold behaved similarly to a pure gold sample. CO had only a single desorption state seen in TPD at low temperatures (110 K), while molecular oxygen could not be adsorbed and no reaction was detected with TPD or SSR between 100 and 700 K. The Pt(335) surface covered with three fourths of a monolayer of gold produced new desorption states of CO not seen on either the pure gold or pure platinum surfaces, while the oxygen recombination and desorption at the step sites on Pt(335) was completely eliminated. While the CO₂ peak desorption rate in TPD moved down from 430 K at low CO coverages to 230 K at high CO coverages, the peak steady state CO₂ reaction rate (SSR) also moved to lower temperatures with increasing CO pressure (and coverage). The peak steady state rate can be higher on this bimetallic surface than on a pure Pt (or pure Au) surface, possibly because of the reduction in CO inhibition of the reaction, while maintaining the ability to adsorb oxygen. The considerations necessary to relate this activity to the selectivity of the Au/Pt sensor will be discussed. [1] G. Baier, V. Schule, and A. Vogel, Appl. Phys. A 57, 51 (1993).
11:20 AM		SS1-MoM-10 The Interaction of Sulfur with Metal/Oxide Surfaces S. Chaturvedi, J.A. Rodriguez, J. Hrbek, M.A. Kuhn (Brookhaven National Laboratory) Probably the most severe poisoning encountered in catalytic systems is that induded by sulfur on metal/oxide catalysts. A fundamental knowledge of the factors that control the interaction of sulfur with metal/oxide interfaces can provide novel ideas for enhancing the sulfur tolerance of industrial catalysts. The adsorption of S₂ on Cu/ZnO and AM/Al₂O₃ surfaces (AM= Cu, Ag or Zn) has been studied using TDS, core and valence-level photoemission, and ab initio SCF calculations. The sticking coefficient of S₂ on clean alumina is small (< 0.1) at temperatures between 300 and 700 K. In the S₂/Al₂O₃ system, there is an energy mismatch between the orbitals of the molecule and the bands of the oxide and the reactivity of S₂ on pure alumina is very low. The adsorption of S₂ on AM/Al₂O₃ surfaces induces strong perturbations in the electronic properties of the admetals and oxide support. In the AM/Al₂O₃ systems, the supported metal clusters (or particles) provide a large number of electronic states that are very efficient at donating charge into the S₂(2π_g) orbitals, inducing in this way the breaking of the S-S bond and the formation of admetal sulfides (AMS_x). These sulfides decompose at temperatures between 750 and 1000 K. ZnO exhibits a much larger reactivity toward S₂ than Al₂O₃. The smaller band gap in zinc oxide facilitates S₂-ZnO interactions, and on this oxide sulfur forms very stable adlayers with coverages as large as 0.7 ML. The adsorption of S ₂ on Cu/ZnO surfaces produces drastic changes in the morphology and band structure of the supported copper particles.
11:40 AM		SS1-MoM-11 Structure and Stability of ZrO₂ Surfaces and the Ni/ZrO₂ Interface A. Christensen, E.A. Carter (University of California, Los Angeles) Ceramic thin films grown on metals are important material structures to characterize in order to understand how to create optimal thermal barrier coatings. We have studied various phases of ZrO₂ and the Ni/ZrO₂ interface as prototypes, by density functional calculations within the local density approximation and the pseudopotential formalism. We report the predicted surface energies and relaxations for the principal surfaces of different phases of ZrO₂. Then we discuss the bonding, relative stability and relaxation effects at the Ni/ZrO₂ interface for various interface geometries. In the end, the implications for optimizing stability of metal/metal oxide interfaces will be addressed.