AVS1996 Session SS3-ThA: Metal Oxide Surfaces: Reactions

Thursday, October 17, 1996 1:30 PM in Room 203A

Thursday Afternoon

Start	Invited?	Item
1:30 PM		SS3-ThA-1 Evidence for Weakly Bound Oxygen on Ceria Films E. Putna, J. Vohs, R. Gorte (University of Pennsylvania) Temperature programmed desorption (TPD) has been used to compare the oxygen desorption characteristics of a thin film of ceria prepared by vapor deposition on an alpha-alumina(0001) substrate with that of a ceria(111) surface. On ceria(111), there is no substantial desorption of oxygen below approximately 1300K, while a low-temperature state, between 800 and 1250K, was observed from the ceria film. By adding Rh to the ceria film and then measuring the reaction of adsorbed CO on the Rh, it was demonstrated that this low-temperature feature is responsible for the oxidation of CO during TPD. The implications of these results for understanding the oxygen storage function of ceria-supported catalysts are discussed.
1:50 PM		SS3-ThA-2 Chemisorption Geometry, Vibrational Spectra, and Thermal Desorption of Formic Acid on TiO\sub 2\(110) S. Chambers, M. Henderson, Y. Kim, S. Thevuthasan (Pacific Northwest National Laboratory) We have used high-energy, scanned-angle x-ray photoelectron diffraction (XPD), high-resolution electron energy loss spectroscopy (HREELS), and temperature programmed desorption (TPD) to determine the molecular orientation, vibrational frequencies, and desorption temperatures for formic acid on TiO\sub 2\(110). Upon adsorption at 110 K, formic acid dissociates into formate anions and surface protons, forming a weakly-ordered (1x2) reconstruction. The formate is identified in HREELS by a symmetric O-C-O stretch at 1360 cm\super - 1\ and a C-H stretch at 2940 cm\super -1\. High-energy XPD reveals that the formate binds in a bidentate fashion to Ti cation rows along the [001] direction with an O-C-O bond angle of approximately 120\super o\. Additional details about the chemisorption geometry are currently being sought from low- energy, scanned-angle and fixed-angle, energy-dependent XPD data recently obtained at the Advanced Light Source. The surface proton adsorbs to bridging oxygens to form a hydroxyl group observed in HREELS at about 3600 cm\super -1\ . Some surface protons and formate anions recombine and desorb as formic acid below 500 K. However, most of the decomposition products follow reaction pathways leading to water, carbon monoxide, and formaldyhyde desorption. Water is formed in the desorption process below 450 K via the abstraction of lattice oxygen by deposited acid protons. This process leaves oxygen vacancies in which some formate species react to form formaldehyde at 545 K. The additional hydrogen needed to form this product comes from decomposition of other formate species to CO in a broad TPD state at 555 K.
2:10 PM	Invited	SS3-ThA-3 Ultraviolet Raman Spectroscopy of Catalysts and Other Solids P. Stair (Northwestern University); C. Li (Dalian Institute of Chemical Physics) Raman spectroscopy is a promising method for characterizing catalysts and other solids because it has the potential to study the sample at practical temperatures and pressures. However, the application of conventional Raman spectroscopy has been limited by its low sensitivity and the masking effect of strong fluorescence. Recently an ultraviolet Raman instrument was set up in our laboratory which improves the Raman sensitivity by avoiding the strong fluorescence and by enhancing the Raman cross section. The 514 nm output of a 18-watt Ar\super +\ ion laser is frequency doubled to 257 nm using a temperature-tuned KDP crystal. The 257 nm beam is used to excite Raman scattering. The power delivered to the sample was kept below 5 mW to avoid thermal decomposition of the sample. The Raman scattered light is collected by an AlMgF\sub 2\ coated ellipsoidal reflector and focused into a spectragraph. The detector is an imaging multichannel photomultiplier tube (IPMT). Catalyst samples were formed pressed discs. Thin films were supported on a solid substrate. Measurements can be performed in air, in-situ in a quartz cell, or in a ball-on-disk tribology apparatus. Many solids and films that are difficult to study by conventional Raman have been tested in this new instrument. Among the samples are coked hydrotreating catalysts, diamond films, lubricant films, and macroporous molecular sieves all of which show strong fluorescence using conventional Raman. To our great surprise and delight, beautiful Raman spectra were obtained from all the tough samples. In this talk we will present an overview of the ultraviolet Raman technique including a perspective on the kinds of information that can be obtained and the range of samples that can be studied.
2:50 PM		SS3-ThA-5 The Reactions of Carbonyls on the Oxides of Lanthanides and Actinides P. Buchanan, H. Idriss (University of Auckland, New Zealand) The reactions of carbonyls (acetaldehyde and acetone) were investigated on the surfaces of reduced lanthanides (Cerium and Samarium oxides) and actinides (Thorium and Uranium oxides) by temperature programmed desorption as well as by surface and bulk spectroscopy. Several reactions were observed. Among them the most important are: 1. Carbon- Carbon bond formation to symmetric alkenes, via reductive coupling, and \beta\aldolization to unsaturated aldehydes and ketones. 2. Carbon-Oxygen Bond formation to ethylacetate from acetaldehyde via Tishenko reaction on Thorium and Uranium oxides). Within the U-O system, probably the most complicate known system among oxides, UO\sub2\ (prepared from hydrogen reduction of U\sub3\O\sub8\ or by CO reduction of UO\sub3\) exhibits interesting properties regarding Carbon-Oxygen bond dissociation most likely because of its superstoichiometric characteristics. This is due to its capacity of adsorbing considerable amounts of interstitial oxygen within the frame of the fluorite structure up to UO\sub2.25\ but still contains intact oxygen vacancies. The correlation between the bulk structures and the oxidation states of these oxides from one side and the reaction activity and the reaction selectivity of carbonyls from the other side is also investigated. The similarities and deviations from organometallic chemistry, particularly those related to the effects of cations in low oxidation states (suboxide forms are more stable in the solid state) is also studied.
3:10 PM		SS3-ThA-6 Adsorption of D\sub 2\O on the Surface of a Fe\sub 3\O\sub 4\(001) Epitaxial Film Grown by MBE: A TPD, LEED and AES Study I. Ismagilov, G. Herman, Y. Kim, Y. Gao, S. Chambers, C. Peden (Pacific Northwest National Laboratory) Temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED) have been used to study the interaction of D\sub 2\O with a Fe\sub 3\O\sub 4\(001) epitaxial film. The sample was prepared by molecular beam epitaxy (MBE) and characterized by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and LEED in a separate specially-designed UHV system. Due to a close matching of surface lattice parameters, MgO(001) was selected as a substrate for the Fe\sub 3\O\sub 4\(001) film. The experiments were performed on both ordered and disordered surfaces of the sample, the latter produced by Ar\super +\ bombardment. LEED, Auger electron spectroscopy (AES), and TPD spectra suggest that the nature of D\sub 2\O species formed can noticeably depend on the chemical state and structure of the clean iron oxide surface. For example, the ordered Fe\sub 3\O\sub 4\(001) surface exhibits three main chemisorbed states of D\sub 2\O: T\sub 1\ = 320 K at 0.3 ML coverage, T\sub 2\ = 280 K at 0.6 ML coverage, and T\sub 3\ = 225 K at 1.0 ML coverage. In addition, LEED is being used to study the adsorbate structure corresponding to the distinct states indicated by TPD. Based on these results, coverage-dependent mechanisms of D\sub 2\O interaction with Fe\sub 3\O\sub 4\(001) are proposed.
3:30 PM		SS3-ThA-7 Reaction of Hydrogen with Oxide and Sulfide Surfaces: Formation of Water and Hydrogen Sulfide S. Li (National University of Singapore); J. Rodriguez, J. Hrbek (Brookhaven National Laboratory); H. Huang, G. Xu (National University of Singapore) The reaction of hydrogen ( H/sub 2/, D/sub 2/ and D) with surfaces of oxides and sulfides of several metals (K, Cs, Mo, Ru, Cu and Zn) has been investigated at temperatures between 120 and 500 K. All the surfaces were unreactive toward molecular hydrogen under UHV conditions at temperatures bellow 300 K. However, most of these systems showed a large reactivity toward atomic hydrogen. As gas-phase atomic hydrogen (D) impinged on the oxides and sulfides, surface OD and SD species, plus gaseous D/sub 2/O and D/sub 2/S were formed. Upon exposing O/Ru(001) and RuO/sub x/ surfaces to D, atomic hydrogen reacted with oxygen with no induction period for the production of D/sub 2/O even at 120 K. At low temperatures (<200 K) the desorption of water from the sample is the rate-limiting step for removal of oxygen, while formation of hydroxyl and water is the rate determining step for the removal of oxygen at higher temperature (>250 K). The addition of potassium to these surfaces induced a reduction in the rate of hydrogenation of oxygen due to the formation of very stable KO/sub x/. For the sulfide surfaces the 2D/sub (gas)/ + S/sub (solid)/ --> D/sub 2/S/sub (gas)/ reaction was very effective for the removal of sulfur atoms (100 - 300 K). A good correlation was found between the rate of formation of gaseous hydrogen sulfide and the stability of S-S and S-metal bonds in a surface. The behavior of oxide and sulfide catalysts in hydrogenation and hydrodesulfurization processes will be discussed in light of these results. This work was supported by the US Department of Energy (DE-AC02-76CH00016.
3:50 PM		SS3-ThA-8 Electron Stimulated Desorption Studies of the Reaction of MgO(100) Surface with D\sub 2\O I. Colera (Universidad Carlos III, Spain); E. Soria, J. de Segovia (Instituto de Fisica Aplicada, CSIC, Spain); E. Roman (Instituto de Ciencia de Materiales, CSIC, Spain); R. Gonzalez (Universidad Carlos III, Spain); Y. Chen (U.S. Department of Energy) In this work, the reaction of D\sub 2\O with MgO(100) surfaces is investigated by electron stimulated desorption, (ESD). X-ray electron spectroscopy (XPS) and secondary electron emission are also used as complementary techniques. It was previously found\super 1\ that O\super +\ desorbed ions on a clean MgO(100) surface show an energy threshold at 55 eV which coincides with the excitation energy of the Mg 2p core level. The secondary electron emission also shows a threshold at about 55 eV which suggests that a Feibelman-Knotek mechanism could be responsible for the ion desorption. When the surface was covered with a layer of D\sub 2\O, D\super + \ and minor amounts of OD\super +\ ions were desorbed at the same energy threshold. The O\super+\ signal from the oxide decreases upon D\sub 2\O adsorption. The kinetic energy distribution of the O\super +\ desorbed ions presents the two most probable kinetic energies at 5.4 eV and 8.7 eV, and the intensity ratio of the higher energy to the lower energy decreases with D\sub 2\O adsorption. The kinetic energy distribution of the D\super +\ desorbed ions has a main peak at 2.5 eV and a satellite at about 6.5 eV. The intensities of the O\super+\, D\super +\ and OD\super +\ as a function of the D\super +\O exposure are presented. The XPS studies are compared with the ESD results. These findings suggest that the D and O dissociated species are probably bonded to the Mg atoms. 1. I. Colera et al. Paper presented at the 5th European Vacuum Conference, Salamanca, 23-29 September 1996. To appear in Vacuum.
4:10 PM		SS3-ThA-9 The Growth, Structure and Reactivity of Au on TiO\sub 2\(110) L. Zhang, R. Persaud, T. Madey, F. Cosandey (Rutgers University) We have studied the growth mode of ultrathin Au films on the TiO\sub 2\(110) surface using Low Energy Ion Scattering (LEIS), X-ray Photoelectron Spectroscopy (XPS), High Resolution Scanning Electron Microscopy (HR SEM) and Low Energy Electron Diffraction (LEED). In the substrate temperature range between 160K and 475K, Au evaporated under UHV conditions grows on the stoichiometric TiO\sub 2\(110) surface as three-dimensional islands with the size of islands increasing as the surface temperature increases. Analysis of XPS data obtained over a wide range of Au coverages indicates that the Au islands have droplet-like shapes. This result has been verified by our Scanning Tunneling Microscopy (STM) and SEM studies. An independent SEM estimate of the substrate area covered by Au is in excellent agreement with our LEIS results. No evidence of significant chemical interactions has been found between the Au and TiO\sub 2\ substrate. By annealing Au/TiO\sub 2\ to temperatures up to 775K, the Au islands continue to grow; encapsulation of the Au islands by Ti suboxides is not observed.
4:30 PM		SS3-ThA-10 An In Situ Xps Study of Gold Oxidation by UV/Ozone A. Krozer, M. Rodahl (Chalmers University of Technology, Sweden) UV/ozone cleaning is an effective and established method of removing contaminants from surfaces [1]. Contrary to our expectations, we found that, in some cases, the gold electrodes of a QCM device became HEAVIER after UV/ozone cleaning and become LIGTHER after subsequent water rinse. It has recently been reported that UV/ozone treatment in lab air oxidizes gold [2]. This has motivated us to perform an in situ XPS study of gold oxidation by UV/ozone. UV/ozone treatment of the gold sample was performed in an UHV preparation chamber and it was transferred to the analysis chamber without breaking vacuum. UV/ozone treatment of initially clean gold surfaces (< 5% of a ML of contaminants) resulted in two different O 1s peaks (529.5 and 530.5 eV), and a split of each of the two Au 4f peaks. All additional peaks were stable at RT, but disappeared after ~0.5 hr heating to 180 C. The two O1s components suggest the formation of gold oxide thicker than 1 ML. Most of this gold oxide was removed after gentle rinse with pure water in air, consistent with the QCM measurements. Prolonged UV/ozone treatment of air exposed gold surfaces removed most of the air contamination, but left a residue of ~20% of a ML (C, Si, Cl). The XPS spectra of UV/ozone treated "contaminated" (air exposed) Au produced three O 1s peaks, two characteristic of gold oxide, and a third, new peak at 531.5 eV. The latter feature remained after prolonged heating to >400 C, whereas the other two disappeared within minutes of heating. There is a controversy in the literature whether or not gold oxide is stable at >400 C [3]. Our results indicate that this controversy is probably due to the use of contaminated gold. (In an XPS survey spectrum of Au, an impurity level of ~20% of a ML is barely detectable.) [1] J. R. Vig, in "Surface Contamination: Genisis, Detection, and Control" K. L. Mittal, Ed., v1, p235 (1979). [2] D. E. King, JVST A v13, p1247 (1995). [3] Gmelin Handbook, Au Suppl. B1