Papers

AVS1997 Session SS2-ThA: Reactions on Alloys and Modified Surfaces

Thursday, October 23, 1997 2:00 PM in Room A3/4

Thursday Afternoon

Start	Invited?	Item
2:00 PM		SS2-ThA-1 Atomic View of Corrosion of Strained Metal Films J. de la Figuera, N.C. Bartelt, A.K. Schmid, R.Q. Hwang (Sandia National Laboratories) The interaction of oxygen with the metals is crucial in many technologically fundamental problems such as corrosion and catalysis. This is particularly true in thin metal films since it is well known that stress in the film can greatly affect its corrosion properties as compared to the bulk metals. Using time-resolved STM, we have investigated the atomistic dynamics of the interaction of O with a strained metal film, Cu on Ru(0001). This is a model system for such a study since a well studied series of dislocation patterns form as a function of Cu thickness ¹. Oxygen interacts strongly with these dislocations and through etching of the film, alters and removes them. The detailed effects of O as a function of layer have been studied. In the first layer, O interacts strongly with the metastable dislocation network, removing the dislocations and driving the layer to a pseudomorphic state. In subsequent layers, the interaction diminishes until a bulk-like behavior is reached. We will contrast these results with that of bulk Cu(111). Our data allow us to present a detailed model of the key atomic interactions leading to the strongly reactive behavior. From the comparison of the different layers, some general ideas applicable to the chemisorption on Cu will be forwarded. General mechanisms will be presented as to the relationship between strain and chemical reactivity. This work was supported by DOE under Contract No. DE-AC04-94AL85000. One of us (J. de la F.) thanks the Spanish MEC and the Fulbright Commission for a fellowship. ¹C. Günther et al, Phys. Rev. Lett. 74, 754 (1995)
2:20 PM		SS2-ThA-2 Oxidation and Reduction of Pt-Sn Surface Alloys N.A. Saliba, Y.-L. Tsai, C. Panja, B.E. Koel (University of Southern California) Interactions between Pt, Sn, and oxygen control the reactivity and chemistry of many important systems, ranging from supported Pt-Sn bimetallic catalysts to Pt-doped SnO₂ sensors. We have used epitaxial thin films of ordered Pt-Sn alloys (intermetallic compounds) that can be grown on Pt(111) or Pt(100) single crystal substrates via vapor deposition of Sn to study oxidation and reduction reactions of Pt-Sn alloys. Oxygen (O₂) does not dissociatively chemisorb under UHV conditions on these surfaces. However, we have utilized ozone (O₃) to controllably and cleanly oxidize these Pt-Sn surfaces at room temperature. Ozone exposures on the clean Pt(111) or Pt(100) single crystal substrates also form thin oxide films, and we take oxidation/reduction results for these platinum oxide surfaces as benchmarks to compare to our results on the Pt-Sn alloys. The oxidation and reduction chemistry of these surfaces is characterized by TPD, AES, XPS, ISS, and LEED. The thermal stability and the oxidation resistance of the alloys was also probed. We find that the Sn in these alloys is oxidized at 300 K, disrupting the two-dimensional structure of the Pt-Sn alloy, and small islands of tin oxide (SnO_x) are formed. At higher oxygen concentrations, chemisorbed oxygen and platinum oxide (PtO_x) phases are formed. The O₂ TPD curves indicate a growth in the size of small PtO_x clusters with increasing oxygen concentration. Sn enhances the oxidation of Pt, forming platinum oxide at lower oxygen concentrations, and stabilizes the reduction of platinum oxide, such that decomposition occurs at higher temperatures than in the absence of Sn. In all cases, Pt has a catalytic effect to lower the reduction temperature of tin oxide.
2:40 PM		SS2-ThA-3 Oxidation-Reduction Kinetics of Palladium Catalysts G. Veser (University of Stuttgart, Germany); A.G. Wright, R.A. Caretta (University of Minnesota) Palladium is an important industrial oxidation catalyst which shows very complex behavior, such as hysteresis during catalytic oxidation of hydrocarbons^1-3, an unusually high activity for methane combustion^4,5, and a transition between PdO and Pd as the active phases of the catalyst depending on reaction conditions^1,6. In an approach to understand these phenomena, we investigated the oxidation and reduction of Pd foil catalysts. The foils were oxidized in a reaction chamber in 20 kPa of pure air at different oxidation temperatures and durations. Oxidized samples were then transferred into an attached UHV chamber and analyzed with XPS before and during reduction and by sputtering depth profiling. The observed decomposition kinetics can be separated into three distinct regimes: a low temperature regime (T< 200ºC, E_act= 104 kJ/mol), a high temperature regime (T> 400ºC, E_act= 213 kJ/mol), and an intermediate regime (200ºC< T< 400ºC) which does not follow a simple kinetic rate law and is characterized by the occurrence of a step-like feature in the reduction time trace. The regimes are interpreted as a kinetically controlled regime at low temperatures and a diffusion controlled regime at high temperatures, the latter being confirmed by sputtering depth profile experiments. A simple generic model is able to qualitatively reproduce the behavior in the intermediate region as a transition between reaction and diffusion controlled kinetics. ¹R.J. Farrauto, M.C. Hobson, T. Kennelly, and E.M. Waterman, Appl. Catal. A81 (1992) 227 ²S. Salomonson, S. Johansson, and B. Kasemo, Catal. Lett. 33 (1995) 1 ³J.G. McCarty, Catal. Today 26 (1995) 283 ⁴R. Burch, P.K. Loader, and F.J. Urbano, Catal. Today 27 (1996) 243 ⁵M. Ziauddin, G. Veser, and L.D. Schmidt, to be published ⁶K. Skizawa, M. Machuda, K. Eguchi, and H. Arai, J. Catal. 142 (1993) 655
3:00 PM		SS2-ThA-4 Enhanced Reactivity of Co Deposited on Cu(111) J. Hvolbæk Larsen, I. Chorkendorff (Technical University of Denmark) Recently the shift in the center of d-band was correlated with the CO binding energy for various bimetallic systems ¹, ². These d-band shifts for pseudomorphic overlayers have now been calculated for a large number of systems ³, e.g. a large d-band shift towards the Fermi level is found for the Co-Cu system. Thus pseudomorphic overlayers of Co on Cu is expected to be more reactive than pure Co itself. This interesting result of an enhanced reactivity is tested beyond the CO chemisorption by investigating the dissociative reaction of CH₄. The supersonic CH₄ molecules dissociate on Co but not on Cu under these experimental conditions. The results show that the sticking coefficient of CH₄ as a function of Co coverage initially increases linearly with the Co coverage and then goes through a maximum. By probing the different sites on the surface with temperature programmed desorption of CO and by using recent STM results the Co coverage yielding the maximum reactivity is found to be when the Cu surface is completely covered with the three layer Co structure ⁴. When depositing additional Co layers the sticking coefficient decreases towards the value of pure Co. Counter intuitively, it can from the experimental results presented here be concluded that Co becomes more reactive when adsorbed on the more noble Cu substrate in good agreement with the theoretical predictions. ¹B. Hammer et al., Phys. Rev. Lett. 76, 2141 (1996) ²J. A. Rodriguez et al., Science 257, 897 (1992) ³A. Ruban et al., J. Mol. Catal. A 115, 421 (1997) ⁴M. Ø. Pedersen et al., submitted to Surf. Sci. (1997)
3:20 PM		SS2-ThA-5 Surface Reactivity Studies of an AlPdMn Alloy C.J. Jenks, P.J. Pinhero, J.W. Anderegg, P.A. Thiel (Iowa State University) We present results of ultra-high vacuum studies of the chemical reactivity of an alloy of Al₇₀Pd₂₁Mn₉, a material that is quasicrystalline in structure. Results from low energy ion scattering and calculations based on low energy electron diffraction intensity versus voltage studies suggest that the topmost surface layer of these materials is predominantly aluminum (> 80%). However, most of the practical properties of these materials are known to be quite different from those of metallic aluminum or aluminum oxide. Although these materials are themselves metallic, they are poor electrical and thermal conductors. Quasicrystals are also characterized by low coefficients of friction and high hardness. The apparent surface energy of aluminum-based quasicrystals obtained from wetting experiments is closer to that of pure aluminum rather than aluminum oxide.¹ Furthermore, methanol decomposition is known to be better catalyzed by AlPdMn in its quasicrystalline form than in its crystalline form.² In combination, the properties exhibited by aluminum-based quasicrystals suggest that their reactivity might be different from that of pure aluminum. However, thus far we have found from temperature programmed desorption studies that simple molecules, such as hydrogen, oxygen, methanol and alkyl halides adsorbed onto Al₇₀Pd₂₁Mn₉ behave similarly to the same molecules adsorbed onto pure aluminum. ¹J. M. Dubois et al., to be published. ²T. Masumoto et al., European Patent Application 94115137.5 (1994).
3:40 PM		SS2-ThA-6 Hydrodesulfurization Reactions on Carbon Modified Mo(110) Surfaces C.L. Roe, K.H. Schulz (Michigan Technological University) The role of carbon in the reaction pathways of hydrodesulfurization (HDS) catalytic systems is not well understood. Carbidic surfaces have been proposed to be involved in desulfurization surface reaction pathways. Additionally, carbon-supported HDS catalysts are generally more active than alumina supported materials. Recently, metal surfaces with interstitial carbidic adlayers have shown catalytic properties similar to platinum-based catalysts. In order to investigate the effects of carbon on surface reactivity, we have initiated studies of HDS reactions on clean and carbon-modified Mo(110) surfaces. Initial studies have focused on the decomposition behavior of 1,2-ethanedithiol on Mo(110) surfaces using temperature programmed desorption. On a clean Mo(110) surface, a single product desorption feature was observed at approximately 450 K. Reaction products detected include vinyl thiol (CH2=CH-SH) and acetylene; no desorbing ethanedithiol was detected. Following the TPD experiments, a significant amount of sulfur had been deposited on the surface as determined by Auger electron spectroscopy. Coincident with the addition of sulfur onto the Mo(110) surface, the growth of a second desorption state at approximately 200 K was observed. Complete passivation of the surface by sulfur occurred following several TPD cycles, afterwhich only the desorption of physisorbed ethanedithiol at about 200 K was observed. Significantly different results were observed in the dithiol TPD studies on carbon covered (C/Mo = 0.5) Mo(110) surfaces. In constrast to the clean surface, no acetylene was detected as a desorption product. However, ethylene, vinyl thiol and trace amounts of ethanedithiol were observed during TPD experiments. These product molecules are thought to arise from two distinct surface intermediates: 1) a thiolate species bound through one of the ethanedithiol terminal sulfur atoms to the surface, and 2) a cyclic organosulfur species produced via C-S bond scission through each of the terminal sulfur atoms. We propose that vinyl thiol is produced via C-S bond scission and subsequent hydride elimination of the first intermediate, while ethylene is formed via the second intermediate. This paper will discuss the details of the different reaction pathways for carbon-covered and clean Mo(110) surfaces, and will address the effects of surface carbon on HDS reactions in general.
4:00 PM		SS2-ThA-7 Reconstruction and C-1 Chemistry of Oxygen-Modified Mo(100) S.H. Kim, P.C. Stair (Northwestern University) The surface structure and reactions of oxygen-modified Mo(100) surface have been investigated using LEED, AES, TPD, and HREELS. The vibrational spectra of O-Mo(100) with oxygen coverages between 0.8ML and 1.2ML show gradual changes in the relative intensities of the Mo-O vibrations at different adsorption sites, as a function of time at room temperature in UHV, even though there is no significant chemisorption. The rate of this process increases with molecular chemisorption both at room temperature and low temperature. The chemistry of this surface has been studied using two different approaches: thermal activation of CH₃I on O-Mo(100) and direct dosing of CH₃ from the gas phase. CH₃I adsorbs molecularly on O-Mo(100) at 120K. Upon heating, most of the adsorbed CH₃I molecules desorb intact and only small fraction dissociate to CH₃ and I below room temperature. The CH₃ radical, generated by pyrolysis of (CH₃)₂N₂ in the gas phase, chemisorbs on O-Mo(100). The adsorbed CH₃ groups exhibit complex chemistry that varies with the surface coverages of pre-adsorbed oxygen and CH₃.
4:20 PM		SS2-ThA-8 Bonding Nature between Oxygen and Sodium on Si(113) Surface K.S. An, C.C. Hwang, R.J. Park, J.S. Kim, J.B. Lee, C.Y. Park (Sung Kyun Kwan University, Korea) The oxidation of alkali metal(AM) pre-deposited metal or semiconductor surface has received increasing attention because of technological application of low workfunction and AM-promoted oxidation of the system. AM is known to enhance the sticking coefficient of oxygen molecule on semiconductor surface. But the role of AM as a catalyst is not fully understood because the bonding nature between AM and oxygen is still unclear. To know the bonding nature between oxygen and sodium(Na) in the AM-promoted oxidation, the oxygen adsorption on the Na/Si(113) surface has been studied at room temperature and low temperature using XPS and SRPES. Si 2p, O 1s, Na 1s, Na 2p core level, Na KLL Auger peak and valence band were simultaneously measured with the oxygen exposure and Na coverage. It was observed that the Auger parameter obtained from Na 1s and Na KLL peak decreased drastically by about 3 eV with increasing oxygen exposure. In contrast with the interpretation of previous photoemission spectroscopy studies, the result demonstrates that the bonding nature is undoubtedly ionic. This interpretation is very consistent with recent metastable deexitation spectroscopy measurements. Based on these results, we will discuss the role of AM as a catalyst in the AM-promoted oxidation with several subtle problems such as Si backbond weakening and temperature dependence of dissociation.
4:40 PM		SS2-ThA-9 Study of the Reactions of Li with Tetrahydrofuran and Propylene Carbonate by Photoemission Spectroscopy G.R. Zhuang, K. Wang, P.N. Ross (Lawrence Berkeley National Laboratory and University of California, Berkeley) The reaction of Li with several organic solvents of technical importance in lithium batteries, such as tetrahydrofuran (THF), propylene carbonate (PC), dimethyl carbonate(DMC) and diethyl carbonate(DEC) were studied in UHV by photoemission spectroscopy. The organic condensate layers were formed by dosing thin (6-10 nm) films of Li at 120 - 135 K, with the reaction monitored by XPS and UPS upon subsequent warming of the sample. Activation of the first layer of THF molecule by Li starts at a temperature as low as 120 K. Polymerization of THF (forming poly-THF) occurs upon melting near 180 K, but is accompanied by chain-terminating reactions that form Li alkoxide(s) and hydrocarbon gas(es), such as ethylene and/or propylene. Between 180 and 320 K, there is progressively greater conversion of poly-THF alkoxide such that at 320 K , the surface film is almost entirely composed of alkoxide. At or near its bulk melting temperature of 220 K, essentially all of the PC remaining on the surface has reacted with Li to form an alkyl carbonate. With increasing temperature, part (25 - 33%) of the alkyl carbonate decomposes to form an alkoxide, and in the temperature interval of 270 -320 K there may be a small amount (< 5% of the carbon present) of Li carbonate formed. The alkyl group in the organo-Li compounds derived from PC is most probably propylene. There is no evidence of the formation of any gaseous products at temperature below 320 K under the conditions of these experiments. Of particular relevance to battery technology, however, is the fact that in both cases the organo-Li layers that have formed at 270 K were formed in the presence of excess unreacted Li.
5:00 PM		SS2-ThA-10 Temperature-Dependent Near-Surface Chemistry of BaLi₄ Gettering Alloy Studied by Means of X-ray Photoemission Spectroscopy S. Vandré, J. Kovac, E. Magnano, E. Narducci (INFM, Italy); R.M. Caloi, P. Manini (SAES Getters S.p.A., Italy); M. Sancrotti (INFM, Italy) BaLi₄ is a very peculiar getter material, able to absorb significant amounts of nitrogen at room temperature. Gas absorption efficiency and capacity is such that the alloy has been recently proposed to maintain the required vacuum level for Vacuum Insulated Panels (VIP) used in refrigerators. Most of the very intriguing sorption properties of BaLi₄ seem to rely on the peculiar chemico-physical characteristics of its surface. We have analyzed the surface and sub-surface composition of the BaLi₄ intermetallic alloy using XPS and Ar⁺ ion depth profiling. At each treatment step, significant core levels of the nominal constituents and contaminants (Ba 3d, Li 1s, O 1s, C 1s, and N 1s) were measured. The effects induced by RT gas exposure (air, N₂, and O₂) and vacuum annealing at low temperature (100 C) have been studied in detail. An ingot of BaLi₄ scraped under UHV shows no preferential segregation between Ba and Li. After air exposure the ingot has been heated up to 100 C for 1h under UHV. Both Ba and Li show a high degree of metallicity, comparable to the scraped sample. This finding indicates that the low temperature annealing at 100 C can restore the surface reactivity of the alloy. Dosing the BaLi₄ with nitrogen does not lead to appreciable change in the nitrogen surface concentration. However, the nitrogen concentration has been found to increase after repeated sputtering, thus demonstrating the existence of a strong chemical reaction with the bulk alloy. This is shown to be in good agreement with N₂ sorption tests carried out at RT on BaLi₄ powder.