Papers

AVS1996 Session SS2-TuA: Metal Surface Reactions I

Tuesday, October 15, 1996 2:00 PM in Room 201B

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS2-TuA-1 Formation of Surface Alloys and Adsorption and Reactivity of These Studied by STM F. Besenbacher, I. Stensgaard, P. Murray, M. Pedersen, I. Boenicke (University of Aarhus, Denmark) The formation of surface alloys in heteroepitaxial metal on metal growth has been studied by Scanning Tunneling Microscopy (STM). Two types of surface alloys have been studied: I) Surface alloys for which the bulk heat of alloy formation is negative, that is, the two metals are bulk miscible and a number of stable alloy phases exist and II) Surface alloys for which the two metal constituents are immiscible in the bulk, i.e. an immiscibility gap exists in the bulk phase diagram. Nevertheless for the latter class of metals it has been discovered that 2-D surface alloys can form having apparently no 3-D analogy. I will discuss how these two types of surface alloys nucleate and grow and present results for representative examples of both types, I: Pd-Cu, Pt-Cu and II: Au-Ni, Co-Cu. Finally, I will address the adsorption and reactivity of these surface alloys. The prospects of these studies are the possibility of 'designing' -on the atomic scale- surfaces with particular catalytical properties.
2:40 PM		SS2-TuA-3 Chemical Activity of a Two-dimensional Ni(111)-Au Alloy Towards CH\sub 4\, CO, and D\sub 2\ I. Chorkendorff, J. Larsen, P. Holmblad (Technical University of Denmark) Despite that Au and Ni in principle constitutes an immiscible system, a two-dimensional alloy can be formed by alloying Au into the outermost atomic layer of Ni(111) significantly altering the physical and chemical properties of the surface. The reactivity is investigated by the use of seeded supersonic molecular beams of CH\sub_4\ and it is found that the nobleness of Au reduces the overall reactivity towards CH\sub 4\. This is accounted for in an ensemble model resolving the sticking probability on Ni atoms having different nearest neighbor surroundings. Although a mean field description of site distributions is found to be a very good approximation it is improved by using experimentally determined ensemble statistics from STM images. The strong influence of the vibrational temperature on the sticking coefficients of CH\sub 4\ versus translational energy on the pure Ni(111) is also demonstrated. Desorption energies of CO and D\sub 2\ is observed to decrease approximately 25-30 kJ/mole as the coverage of Au is increased from 0.0 to 0.7 ML. In TPD spectra of deuterium saturated surface alloys a new clearly resolved desorption state is observed at 180-220 K with maximum intensity around \theta\ \sub Au\ = 0.3-0.4 ML. This state is clearly related to chemisorption sites involving both Au and Ni. A site model based on mean field statistics adequately accounts for the appearance of this state. The effect of Au is also evident in the TPD spectra of CO saturated Au/Ni(111) surface alloys where the saturation coveraged by using experimentally determined ensemble statistics from STM images. The strong influence of the vibrational temperature on the sticking coefficients of CH\sub 4\ versus translational energy on the pure Ni(111) is also demonstrated. Desorption energies of CO and D\sub 2\ is observed to decrease approximately 25-30 kJ/mole as the coverage of Au is increased from 0.0 to 0.7 ML. In TPD spectra of deuterium saturated surface alloys a new clearly resolved desorption state is observed at 180-220 K with maximum intensity around \theta\ \sub Au\ = 0.3-0.4 ML. This state is clearly related to chemisorption sites involving both Au and Ni. A site model based on mean field statistics adequately accounts for the appearance of this state. The effect of Au is also evident in the TPD spectra of CO saturated Au/Ni(111) surface alloys where the saturation coverage.
3:00 PM		SS2-TuA-4 Do 'Hot' Oxygen Atoms Exist on Pt(111)? A Study by Scanning Tunneling Microscopy J. Wintterlin, R. Schuster, G. Ertl (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) There are conflicting results in the literature about the existence of transient mobility during adsorption. Adsorption involving 'hot' atoms was claimed from experiments of O\sub 2\ dissociation on Al(111) [1], which was not confirmed by later molecular dynamics studies [2]. To clarify the situation the dissociation of O\sub 2\ on a Pt(111) surface was studied by variable temperature STM at 150 - 160 K for which former TPD experiments indicated the existence of 'hot' oxygen atoms [3]. Using STM we find that the two oxygen atoms created by the dissociation appear in pairs, with an average distance of two lattice constants. Thermal motion, which was determined in addition, sets in only at around 200 K, with a diffusion barrier of 0.43 eV and a preexponential factor of 10\super -6.3\ cm\super 2\s\super -1\. The distance of two lattice constants found at around 160 K is therefore not caused by thermal diffusion. It is concluded that it results from transient ballistic motion, where the short range traveled is in agreement with the molecular dynamics studies. Possible explanations for the difference to aluminum are discussed. [1] H. Brune, J. Wintterlin, R. J. Behm, and G. Ertl, Phys. Rev. Lett. 68, 624 (1992). [2] C. Engdahl and G. Wahnstrom, Surface Sci. 312, 429 (1994). [3] T. Matsushima, Surface Sci. 127, 403 (1983).
3:20 PM		SS2-TuA-5 Kinetics and Thermochemistry of O\sub 2\ Adsorption, Dissociation, and Desorption on Pt(111) P. Grasso, A. Anton (Cornell University) The interaction of O\sub 2\ with platinum involves three distinct adsorbed states: (1) a mobile, physically adsorbed precursor that communicates directly with the gas phase; (2) a chemisorbed, molecular state that mediates dissociation of O\sub 2\; and (3) a chemisorbed atomic state. Kinetic competition among elementary reactions for interconversion of these leads to complex dependencies of overall adsorption and desorption rates on surface coverage and temperature. The situation is complicated further by the presence of lateral interactions that lead to order disorder phenomena and other nonlinearities. We present isothermal measurements of the rates of O\sub 2\ adsorption, dissociation, and desorption on Pt(111) under conditions near thermal equilibrium for broad ranges of surface coverage and temperature. We combine transition state theory with statistical mechanical models for interacting lattice gases to develop rate expressions for all the contributing elementary steps, and we compare the rate expressions to measured data to extract reliable estimates of zero-point energy differences, single-particle partition functions, and pairwise interaction energies. Finally, we use the measured parameters to construct a potential energy diagram for the coverage-dependent interaction of oxygen with Pt(111).
3:40 PM		SS2-TuA-6 Formation of NH\sub 3\ by the Hydrogenation of Atomic Nitrogen on Rh(111) R. Van Hardeveld, R. Van Santen, J. Niemantsverdriet (Eindhoven University of Technology, The Netherlands) Atomic nitrogen layers with well determined coverage were prepared by reaction between NO and H\sub 2\. Secondary Ion Mass Spectrometry (SIMS) spectra showed that the atomic oxygen resulting from dissociated NO could selectively be removed by reaction with H\sub 2\ at 400 K. The rate of NH\sub 3\ formation was derived from the decrease of the atomic nitrogen coverage to which it moreover appeared proportional. The hydrogenation rate was proportional to the H\sub 2\ pressure between 1.10\sup -8\ and 5.10\sup -7\ mbar, indicating that the second or third hydrogenation step is rate limiting. Between 5.10\sup -7\ and 1.10\sup -6\ mbar, the H\sub 2\ pressure dependence decreased considerably. SIMS spectra of the surface under reaction conditions revealed that N and NH\sub 2\ were the predominant surface species and that NH was virtually absent. The fraction of NH\sub 2\ on the surface increased with increasing H\sub 2\ pressure but reached a constant value of 0.3 for H\sub 2\ pressures above 5.10\sup -7\ mbar. Despite a very low coverage, NH\sub 3\ could be detected on the surface by SIMS under reaction conditions. The NH\sub 3\ steady state coverage increased with increasing H\sub 2\ pressure. The temperature dependence of the NH\sub 3\ formation rate was investigated in the range between 325 and 400 K, which yielded an effective preexponential and activation energy of 10\sup 2\ s\sup -1\ and 40 kJ/mole, respectively. From these results we conclude that the third hydrogenation step, the hydrogenation from NH\sub 2\ to NH\sub 3\, is the rate determining step in the NH\sub 3\ formation on Rh(111).
4:00 PM		SS2-TuA-7 The Reactions of Oxygen Adsorbed on Cu(110) with Ammonia R. Madix, X. Guo (Stanford University) The reactions between oxygen adsorbed on Cu(110) and ammonia have been examined at surface temperatures of 300 and 400 K and at high and low oxygen coverage with scanning tunneling microscopy. At low oxygen coverage ammonia readily reacts at either temperature with the isolated Cu-O strings, or pseudomolecules, oriented along the (100) direction on the surface, leaving either N or NH strands oriented along the (110) direction. In the steady state at 300 K a mixture of ammonia and oxygen first deposits NH-containing strings along the (110) direction on the clean surface; the buildup of Cu-O pseudomolecules follows. If the oxgen is removed from the ambient atmosphere, these Cu-O strings react away readily. The (2x1) oxide islands show a different behavior. In the absence of defects the islands react only at the island perimeters; however, extraordinarily rapid reactions are observed due to internal vacancies. In general, the reaction of ammonia with the islands proceeds more rapidly along the (100) direction. Typically, reaction is initiated at the island perimeter and proceeds to thin the island in a row by row fashion. Because N-atom-containing-structures form along the (110) direction, they can block the ends of Cu-O chains and inhibit further reaction, rendering the islands unreactive. Overall, these experiments clarify differences in the reactivity of oxygen on Cu(110) previously attributed to a differing oxygen species. Clear evidence is also observed for initiation of the reaction at step defects of certain orientation.
4:20 PM		SS2-TuA-8 Observation and Characterization of a High Temperature Desorption State of NH\sub 3\ when Co-adsorbed with H on Al(111) C. Kim, V. Bermudez, J. Russell, Jr. (Naval Research Laboratory) Group III nitrides, such as AlN, are receiving increased atten- tion due to their potential applications in optoelectronic de- vices, high power electronics and sensors. High quality films of AlN have been produced by MOCVD using NH\sub 3\ and R\sub 3\Al (R = CH\sub 3\ or C\sub 2\H\sub 5\) as precursors. We have investigated the chemical interaction of ammonia with clean and hydrogenated Al(111) surfaces, as a model system, in a UHV environment using TPD, IRRAS, AES and LEED. On clean Al(111), NH\sub 3\ adsorbs molecularly and desorbs by 150 K. In contrast, on the H-covered surface, a new NH\sub 3\ desorp- tion state is observed at 250 K. Thus, the presence of surface H increases the NH\sub 3\ adsorption bond energy by about 10 Kcal/mol. On the fully H-covered Al(111) surface, the high-temperature de- sorption state saturates with one monolayer of NH\sub 3\, and the NH\sub 3\ and H coverage dependences of this state indicate that NH\sub 3\ binds to surface H sites. IRRAS shows that surface H causes appre- ciable change in the NH\sub 3\ adsorption structure. Only the NH\sub 3\ sym- metric deformation mode, at 1170 cm\super -1\, is observed on the clean surface since the molecule is oriented with the 3-fold axis normal to the surface. However, on the H covered surface, the N-H stretch (3200 cm\super -1\), the asymmetric deformation (1610 cm\super -1\) and a 130 cm\super -1\ blue shift for the symmetric deformation are seen, indica- ting that the NH\sub 3\ three-fold axis is tilted away from the surface normal. IRRAS also shows that NH\sub 3\ interacts selectively with terminally-bonded surface H. From this work we develop a model for the bonding and geometry of NH\sub 3\ on the H-covered surface and account for the physical basis of this behavior.
4:40 PM		SS2-TuA-9 Orientation Differences of Resolved Enantiomers on a Chiral Surface M. Buelow (Carnegie Mellon University); C. McFadden (University of Illinois, Urbana-Champaign); A. Gellman (Carnegie Mellon University) The enantiomers of chiral organic molecules are anticipated to interact differently with chiral metal surfaces. Infrared Reflection Absorption Spectroscopy (IRAS) has been used to probe differences in the orientation of r- and s-2-butanoxide, the reaction intermediates for r- and s-2-butanol dehydrogenation to produce 2-butanone on silver surfaces. The Ag(643) surface has been previously determined to have a structure with short hexagonal close packed terraces separated by kinked steps. This surface has a non-superimposable mirror image and can thus be considered chiral. Temperature Programmed Reaction (TPR) spectroscopy was used to characterize the chemistry of the system and to verify the presence of 2-butanoxide on the surface, but was unable to resolve any difference in the reaction kinetics of r- and s-2-butanol, which would be expected if the enantiomers had a different interaction with the surface. IRAS spectroscopy, however, does show that there is a difference between the orientation of r- and s-2-butanoxide on the Ag(643) surface. A difference in orientation can cause changes in absorption intensities. The primary distinction can be seen in the peaks that represent the symmetric C-H stretches at 2855 cm-1 and 2875 cm-1, the relative intensities of which reverse in magnitude for r- and s-2-butanoxide. There does not appear to be significant change in the relative magnitudes of the low frequency vibrations below approximately 1300 cm-1. This is the first observation of a difference in the interaction of enantiomers on a chiral surface. This may have important implications to the pharmaceutical industry for improved enantiomer selectivity or separation using heterogeneous chiral catalysis.