Papers

AVS2004 Session SS2-FrM: Bimetallic Surface Chemistry and Structure

Friday, November 19, 2004 9:00 AM in Room 210C

Friday Morning

Start	Invited?	Item
9:00 AM		SS2-FrM-3 Tuning the Electronic and Chemical Properties of Bimetallic Surfaces J.G. Chen, J.R. Kitchin, N.A. Khan, M.A. Barteau (University of Delaware) It is well known that bimetallic surfaces often show novel properties that are not present on either of the parent metal surfaces. However, it is difficult to know a priori how the chemical properties of a particular bimetallic surface will be modified relative to the parent metals. There are two critical factors that contribute to the modification of the chemical properties of a metal in a bimetallic surface. First, the geometry of the bimetallic structure is typically different from that of the parent metals, e.g. the average metal-metal bond lengths change. This gives rise to strain effects that are known to modify the electronic structure of the metal through changes in orbital overlap. Second, the presence of other metals around a metal atom also changes its electronic environment, giving rise to further modifications of its electronic structure through the ligand effect. We have investigated the electronic and chemical properties of model bimetallic surface structures, epitaxial monolayers and subsurface epitaxial monolayers, using a combination of experimental and theoretical modeling to gain further insights into these factors. In the current presentation we will first utilize the adsorption and desorption of hydrogen to demonstrate the correlation between the hydrogen binding energy and the center of the d-band in various bimetallic surfaces. We will also provide a general equation that allows one to predict how the electronic properties, especially the d-band center, will be affected in bimetallic systems. We will then use the hydrogenation of cyclohexene to demonstrate the effect of weakly-bonded hydrogen on the novel low-temperature hydrogenation activities on the bimetallic surfaces. Finally, we will use the results from the hydrogenation of cyclohexene to demonstrate a strong correlation between UHV studies on model bimetallic surfaces and reactor studies on corresponding supported bimetallic catalysts.
9:40 AM		SS2-FrM-5 The Study of Electronic Structures and Surface Segregation of Pt₃M Alloy (M= Ti, V, Cr, Mn, Fe, Co, Ni, and Zr) B.S. Mun (Lawrence Berkeley National Laboratory); M. Watanabe (Lawrence Berkeley National Laboratory & SPring-8 Project Team (RIKEN)); V. Stamenkovic, N.M. Markovic, P.N. Ross Jr. (Lawrence Berkeley National Laboratory) The systematic study of surface electronic structures of Pt₃M (M= Ti, V, Cr, Mn, Fe, Co, Ni, and Zr) polycrystalline alloys are presented with high resolution photoemission spectroscopy. The positions of local d-band center from valence band density of state (DOS) measurements are carefully monitored before and after annealing process. Concurrently, the presence and magnitude of surface segregation are measured with angle resolved core-level photoemission spectroscopy. The correlation between the d-band center of DOS and its chemical properties are discussed.
10:20 AM		SS2-FrM-7 The Study of Electronic Structures and Surface Segregation of Single Crystalline Pt₃Ni(100),(100),and (111) M. Watanabe, B.S. Mun, V. Stamenkovic, N.M. Markovic, P.N. Ross Jr. (Lawrence Berkeley National Laboratory) The surface electronic structures of Pt₃Ni(100), (110), and (111) single crystalline samples are investigated with synchrotron-based photoemission spectroscopy. From the measurement of valence band spectrum, the d-band centers of density of state are estimated and compared before and after the annealing treatment. In addition, the core-level angle resolved photoemission technique is applied to estimate the segregation of Pt and Ni at the surface. The correlation between electronic structures of each index and its chemical properties are discussed.
10:40 AM		SS2-FrM-8 Chemisorption and Quantum Size Effects on Pseudomorphic fcc-Co and fcc-Fe Films Grown on Cu(100) H. Yao, A.G. Danese (Rutgers University); C.J. Bosco, F.G. Curti (Seton Hall University); R. Bartynski (Rutgers University) Nanoscale metal thin films can exhibit quantum size effects (QSE) whereby their electronic, structural, magnetic and chemical properties may differ greatly from those of the bulk. The quantum confinement of electrons due to high reflectivity scattering at the film's interfaces forms so-called metallic quantum well (MQW) states which give rise to many of these interesting phenomena. In previous studies we have examined the role of MQW states in the chemisorption of CO on Cu MQW overlayers on pseudomorphic fcc-Co and fcc-Fe films grown on Cu(100). In the current work, we report on the adsorption properties of the pseudomorphic transition metal layers themselves. In addition to the fact that the pseudomorphic films represent metastable structures, these systems have partially filled d-bands that change significantly as a function of film thickness. For a Co film with an approximate coverage of 5 monolayers of Co we find that a saturation dosage of CO leads to the formation of a c(2x2) structure as observed in low energy electron diffraction. Inverse photoemission spectra of this system exhibits a two peaked structure centered at 3.8 eV above the Fermi level and is assigned to the unoccupied CO 2π* orbital. CO is found to adsorb molecularly at room temperature and in TPD measurements we find a desorption temperature of approximately 400 K, a temperature in the range previously seen for CO adsorbed on hcp Co surfaces. For saturation doses carried out at lower temperatures we find a second lower temperature desorption peak around 270 K in the desorption spectra. These results, and those from fcc-Fe, are compared to the parent single crystal systems.
11:00 AM		SS2-FrM-9 Probing the Mechanism of n-hexane Dehydrocyclization over Pt-Sn Catalysts: Adsorption and Reactivity of n-hexane, 1-hexene, and 1,5-hexadiene on Pt(111) and Sn/Pt(111) Surface Alloys H. Zhao, B.E. Koel (University of Southern California) Supported bimetallic platinum catalysts are widely used to carry out skeletal reactions of alkanes, and such processes including dehydrocyclization are important for naphtha reforming. In order to clarify discussions of the mechanisms of these reactions, we have investigated the adsorption of n-hexane and the reactivity of several likely intermediates in such processes, i.e., 1-hexene and 1,5-hexadiene on well-defined, single-crystal surfaces of Pt(111) and two, ordered Sn/Pt(111) alloys under UHV conditions by using primarily TPD, AES, and LEED. Alloying caused a small decrease in the adsorption energy for all adsorbates. However, alloying decreased strongly the reactivity of 1-hexene and 1,5-hexadiene such that no dehydrogenation occurred during TPD on the (√3*√3)R30°-Sn/Pt(111) alloy with Θ_Sn=0.33. This alloy surface does not contain pure-Pt 3-fold sites which can be inferred to be particularly reactive sites. Alloying did not decrease the saturation coverage of any of these molecules.
11:20 AM		SS2-FrM-10 Experimental and Theoretical Characterization of the Structure of Defects at the Pyrite FeS2 (100) Surface K. Andersson, M. Nyberg (Stockholm University, Sweden); O. Ogasawara (Stanford Synchrotron Radiation Laboratory); D. Nordlund (Stockholm University, Sweden); T. Kendelewicz, C.S. Doyle, G.E. Brown, Jr (Stanford University); L.G.M. Pettersson (Stockholm University, Sweden); A. Nilsson (Stanford University) Defect-free pyrite FeS2 (100) surfaces were generated and a controlled manipulation of sulfur defect density at these surfaces was performed. Sulfur species of different coordination and environments at the surface were probed by S 2p photoemission in combination with theoretical modeling of S 2p core-level shifts. A strict structural assignment of S 2p peaks at the FeS2 (100) surface in the low defect density regime was achieved. Based on our results, a defect that is related to a surface sulfur vacancy is confirmed to provide the active site for the rapid initial oxidation stage at the pyrite surface.
11:40 AM		SS2-FrM-11 H₂S Adsorption and Dissociation on Fe-Al and Fe-Si Alloy Surfaces from First Principles D.E. Jiang, E.A. Carter (UCLA) H₂S attacks iron aggressively and causes the formation of iron sulfide. The sulfidation process is more kinetically favorable than other processes such as carburization. We are interested in determining a way to chemically pretreat iron surfaces to improve resistance to H₂S. One strategy is to alloy the surface. Here we investigate whether alloying an Fe surface with Al or Si might improve such resistance, via periodic density functional theory calculations of H₂S adsorption and dissociation on low-index surfaces of the intermetallic compounds FeAl and Fe₃Si. Stable adsorption structures and relative stabilities of H, S, HS, and H₂S on FeAl(110), FeAl(100), Fe₃Si(110), and Fe₃Si(100) are elucidated and compared with those on Fe(110) and Fe(100). We also present predicted minimum energy paths and energy barriers for the sequential dehydrogenation steps: H₂S to HS + H and HS to H + S. Our study shows that FeAl is indeed more resistant to H₂S attack than Fe, while Fe₃Si is as susceptible as Fe.