AVS 59th Annual International Symposium and Exhibition
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP1
MORTON S. TRAUM AWARD FINALIST: Quantum Tunneling Driven Assembly and Diffusion of Hydrogen and Deuterium on Cu(111)

Tuesday, October 30, 2012, 6:00 pm, Room Central Hall

Session: Surface Science Poster Session
Presenter: A.D. Jewell, Tufts University
Authors: A.D. Jewell, Tufts University
G. Peng, University of Wisconsin Madison
G. Kyriakou, Tufts University
M. Mavrikakis, University of Wisconsin Madison
C.H. Sykes, Tufts University
Correspondent: Click to Email
Hydrogenation reactions are central to the petrochemical, fine chemical, pharmaceutical, and food industries and are of increasing interest in energy production and storage technologies . The processes of molecular adsorption, dissociation, diffusion, association, and desorption are important surface phenomena in heterogeneous catalysis. Typical heterogeneous catalysts often employ alloys based on platinum, palladium, rhodium and ruthenium. While these metals are active at modest temperature and pressure, they are not always 100% selective and are expensive. Given that molecular hydrogen (H2) dissociation is often the rate limiting step, one strategy is to engineer the minimal catalytic ensemble that will activate H2 but leave the other reactants untouched. We describe a system which offers low dissociation barriers at one location on the surface and weaker binding in other regions. The Pd/Cu surface alloy was prepared in the dilute limit in which 1% Pd resides as individual, isolated substitutional atoms in a 99% Cu(111) surface. In terms of adsorption, these Pd atoms significantly lower the barrier to H2 dissociation and allow the spillover of H atoms onto the Cu surface.[1] This system also offers the opportunity to study the diffusion, association, and assembly of large quantities of H and D on the Cu(111) surface. Through careful low-temperature scanning tunneling microscopy (STM) tracking experiments we show that quantum tunneling effects dominate the diffusion properties of H and D on the Cu surface.[2] With this direct visualization and quantification of quantum tunneling effects in adatom diffusion, we reveal two types of weak interactions between H adatoms, which lead to assembly into small clusters and larger assemblies of small clusters. We show that the self-assembly of H into large islands is, in fact, a tunneling effect resulting from inter-atom energy being much smaller than the diffusion barrier. We further demonstrate that these latter effects are not at play for D. Density Functional Theory (DFT) calculations provide estimates for both diffusion and interaction energies. Theory also provides quantum tunneling probabilities that agree well with experiment.[2] References: [1] G. Kyriakou, M.B. Boucher, A.D.Jewell, E.A. Lewis, T.J. Lawton, A.E. Baber, H.L. Tierney, M. Flytzani-Stephanopoulos, and E.C.H. Sykes, Science 335, 1209 (2012). [2] A.D. Jewell, G. Peng, G. Kyriakou, M. Mavrikakis, E.C.H. Sykes, in preparation.