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metal homo-epitaxy on fcc (001) surfaces: Is there transient mobility of adsorbed atoms?
A485 The structure and electronic properties of MoC have been characterized using ASED bulk and slab band and cluster MO calculations. Singly bonded adsorbates, H and CH3, are found to cause minor Mo and large C relaxations, the latter a result of breaking a relatively weak subsurface C - M o bond to form stronger C - H or C - C H 3 bonds. Doubly bonded adsorbates, CO and CH 2, on the other hand, cause large relaxations of Mo upward and this frees up d-orbitals which are involved in or-bonding with the adsorbate. When ~r complexed, C2H 4 causes large Mo relaxations but when di-o-bonded to 2 Mo the relaxations are small. The adsorption energies parallel those found in our recent study of these species on MoS2, except for the weaker C2H 4 adsorption energy calculated for the carbide, which is consistent with the greater ability of molybdenum carbide catalysts to generate C2H 4.
Surface Science 254 (1991) 329-340 North-Holland
329
Theoretical study of physisorption and chemisorption of hydrogen on Ag(111) from LSD calculations C.
Mijoule and V. Russier
Laboratoire de Dynamique des Interactions Mol~culaires, Universitd P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 7 June 1990~ accepted for publication 5 March 1991 In this paper we present some calculations which describe the process of physisorption and dissociation of molecular hydrogen on a A g ( l l l ) surface. The stability of chemisorbed atomic hydrogen with respect to physisorbed molecular hydrogen is discussed. We use a cluster type approach in the framework of the local approximation of the density functional theory. Size effects are studied by considering Ags, AgT, Ag]0 and Ag12 dusters. It is shown that localized electronic states contamine the physisorption region. On the other hand the physisorption energy is roughly independent of the size of the system. This is not the case for the chemisorption where it is well known that binding energies vary significantly with the size of the cluster. In order to compare the relative energies of both chemisorbed and physisorbed states, two kinds of treatments are needed. While the binding energy may be obtained directly when the adsorbed molecule is above the jellium edge, the electronic delocaliTation effects have to betaken into account when the hydrogen atoms are embedded in the electronic sea of the surface. In this way, it is shown that the dissociation process is not allowed for molecular H 2 on a A g ( l l l ) surface.
Surface Science 254 (1991) 341-353 North-Holland
341
Metal/metal homo-epitaxy on fcc (001) surfaces: Is there transient mobility of adsorbed atoms? David E. Sanders
*
and Andrew E. DePristo
Department of Chemistry, Iowa State University, Ames, 1,4 50011, USA Received 26 December 1990; accepted for publication 19 February 1991 We have investigated the dynamics of adsorption of a metal atom on a fcc (001) metal substrate of the same atom type. Two points were considered in detail: (1) the extent of ballistic or transient mobility (i.e., ability of the adatom to transform kinetic energy gained by adsorption into motion parallel to the subgtrate); (2) convergence of results with the number of moving atoms retained in the simulation. Using the molecular dynamics technique with four different types of potential energy surfaces (PES), we have discovered there is no transient mobility for the Cu/Cu(001) system at a surface temperature of 80 K and an incident gas atom Kinetic energy of 0.25 eV. We have also discovered an important pitfall in such simulations: use of an insufficient number of active substrate layers leads to incorrect results displaying significant ballistic motion of the adsorbate. Using our most accurate many-body density functional based MD/MC-CEM PES and retaining sufficient number of active layers, we have also found an absence of transient mobility for the Ni/Ni(001), Rh/Rh(001), Pd/Pd(001), Pt/Pt(001), and Au/Au(001) systems. Only the Ag/Ag(001) system, with a very small diffusion barrier, was found to display any mobility.
Surface Science 254 (1991) 329-340 North-Holland
329
Theoretical study of physisorption and chemisorption of hydrogen on Ag(111) from LSD calculations C.
Mijoule and V. Russier
Laboratoire de Dynamique des Interactions Mol~culaires, Universitd P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 7 June 1990~ accepted for publication 5 March 1991 In this paper we present some calculations which describe the process of physisorption and dissociation of molecular hydrogen on a A g ( l l l ) surface. The stability of chemisorbed atomic hydrogen with respect to physisorbed molecular hydrogen is discussed. We use a cluster type approach in the framework of the local approximation of the density functional theory. Size effects are studied by considering Ags, AgT, Ag]0 and Ag12 dusters. It is shown that localized electronic states contamine the physisorption region. On the other hand the physisorption energy is roughly independent of the size of the system. This is not the case for the chemisorption where it is well known that binding energies vary significantly with the size of the cluster. In order to compare the relative energies of both chemisorbed and physisorbed states, two kinds of treatments are needed. While the binding energy may be obtained directly when the adsorbed molecule is above the jellium edge, the electronic delocaliTation effects have to betaken into account when the hydrogen atoms are embedded in the electronic sea of the surface. In this way, it is shown that the dissociation process is not allowed for molecular H 2 on a A g ( l l l ) surface.
Surface Science 254 (1991) 341-353 North-Holland
341
Metal/metal homo-epitaxy on fcc (001) surfaces: Is there transient mobility of adsorbed atoms? David E. Sanders
*
and Andrew E. DePristo
Department of Chemistry, Iowa State University, Ames, 1,4 50011, USA Received 26 December 1990; accepted for publication 19 February 1991 We have investigated the dynamics of adsorption of a metal atom on a fcc (001) metal substrate of the same atom type. Two points were considered in detail: (1) the extent of ballistic or transient mobility (i.e., ability of the adatom to transform kinetic energy gained by adsorption into motion parallel to the subgtrate); (2) convergence of results with the number of moving atoms retained in the simulation. Using the molecular dynamics technique with four different types of potential energy surfaces (PES), we have discovered there is no transient mobility for the Cu/Cu(001) system at a surface temperature of 80 K and an incident gas atom Kinetic energy of 0.25 eV. We have also discovered an important pitfall in such simulations: use of an insufficient number of active substrate layers leads to incorrect results displaying significant ballistic motion of the adsorbate. Using our most accurate many-body density functional based MD/MC-CEM PES and retaining sufficient number of active layers, we have also found an absence of transient mobility for the Ni/Ni(001), Rh/Rh(001), Pd/Pd(001), Pt/Pt(001), and Au/Au(001) systems. Only the Ag/Ag(001) system, with a very small diffusion barrier, was found to display any mobility.