S. P. Mehandru

Dopant Effect of Yttrium and the Growth and Adherence of Alumina on Nickel‐Aluminum Alloys

The atom superposition and electron delocalization molecular orbital theory and large cluster models have been employed to study cation vacancy diffusion in alpha-Al2O3 and the bonding of alpha-Al2O3 to nickel, aluminum, and yttrium surfaces. Al(3+) diffusion barriers in alpha-Al2O3 by the vacancy mechanism are in reasonable agreement with experiment. The barrier to Y(3+) diffusion is predicted to be much higher. Since addition of yttrium to transition metal alloys is known to reduce the growth rate and stress convolutions in protective alumina scales, this result suggests the rate-limiting step in scale growth is cation vacancy diffusion. This may partially explain the beneficial effect of yttrium dopants on scale adhesion. The theory also predicts a very strong bonding between alumina and yttrium at the surface of the alloy. This may also be important to the adhesion phenomenon. It is also found that aluminum and yttrium atoms bond very strongly to nickel because of charge transfer from their higher lying valence orbitals to the lower lying nickel s-d band.

Adsorption of H, CH3, CH2 and C2H2 on 2 × 1 restructured diamond (100): Theoretical study of structures, bonding, and migration

Abstract Surface properties of clean and hydrogenated diamond (100) have been calculated using the atom superposition and electron delocalization molecular orbital (ASED-MO) and ASED-band methods. For the clean surface, dimerization and 2 × 1 restructuring are predicted. The monohydrogenated surface maintains the 2 × 1 structure but with elongated surface CC dimer bonds. The dihydrogenated surface takes a 1 × 1 structure but, because of steric crowding, is not as stable. These findings support the interpretations of recently obtained experimental results of Hamza et al. They are also analogous to the well established properties of clean and hydrogenated Si(100). CH3 and CH2 migration energy barriers in the presence of H vacancies on the monohydrogenated surface are calculated to be high for CH3 but low enough for CH2 that surface migration might occur under low pressure diamond growth conditions. Because of electron promotion to the π∗ orbital, acetylene does not form a strong bond to a surface radical site, but it can bind strongly by bridging two adjacent hydrogen vacancy sites on the monohydrogenated surface. This structure is not likely to be involved in the growth mechanism.

Binding and orientations of CO on Fe(110), (100), and (111): A surface structure effect from molecular orbital theory

Abstract Calculations based on the atom superposition and electron delocalization molecular orbital method show that at low coverage CO binds on the high-coordinate site in the perpendicular orientation on Fe(110), on the high-coordinate site in the lying-down orientation on Fe(100), and on the di-σ bridging site in the lying-down orientation on Fe(111). These findings confirm the earlier suggestions regarding CO binding site and orientation on the (110) and (100) surfaces. However, they do not support the previously proposed deep-hollow binding site for CO on the (111) surface. The differences for thee favored CO binding site and orientation on Fe(110), (100), and (111) surfaces are explained on the basis of surface-atom coordinations and atom-atom spacings. In the favored lying-down CO orientations on the (100) and (111) surfaces, 4σ and π donation interactions coupled with the familiar 5σ donation to the surfaces, and back-donation to the CO π ∗ orbitals are responsible for binding to the surfaces.

Hydrogen binding and diffusion in diamond

We have investigated the binding and diffusion pathways for atomic hydrogen in diamond using the semiempirical atom superposition and electron delocalization molecular orbital (ASED-MO) theory. The bond-centered site has been found to be more stable than the tetrahedral and hexagonal interstitial sites due to the formation of a low-lying band-gap orbital which takes the promoted electron. A second hydrogen binds even more stably to the nearby antibonding site with additional stabilization of the now doubly occupied band gap orbital. The bond-centered hydrogen is predicted to migrate along the high-density (110) planes in the diamond lattice with an activation barrier of 1.9 eV. A carbon atom vacancy is found to attract interstitial H which bind to dangling orbitals on the surrounding C atoms. These bond strengths decrease as up to a maximum of four H atoms enters the vacancy. A hydrogen atom in a vacancy is found to increase the activation energy for vacancy migration.

Adsorption and bonding of C 1 H x and C 2 H y on unreconstructed diamond(111). Dependence on coverage and coadsorbed hydrogen

The adsorption and bonding of CH 3 , CH 2 , CH, C 2 H, and C 2 H 2 fragments to clean and hydrogenated diamond(111) surfaces are investigated in the framework of the atom superposition and electron delocalization molecular orbital method. Low coverage calculations are performed using large cluster models for the surfaces, and high coverages are examined with band calculations on thick two-dimensional slabs with every surface carbon covered by a hydrocarbon fragment (i.e., 1:1 surface coverage). For low coverage adsorption on clean and H-covered surfaces the adsorption energies are in the order C 2 H>CH ≃ CH 2 >CH 3 . In each case, the predominant component of bonding is covalent in character and is a result of overlaps between the sp -hybridized singly occupied dangling surface state orbital on the surface carbon and the sp -hybridized orbital on the fragment carbon atom. While the charge transfer contribution to bonding is nearly the same for CH 3 , CH 2 , and CH fragments, it is significantly larger for C 2 H due to a comparatively stable radical orbital on C 2 H. C 2 H 2 binds to the surface on the di-σ site where both its ends form bonds to the surface atoms. Onefold adsorption to a H-covered surface is predicted to be unstable. The 1:1 CH 3 coverage on diamond(111) is highly unstable because of steric repulsions between adsorbate fragments due to their spacial proximity. This finding is supported by a calculation of the cis-trans isomerization energy of di-t-butyl ethylene, including full structure relaxations. At low coverage CH 3 can bind on adjacent surface sites by tilting away from one another. The 1:1 coverage for CH 2 , CH, and C 2 H fragments is predicted to be stable on this surface.

Bonding at the α-Al2O3(001)/Pt(111) interface: Molecular orbital theory

Cluster models of the α-Al 2 O 3 (001) and the Pt(111) surfaces have been used in an atom superposition and electron delocalization molecular orbital study of interfacial bond strengths and the bonding theory of platinum-alumina seals. It is found that Al 3+ centers of the Al 2 O 3 (001) surface have empty dangling surface orbitals into which surface Pt atoms can donate to form strong bonds. O 2− centers of Al 2 O 3 are shown to bond more weakly to Pt because of the nearly closed-shell nature of the interaction. Bonds between O 2− and oxidized surface Pt atoms to give a … Al 3+ O 2− Pt + Pt… interface are calculated to be strong.

CO adsorption on (111) and (100) surfaces of the Pt3Ti alloy: Evidence for parallel binding and strong activation of CO

Abstract An atom superposition and electron delocalization molecular orbital (ASED-MO) study has been made of CO adsorption on a 40-atom cluster model of the (111) surface and a 36-atom cluster model of the (100) surface of the Pt 3 Ti alloy. Parallel binding to high-coordinate sites associated with Ti and low CO bond scission barriers are predicted for both surfaces. The preference for parallel adsorption is a consequence of the nature of the CO π-to-surface donation interactions. On Ti sites the π orbitals donate to the nearly empty Ti 3 d band and the antibonding counterpart orbitals are empty. Thus the π donation makes substantial contributions to the adsorption bond order that are in addition to the contributions from 5σ donation and metal backbonding to the π ∗ orbitals. Altogether these bonding interactions favor the lying down orientation. On Pt sites, on the other hand, the π donation antibonding counterpart orbitals are occupied so that the net interaction with Pt is a closed-shell repulsion. CO bonds upright in order to minimize the π interaction and, concomitantly, the closed-shell repulsion, while maintaining 5σ donation and π ∗ backbonding stabilizations. Comparisons are made with the results for a 40-atom cluster model of the unalloyed Pt(111) surface. It is shown that the extended Huckel parameterization is inappropriate for studying CO adsorption to Pt with the ASED-MO theory because it incorrectly favors adsorption bonding through the oxygen end.

Acetylene adsorption to Fe(100), (110), and (111) surfaces; Structures and reactions

Abstract We have made full structure determinations for acetylene on small and large cluster models of Fe(100), (110), and (111) surfaces using the atom superposition and electron delocalization molecular orbital theory. Four-fold sites are found favored on the (100) and (110) surfaces and the di-σ bridging site is favored on Fe(111). We calculate carbon bond stretches of 0.36, 0.30, and 0.26 A, CH bending angles away from the surfaces of 65°, 60°, and 65°, decreasing adsorption energies, and increasing carbon bond scission energies for Fe(100), (110), and (111), respectively. The bonding in chemisorbed acetylene is compared with the ethylene molecule. Conditions for twisting of the acetylene molecule are discussed, but no twisting is observed at the most stable chemisorption sites on these surfaces. The relative activities of these surfaces toward acetylene are analyzed and a relationship is found between surface atom density and activity, such that the (110) and (111) surfaces are less active than the (100) surface because of their respective higher and lower surface iron atom packing densities. Finally, probable surface coverages and structures are discussed on the basis .of calculated interactions between two adsorbed acetylene molecules and adsorbed acetylene and a CH fragment on the large clusters.

Formate adsorption and azimuthal orientation on Cu(100) from molecular orbital theory

Abstract The most stable binding site and azimuthal orientation for formate species on Cu(100) has been investigated by using the semi-empirical atom superposition and electron delocalization molecular orbital calculations and a large cluster model for the surface. Chemisorption is most stable on the short-bridge site, where the carbon atom lies above the twofold-coordinated bridging site and the two oxygen atoms point towards the on-top sites of neighboring copper atoms, with the molecular plane perpendicular to the surface. These calculations support the most recent photoelectron diffraction investigation results regarding the binding site and orientation of the molecule on this surface.

Binding and orientations of O2 on Ag(100) and Pb/Ag(100)

Molecular orbital calculations using the atom superposition and electron delocalization method predict that O2 favors binding parallel to the Ag(100) surface on the high-coordinate 4-fold site with its axis oriented in such a way that the O-ends point toward the neighboring 2-fold surface sites. In this orientation, the filled O2 5σ and πu Orbitals donate to the Ag sd-hybrid orbitals and the surface back-donates to the half-filled O2 πg orbitals. For the Pb-covered Ag(100) surface, the calculations show that the favored binding site and orientation for O2 depend on the adatom coverage. At low coverages (θpb≈14), O2 is predicted to bind perpendicular to the surface on the top of a Pb atom, whereas at higher coverages (θpb≳12), a lying-down O2 with its axis bridging two nearest-neighbor Pb atoms on the surface is favored. These results for O2 adsorption are related to the electrochemical results for O2 reduction on the bare and UPD Pb-covered silver electrodes.