Theoretical studies of chemisorption processes on nickel surfaces

Citation

Upton, Thomas Hallworth (1980) Theoretical studies of chemisorption processes on nickel surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bvbb-m935. https://resolver.caltech.edu/CaltechTHESIS:11192009-140133915

Abstract

Part I: Generalized Valence Bond and Configuration Interaction calculations are reported for the zero valent nickel complexesNiC_2H_2, NiC_2H_4, Ni_2C_2H_4, and Ni_2C_2H_2. It is found that the NiC_2H_2 and NiC_2H_4 a coordination complex is formed in which the ligand π orbital delocalizing into an empty Ni 4sp orbital. The 4s^13d^9 configuration of the Ni atom is stabilized. Bond energies of 16.7 and 14.2 kcal are found for the two complexes, respectively. In both complexes, the ligand is very weakly distorted, a result that is supported by complementary experimental data characterizing the IR and UV-visible spectral properties of NiC_2H_4 and thNi_2C_2H_4 π-complex. Acetylene is coordinated to Ni_2 in both di-σ and di-π bonded form, for which bond energies of 23 and 60 kcal (relative to C_2H_2 + 2Ni) are found, respectively. Configuration interaction calculations are also reported characterizing all ligand valence ionization levels for theNiC_2H_2, NiC_2H_4 and Ni_2C_2H_2 complexes. Excellent agreement is found between the UPS results for chemisorbed ethylene and the NiC_2H_4 calculated spectrum. "Bonding shifts" are found to result from a differential screening effect. Part II: Extensive generalized valence bond (GVB) and configuration interaction calculations (POL-CI) have been carried out for the lowest states of Ni_2 and Ni_2^+ for bond lengths from 1.6 to 4.0 Å. The six lowest states of Ni_2 are found to be essentially degenerate with an average equilibrium bond length r_e = 2.04 Å and D_e = 2.92 eV. A ^4Σ^+_g ground state is found for the ion with a bond length R_e = 1.96 Å and dissociation energy D_e = 4.14 eV. The bonding of Ni is dominated by the interactions of the 4s orbital on each Ni with each Ni of Ni_2 corresponding to a (4s)^1(3d)^9 configuration. The lowest states lead to singly occupied σ orbitals on each center with other 3d occupations leading to 100 electronic states within about 1.0 eV of the ground state. Hartree-Fock calculations are also reported, characterizing the low-lying states of an Ni_8 cluster. It is found that the 4s^13d^9 valence configuration of the Ni atom is strongly stabilized, and that here too, the 3d orbitals remain localized and are of secondary importance in the bonding. As a result of these findings, further first principles calculations have been carried out characterizing the "conduction band" properties of high and low symmetry clusters up to Ni_(87). Macroscopic properties [ionization potential (IP), electron affinity (EA), bandwidth, and cohesive energies are not sensitive to cluster geometry, and except for EA, show definite convergence towards the bulk limit by Ni_(87) . Even for Ni_(87), the EA is over 2.5 eV smaller than the IP, and the origin of this effect is discussed. Part III: First Principles Hartree-Fock and Generalized Valence Bond Calculations are reported for the bonding of atomic H, Cl, Na, 0, and S on high symmetry sites of the Ni(100) face using an Ni_(20) cluster as a model. All of the adsorbates are found to prefer the fourfold site, with bond energies (D_e) of 3.04, 4.9, 1.3, 3.63, and 4.34 eV, respectively. Bond distances are 0.78, 1.38, 2.7, 0.88, and 1.24 Å above the surface, which are (except for Na) in excellent agreement with available experimental data. Vibrational frequencies of 73, 17, 30, 46, and 37 meV are found for each adsorbate, respectively. Decreasing site coordination is found to uniformly increase vibrational frequencies, and bond distances while decreasing bond energies. The data are analyzed through the introduction of the concepts of site acidity and basicity, and it is found that site basicity increases with increasing coordination. This trend is responsiale for the observed preference of each adsorbate for high coordinate sites, and it is expected that donor adsorbates (such as CO) would show a reverse trend. The data for Na are found to be in poor agreement with the analogous bulk data, confirming the prediction from Part II.0 that small particles should behave differently from the bulk when bonded to highly electropositive species. Part IV: A technique is presented for carrying out ab initio Hartree-Fock calculations on systems of infinite three-dimensional periodicity. The method represents an adaptation of standard molecular basis set expansion techniques and fully utilizes translational and point group symmetry to simplify the calculations. It is shown that the expression for total energy may be written as a sum of pairwise interactions between neutral charge units consisting of a nucleus and a localized compensating electronic charge. The resulting sums are rapidly convergent. The technique is illustrated with sample calculations on face-centered cubic lattices of hydrogen, lithium, and sodium. Generalization to systems of lower symmetry is discussed.

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