Theoretical studies of chemisorption

Citation

Walch, Stephen Perry (1977) Theoretical studies of chemisorption. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/TDQT-V666. https://resolver.caltech.edu/CaltechTHESIS:11092009-084139276

Abstract

PART A: GVB and GVB-Cl wavefunctions (using a double zeta basis) have been obtained as a function of internuclear distance for the lowest three states of NiCO. The wavefunctions lead to a qualitative description in which the Ni atom is neutral with a (4s)^1(3d)^9 atomic configuration. The CO lone pair delocalizes slightly onto the Ni, leading to the 4s-like orbital hybridizing away from the CO. The dir pairs on the Ni are slightly back-bonding to the CO. The three bound states are ^3Σ^+, ^3∏, and ^3Δ consisting of the singly occupied 4s-like orbital plus a single d hole in a σ, π, or δ orbital, respectively. The ground state is found to be ^3Δ with calculated R_e = 1.90 Å, D_e = 1.15 eV = 26.5 kcal/mol, and ω_c(Ni-C) = 428 cm^(-1), all reasonable values, although direct information on NiCO is not yet available. The adiabatic excitation energies are calculated as 0.240 eV to ^3Σ^+, and 0.293 eV to ^3∏. The states with (4s)^2(3d)^8 configurations on the Ni lead to repulsive potential curves with vertical excitation energies in the range of 3.0 to 5.0 eV. PART B: Configuration interaction calculations have been carried out for a number of positive ion states of NiCO. These calculations indicate that there are two distinct groups of ionization potentials. The first group involves ionizations out of Ni-like orbitals. The lowest states of this group involve ionization out of a Ni 4s-like orbital leading to a 3d^9 configuration and states of symmetry ^2Σ^+, ^2∏, and ^2Δ depending on whether the 3d-hole is taken in a σ , π) or δ orbital. At the optimum geometry of NiCO, the dissociation energy of NiCO^+ to Ni^+(^2D) and CO is calculated to be 2.26, 2.03 and 2.50 eV for the ^2Σ^+, ^2∏, and ^2Δ states, respectively, in reasonable agreement with the value of 2.10 eV calculated from the experimental heat of formation of NiCO^+. Other states in the first group involve ionization out of Ni 3d orbitals leading to a group of ion states with a width of 3.1 eV. This is in good agreement with the Ni d bandwidth as observed in photoemission experiments. The second group of ion states correlates at large Ni-C separation with the ground state of the Ni atom and various states of CO^+. The principal change as compared with free CO is that the 5σ ionization (lone pair on the CO) increases in energy by about 2.5 eV, whereas the 4σ and 1π ionizations change only slightly. This leads to the 5σ and 1π ionizations being nearly degenerate, with the 4σ ionization about 3.0 eV higher, in agreement with the currently accepted interpretation of the photoelectron spectrum of CO chemisorbed on Ni. PART C: Geometries for 0 and S overlayers on the (100) and (110) surfaces of Ni have been calculated using ab initio wavefunctions for 0 and S bonded to small clusters of Ni atoms (1 to 5 Ni atoms). The calculated geometries are within 0.07Å of the results of dynamic LEED intensity calculations, indicating that accurate geometries of chemisorbed atoms may be obtained from calculations using clusters including only those metal atoms within bonding distance. PART D: Electronic wavefunctions have been obtained as a function of geometry fora S atom bonded to Ni clusters consisting of l to 4 atoms de-signed to model bonding to the Ni(100) and Ni(110) surfaces. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. Modeling the (100) surface with four Ni atoms, we find the optimum S position to be 1.33Å above the surface, in good agreement with the value (1.30 ± 0.10Å) from dynamic LEED intensity calculations. The bonding is qualitatively like that in H_2S with two covalent bonds to one diagonal pair of Ni atoms. There is a S pπ pair overlapping the other diagonal pair of Ni atoms. [Deleting this pair the S moves in to a position 1.04Å from the surface.] There are two equivalent such structures, the resonance leading to equivalent S atoms and a c(2x2) structure for the S overlayer. The Ni in the layer beneath the surface seems to have little effect (~0.03Å) on the calculated geometry. The above model of the bonding suggests that for the (110) surface the S lies along the long edge of the rectangular unit cell (2 coordinate) rather than at the four coordinate site usually assumed. Our calculated position for the S of 1.04Å is in reasonable agreement with the value from dynamic LEED intensity calculations, 0.93 ±0.10Å. Bonding the S directly above a single Ni atom leads to a much weaker bond (D_e = 3.32 eV) than does bonding in a bridge position (D_e = 5.37 eV). PART E: Electronic wavefunctions have been obtained as a function of geometry for an 0 atom bonded to Ni clusters (consisting of 1 to 5 atoms) designed to model bonding to the Ni(100) and Ni(110) surfaces. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. For the (100) surface, we find that the charge distribution for the full 0 overlayer is consistent with taking a positively charged cluster. The four surface atoms in the surface unit cell and the atom beneath the surface are important in determining the geometry, leading to a Ni^+_50 cluster as the model for the (100) surface. The optimum oxygen position with this model is 0.96Å above the surface (four-fold coordinate site) in good agreement with the value (0.90± 0.10Å) from dynamic LEED intensity analysis. The atom beneath the surface allows important polarization effects for the positively charged cluster. The bonding to the surface involves bridging two diagonal surface Ni atoms. There is an 0(2pπ)pair which overlaps the other diagonal pair of Ni atoms leading to nonbonded repulsions which increase the distance above the surface. There are two equivalent such structures, the resonance leading to a c(2 x 2) structure for the 0 over-layer. The above model suggests that for the (110) surface the 0 lies along the long edge of the rectangular unit cell. For this registry with the surface, calculations based on Ni_20 and Ni_30 models indicate that the oxygen is only 0.1Å above the plane of the surface. PART F: Generalized valence bond and configuration interaction wave-functions have been obtained as a function of R for numerous electronic states of NiO. All the lower states are found to involve the (4s)^1(3d)^9 Ni atom configuration and 0 in the (2s)^2(2p)^4 configuration. There are two groups of states. The lower group of states involves pairing singly occupied Mi(4s).and 0(2p σ) orbitals into a (somewhat ionic) sigma bond pair with various pairings of the Ni(3d)^9 and 0(2pπ)^3 configurations. This leads to a number of states including the ground state which we find to be x^3Σ^-. (The electronic structure is analogous to that of O_2.) The calculated D_o and R_e for the x^3Σ^- state of Ni0 are 89.9 Kcal/mole and 1.60 Å respectively. The bond energy is in good agreement with the experimental value 86.5 ±5 Kcal/mole, while the R_e value is not known experimentally. The higher group of states involve a doubly occupied 0(2p σ) orbital., The Ni(4s) orbital in this case is non-bonding and builds in 4p character to move away from the oxygen orbitals. The bonding mainly involves stabilization of the oxygen orbitals by the Ni(3d)^9 core (somewhat analogously to the bonding in NiC0). Numerous allowed transitions between these states and the states of the lower group are calculated to be in the range 1.0 to 3.0 eV where numerous bands are seen in emission.

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