S. F. Ruzankin

Nest defect as the site of stabilization of transition element ions implanted in high-silica zeolites. Cluster calculation of Fe(II) and Fe(III) ions entrapped by the zeolite matrix

Cluster calculations of the structure of the nest defect in zeolites were carried out by the density functional theory (DFT) method. The hydroxyl groups of the defect form an ordered structure stabilized by hydrogen bonds. Immobilization of Fe(II) and Fe(III) ions by the nest defect from iron chlorides is discussed for the two limiting cases of “rigid” and “labile” lattices, and the resulting structures are compared. The implanted Fe(II) and Fe(III) ions are considered as the precursors of the selective oxidation centers formed by N2O decomposition; the difference between the stabilization energies of oxygen on these centers is evaluated.

Theoretical analysis of thermally stable adsorption forms of oxygen on silver

Theoretical analysis of the properties of oxygen in the surface layer of silver is performed in a cluster approximation. The oxygen molecule is placed in an octahedral cavity of the Ag14 cubic cluster. The O2/Ag14 system is optimized and structural parameters determined by the ab initio Hartree- Fock method. The energies are estimated by methods taking into account electron correlation in the Möller- Plesset and density functional theory approximations. For the three nonequivalent positions of O2 in the Ag14 cluster,it is shown that the molecular forms of oxygen may be stabilized. The calculations showed that the ground state of the O2/Ag14 system is a triplet but the spin density is delocalized over the silver atoms. Spin polarization leads to splitting of the 3Σg- level. The oxygen molecule has intermediate parameters between the superoxide O21− and peroxide O22− with the bond length R(O- O) 1.41 å.

Oxygen diffusion through the Ag(111) surface: A quantum chemical study by the NDDO/MC method

Atomic forms of oxygen on the (111) face of metallic silver are studied by the NDDO/MC semiempirical method. The surface (above the octahedral void) and subsurface (within the void) positions of oxygen between the first and second layers of the (111) face are investigated. The potential surface cross section is calculated for the subsurface position of oxygen. A new approach is used to take into account surface relaxation due to reaction with adsorbate. The barrier of atomic oxygen diffusion through the surface is much lower than the banier of its desorption from the surface. The correlation correction to the diffusion and desoption barrier energies is estimated by the double CI (DCIP) method. The greatest correlation effects are obtained in the desorption banier calculation.

On the molecular models of lewis acid sites on the surface of γ-Al2O3 and in zeolites: a density functional study of CO adsorption

Adsorption complexes of CO-Lewis acid sites with 3- and 5-coordinated Al3+ are modeled by the density functional method using the cluster approach. Cluster models of the site with 4-coordinated Al3+ on the surface of γ-Al2O3 and in zeolites are suggested. For these models of adsorption complexes, C-O vibration frequencies are calculated and the energetics of CO adsorption is evaluated.

Coordination behavior of the core and valence levels of low-coordinated oxygen ions on the surface of magnesium oxide

For low-coordinated (N=2,3,4,5) oxygen ions on the surface and in the bulk of magnesium oxide, the behavior of the core O1s- and valence O2s-, O2p-levels is considered. The MgO610−, OMg610+, Mg13O142−, O13Mg142+, Mg4O76−, and OMg22+ clusters are calculated by the SCF-Xα-SW method. The binding energies of oxygen 1s-, 2s-, and 2p-states decrease with coordination. This coordination dependence is explained by the greater change of the Madelung potential as compared to variations of the purely electronic terms of the binding energies of oxygen ions. The dependence of oxygen atomic charges and relaxation energies on coordination is also discussed.

Modeling the photoelectron spectra of the valence O2p-band of zinc oxide by the Xα-scattered wave method

The photoelectron spectra (PES) of the valence O2p-band of zinc oxide are modeled by Xα-scattered wave cluster calculations in a wide range of incident quantum energies hv (from 30 to 150 eV and 1253.6 eV). For the Zn10O10 cluster, the calculated intensities of PES reproduce well the specific features of the experimental spectra. It is shown that Zn3d-electrons participate in covalent binding of zinc and oxygen. The admixture of the Zn3d-states in the hybrid orbitals of the valence band is ≈7%.