No abstract

A theoretical investigation of transition-metal interaction with a silica surface is reported herein. The study employs periodic density functional theory at the full-potential linearized augmented plane wave (FP-LAPW) level with spin polarization taken into account. Initial low coverages of Co and Ni metal are examined on a siloxane surface of a 2-dimensional periodic slab model with hexagonal unit cell of composition O 3(top) Si 2 -(OH) 2 . The geometry of the top oxygen layer is optimized before and after the metal adsorption along with the position of the metal atom. The preferred adsorption site is found to be a 3-fold hollow relaxed structural feature formed by the top layer O atoms without Si along inverted surface normal in the second layer. The calculated adsorption energies for Co are slightly larger than for Ni on all sites, while the differences among sorption sites are quite marked for both metals. The patterns of total energy are replicated by the stabilization of the occupied metal 4s orbital in forming a surface bond with the primary participation of the Si3s (and to smaller extent surface O2sp) empty antibonding orbital of the silica. The third layer of O atoms remains unaffected. The calculated energy band structure and densities of states yield useful insight into the detailed bonding, show a significant dispersion of the stabilized metal 4s orbital with average energy below the Fermi level, and symmetry splitting of the flat 3d band in the trigonal site. Partial occupancy of the 3d levels provides contributions to the adsorption energy which are much smaller than those due to the metal 4s-Si 3s/O 2sp interaction.

The structure of perchlorotriphenylamine has been determined by X-ray analysis. Because this molecular propeller occupies a crystallographic site of C2 symmetry, the nitrogen atom is located in the plane of the neighboring carbon atoms. Partial resolution was achieved by chromatography on microcrystalline cellulose triacetate. Perchlorotriphenylamine thus becomes the first example of an optically active compound of the type 3 or A^ZX in which Z is not a chiral center. The compound is optically stable under ordinary conditions, and elevated temperatures are required for racemization. The activation energy (£a) for this process, 28.0 kcal/mol, slightly exceeds an empirical force field estimate of 25-27 kcal/mol calculated for the racemization barrier in the sterically similar trimesilylamine.

The electronic and geometric structure of α-Al2O3 (0001) surface with and without adsorbed Co and Ni atoms has been investigated using the full-potential linearized augmented plane-wave density-functional theory method. It has been found that the truncated α-Al2O3 (0001) surface undergoes a large surface reconstruction from its bulk structure, which is further changed upon the metal atom adsorption. Geometries, energies, and electronic properties of the partially optimized and the truncated undistorted α-Al2O3 slabs are compared. Electronic “surface state levels” due to the unsatisfied bonding of the Al atoms at both ends of the slab are identified. Among several geometries, the 3-fold oxygen site has been found to be the only stable adsorption site for both Co and Ni atoms. Several factors determine the metal−support interaction between the Co (or Ni) atom and the α-Al2O3 substrate. Among these factors, the “screened ligand field” effects of partially occupied 3d electrons and the further relaxation of the α-Al2O3 substrate are shown to have the largest contributions to the adsorption energy.

Nucleation, clustering, and multilayer growth of metals on oxide surfaces are processes which influence the stability and function of many catalysts. In this work, we have examined initial stages of these processes by means of modeling involving calculations of energies and optimum structures of Co and Ni on a silica surface. Surface concentrations of Co and Ni with metal-to-oxygen ratios ranging from 1:3 to 5:3 (one-third of a monolayer to multilayer) were investigated. The positions of the metal, surface oxygen, and subsurface silicon atoms were optimized, and energies of the metal-oxide and metal−metal interactions in each optimization step as well as in stable geometries were calculated on a two-dimensional periodic slab model with hexagonal unit cell of composition Co n (Ni n )O3(top)Si2(OH)2. The methodology employed the periodic density functional theory (DFT) at the full-potential linearized augmented plane wave (FP-LAPW) level with spin-polarization taken into account. At the lower coverage with n = 1, the Co and Ni metals were bonded to various adsorption sites with energies ranging between 1.0 and 2.0 eV. The 3-fold oxygen hollow sites of the siloxane 6-ring were found to be energetically most favored for the adsorption of either metal, more so than 3-fold sites with Si underneath. For n = 2−3, several nearly equally stable configurations were identified. A further increase of metal coverage resulted in metal clustering due to the stronger metal−metal interaction that ranges around 4.0 eV per metal atom. With n > 3, i.e., metal:oxygen ratios exceeding 1:1, the Co and Ni metals formed layered structures with strong metal−metal bonds and relatively weak metal-silica surface bonds.