Interaction between Catalyst and Support. 1. Low Coverage of Co and Ni at the Silica Surface

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 O3(top)Si2(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.