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 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.

Methyltrioxorhenium Interactions with Lewis Acid Sites of an Amorphous Silica−Alumina

“…39 Periodic structures for crystalline silica have been used to represent amorphous silica in calculations. 46 However, we and others 47 have observed experimentally that CH 3 ReO 3 does not adsorb strongly onto silica, nor does the resulting physisorbed material catalyze olefin metathesis. Crystalline aluminas are also inappropriate models because they lack the strong acidity of silica-alumina, which appears to be necessary for high catalytic activity in olefin metathesis.…”

“…Crystalline aluminosilicate frameworks, such as the zeolites, are not appropriate models, because their structures contain charge-compensating alkali cations and/or strongly acidic, bridging hydroxyl sites that are absent in amorphous silica−aluminas . Periodic structures for crystalline silica have been used to represent amorphous silica in calculations . However, we and others have observed experimentally that CH 3 ReO 3 does not adsorb strongly onto silica, nor does the resulting physisorbed material catalyze olefin metathesis.…”

Modeling of Syngas Reactions and Hydrogen Generation Over Sulfides

“…All other properties obtained with this method are of course also in doubt. In contrast, our experience with methods A-C above [9][10][11][12][13][14] permits the conclusion that results based on these methodologies are valid and reliable, albeit always amenable to refinement.…”

Interaction between Catalyst and Support. 4. Periodic Trends and Patterns in Interactions of First-Row Transition Metals with the Silica Surface

“…The internal atomic coordinates were optimized using a damped Newton scheme under the criterion that requires all the forces acting upon the atoms be less than 10 mRy/au. 10 The all-electron DFT calculations were performed using the WIEN97.9 code with the full-potential linearized augmented plane wave (FP-LAPW) method. 13 The Kohn-Sham (KS) equations 14 are solved self-consistently in an iterative process under the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional for the exchangecorrelation energy, 15,16 subject to periodic lattice boundary conditions.…”