Interaction between Catalyst and Support. 3. Metal Agglomeration on the Silica Surface

Abstract

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.

Theoretical identification of structural heterogeneities of divalent nickel active sites in NiMCM-41 nanoporous catalysts

“…Many references [42,43,[57][58][59][60] have then applied this model to ordered silica mesoporous materials such as MCM-41. The optimizations and single-point computations were implemented using hybrid functional M06 [61] and the Def2-TZVP basis set [62][63][64].…”

“…binding in FER [37], MFI [38,39], AFI [40], and silica [4,41] have been reported earlier. Analogous reports have addressed neutral atoms [42,43]. The present study aims at a systematic theoretical modeling of the locations of Ni 2?…”

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.…”

Synthesis and Study of Plasmon‐Induced Carrier Behavior at Ag/TiO2 Nanowires

“…Metal/semiconductor oxide composite systems, an efficient bifunctional catalyst, are extremely attractive owing to their unusual optoelectronic properties and photocatalytic activity 1–5. Of the various oxide semiconductor photocatalysts, TiO 2 stands out for its high incident photon‐to‐electron conversion efficiency and remarkable chemical stability.…”