Structural and Electronic Properties of Pt₁₃ Nanoclusters on Amorphous Silica Supports

Abstract

An accurate description of metal nanoparticle (NP)−support interactions is required for designing and optimizing NP catalytic systems because NP−support interactions may significantly impact NP stability and properties, such as catalytic activity. The ability to calculate NP interactions with amorphous supports, which are commonly used in industrial practice, is hampered because of a general lack of accurate atomically detailed model structures of amorphous surfaces. We have systematically studied relaxation processes of Pt 13 NPs on amorphous silica using recently developed realistic model amorphous silica surfaces. We have modeled the NP relaxation process in multiple steps: hard-sphere interactions were first used to generate initial placement of NPs on amorphous surfaces, then Pt−silica bonds were allowed to form, and finally both the NP and substrate were relaxed with density functional theory calculations. We find that the amorphous silica surface significantly impacts the morphology and electronic structure of the Pt clusters. Both NP energetics and charge transfer from NP to the support depend linearly on the number of Pt−silica bonds. Moreover, we find that the number of Pt−silica bonds is determined by the silica silanol number, which is a function of the silica pretreatment temperature. We predict that catalyst stability and electronic charge can be tuned via the pretreatment temperature of the support materials. The extent of support effects suggests that experiments aiming to measure the intrinsic catalytic properties of very small NPs on amorphous supports will fail because the measurable catalytic properties will depend critically on metal−support interactions. The magnitude of support effects highlights the need for explicitly including amorphous supports in atomistic studies.

Computational Design of Clusters for Catalysis

“…This effect was correlated to the oxidation state of the metal. Similar studies were performed for Ag clusters catalyzing the epoxidation of propylene (110). In this case, however, it was found that whereas Ag nanoclusters are highly efficient catalysts when supported on alumina, they become inactive when deposited on UNCD.…”

“…The structural transition from two-dimensional (2D) to three-dimensional (3D) clusters also leads to completely different activities and selectivities, although there is no general or simple way to predict how catalytic properties will be affected. For Pt n /rutile TiO 2 (110), scanning tunneling microscopy showed a structural transition from 2D to 3D at n = 8, and this transition goes together with a decrease in the activation energy for CO oxidation (52). Similarly, 2D and 3D Pt 4,6,8 clusters supported on anatase TiO 2 (101) were compared using DFT (53,54).…”

Controlling Ethylene Hydrogenation Reactivity on Pt13 Clusters by Varying the Stoichiometry of the Amorphous Silica Support

“…In particular, early experiments demonstrated three types of a‐SiO 2 films that can be grown under UHV conditions on a metal single crystal: silicon‐rich, stoichiometric, and oxygen‐rich; initial investigations showed no differences in their interaction with adsorbed silver atoms . The silicon‐rich a‐SiO 2 film represents a support which has electron‐donating properties; that is, the surface is expected to donate electronic charge to the deposited metal clusters . The platinum clusters on the stoichiometric film are expected to remain charge‐neutral overall, whereas the oxygen‐rich surface is expected to deplete electronic charge from the deposited Pt clusters (that is, partial positive charging), as is the case for platinum adsorbed on oxide surfaces …”

Effect of Support Preparation and Nanoparticle Size on Catalyst–Support Interactions between Pt and Amorphous Silica

“…36 Our earlier studies show that, while these structures are only local minima, results obtained for cuboctahedral structures and gas-phase global minima structures are in qualitative agreement. 22 We divided each of the silica surfaces for Pt 13 , Pt 55 , and Pt 147 into grids of 20 (5 × 4), 9 (3 × 3), and 6 (3 × 2) adsorption locations, respectively. Taking into account the 4 different sets of initial conditions (two NP orientations and two silica surface structures), we generated 80, 36, and 24 systems for DFT calculations of Pt 13 , Pt 55 , and Pt 147 , respectively.…”