Solving the Structure of Size-Selected Pt Nanocatalysts Synthesized by Inverse Micelle Encapsulation

Abstract

The structure, size, and shape of gamma-Al(2)O(3)-supported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for subnanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H(2) atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods.

Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects

“…The encapsulating ligands are eliminated by annealing in O 2 at 648 K for 24 hours. Different NP sizes can be obtained by changing the molecular weight of the head (P2VP) of the encapsulating polymer, the metal/P2VP ratio (micelle loading), and the post-preparation annealing treatment and atmosphere 38 . Our micellar synthesis normally leads to 3D-like NP structures.…”

“…Further details on the sample preparation method and synthesis parameters can be found in Refs. 28,38,[41][42][43][44] and Table I. Table I.…”

“…50 Quantitative determination of the NP shape was carried out by analyzing low temperature EXAFS data up to the 4 th nearest neighbor contribution, including multiple scattering (MS), as described in Refs. 38,43 and references therein. The data were acquired in H 2 after NP reduction.…”

“…Examples of the NP shapes typical of similarly prepared but larger micellar Pt NPs resolved by STM can be found in Refs. 38,43,51 Table I also contains information on the total number of atoms within each NP (N t ), the ratio of the number of Pt atoms at the NP surface and perimeter to the total number of atoms (N s /N t ), and the ratio of the number of Pt atoms in contact with the support to the total number of atoms within a NP (N c /N t ) extracted from the selected model NP shapes. The distinction between 2D-and 3D-like NPs obtained from the former analysis is in agreement with the general trends observed here for the EXAFS static disorders, with the highest values corresponding to the 2D NPs (S1 and S4, 0.0019 and 0.0028 Å 2 , respectively), i.e.…”

“…It is well known that H lifts the contraction that the bonds of Pt NPs undergo because of low coordination either partially 6,28,37,38 or almost totally. 39 On the other hand, the effective hydrogen coverage on the NP surface might vary in the course of an experimental thermal cycle.…”