The Emergence of Nonbulk Properties in Supported Metal Clusters: Negative Thermal Expansion and Atomic Disorder in Pt Nanoclusters Supported on γ-Al₂O₃

Abstract

The structural dynamics-cluster size and adsorbate-dependent thermal behaviors of the metal-metal (M-M) bond distances and interatomic order-of Pt nanoclusters supported on a gamma-Al(2)O(3) are described. Data from scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS) studies reveal that these materials possess a dramatically nonbulklike nature. Under an inert atmosphere small, subnanometer Pt/gamma-Al(2)O(3) clusters exhibit marked relaxations of the M-M bond distances, negative thermal expansion (NTE) with an average linear thermal expansion coefficient alpha = (-2.4 +/- 0.4) x 10(-5) K(-1), large static disorder and dynamical bond (interatomic) disorder that is poorly modeled within the constraints of classical theory. The data further demonstrate a significant temperature-dependence to the electronic structure of the Pt clusters, thereby suggesting the necessity of an active model to describe the cluster/support interactions mediating the cluster's dynamical structure. The quantitative dependences of these nonbulklike behaviors on cluster size (0.9 to 2.9 nm), ambient atmosphere (He, 4% H(2) in He or 20% O(2) in He) and support identity (gamma-Al(2)O(3) or carbon black) are systematically investigated. We show that the nonbulk structural, electronic and dynamical perturbations are most dramatically evidenced for the smallest clusters. The adsorption of hydrogen on the clusters leads to an increase of the Pt-Pt bondlengths (due to a lifting of the surface relaxation) and significant attenuation of the disorder present in the system. Oxidation of these same clusters has the opposite effect, leading to an increase in Pt-Pt bond strain and subsequent enhancement in nonbulklike thermal properties. The structural and electronic properties of Pt nanoclusters supported on carbon black contrast markedly with those of the Pt/gamma-Al(2)O(3) samples in that neither NTE nor comparable levels of atomic disorder are observed. The Pt/C nanoclusters do exhibit, however, both size- and adsorbate-induced trends in bond strain that are similar to those of their Pt/gamma-Al(2)O(3) analogues. Taken together, the data highlight the significant role that electronic effects--specifically charge exchange due to both metal-support and metal-adsorbate interactions--play in mediating the structural dynamics of supported nanoscale metal clusters that are broadly used as heterogeneous catalysts.

Thermal properties of nanoporous gold

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

“…Metal nanoparticles (NPs) have been shown to display anomalous electronic and thermodynamic properties, including metal to non-metal transitions, 1, 2 superheating, 3 and negative thermal expansion [4][5][6][7] . However, despite decades of intense research, the origin of these effects is still heavily debated.…”

“…TEM, X-ray diffraction, differential scanning calorimetry, EXAFS, etc.) for several nanoscale material systems, including supported or embedded Pt 5,6,28,29 , Ag 17 , and Ge NPs 16,30,31 , as well as unsupported nanocrystalline agglomerates such as Ag 32 , Au 32 , ZnS 12 , and thiol-capped CdS 13 and CdTe 14 nanocrystals. A comparison between the different references is challenging due to the distinct sample preparation and characterization conditions, and in some cases, the lack of detail on the specific structure of the systems investigated.…”

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