Tuning the Structure of Pt Nanoparticles through Support Interactions: An in Situ Polarized X-ray Absorption Study Coupled with Atomistic Simulations.

Interactions of nanoparticles (NPs) with their environment may have a pronounced effect on their structure and shape as well as on their functionality in applications such as catalysis. It is therefore crucial to disentangle the particle-adsorbate and particle-support interaction effects on the particle shape, its local structure, atomic dynamics, and its possible anisotropies. In order to gain insight into the support effect, we carried out an X-ray absorption fine-structure spectroscopy (XAFS) investigation of adsorbate- and ligand-free size-selected Pt NPs deposited on two different supports in ultrahigh vacuum. Polarization-dependent XAFS measurements, neural network-based analysis of X-ray absorption near-edge structure data, and reverse Monte Carlo (RMC) simulations of extended X-ray absorption fine structure (EXAFS) were used to resolve the 3D shape of the NPs and details of their local structure. A synergetic combination of advanced in situ XAFS analysis with atomic force microscopy and scanning tunneling microscopy (STM) imaging provides uniquely detailed information about the particle-support interactions and the NP/support buried interface, not accessible to any experimental technique, when considered alone. In particular, our combined approach reveals differences in the structure of Pt NPs deposited on TiO2(110) and SiO2/Si(111). Pt NPs on SiO2 assume a spherical-like 3D shape and weakly interact with the support. In contrast, the effective shape of analogously synthesized Pt NPs on TiO2(110) after annealing at 600 °C is found to be a truncated octahedron with (100) top and interfacial facets that are encapsulated by the TiO2 support. Modeling disorder effects in these NPs using an RMC approach reveals differences in bond-length distributions for NPs on different supports and allows us to analyze their anisotropy, which may be crucial for the interpretation of support-dependent atomic dynamics and can have an impact on the understanding of the catalytic properties of these NPs.

Tailoring the Catalytic Properties of Metal Nanoparticles via Support Interactions.

The development of new catalysts for energy technology and environmental remediation requires a thorough knowledge of how the physical and chemical properties of a catalyst affect its reactivity. For supported metal nanoparticles (NPs), such properties can include the particle size, shape, composition, and chemical state, but a critical parameter which must not be overlooked is the role of the NP support. Here, we highlight the key mechanisms behind support-induced enhancement in the catalytic properties of metal NPs. These include support-induced changes in the NP morphology, stability, electronic structure, and chemical state, as well as changes in the support due to the NPs. Utilizing the support-dependent phenomena described in this Perspective may allow significant breakthroughs in the design and tailoring of the catalytic activity and selectivity of metal nanoparticles.

Element-resolved thermodynamics of magnetocaloric LaFe(13-x)Si(x).

By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe(13-x)Si(x). These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.

In situ coarsening study of inverse micelle-prepared Pt nanoparticles supported on γ-Al2O3: pretreatment and environmental effects.

The thermal stability of inverse micelle prepared Pt nanoparticles (NPs) supported on nanocrystalline γ-Al(2)O(3) was monitored in situ under different chemical environments (H(2), O(2), H(2)O) via extended X-ray absorption fine-structure spectroscopy (EXAFS) and ex situ via scanning transmission electron microscopy (STEM). Drastic differences in the stability of identically synthesized NP samples were observed upon exposure to two different pre-treatments. In particular, exposure to O(2) at 400 °C before high temperature annealing in H(2) (800 °C) was found to result in the stabilization of the inverse micelle prepared Pt NPs, reaching a maximum overall size after moderate coarsening of ∼1 nm. Interestingly, when an analogous sample was pre-treated in H(2) at ∼400 °C, a final size of ∼5 nm was reached at 800 °C. The beneficial role of oxygen in the stabilization of small Pt NPs was also observed in situ during annealing treatments in O(2) at 450 °C for several hours. In particular, while NPs of 0.5 ± 0.1 nm initial average size did not display any significant sintering (0.6 ± 0.2 nm final size), an analogous thermal treatment in hydrogen leads to NP coarsening (1.2 ± 0.3 nm). The same sample pre-dosed and annealed in an atmosphere containing water only displayed moderate sintering (0.8 ± 0.3 nm). Our data suggest that PtO(x) species, possibly modifying the NP/support interface, play a role in the stabilization of small Pt NPs. Our study reveals the enhanced thermal stability of inverse micelle prepared Pt NPs and the importance of the sample pre-treatment and annealing environment in the minimization of undesired sintering processes affecting the catalytic performance of nanosized particles.