Atomically resolved structures and compositions of Pt alloy nanoparticles were obtained using aberration-corrected high-angle dark field imaging, which was correlated to specific ORR activity based ...on a Pt surface area. The enhanced specific ORR activity (∼2 times relative to Pt) of acid-treated “Pt3Co” nanoparticles can be related to composition variations at the atomic scale and the formation of percolated Pt-rich and Pt-poor regions within individual particles. Upon annealing, we show direct evidence of surface Pt sandwich-segregation structures, which correspond to a specific ORR activity ∼4 times relative to Pt.
The present study stages a comparative evaluation of microstructure and associated mechanical and thermal response for common cast aluminum alloys that are used for manufacturing automotive cylinder ...heads. The systems considered are Al-Cu (206-T6), Al-Si-Cu (319-T7), and Al-Si (356-T6, A356-T6, and A356 + 0.5Cu-T6). The focus of the present manuscript is on the evaluation of microstructure at various length scales after aging, while the second manuscript will deal with the mechanical and thermal response of these alloys due to short-term (aging) and long-term (pre-conditioning) heat treatments. At the grain-scale, the Al-Cu alloy possessed an equiaxed microstructure as opposed to the dendritic structure for the Al-Si-Cu or Al-Si alloys which is related to the individual solidification conditions for these alloy systems. The composition and morphology of intermetallic precipitates within the grain and at the grain/dendritic boundary are dictated by the alloy chemistry, solidification, and heat treatment conditions. At the nanoscale, these alloys contain various metastable strengthening precipitates (GPI and
θ
″
in Al-Cu alloy,
θ
′
in Al-Si-Cu alloy, and
β
′
in Al-Si alloys) with varying size, morphology, coherency, and thermal stability.
Rotating disk electrode measurements of acid-treated “Pt3Co” nanoparticles showed specific oxygen reduction reaction (ORR) activity (∼0.7 mA/cmPt 2 at 0.9 V vs RHE in 0.1 M HClO4 at room ...temperature), twice that of Pt nanoparticles. Upon annealing at 1000 K in vacuum, the ORR activity at 0.9 V was increased to ∼1.4 mA/cmPt 2 (four times that of Pt nanoparticles). High-resolution transmission electron microscopy and aberration-corrected high-angle annular dark-field in the scanning transmission electron microscope was used to reveal surface atomic structure and chemical composition variations of “Pt3Co” nanoparticles on the atomic scale. Such information was then correlated to averaged Pt−Pt distance obtained from synchrotron X-ray powder diffraction data, surface coverage of oxygenated species from cyclic voltammograms, and synchrotron X-ray absorption spectroscopy. It is proposed that ORR activity enhancement of acid-leached “Pt3Co” relative to Pt nanoparticles is attributed to the formation of a percolated structure with Pt-rich and Pt-poor regions within individual particles, while the increase in the specific ORR activity of annealed “Pt3Co” nanoparticles relative to Pt can be attributed to the presence of surface Pt segregation.
The microstructural and strength evolution of an additively manufactured Al-8.6Cu-0.5Mn-0.9Zr alloy upon aging at 300, 350, and 400 °C is investigated. The strengthening phases of the alloy evolve ...significantly upon aging, with breakdown and spheroidization of the interconnected θ-Al2Cu network, dissolution of metastable θ'-Al2Cu precipitates, and precipitation of nanometric L12-Al3Zr from a matrix supersaturated in Zr. In the peak-aged states, the alloy displays a favorable combination of strength and ductility, with a room-temperature yield strength of 314–341 MPa and ductility of 11–13%. The measured yield strengths for microstructures with different aging treatments are compared to predictions of yield strengths from grain boundary, solid solution, and particle strengthening contributions. The observed strain hardening behavior is related to fundamental precipitate and dislocation interactions. Comparison between predicted and measured strength values indicates a continued need for strengthening models specifically developed for the heterogeneous microstructures of additively manufactured alloys.
The catalytic performance of nanoparticles is primarily determined by the precise nature of the surface and near-surface atomic configurations, which can be tailored by post-synthesis annealing ...effectively and straightforwardly. Understanding the complete dynamic response of surface structure and chemistry to thermal treatments at the atomic scale is imperative for the rational design of catalyst nanoparticles. Here, by tracking the same individual Pt3Co nanoparticles during in situ annealing in a scanning transmission electron microscope, we directly discern five distinct stages of surface elemental rearrangements in Pt3Co nanoparticles at the atomic scale: initial random (alloy) elemental distribution; surface platinum-skin-layer formation; nucleation of structurally ordered domains; ordered framework development and, finally, initiation of amorphization. Furthermore, a comprehensive interplay among phase evolution, surface faceting and elemental inter-diffusion is revealed, and supported by atomistic simulations. This work may pave the way towards designing catalysts through post-synthesis annealing for optimized catalytic performance.
Creep behavior of three cast Al–Cu alloys at 300 °C was studied by measuring their steady state creep rates as a function of stress. Microalloying additions in two alloys stabilized the θ' (Al2Cu) ...precipitates to 300 °C, which allowed grain boundary-controlled creep deformation to dominate at low stresses in these alloys. In contrast, the instability of the microstructure at 300 °C in a conventional Al–Cu 206 alloy led to the majority of θ′ precipitates transform to the θ phase. The 206 alloy displayed diminished resistance to dislocation motion and a dislocation creep mechanism dominated from the lowest stresses. A modified Coble creep model was developed to describe the experimental low-stress creep rates in the alloys with thermally stable precipitate structures. It is concluded that increasing the thermal stability of precipitates in Al–Cu alloys can provide significant improvement in their creep performance.
In many heterogeneous catalysts, the interaction of metal particles with their oxide support can alter the electronic properties of the metal and can play a critical role in determining particle ...morphology and maintaining dispersion. We used a combination of ultrahigh magnetic field, solid-state magic-angle spinning nuclear magnetic resonance spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy coupled with density functional theory calculations to reveal the nature of anchoring sites of a catalytically active phase of platinum on the surface of a gamma-Al2O3 catalyst support material. The results obtained show that coordinatively unsaturated pentacoordinate Al3+ (Al3+penta) centers present on the (100) facets of the gamma-Al2O3 surface are anchoring Pt. At low loadings, the active catalytic phase is atomically dispersed on the support surface (Pt/Al3+penta = 1), whereas two-dimensional Pt rafts form at higher coverages.
In an effort to obtain the maximum atom efficiency, research on heterogeneous single-atom catalysts has intensified recently. Anchoring organometallic homogeneous catalysts to surfaces creates issues ...with retaining mononuclearity and activity, while the several techniques developed to prepare atomically dispersed precious metals on oxide supports are usually complex. Here we report a facile one-pot synthesis of inorganometallic mononuclear gold complexes formed in alkaline solutions as robust and versatile single-atom gold catalysts. The complexes remain intact on impregnation onto supports or after drying in air to give a crystalline powder. They can be used to interrogate the nuclearity of the catalytically active gold site for reactions known to be catalysed by oxidized gold species. We show that the Au
-O
- cluster directs the heterogeneous coupling of two methanol molecules to methyl formate and hydrogen with a 100% selectivity below 180 °C. The reaction is industrially important as well as the key step in methanol steam reforming on gold catalysts.
Investigation of atomically precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects on Au catalysis. Herein, we explored the catalytic behavior of a ...newly synthesized Au
(L
)
nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO
, CeO
, and Al
O
. Stability of the supported Au
nanoclusters was probed structurally by in situ extended X-ray absorption fine structure (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and their ability to adsorb and oxidize CO was investigated by IR absorption spectroscopy and a temperature-programmed flow reaction. Low-temperature CO oxidation activity was observed for the supported pristine Au
(L
)
nanoclusters without ligand removal. Density functional theory (DFT) calculations confirmed that the eight uncoordinated Au sites in the intact Au
(L
)
nanoclusters can chemisorb both CO and O
. Use of isotopically labeled O
demonstrated that the reaction pathway occurs mainly through a redox mechanism, consistent with the observed support-dependent activity trend of CeO
> TiO
> Al
O
. We conclude that the uncoordinated Au sites in the intact Au
(L
)
nanoclusters are capable of adsorbing CO, activating O
, and catalyzing CO oxidation reaction. This work is the first clear demonstration of a ligand-protected intact Au nanocluster that is active for gas-phase catalysis without the need of ligand removal.
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