Faraday Discussions最新文献

In order to extend catalysis theory to complex alloys and multiple adsorbates, we have to face the fact that the number of possible surface site-adsorbate combinations gets too large to calculate. We, instead, define rules for adsorbate-adsorbate interactions; specifically, blocking rules in terms of disallowed local adsorbate-adsorbate configurations. We then conduct simple simulations to investigate how different rules entail certain outcomes. For the PdAg intermetallic and PdAg solid solutions, we find that the presence of Ag atoms hinders O* species from covering the whole (111) surface, which is the case for unary Pd(111), and instead allows for adsorbed *OH species. We predict that the adsorbed *OH species improves the oxygen reduction reaction activity because they have adsorption energies at the top of the activity volcano. Experiments can use our results to distinguish between the different possible PdAg(111) alloy surface manifestations, and to better understand adsorbate coverage on complex alloys. Lastly, we use our approach on Ag₁₄Ir₁₆Pd₃₀Pt₁₄Ru₂₆ high-entropy alloys, but find that the choice of adsorbate-adsorbate interaction rules affects the oxygen reduction in less distinguishable ways compared to the binary PdAg alloys.

High-entropy Ag-Au-Cu-Pd-Pt nanoparticles in the 2400-6300-atom size range were computationally studied at thermodynamical equilibrium and room temperature using a combination of well established many-body potentials and Monte Carlo methods. Tools from percolation theory are used to further quantify the deviations to ideal behavior from noninteracting solid solutions. Upon varying the concentration of each element one at a time, the possible surface enrichment in the various metals is determined and the fragment statistics provide insight into the spatial distribution of atoms within the nanoparticles and their tendency for mixing or segregation. The effects of size and dimensionality are addressed separately, by comparing the results obtained for the 0D (nanoparticle) system with those for the 2D (slabs) and 3D (periodic) samples. Although these properties are found to depend on the underlying many-body potential to some extent, some robust trends are predicted, notably for silver and platinum, which strongly segregate and preferentially reside at the surface and in the core of the nanoparticles, respectively.

Recent breakthroughs in the field of high-entropy alloy nanoparticles (HEA NPs) have significantly expanded their potential applications (such as catalysis or energy storage) making them promising candidates for use over a wide temperature range. However, their thermal stabilities are not yet fully understood, which is crucial to their future development. To better understand these phenomena and the underlying mechanisms, we performed molecular dynamics simulations by adopting an incremental approach to investigate the structural and thermal stability of CoNiPtCuAu HEA NPs, as well as their ternary and quaternary sub-alloys. More precisely, CoNiPt ternary system is first considered and then Cu and Au atoms are progressively introduced with the aim to analyse and quantify the thermal stability of HEA NPs in terms of their melting temperature and segregation mechanisms. Through our atomic-scale simulations, we demonstrate the negative impact of Au and Cu atoms on thermal stabilisation, whose presence at the surface tends to favour melting of the NPs because of their low melting point. These detailed analyses provide a robust and relevant research approach for identifying the key parameters influencing the thermal stability of HEA NPs, which is essential for obtaining such nano-objects with optimised structural and thermal properties.

High-entropy alloys have great potential as electrocatalysts for water-splitting reactions. Benefiting from the cocktail effect and lattice distortion, high-entropy alloys exhibit relatively low overpotentials and significant stability, making them excellent candidates for electrocatalytic water splitting. These materials offer a cost-effective and abundant alternative to conventional noble-metal catalysts such as Pt and IrO₂, which are limited by high costs and scarcity. This study investigates the electrocatalytic performance of high-entropy alloy powders prepared with equimolar ratios of Ni, Cu, Mn, and W, with additional elements (Co, Fe, or Mo) introduced to optimize their activity for the hydrogen evolution reaction and oxygen evolution reaction. The high-entropy alloy powders are synthesized via ball milling, involving both dry milling and wet milling in ethanol, followed by washing and drying at room temperature. Comprehensive characterization techniques, including X-ray diffraction, field-emission scanning electron microscopy, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, are employed to analyze their structure and properties. Electrochemical studies reveal that Fe and Mo significantly enhance hydrogen evolution reaction activity, achieving overpotentials of 301 mV and 305 mV, respectively, with corresponding Tafel slopes of 200.9 mV dec^-1 and 153.3 mV dec^-1. Meanwhile, Co incorporation improves oxygen evolution reaction performance, reducing the overpotential to 326 mV with a Tafel slope of 143.7 mV dec^-1. These findings underscore the potential of high-entropy alloy powders for advancing renewable energy technologies.

The influence of chemical ordering and its length scale on the magnetic behaviour of the Al_0.2Ti_0.3Co_1.5CrFeNi_1.5 high entropy alloy (HEA) was investigated across three distinct microstructural conditions, including a solution annealed state, an annealed condition at 750 °C, and a cold rolled plus annealed condition. These processing routes produced changes in the volume fraction, size, and morphology of L1₂ ordered precipitates along with the formation of a minor L2₁ phase in the annealed states. Magnetic measurements performed between 2 K and 300 K revealed clear differences in saturation magnetization, coercivity, and magnetic transition temperatures across the three conditions. The solution annealed condition exhibited a single magnetic transition near 48 K and low coercivity, consistent with a fine dispersion of coherent L1₂ precipitates. In the annealed and cold rolled conditions, coarsening and morphological evolution of the L1₂ phase led to the emergence of a second magnetic transition at lower temperatures, attributed to partial magnetic decoupling of the precipitates. Coercivity increased significantly in these conditions due to enhanced domain wall pinning, while residual hysteresis at 300 K is attributed to a combination of minor L2₁ phase at grain boundaries and microstructural features in the cold-worked condition. These results demonstrate that chemical ordering and its structural evolution play a central role in governing low temperature magnetic behaviour in this alloy system. The findings contribute to the broader understanding of multifunctional HEAs that combine tunable magnetic properties with excellent mechanical performance.

Multi-metallic alloys such as high entropy alloys (HEAs) span an extensive compositional space, potentially offering materials with enhanced activity and stability for various catalytic reactions. However, experimentally identifying the optimal composition within this vast compositional space poses significant challenges. In this study, we present a medium-throughput approach to screen the composition-activity correlation of electrodeposited multi-metallic and HEA nanoparticles. We apply the approach for exploring the Pd-Ag-Au composition subspace for the alkaline Oxygen Reduction Reaction (ORR). The Pd-Ag-Au alloy nanoparticles were synthesized electrochemically, characterized and evaluated for the ORR using a rotating disk electrode (RDE) setup. From 107 individual measurements, a composition-activity correlation model was constructed using Gaussian Process Regression (GPR), pinpointing the optimal composition around Pd₈₅Ag₁Au₁₄. The experimental results are then compared to theoretical predictions based on the well-established descriptor approach utilizing density functional theory (DFT) calculations. While some discrepancies exist, the experimental DFT-derived models show partial overlap, validating the utility of computational screening for multi-metallic systems. This work provides valuable insights for the efficient screening of multi-metallic catalysts for catalytic applications and exemplifies advanced pathways on how to compare and analyze experimental data to simulations based on well-defined hypotheses.

The quaternary alloy AlCrTiV has been proposed as both a lightweight high-entropy alloy and also a functional spin filter material based on the Heusler structure. Experimental investigations to-date, based on X-ray diffraction, offer conflicting interpretations of the structure. Here we simulate diffraction patterns of the various proposed structures to show that neutron diffraction, in particular, can reveal the nature of long-range chemical order and discriminate among distributions of the refractory transition metals. Magnetic contributions to the neutron diffraction are also discussed.

The gas reactivity of high-entropy nanoalloys (HENAs) is an emerging area of research with significant potential for applications in catalysis, gas sensing, hydrogen storage, and corrosion resistance. Insights into the structure-reactivity relationships that dictate the behavior of HENAs in reactive gas environments are critical for optimizing their performance across these applications. However, understanding the complex structural attributes of HENAs, such as size, shape and structure in response to a gas stimulus, remains challenging because of the limited accessibility to methods capable of probing these attributes under in situ or operando conditions. Here, we performed aberration-corrected environmental gas scanning transmission electron microscopy (STEM) observations to investigate the atomic and chemical structures of quinary CoNiCuPtAu HENAs in response to pure oxygen exposure at atmospheric pressure and elevated temperatures. The nanoparticles were fabricated by pulsed laser deposition with a high degree of control over both size and composition. Atomic-scale STEM imaging combined with energy dispersive X-ray (EDX) spectroscopy at the single particle level revealed a complex structural and chemical evolution pathway for CoNiCuPtAu HENAs under oxygen at atmospheric pressure during progressive heating up to 700 °C. Notably, we have identified substantial mass transfers between nanoparticles accompanied by oxygen-induced demixing of components, nanovoid formation and the stabilization of platelet-like nanostructures crystallizing as a Co-Ni oxide solid solution.