Quantitative Characterization of the Thermally Driven Alloying State in Ternary Ir–Pd–Ru Nanoparticles

Abstract

Compositional and structural arrangements of constituent elements, especially those at the surface and near-surface layers, are known to greatly influence the catalytic performance of alloyed nanoparticles (NPs). Although much research effort often focuses on the ability to tailor these important aspects in the design stage, their stability under realistic operating conditions remains a major technical challenge. Here, the compositional stability and associated structural evolution of a ternary iridium–palladium–ruthenium (Ir–Pd–Ru) nanoalloy at elevated temperatures have been studied using interrupted in situ scanning transmission electron microscopy and theoretical modeling. The results are based on a combinatory approach of statistical sampling at the sub-nanometer scale for large groups of NPs as well as tracking individual NPs. We find that the solid solution Ir–Pd–Ru NPs (∼5.6 nm) evolved into a Pd-enriched shell supported on an alloyed Ir–Ru-rich core, most notably when the temperature exceeds 500 °C, concurrently with the development of expansive atomic strain in the outer surface and subsurface layers with respect to the core regions. Theoretically, we identify the weak interatomic bonds, low surface energy, and large atomic sizes associated with Pd as the key factors responsible for such observed features.

The Thermal Stability of Asymmetric Separated Configurations inside Alloy Nanoparticles: Atomic-Scale Modeling of Pd–Ir Nanophase Diagrams

“…The QJ configuration induced by CBEV effects in Pd-Ir NPs can be expected also in ternary Pd-Ir-Ru NPs, as reasoned in the Supporting Information. It is reflected in recently reported atomic-scale STEM-high-angle annular dark-field (HAADF) image and corresponding energy-dispersive X-ray spectroscopy (XEDS) line profile, which are fairly comparable to FCEM/CBEV computed results (Supporting Information). Additionally, the general prediction regarding CBEV induced subsurface enrichment by the element with a higher surface-subsurface strengthening was supported before by a test case that used this approach for semi-infinite Pt–Rh. , In particular, the computed oscillatory segregation profile was in good agreement with previously reported experimental data …”

“…Therefore, the O h surface sites are grouped according to only four coordinations (vertices, edges, and (100) and (111) faces, Figure a). Regarding the core, the full atomic-site model (FASM) can handle only O h (147), whereas the rotational-symmetric site grouping model is used for the larger O h (309) and O h (561) (Figure ). Namely, in the RSSM, sites having equal distances from the NP center and from a chosen axis are grouped together (reported electron microscopy images of phase-separated NPs seem to exhibit such symmetry , ).…”

“…Binary alloy nanoparticles (NPs or “nanoalloys”, 1–100 nm in size) have received during the last 2–3 decades growing scientific attention, both experimentally (e.g., by scanning transmission electron microscopy, STEM , ) and theoretically. , Compared to the corresponding bulk alloys, these small particles, having a relatively high surface-area to volume ratio and various inequivalent atomic sites, typically exhibit modified structural and compositional configurations (“chemical order”), which lead to altered chemical and physical properties that can be utilized in various applications (e.g., in heterogeneous catalysis, magnetic storage, optical devices, and in nanomedicine). Such potentially practical aspects had a significant role in inspiring the emergence of comprehensive basic research.…”

Chemical Synthesis, Characterization, and Properties of Multi‐Element Nanoparticles

“…Recently, a quantitative characterization of the alloying state in ternary alloy nanoparticles succeeded by a combination of STEM-EDS analysis and a statistical approach. [112] However, for multi-element NPs, instruments with much higher detection sensitivity would be required because of the low-intensity signals from a fewer number of multi-elements in nano(angstrom)-scale space.…”

“…In addition to the synthesis, the development in the structural analysis, especially surface structure at the atomic scale, is also required to further elucidate their catalytic mechanism. Recently, a quantitative characterization of the alloying state in ternary alloy nanoparticles succeeded by a combination of STEM‐EDS analysis and a statistical approach [112] . However, for multi‐element NPs, instruments with much higher detection sensitivity would be required because of the low‐intensity signals from a fewer number of multi‐elements in nano(angstrom)‐scale space.…”