In Situ Solid–Gas Reactivity of Nanoscaled Metal Borides from Molten Salt Synthesis

Abstract

Metal borides have mostly been studied as bulk materials. The nanoscale provides new opportunities to investigate the properties of these materials, e.g., nanoscale hardening and surface reactivity. Metal borides are often considered stable solids because of their covalent character, but little is known on their behavior under a reactive atmosphere, especially reductive gases. We use molten salt synthesis at 750 °C to provide cobalt monoboride (CoB) nanocrystals embedded in an amorphous layer of cobalt(II) and partially oxidized boron as a model platform to study morphological, chemical, and structural evolutions of the boride and the superficial layer exposed to argon, dihydrogen (H), and a mixture of H and carbon dioxide (CO) through a multiscale in situ approach: environmental transmission electron microscopy, synchrotron-based near-ambient-pressure X-ray photoelectron spectroscopy, and near-edge X-ray absorption spectroscopy. Although the material is stable under argon, H triggers at 400 °C decomposition of CoB, leading to cobalt(0) nanoparticles. We then show that H activates CoB for the catalysis of CO methanation. A similar decomposition process is also observed on NiB nanocrystals under oxidizing conditions at 300 °C. Our work highlights the instability under reactive atmospheres of nanocrystalline cobalt and nickel borides obtained from molten salt synthesis. Therefore, we question the general stability of metal borides with distinct compositions under such conditions. These results shed light on the actual species in metal boride catalysis and provide the framework for future applications of metal borides in their stability domains.

Beyond the Compositional Threshold of Nanoparticle-Based Materials

“…In situ characterizations, like NAP-XPS and TEM 31 will be pertinent in this context, but again the example of gold particles draws future guidelines to understand hybrid interfaces, especially the organic layer composition and the ligands distribution, by Nuclear Magnetic Resonance 56 and electron tomography 57 coupled to Density Functional Theory calculations and molecular dynamic simulations. 56,57…”

“…-Decreasing synthesis temperatures through liquid-phase pathways, especially with high temperature liquids. 27,[30][31][32]41 Different stimuli can trigger "nano-crystallization" in liquids, including microwaves 36,37 or, on the opposite, melt solidification. 58 Frequently raised issues of the aforementioned methods are scalability, environmental impact, cost and size distribution width.…”

Designing Nanoparticles and Nanoalloys with Controlled Surface and Reactivity

“…Future of the field probably lays in increasing the complexity of these objects, eg. by pursuing ternary compounds and the insertion of lighter elements in alloys, such as carbon or boron,, through innovative routes. In the development of catalysts and devices, coupling of environmental techniques described here with others, more sensitive to organic surface species, such as infra‐red spectroscopies (eg.…”

“…While each environmental technique described above provides invaluable insights on the surface and core of nanoparticles, combining local (TEM) and ensemble (XAS and XPS) measurements is best suited to fully understand the transformations of nanoalloys …”

Metal Oxysulfides: From Bulk Compounds to Nanomaterials

“…An eutectic mixture can even be used to benefit from a lower melting point. Molten salts are typically suitable for reaction temperatures between 300 and 1,000 • C, which enable the formation of nanoparticles while avoiding their sintering (Portehault et al, 2011;Gouget et al, 2017). After cooling, the particles are obtained in a matrix composed by the salts that are washed with water or alcohols.…”