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

Guillaume Gouget(1), Damien P. Debecker(2), Ara Kim(1,2), Giorgia Olivieri(3), Jean-Jacques Gallet(3,4), Fabrice Bournel(3,4), Cyril Thomas(5), Ovidiu Ersen(6), Simona Moldovan(4), Clément Sanchez(1), Sophie Carenco(1), and David Portehault(1), 2017

Image courtesy of Inorganic Chemistry

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 (H2), and a mixture of H2 and carbon dioxide (CO2) 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, H2 triggers at 400 °C decomposition of CoB, leading to cobalt(0) nanoparticles. We then show that H2 activates CoB for the catalysis of CO2 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.

Impact Statement

The reactivity of metal boride nanocrystals versus reducing and oxidizing atmospheres at 400 °C is probed by in situ electron microscopy and near-ambient pressure X-ray photoelectron and near-edge X-ray absorption spectroscopies. This combination unveils a decomposition process of the borides and sheds light on the strong dependency of their stability on the nature of the surrounding medium.