Nanoscale High-Entropy Alloy Catalysts from a Flame Aerosol Process
Shuo Liu, Chaochao Dun, Jeffrey Urban, MARK SWIHART, University at Buffalo - SUNY
Abstract Number: 72
Working Group: Nanoparticles and Materials Synthesis
Abstract
In metal alloys combining five or more elements in near equiatomic concentrations, the entropic contribution to the total Gibbs energy overcomes the enthalpic contribution to stabilize a solid solution structure. Thus, this class of materials has become known as high-entropy alloys. Historically, most research on high-entropy alloys focused on mechanical property enhancement, in part because conventional high-entropy alloy fabrication methods produce powders or bulk materials. More recently, novel synthesis methods have enabled production of high-entropy alloy nanomaterials with outstanding performance in energy-related applications, particularly in catalysis where the catalyst activity and stability can be rationally modified by the interaction among multiple elements.
Here, we present a one-step, continuous, and scalable flame aerosol synthesis route to produce supported high-entropy alloy nanoparticles. In contrast to conventional flame spray pyrolysis, this method separates the flame and particle formation processes. This allows it to provide a relatively lower reaction temperature at which nanoparticles form by a droplet to particle, rather than gas to particle, route. The droplet drying process is extremely fast, so the metallic elements can be combined uniformly in a single FCC alloy phase independent of their thermodynamic miscibility. Excess H2 is supplied to the flame, providing a reducing atmosphere in the reaction chamber. Moreover, a support material can be dispersed in the precursor solution, to introduce heterogeneous nucleation sites for alloy particle growth. The resulting high density of nuclei allows high mass loadings of sub-5 nm nanoparticles on supports. We show that highly dispersed high-entropy alloy (e.g. RuRhPdIrPt) nanoparticles can be deposited on various support materials, such as graphene, MOFs, carbon nanotubes, and metal oxides. We also present an in situ strategy to form the high-entropy alloy nanoparticles and mesoporous silica support simultaneously. We show that less reducible metals (e.g. W) can be incorporated into the high entropy alloy nanoparticles, even when that metal alone could not be reduced by hydrogen in the presence of water. Finally, we show that graphene-supported high-entropy nanoparticles outperform their single-component counterparts in a representative electrocatalysis application (hydrogen oxidation reaction).