Sub-nanometric high entropy alloy cluster: hydrogen spillover-driven synthesis on CeO2 and structural reversibility

Abstract

High-entropy alloy (HEA) nanoparticles (NPs) have attracted significant attention as promising catalysts, as a consequence of the various unique synergistic effects originating from the nanometer-scale, near-equimolar mixing of five or more components to produce single-phase solid solutions. However, the study of sub-nanometer HEA clusters having sizes of less than 1 nm remains incomplete despite the possibility of novel functions related to borderline molecular states with discrete quantum energy levels. The present work demonstrates the synthesis of CeO2 nanorods (CeO2-NRs) on which sub-nanometer CoNiCuZnPd HEA clusters consisting of 13 atoms were formed with the aid of a pronounced hydrogen spillover effect on readily reducible CeO2 (110) facets. The CoNiCuZnPd HEA sub-nanoclusters exhibited higher activity during the reduction of NO by H2 even at low temperatures compared with the corresponding monometallic catalysts. These clusters also showed a unique structural reversibility in response to repeated exposure to oxidative/reductive conditions, based on the sacrificial oxidation of the non-noble metals. Both experimental and theoretical analyses established that multi-element mixing in quantum-sized regions endowed the HEA clusters with entirely novel catalytic properties.

Specific Hydrogen Spillover Pathways Generated on Graphene Oxide Enabling the Formation of Non‐Equilibrium Alloy Nanoparticles

“…[10,18] Moreover, the hydrogen spillover pathway can be completely different depending on the reducibility of the metal oxide and exposed crystal facets. [4,[19][20][21] Recently, hydrogen spillover on carbon-based materials such as graphene, [22,23] carbon nanotubes, [24,25] and fullerene [26] has been investigated as a means of developing high-capacity hydrogen storage materials. These materials provide high electronic and ionic conductivities that are indispensable to the occurrence of hydrogen spillover.…”

“…[4,41,42] In such processes, the spilled hydrogen generates a non-equilibrium field over the support material that simultaneously reduces multiple deposited metal ions having different redox potentials. [21,43] This hydrogen spillover-driven method is relatively simple but to date has only been applicable to TiO 2 and CeO 2 supports. [4] As such, the synthesis of non-equilibrium solid-solution alloy nanoparticles using carbon-based materials will not only expand the design range of catalysts but also provide potential new applications for hydrogen spillover techniques.…”