Formation of Hierarchical Nanoporous Gold for Enhanced Catalytic Activity via Ternary Au-Ag-Ge Two-Phase Eutectic Microstructure

Nanoporous (np) metal foams are three-dimensional bicontinuous structures consisting of nanometric pores and ligaments. They possess a high surface area, exhibit good chemical reactivity and catalytic activity, and demonstrate excellent mechanical properties considering their volume-to-density ratio. The most common synthesis method for np-foams involves the selective dissolution of at least one less-noble element or phase from metal alloys through chemical or electrochemical selective corrosion. This process enables the reorganization of the more noble metal into nanometric ligaments. Np-Au, a type of metallic foam, can be synthesized by dealloying Ag from Au-Ag alloys to create a bicontinuous porous morphology typically on the scale of 30-50 nm. It is utilized as a supported or unsupported catalyst in a wide range of chemical reactions, including low-temperature CO oxidation and H2O2 decomposition. In this study, we utilized ternary systems with two-phase eutectic microstructures to produce hierarchical bicontinuous np-Au crystals via selective dissolution. By employing different dissolution methods and varying dealloying durations, we were able to tune the morphology, reducing the size of pores and ligaments to an average of 20 nm. Our results demonstrate that the catalytic activity of hierarchical np-Au crystals can be increased by up to an order of magnitude compared to that of non-hierarchical np-Au formed from the (Au-Ag)-Ge hypereutectic melt and twofold compared to conventional np-Au. The morphology tuning of np-metal foams, achieved through the two-step dissolution of ternary eutectic microstructures, may offer a promising avenue for enhancing catalytic activity across a wide range of metal catalysts.