Shape Controlled Synthesis of Au/Ag/Pd Nanoalloys and Their Oxidation-Induced Self-Assembly into Electrocatalytically Active Aerogel Monoliths

Abstract

The synthesis of size and shape controlled Au/Ag/Pd alloy nanoparticles (NPs) and their self-supported assembly into monolithic aerogels for electro-oxidation of ethanol is reported. Two distinct morphologies of ultrasmall (3–5 nm) Au/Ag/Pd alloy NPs were produced via stepwise galvanic replacement of thiol-coated Ag NPs. The resultant nanoalloys were self-assembled into large, free-standing, aerogel superstructures that exhibit direct NP connectivity, high surface area (269 ± 18.1–76 ± 6.4 m2/g) and mesoporosity (2–50 nm), and high electrocatalytic activity via controlled oxidation of the surfactant ligands. The gelation kinetics have been tuned by varying the oxidant/surfactant molar ratio that governs the acidity of sol–gel reaction and consequently the extent of Ag dealloying with in situ generated HNO3. As-synthesized Au/Ag/Pd aerogels exhibit polymeric or colloidal gel morphology that can be manipulated by varying the shape and composition of precursor NPs. The electrocatalytic activity of ternary alloy aerogels for oxidation of ethanol was investigated using cyclic voltammetry and chronoamperometry. The monolithic aerogels exhibit high catalytic activity and durability, which is ∼20–30 times greater than those of the discrete Au/Ag/Pd alloy NPs. The polymeric morphology of high Pd-containing alloy aerogels resulted in ∼1 order of magnitude higher current density and mass activity in comparison to low Pd-containing colloidal aerogels. The synergistic effect of trimetallic alloy mitigates the catalyst poisoning effects and increases the stability and durability while the self-supported superstructure with direct NP connectivity, high surface area, and mesoporosity offers a facile conduit for both molecular and electron transport, enabling Au/Ag/Pd aerogel as a high-efficiency electrocatalyst.

Engineering Self‐Supported Noble Metal Foams Toward Electrocatalysis and Beyond

“…Later on, composition and microstructure modulation of aerogels have been demonstrated to enhance the EOR performance. As displayed in Figure 10 a–c, a stepwise GRR on Ag NPs followed by oxidant‐induced gelation produced ternary alloy aerogels, displaying an I f of about two times higher than that of Pd/C . By incorporating Cu and Au/Pt into palladium aerogels, I f of about ≈3 and 6.1 times higher than that of Pd/C for EOR were reported, respectively.…”

“…For certain low‐SSA NMFs, krypton molecules are used as probes, due to their lower saturation pressure (≈2.63 Torr for supercooled liquid krypton at 77K) compared to that of nitrogen molecules (≈760 Torr at 77K), thus allowing to acquire a much more accurate result ascribed to a large relative pressure change with a small amount of gas adsorption/desorption. Depending on composition and structure, SSAs ranging from 0.1 to 269 m 2 g −1 have been reported for NMFs . Since significantly large atomic weight of noble metals lead to quite low mass‐normalized SSAs compared to that of other materials like silica aerogels and nanocarbon foams, SSAs normalized by molar quantity have also been adopted, which are typically located in the range between 100 to 20 000 m 2 mol ‐1 .…”

“…Impressively, an ultrahigh specific surface area of up to 160 m 2 g −1 was obtained for the resulting Ag–nanoshell‐based aerogels, outperforming the SSAs of most NMFs. Several following studies further expanded the system to Au–Ag and Au–Ag–Pd aerogels . By engineering the composition of ternary Au–Ag–Pd alloy NPs by the stepwise galvanic replacement reaction (GRR) of l ‐glutathione‐stabilized Ag NPs, Nahar et al observed that spherical and anisotropic NPs were obtained with higher and lower Pd proportion, respectively, further resulting in gels with different microstructures.…”

“…Au–Ag–Pd alloy aerogels, a) synthesis process, b,c) CVs and chronoamperometric scans of Au–Ag–Pd alloy NPs, the corresponding aerogel, and commercial Pd/C catalyst conducted in 1 m KOH + 1 m ethanol solution. Adapted with permission . Copyright 2017, American Chemical Society.…”

“…Depending on composition and structure, SSAs ranging from 0.1 to 269 m 2 g −1 have been reported for NMFs. [16,38] Since significantly large atomic weight of noble metals lead to quite low mass-normalized SSAs compared to that of other materials like silica aerogels and nanocarbon foams, SSAs normalized by molar quantity have also been adopted, which are typically located in the range between 100 to 20 000 m 2 mol −1 . For example, a massnormalized SSA of 100 m 2 g −1 converts to a molar quantity normalized SSA for a Pd aerogel of about 1.1 × 10 4 and for a carbon aerogel of about 1.2 × 10 3 m 2 mol −1 , respectively.…”