Protein-assisted formation of Au single-atom alloys in Pd-rich nanoparticles

Protein-protected monometallic nanoparticles (NPs) have been extensively investigated due to their promising applications, but are not often used to produce alloy NPs. This study shows how protein-assisted NP synthesis can be used to produce bimetallic AuPd alloy NPs, particularly single-atom alloys (SAAs). Detailed characterization of alloy NPs with a variety of Au/Pd compositions provides insights into their local structural and electronic properties. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements revealed substantial Pd enrichment in the recovered products relative to the precursor compositions. Transmission electron microscopy (TEM) imaging studies indicated an ultra-small NP size of approximately 1.6 nm in diameter. X-ray absorption near edge structure (XANES) measurements demonstrated that decreasing Au content led to an increased Au d-electron density, indicating d-electron transfer from Pd to Au. Furthermore, extended X-ray absorption fine structure (EXAFS) analysis identified three distinct structural regimes within the series of protein-protected AuPd NPs: a random-alloy structure at moderate (∼50%) Au content, Au-rich clustered environments at low (∼10-25%) Au content, and an Au SAA-type local environment at Au content below 5%. A preliminary catalytic comparison using 4-nitrophenol reduction showed a substantially higher apparent reaction rate for Au₃Pd₉₇ than for Pd₁₀₀ under the tested conditions. Collectively, these findings provide insights into the formation mechanism and structural evolution of protein-protected AuPd nanoparticles, underscoring their potential for Au SAA catalyst development.