Bimetallic nanoparticles can lead to catalysts with improved turnover rates and selectivity, but many synthetic protocols, such as impregnation or precipitation, typically form particles non-uniform in size and composition. Colloidal methods, which can potentially avoid such non-uniformity, often require heteroatoms or reagents that poison the catalyst surface. We describe a sequential displacement-reduction colloidal synthesis method that forms uniform bimetallic AuPt, AuRh, and AuPd nanoparticles (3-4 nm diameter) with a narrow size distribution. These particles are prepared using ethanol or ethylene glycol as reducing solvents and polyvinylpyrrolidone or polyvinylalcohol as protecting polymers. Such materials contain only C, H, O, and N, and the resulting residues can be removed from particle surfaces by post-synthetic H2 treatment at low temperatures without detectable sintering.The mechanism for bimetallic formation was examined by in-situ UV-visible spectroscopy using Au-derived plasmon bands, and by transmission electron microscopy. The surface atoms of a seed nanoparticle (Pt, Rh, or Pd) first reduce Au+ ions in solution onto the particle surface as Au0 due to the higher reduction potential of Au relative to  Pt, Rh, or Pd. Pt2+, Rh2+, or Pd2+ ions generated by the reaction are then re-deposited onto the surface by the reducing solvent.Differences in the enthalpy of bulk alloy formation influence the ease of synthesis, with AuPt, a thermodynamically unfavorable alloy, requiring additional synthesis considerations to prevent monometallic Au formation compared to AuPd, which is favorable. These include minimizing solvent reduction strength and metal precursor concentration, and maximizing Pt surface concentration. Developing rigorous conclusions about the size and composition effects of bimetallic clusters is key for designing valued catalysts for reactions such as methanol oxidation in fuel cells and COoxidation in automobile catalytic converters. Mechanistic knowledge of how to produce uniform clusters, such as those made in this study, is crucial to make such conclusions.

Colloidal methods for the synthesis of Au-containing bimetallic nanoparticles as model catalysts

248th National Meeting (2014)

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