Mechanistic Evidence for Sequential Displacement–Reduction Routes in the Synthesis of Pd–Au Clusters with Uniform Size and Clean Surfaces

Abstract

Bimetallic Pd–Au clusters with (Pd/Au)at compositions of 0.5, 1.0, and 2.0 narrowly distributed in size were prepared using colloidal methods with reagents containing only C, H, and O atoms, specifically polyvinyl alcohol (PVA) as protecting species and ethanol as the organic reductant. Synthesis protocols involved contacting a solution of Au precursors with nearly monodisperse Pd clusters. The formation of Pd–Au clusters was inferred from the monotonic growth of clusters with increasing Au content and confirmed by the in situ detection of Au plasmon bands in their UV–visible spectra during synthesis. Specifically, transmission electron microscopy (TEM) showed that growth rates were proportional to the surface area of the clusters, and rigorous deconvolution and background subtraction allowed for determination of the intensity and energy of Au-derived plasmon bands. This feature emerged during initial contact between Au precursors and Pd clusters apparently because Au3+ species deposit as Au0 using Pd0 as the reductant in a fast galvanic displacement process consistent with their respective redox potentials. The plasmon band ultimately disappeared as a result of the subsequent slower reduction of the displaced Pd2+ species by ethanol and of their deposition onto the bimetallic clusters. Such displacement–reduction pathways are consistent with the thermodynamic redox tendencies of Au, Pd, and ethanol and lead to the conclusion that such triads (two metals and an organic reductant) can be chosen from thermodynamic data and applied generally to the synthesis of bimetallic clusters with other compositions. These bimetallic clusters were dispersed on mesoporous γ-Al2O3 supports, and PVA was removed by treatment in ozone at near-ambient temperature without any detectable changes in cluster size. The absence of strongly bound heteroatoms, ubiquitous in many other colloidal synthesis protocols, led to Al2O3-dispersed clusters with chemisorption uptakes consistent with their TEM-derived cluster size, thus demonstrating that cluster surfaces are accessible and free of synthetic debris. The infrared spectra of chemisorbed CO indicated that both Pd and Au were present at such clean surfaces but that any core–shell intracluster structure conferred by synthesis was rapidly destroyed by adsorption of catalytically relevant species, even at ambient temperature; this merely reflects the thermodynamic tendency and kinetic ability of an element to segregate and to decrease surface energies when it binds an adsorbate more strongly than another element in bimetallic particles.