Displacement-reduction routes to PtPd clusters and mechanistic inferences for the synthesis of other bimetallic compositions

Abstract

Bimetallic PtPd clusters (2.1–2.9nm) dispersed on SiO2 and uniform in composition and size were prepared using colloidal methods with reagents containing only C, O, H, and N atoms. These synthetic protocols extend galvanic displacement-reduction (GDR) processes previously used to prepare AuPd and AuPt clusters. Such processes exploit the different redox potentials of two elements to encourage their deposition within the same cluster. The size, composition, and formation mechanism of PtPd clusters were probed using transmission electron microscopy, UV–visible spectroscopy, energy-dispersive X-ray spectroscopy, and high-angle annular dark-field imaging. Taken together with previous data for AuPd and AuPt systems, these findings highlight key general features, properties, and protocols required to form uniform bimetallic clusters. Exothermic alloys, such as PtPd and AuPd, form predominantly via selective GDR routes; in contrast, alternate routes become significant for endothermic alloys, such as AuPt. Bimetallic clusters grow via GDR processes (PtPd, AuPd) at rates proportional to the surface area of each cluster; therefore, compositional uniformity is dictated by the size distribution of the seed clusters. The rate of GDR processes reflects the difference in reduction potentials of the two components, as shown by more facile formation of AuPd than PtPd clusters. These considerations and experimental evidence provide useful guidance for conditions and protocols likely to succeed for other bimetallic pairs. Low-temperature (≤423K) reductive treatments (in H2 or EtOH) successfully removed all synthetic detritus from Pd and PtPd clusters dispersed on SiO2, without significant coalescence. Such removal strategies are more challenging for Pd than for Pt clusters because of stronger Pd-polymer bonds and the greater sintering tendency of Pd clusters.