Structure and Chemical Composition of a Surfactant-Stabilized Pt3Sn Alloy Colloid

A tetraoctylammonium-stabilized PtSn colloid with nominal composition Pt 3 Sn has been prepared by coreduction of the metal salts. This colloid, which is used in the manufacture of anode catalysts for low-temperature polymer membrane fuel cells, was supported on silica and the structure studied by in situ X-ray diffraction and Debye function analysis. Results indicate that the supported colloid is best described as a highly disordered bimetallic fcc cluster with a particle size of ∼1.3 nm. From TEM images a slightly larger size of 1.5 ± 4 nm is derived. Point-resolved EDX (energy-dispersive X-ray spectroscopy) confirms the elemental ratio of 3:1 in the PtSn particles. However, the XRD simulation indicates some deviations from uniform alloy formation. On removal of the stabilizing tetraoctylammonium ligands by heating the samples in He to ∼200 °C, coalescence of neighboring particles occurs. The new alloy formed consists of a majority of cubic Pt 0.81 Sn 0.19 particles, with average size 3.1 nm, and a minority phase of stoichiometric hexagonal PtSn, 4.4 nm in size. The former phase can be considered as a metastable supersaturated solid solution of Sn in Pt. The two phases are stable even on heating to 375 °C for 1.5 h in He.