Carbon-supported Pt-Sn electrocatalysts for the anodic oxidation of H2, CO, and H2/CO mixtures.: Part I.: Microstructural characterization

High-resolution transmission electron microscopy (HRTEM), microchemical analysis by X-ray emission spectroscopy (EDS), and X-ray diffraction (XRD) were used to characterize the composition, size, distribution, and morphology of the phases present in three different carbon-supported Pt-Sn catalysts. The catalysts had nominal Pt/Sn atomic ratios of 1: 1 (500N and 900N samples) and 3:1 (E270 sample). The 1: 1 catalyst sample heat-treated at 500 degrees C (500N) contained a Pt-rich fcc alloy phase (a(0) = 0.3965 nm) and tetragonal SnO2. The sample from the same precursors heat treated at 900 degrees C (900N) consisted of stoichiometric hexagonal PtSn, a nearly stoichiometric Pt3Sn fcc phase (a(0) = 0.3988 nm), and tetragonal SnO2. Neither pure tin nor pure platinum particles were detected in any of the catalysts. The 3 : 1 catalyst sample reduced in H-2 at 270 degrees C (E270) was composed entirely of the stoichiometric Pt3Sn cubic phase (a(0) = 0.3998 nm). The Pt3Sn-type particles are predominantly single nanocrystals with mean diameters of similar to 4 nm (for the catalysts E270 and 500N) and similar to 5 nm (for the catalyst 900N), whereas the PtSn particles were larger. The Pt3Sn-type particles in 500N and 900N were generally highly faceted, with {111} and {200} facets, whereas the E270 catalyst had generally a smooth spheroidal outline. Whereas the Pt3Sn particles in the E270 and 900N catalysts were equiaxed, in the 500N catalyst the alloy particles had uniquely elongated (e.g., ellipsoidal) shapes. (c) 2005 Elsevier Inc. All rights reserved.