For supported bimetallic catalysts it is important to know if the metal components are present in the form of ordered alloys or metal mixtures. For metals with similar crystal structure, Vegard's law predicts a simple relationship between alloy composition and lattice dimensions [1]. It is therefore possible to use high-resolution electron microscopy (HREM) to provide both structural and compositional information. in nanometer sized particles, accurate determination of the lattice spacing from fringes in an HREM image is difficult because of errors due to signal-to-noise, particle tilt and edge effects [2,3]. However, statistical distributions of fringe spacing can be used to determine the lattice spacing of particle ensembles to within 1.5% when measurements are carried out on at least 100 particles [2]. We extend this technique with model AuPd bimetallic catalysts and show that Vegard's law applies to nanometer-sized particles.

Model bimetallic catalysts were prepared by sputtering metal from AuPd alloy targets and/or a Pd target onto thin amorphous carbon substrates.