Structural and morphological characterization of alumina supported Pd nanoparticles obtained by colloidal synthesis

Alumina supported Palladium nanoparticles catalysts have been prepared by colloidal synthesis. The characterization of the structure and the morphology of the obtained nanoparticles is done by a two step procedure. In a first step, the possible morphologies of the particles are derived from physical principles. High Resolution Transmission Electron Microscopy (HRTEM) images and X-Ray Diffraction (XRD) diagrams are simulated. Simulations allows us to evaluate the limits of both methods to characterize such samples. We show that HRTEM is able to distinguish between the possible morphologies but is not sufficient to characterize vacancies or surface defects. We demonstrate that XRD is a good complementary technique, able also to obtain information about structure, morphology and size even for such small objects. In a second step, simulations are compared with experimental results. We show that two modes of synthesis, acidic and basic, lead to the same structure and morphology of nanoparticles. PdO nanoparticles present an octahedral shape close to 2 nm. The metallic Pd nanoparticles are always observed as cuboctahedra close to 2 nm. This same structure for both modes of synthesis cannot explain the differences of catalytic activity observed in the literature [Didillon B. et al. (1998) Stud. Surf. Sci. Catal., 118, 41-54]. The aggregation state of the individual synthesized particles is thus introduced to account for the differences in the catalytic performance.