New Palladium Nanomaterials for Catalysis: Mechanisms Controlling Formation and Evolution of Nanostructures in a Seed-Me
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New Palladium Nanomaterials for Catalysis : Mechanisms Controlling Formation and Evolution of Nanostructures in a Seed-Mediated Synthesis Laure Bisson1,2, CÈdric BoissiËre1, ClÈment Sanchez1, CÈcile Thomazeau2, and Denis Uzio2 1 Laboratoire de Chimie de la MatiËre CondensÈe de Paris, UPMC, Paris, 75005, France 2 Catalyse et SÈparation, Institut FranÁais du PÈtrole, BP n∞3, Vernaison, 69390, France
ABSTRACT Metallic nanoparticles are commonly used as the active phase of heterogeneous catalysts. Today, a current issue in catalysis research is to determine whether a specific crystallographic plane of the metallic nanoparticle is responsible for activity and selectivity properties in a structure sensitive reaction. Following this purpose, metallic nanoparticles with specific morphologies have been studied. In the present work, palladium nanostructured particles are prepared in a surfactant mediated aqueous medium by a seeding growth approach, and present various morphologies : rods, tetrahedra and/or bipyramides, cubes, icosahedra... Synthesis of these nanoparticles and observation of their growth by a detailed Transmission Electron Microscopy study will be presented. We provide evidence that nucleation and growth of these particles are dominated by an aggregative mechanism. Moreover, upon ageing nanostructured particles undergo a ripening process to spherical morphology, attributed to an oxidative etching.
INTRODUCTION Recently, extensive efforts have been devoted to the synthesis of metallic anisotropic nanoobjects, in order to develop new nanomaterials, exhibiting promising applications in photonics, electronic devices, catalysis... In heterogeneous catalysis, metallic particles used as the active phase of catalysts have unique catalytic properties linked to their size. Nevertheless a current issue in catalysis research is to correlate particles morphology and specific surface sites exposed, to catalytic activity and selectivity in a structure sensitive reaction. Chemical synthesis seem to be a simple way to control metallic nanoparticles (NPs) dimension, morphology and structure. Synthesis of NPs using templates such as surfactant micelles, polymers matrices or mesoporous materials have been of growing interest. Some stabilizing agents used to protect particles from aggregation are also reported to adsorb selectively onto a specific crystallographic facet and to favor an anisotropic growth. Selective adsorption agents may be molecular ligands, polymers or surfactants. Among these methods, the seed-mediated approach was developed to obtain various nanostructures. It is based on decoupling nucleation and growth steps, allowing a separate control of their kinetics. A seed-mediated synthesis of gold NPs exhibiting specific morphologies (rods, cubes, tetrahedra, spheresÖ), using CTAB (CetylTrimethylAmmonium Bromide) as a selective adsorption agent, has been recently developed by Murphy and co-workers [1, 2]. Numerous parameters seem to play a critical role in the formation of these particles [3, 4].
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