Halide-Induced Leaching of Pt Nanoparticles - Manipulation of Particle Size by Controlled Ostwald Ripening

The synthesis of surfactant-free Pt nanoparticles (NPs) in alkaline ethylene glycol (EG), initially introduced in 2000, is of fundamental interest for the preparation of catalysts. Often a suspension of colloidal Pt NPs prepared by this method is stored and upon demand further processed to obtain supported catalysts such as Pt-C or Pt-Al2O3. In this study it is shown that in such colloidal suspensions in the presence of halides, e.g. from the metal precursor salt, leaching of Pt NPs takes place. Metal leaching poses a significant challenge for catalyst stability. However, if the reaction conditions are chosen appropriately, the here identified leaching process can be utilized to achieve particle growth by Ostwald ripening in a controlled manner. By changing the chemical nature of the metal precursor salt and variation of the reaction time, Pt NPs with a narrow size distribution between 1 to 4nm are obtained without any need of surfactants. Subsequently, the obtained particles can be transferred into low-boiling organic solvents, which then allows for coating of deliberately chosen supports to prepare e.g. model catalyst which are tailored in terms of particle size and support material. The suitability of such materials for catalytic investigations is demonstrated by using CO oxidation as a model reaction and it is shown that the catalytic activity (surface normalized) depends on the size of Pt NPs under the applied reaction conditions.