(105c) The Influences of Anions On the Formation of Pt Nanoparticles

The development of synthetic methods in which the control of size and the shape of metal nanoparticles can be achieved is important for the production of active and selective heterogeneous catalysts.  Nanoparticles with controlled shape and size can be prepared by the versatile polyol method, where metal precursors are reduced to form metal nanoparticles in the presence of surface stabilizing agents. Anions, which have been used for controlling the shape and size distribution of nanoparticles, are common additives during the synthesis of nanoparticles, but their exact role in the nanoparticle formation is still unclear.

We present on the influence of halide ion identity and source (endogenous/exogenous) on the formation of platinum nanoparticles by the polyol method.  We prepared platinum nanoparticles with halide-containing and halide-free precursors in the presence/absence of halides in the bulk solution.  The kinetics of reduction, nucleation and particle growth has been studied by the direct measurement of oxidation state speciation with in-situ X-ray absorption spectroscopy. The rate of precursor reduction in the absence of free halide ions decreases in the order of OH^- > Br^- > Cl^-; where the ions represent the endogenous platinum precursor ligand.  The addition of exogenous chloride and bromide ions in the form of the acid hinders the rate of precursor reduction of Cl^- and Br^- containing platinum precursors while free halide ions enhanced the rate of reduction of hydrogen hexahyroxyplatinate.  The kinetic parameters for various platinum precursors have been evaluated in the presence and absence of free halide ions.