Effect of colloidal nanocatalysis on the metallic nanoparticle shape: the Suzuki reaction.

Dominantly tetrahedral shaped poly(vinylpyrrolidone)-platinum (PVP-Pt) nanoparticles are shown to catalyze the Suzuki reaction between phenylboronic acid and iodobenzene but are not as active as the spherical palladium nanoparticles studied previously. The dominantly tetrahedral PVP-Pt nanoparticles (55 +/- 4% regular tetrahedral, 22 +/- 2% distorted tetrahedral, and 23 +/- 2% spherical nanoparticles) are synthesized by using the hydrogen reduction method. The transmission electron microscopy (TEM) results show that a transformation of shape from tetrahedral to spherical Pt nanoparticles takes place 3 h into the first cycle of the reaction. After the first cycle, the spherical nanoparticles have a similar size distribution to that of the tetrahedral nanoparticles before the reaction and the observed shape distribution is 18 +/-6% regular tetrahedral, 28 +/- 5% distorted tetrahedral, and 54 +/- 5% spherical nanoparticles. After the second cycle of the Suzuki reaction, the shape distribution is 13 +/- 5% regular tetrahedral, 24 +/- 5% distorted tetrahedral, and 63 +/- 7% spherical nanoparticles. After the second cycle, the transformed spherical nanoparticles continue to grow, and this could be due to the strong capping action of the higher molecular weight PVP (M(w) = 360 000), which makes the nanoparticles more resistant to aggregation and precipitation, unlike the Pd nanoparticles capped with the lower molecular weight PVP (M(w) = 40 000) used previously. The transformation in shape also occurs when the nanoparticles are refluxed in the presence of the solvent, sodium acetate, and iodobenzene and results in spherical nanoparticles with a similar size distribution to that of the tetrahedral nanoparticles before any perturbations. However, in the presence of phenylboronic acid, the regular tetrahedral nanoparticles remain dominant (51 +/- 6%) and maintain their size. These results support our previous studies in which we proposed that phenylboronic acid binds to the nanoparticle surface and thus acts as a capping agent for the particle and reacts with the iodobenzene. Recycling the nanoparticles results in a drastic reduction of the catalytic activity, and this must be due to the transformation of shape from the dominantly tetrahedral to the larger dominantly spherical nanoparticles. This also supports results in the literature that show that spherical platinum nanoparticles do not catalyze this reaction.