(107e) More Benign Synthesis and Extraction of Ligand-Free Palladium Nanoparticles

We demonstrate a more benign synthesis method for small, relatively nondisperse, metal nanoparticles for catalysis. By eliminating separate capping agents and using a recyclable solvent, this technique avoids the massive waste steams associated with current processes.

Metal nanoparticles offer size-dependent optical, electronic, magnetic, catalytic and other properties. But their syntheses generally require reduction of metal salts in aqueous or organic solutions with capping ligands. These caps serve to passivate the surface of the formed particles and to suppress the growth of the metal clusters that would otherwise aggregate due to the high surface energy of the system. However, the capping ligands may reduce the surface area of the nanoparticles available for catalytic applications. Synthesis often uses excess capping ligands, which then require downstream separations generating waste streams.

In general, a fast, homogeneous reduction reaction is a prerequisite to produce small metal nanoparticles. And it would be desirable to produce small, uniform metal nanoparticles in a solvent medium without the need for the addition of capping ligands and the resulting purification processes. An ideal solvent medium would not only exhibit favorable solvation of a metal salt (e.g. Na₂PdCl₄) and of a reducing agent (e.g. NaBH₄), but would also exhibit sufficient interaction with the surface of the formed metal nanoparticles to passivate and stabilize the nanoparticle dispersion as created within this solvent medium effectively. Dimethyl sulfoxide (DMSO) is a favorable solvent for ionic solids, as well as both polar and polarizable molecules. In addition, it is relatively inexpensive, stable, and exhibits low vapor pressure and a high boiling point of 189 ºC. Here we present the first successful synthesis and stabilization of 3.5 nm Pd nanoparticles (standard deviation of 0.5 nm) within DMSO via a fast, homogeneous reduction of a Pd salt using NaBH₄ in the absence of traditional capping ligands. These Pd nanoparticles were found to be extremely stable and did not exhibit precipitation and/or agglomeration within the DMSO solvent after more than 9-months of aging time. Moreover, these Pd nanoparticles were conveniently separated from the DMSO solvent medium via vacuum freeze drying by taking advantage of the high freezing point of DMSO (18.45 ºC). We have also successfully extracted the Pd metal nanoparticles from the DMSO phase into an organic phase, thereby providing a means of recycling the DMSO solvent.