6834
J. Phys. Chem. B 1997, 101, 6834-6838
In Situ Preparation of Amorphous Carbon-Activated Palladium Nanoparticles N. Arul Dhas,† H. Cohen,‡ and A. Gedanken*,† Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan, 52900 Israel, and Chemical SerVices Unit, Weizmann Institute of Science, RehoVot, 76100 Israel ReceiVed: April 16, 1997; In Final Form: June 3, 1997X
Nanostructured particles of amorphous carbon-activated palladium metallic clusters have been prepared (in situ) at room temperature by ultrasound irradiation of an organometallic precursor, tris-µ-[dibenzylideneacetone]dipalladium [(φ-CHdCH-CO-CHdCH-φ)3Pd2] in mesitylene. Characterization by elemental analysis, transmission electron microscopy, differential scanning calorimetry, X-ray diffraction, X-ray photoelectron spectroscopy, and BET surface area measurements shows that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 Å. Each particle is found to have a metallic core, covered by a carbonic shell that plays an important role in the stability of the nanoparticles. The catalytic activity in a Heck reaction, in the absence of phosphine ligands, has been demonstrated.
1. Introduction Nanostructured noble metallic clusters have gained tremendous attention in a wide variety of fields of chemistry because of the unusual physicochemical properties.1,2 The nanoscale metal particles bridge the gap between the molecular and the solid state and display novel properties resulting from surface or quantum-size effects.1,3 Currently, considerable effort has been focused on the development of synthetic techniques, such as chemical reduction,2,4 photochemical reduction,5 and metal vaporization,6 to generate nanophasic Pd materials and tailor their properties. In order to prevent the formation of undesired agglomeration, these processes are often performed in the presence of stabilizing ligands, polymers, or various surfactants.1,2,7 The stability of these Pd metallic colloids depends on the characteristics of the special protecting agents, which are generally stripped off from the metal surface in vacuum and/or with temperature, resulting in agglomerated palladium metal powders or mirrors. Thus for any application, the originally formed solutions (in liquid phase) need to be used. The common way of immobilizing the palladium particle is to prepare it in the presence of a neutral solid support such as carbon-black, alumina, or silica, which improves the nanoparticle stability; this is known as activated or stabilized palladium. Irrespective of the mode of formation, these materials are shown to catalyze hydrogenation reactions.1,2 However, the liquidphase Pd colloids are reported to show poor catalytic activity toward carbon coupling, Heck reactions,8 due to the formation of an inactive precipitate. Unfortunately, a facile in situ synthesis of solid carbon-activated palladium nanoclusters has not yet been reported. Herein, we describe a simple in situ process for the preparation of carbon-activated palladium nanoparticles and the solid product serves as a catalyst for the Heck reaction. The preparation is based on ultrasonic irradiation of a clear homogeneous solution of a new organometallic precursor at room temperature, where acoustic cavitation, namely, the formation, growth, and implosive collapse of bubbles in a liquid medium, is believed to play a major role in the metallic particles synthesis.9 The extremely high temperatures (several thousands of degrees) and pressures (hundreds * Corresponding author. E-mail:
[email protected]. Fax: +972-3-535 1250. † Bar-Ilan University. ‡ Weizmann Institute of Science. X Abstract published in AdVance ACS Abstracts, August 1, 1997.
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of atmosphere) and the very high cooling rates (≈1010 K/s) attained during acoustic cavitation lead to many unique properties in the irradiated solution.10 2. Experimental Section Ultrasound irradiation of a solution of tris-µ-[dibenzylideneacetone]dipalladium (C51H42O3Pd2) precursor in mesitylene yields a black powder. The precursor is highly soluble in the mesitylene solvent, whose vapor pressure is low at the sonication temperature. Mesitylene (Aldrich) and the precursor (Alpha) were used as received. The purple-colored homogeneous solution of the precursor (300 mg) in mesitylene (50 mL) was first sonicated using high-intensity ultrasound radiation for 3 h by employing a direct immersion titanium horn (Vibracell, 20 kHz, 100 W/cm2) under argon at a pressure of approximately 2 atm. The solution to be sonicated was purged with argon and sonicated under argon atmosphere. The sonication cell was kept immersed in a cold bath containing a dry ice-acetone mixture during the entire sonication. The formation of fine solid particles during sonication was observed by the scattering of light from a He-Ne laser. The resulting black-colored colloidal solution was taken into the glovebox (Ar atm; O2 level less than 10 ppm) and carefully transferred into a centrifuge tube. The black powder was separated by centrifuging (9000 rpm for 30 min), then washed thoroughly with mesitylene, THF, and ethanol, and dried in vacuum. The infrared (IR) spectrum of the resulting residue did not show any characteristic absorption of the organometallic precursor, indicating the complete decomposition of the precursor. Elemental analysis of the black residue gave >41% carbon weight, with a trace amount of hydrogen (
