Amorphous-carbon-activated palladium

Dhas, N. A., Cohen, H., and Gedanken, A., In situ preparation of amorphous carbon-activated palladium nanoparticles. J. Phys Chem. B 101, 6834 (1997a). 

Similar studies in an organic solvent yielded almost the same product [66]. 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-//-[dibenzylideneacetone]dipalladium [(p-CH= CH-CO-CH=CH-5 )3Pd2] in mesitylene. Characterization studies show that the product powder consists of nanosize particles, agglomerated in clusters of approximately 800 A. 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. 

Gedanken and coworkers [194] exploited power ultrasound to generate in situ amorphous-carbon-activated palladium metallic clusters that proved to be a catalyst for Mizoroki-Heck reactions (without phosphine ligands) of bromobenzene and styrene (yield to an appreciable extent of 30%). The catalyst is stable in most organic solvents, without showing any palladium powder segregation, even after heating them to 400 °C. 

In situ preparation of amorphous carbon-activated palladium nanopartides. Journal of Physical Chemistry B, 101, 6834-8. 

The many forms of so-called amorphous carbon, such as charcoals, soot, and lampblack, are all actually microcrystalline forms of graphite. In some soots the microcrystals are so small that they contain only a few unit cells of the graphite structure. The physical properties of such materials are mainly determined by the nature and magnitude of their surface areas. The finely divided forms, which present relatively vast surfaces with only partially saturated attractive forces, readily absorb large amounts of gases and solutes from solution.9 Active carbons impregnated with palladium, platinum, or other metal salts are widely used as industrial catalysts. 

Gedanken et al. prepared palladium nanoparticles by decomposition of [Pd2(dba)3] or [Pd(OAc)2]/Me3(Cj4H29)NBr by extended exposure to ultrasound. In the case of [Pd2(dba)3], the colloidal metal particles were stabilized by amorphous carbon formed from dba. Activities of the soluble colloids in the Heck reaction were comparable to those of the heterogeneous catalyst palladium on charcoal (Table 2) [30, 31]. 

Total combustion of m-xylene was studied over palladium catalysts in various forms (amorphous alloys, deposited over clinoptilolite and over stainless steel flakes). Low/high activity states were observed due to the different oxidation states of palladium measured by XPS method. Homogeneous chain reaction initiated by the catalyst surface was found to be responsible for the high activity state and involvement of the surface carbon was the key issue in the self-sustained and damped oscillations. 

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