Pd cluster

As catalyst, a Pd/Fe system is used, having finely dispersed Pd clusters (< 1 pm) on the Fe surface [20] (see also original citations in [20]). A considerable portion of the surface remains uncovered, exposing Fe for reaction. 

Kan et al. reported preparation of Au-core/Pd-shell bimetallic nanoparticles by successive or simultaneous sonochemical irradiation of their metal precursors in ethylene glycol, respectively. In the successive method, Pd clusters or nanoparticles are first formed by reduction of Pd(N03)2, followed by adding HAUCI4 solution. As a result, Au-core/Pd-shell structured particles are formed, although Pd-core/Au-shell had been expected. In their investigations, the successive method was more effective than the simultaneous one in terms of the formation of the Au-core/Pd-shell nanoparticles [143]. 

Figure 12. Schematic representation of the setup for single particle measurements by electrostatic trapping (ET). Pt denotes two freestanding Pt electrodes (dashed region). A ligand-stabilized Pd cluster is polarized by the applied voltage and attracted to the gap between the Pt electrodes. (Reprinted with permission from Ref. [29], 1997, American Institute of Physics.)...

Low Pd concentrations are beneficial in preventing precipitation of inactive Pd metal.144 Small Pd clusters can be observed in phosphine-free systems,145 and these particles may serve as catalysts or, alternatively, as reservoirs of Pd for formation of soluble reactive species. 

Two reviews18,19 with 20 references are presented. A series of Pd clusters containing from four to several hundred Pd atoms in the metal skeleton was prepared and characterized with structural data and by chemical properties including catalytic activity. 

Substrate reactivity was as expected (Arl > ArBr ArCl). In contrast to the Suzuki cross-coupling, however, Cu and Ru clusters were not active in the Heck reactions, and the activity of Cu/Pd clusters was lower than that of pure Pd clusters. Note the higher activity of Pd clusters prepared in situ (row F) compared to pre-prepared clusters (rows B and G). This increased activity tallies with our findings for Suzuki cross-coupling (7). After reaction, palladium black was observed in all the vials in rows B and G, but not in row F. 

Control experiments confirmed that DMF solutions containing small amounts of PVP-stabilised metal clusters are heated much faster under m/w irradiation than pure DMF or DMP/PVP solutions. Thus, pure DMF and DMF/PVP took 7 min to reach boiling (153 °C), while a 10 mM soln of PVP-stabilised Pd clusters in DMF boiled after 2 min (all other parameters were identical, including reaction vessel position in the oven, vide infra). 

Further, we examined the Heck reaction between w-butyl acrylate and 4-bromobenzotrifluoride 5 in the presence of 2 mol% Pd clusters in a singlevessel monomode m/w oven fitted with an infrared thermometer. 100% conversion with quantitative yield to the cinnamate was obtained after 5 min irradiation at 75 W/240 °C. We then repeated the reaction under conventional heating at 240 °C. After 3.5 min a black tarry gel formed. Extraction followed by GC analysis showed only cinnamate, but the tarry material (probably acrylate polymers/oligomers) could not be analysed. These experiments show that when clusters are present different results are obtained depending whether m/w heating or conventional heating is used. In principle, this could be the result of hot spots created on the metal clusters. 

Figure 2.17 STM images of Pd clusters grown on AI203/Ni3AI(l 1 1) at different substrate temperatures. The image size is 100 nm x 100 nm and the bias voltage is 0.7 V. (Reproduced with permission from Ref. [54].)...

Figure 3.25 A sequence of STM images taken at 673 K of oxygen spillover from Pd clusters in an ambient pressure of 5 x 10-8 mbar of 02 (doubled before image (f)).The total exposures to oxygen from image (a)-(f) were (in units of L) 114, 178, 237, 282, 344, and 531. The spillover begins with the formation of one layer...

Figure5.21 STM imageof 12 clusterfleldson Au(l 1 l),eachfield containing 2500 Pd clusters. Electrolyte 0.1 M H2S04 + 1 mM PdS04.

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

A. G. Zacarias, M. Castro, J. M. Tour, J. M. Seminario, Lowest Energy States of Small Pd Clusters Using Density Functional Theory and Standard ab Initio Methods. A Route to Understanding Metallic Nanoprobes, J. Rhys. Chem. A. 1999,103,7692-7700. 

If we refer back to figure 2, it is clear that although particle contrasts are usefully enhanced in the ratio image, it is impossible to deduce from this image alone whether the sample is a mixture of Pt and Pd particles or comprises alloyed Pt/Pd clusters. 

Vargaftik, M.N. et al., Giant Pd clusters as catalysts of oxidative reactions of olefins and alcohols, J. 

Volkov, V. V. et al., Long- and short-distance ordering of the metal cores of giant Pd clusters, J. Cryst. 

Ozensoy et al.127 also used PM-IRAS to study the CO adsorption behavior on Si02-supported Pd clusters. As mentioned above, these crystalline, ultrathin silica films possess the structural and electronic properties of the bulk analogues, but are thin enough to permit the use of vibrational and electronic spectroscopic techniques (and tunneling microscopy) without charging.39 40 As with the... 

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