Silver-palladium clusters

The reaction of these platinum or palladium clusters with transition metal compounds has also been studied and many cluster derivatives have been prepared for this method, which included reactions with gold,3 02-3306 silver, 302,33 3,3307-3310 copper 3302,3303,3309-3311 Qr mercury com ... 

Zeolite channels have provided sites for silver [555, 556], silicon [557], and selenium [558, 559] clusters copper clusters have been generated between the layers of montmorillonite [560] and copper, platinum, and palladium clusters were formed in silicon dioxide matrices [561]. 

Recently, novel nanomaterials have become a new frontier for SERS experiments, where different metals are collected together to form, for example, bimetallic particles. Thus, the same nanoparticle could be responsible for both SERS effect and catalytic activity. This is the case of the Ag/Pd colloids synthesized by chemical reduction with sodium borohydride (NaBH4) of silver nitrate (AgNOs) and palladium nitrate (Pd(N03)2), with a 96 4 Ag/Pd molar ratio [11]. The silver nanoparticles provide the SERS enhancement for the ligand molecules, while palladium may induce catalytic reactions. Also, in this case, TEM microscopy provides an important help to characterize these composite materials. In Fig. 20.6 TEM images at different magnifications are reported for bimetallic Ag/Pd particles, in comparison with those constituted by pure silver. While these latter present spheroidal shapes, bimetallic particles show more irregularities, due to palladium clusters in contact with the silver core surface. 

Early calculations for copper, palladium, and silver clusters were carried out by various investigators using the EH and CNDO methods, and among these is an attempt by Baetzold to take into account the effect of a carbon support on the electronic structure of a palladium cluster.In 1976, Messmer etal. compared the efficacies of the three methods of calcu-... 

D. E. CUffel, F. P. Zamborini, S. M. Gross, and R. W. Murray, Mercaptoammo-nium-monolayer-protected, water-soluble gold, silver, and palladium clusters, Langmuir, 16 (2000) 9699-9702. 

Lower stability of palladium clusters compared to nickel and platinum ones is explained by lower energy of the Pd —Pd bond. It is assumed that the energy of a M — M bond can be measured by the energy of sublimation, which for Ni, Pd, and Pt is 427, 354, and 565 kJ mol respectively. For Cu, Ag, and Au, the energy of sublimation is 339, 285, and 364 kJ mol respectively. It turns out that in the case of group 11 metals, according to predictions, the number of silver clusters is smaller than that of copper and gold clusters. 

II. The change of the sign between these two models may be also important for clustering and the surface growth mechanism. In the selfconsistent theory, the silver atoms tend to establish bonds with palladium atoms, while in the non-selfconsistent theory, the Ag-Ag bonds are favored. This explains the differences between the concentration profiles calculated within the models I and II and the fact that they do not simply follow the variations of the on-site terms. 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

Further annealing induces additional Ag overlayer enrichment with Pd atoms, causing a substantial intensity increase of the Pd resonant state, while the intensity at the Fermi level remained very small. This is a clear indication of the localized character of the Pd 4d state. The annealing of the Ag multilayer produces a surface alloy with a composition very close to Ago.sPdo.s which has a DOS at the Fermi level substantially smaller than the pure palladium. The annealing at higher temperature produces a Pd(l 10) surface with very small but very persistent amount of silver, which is in the form of three-dimensional clusters, located most probably below the first Pd(l 1 0) layer. 

Nanosized cobalt, copper, gold, nickel, rhodium, and silver particles have been stabilized by polyions and polymers [514, 549-553]. Particularly significant has been the simultaneous reduction of HAuC14 and PdCl2 in the presence of poly(iV-vinyl-2-pyrrolidine) to give relatively uniform, 1.6-nm-diameter, palladium-coated gold bimetallic clusters [554]. 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

Other metallic clusters that have been demonstrated to show the QDL effect are palladium [116, 117], silver [118] and copper [119]. Palladium MFCs capped with mixed monolayers of hexanethiolate/dodecanethiolate and ferrocene thiolate ligands are prepared in a manner similar to that employed for gold MFCs. The DPV studies exhibit a quantized charging effect but the current peaks are not as well defined as those observed for Au-MPCs. Capacitance values of the order of 0.35 aF are obtained, indicating smaller core sizes or thicker monolayer dielectrics [116]. 

In a similar vein, Sakai, Takenaka and Torikai produced Nafion/metal microcomposites by reducing palladium, rhodium, platinum and silver ion-exchanged membranes with H, at high temperatures (100-300°C) (26). The affected metal clusters, distributed homogeneously, were about 50 A in size. As in Risen s work, a polymer matrix template effect appears to be operative. These systems were considered and evaluated within the context of gas separations technology. 

Figure 11. (a) The conduction gap observed in small clusters of gold, palladium, cadmium, and silver as a function of cluster volume, (b) Normalized slope of the /-K curves (the conductance) as a fraction of cluster volume for the four metals studied. Above a critical volume of ca 4nm the slope becomes size-independent. This is possibly a direct indication of a size-induced-metal-insulator transition, as indicated. Taken from Vinod et a/. ... 

Figure 5.7. TEM micrographs of polystyrene-embedded metal clusters (a) Palladium (Pd(SCi2H25)2-polystyrene was annealed at 170°C for 5min), (b) gold (AuSC,2H25-polystyrene was annealed at 180°C for 5min), and (c) silver (AgSQjHjy-polystyrene was annealed at 150°C for 1 min).

Keywords Aluminum Capping principle Closed-shell principle Clusters Condensed clusters Copper Electron-counting rules Gallium Gold Jellium model Nanoclusters Palladium Polyhedral skeletal electron pair theory Silver Tensor surface harmonic... 

Palladium and silver, which have very low metal-metal bond energy values, form considerably fewer clusters than their homologs, and their stable polynuclear carbonyls are not known. 

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