Metal palladium clusters

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

Fig. 51. Scanning electron microscope image of different stages of metalization of DNA. (a) Linear chain of separated palladium clusters connecting two gold contacts (b) magnification of (a) showing clusters with diameter > 40 nm (c) continuous coated DNA strand after one development step with a diameter larger than 40 nm. Reproduced with permission from Ref. (175). Copyright 2001, American Institute of Physics.

Palladium dimethylsilanedithiolato complex is a precursor for Ti—Pd and Ti—Pd2 heterometal-lic complexes.311 The bis-silyl sulfides (R3Si)2S have been widely used to prepare a variety of metal sulfide clusters, because these reagents exploit the strength of the Si—O and Si—Cl bond to... 

The reaction with [Mn(CO)5]2 or [Co(CO)4]2 also led to mixed-metal clusters.962,963 Osmium-palladium mixed-metal carbonyl clusters were made using the unsaturated cluster [Os3(CO)io(/r-H)2] and the carbido cluster [Os5(/U5-C)(CO)15].964 970 Treatment of [Os3(CO)10Gu-H)2] with [Pd2(/u-dppm)2Cl2] afforded the novel high-nuclearity osmium-palladium mixed-metal carbonyl clusters [Os5Pd6(CO)13( -CO)5(/u-H)2(/u-dppm)2], [Os5Pd6(CO)13(//-CO)6(/u-dppm)2], and... 

Moiseev, I. I. Vargaftik, M. N. Catalysis with palladium clusters. In Catalisis by Di- and Polynuclear Metal Cluster Complexes Adams, R. D. Cotton, F. A. Eds. Wiley VCH New York, 1998, p 395. 

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 ... 

In contrast to the usual Wacker-conditions, optimum rates and catalyst stability in the Pd/batophenanthroHne-catalyzed olefin oxidations was observed in the presence of NaOAc (pH s 11.5). Under such conditions, the catalyst-containing aqueous phase could be recycled with about 2-3 % loss of activity in each cycle. In the absence of NaOAc precipitation ofPd-black was observed after the second and third cycles. Nevertheless, kinetic data refer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallic palladium. 

Comparing these results for CO bonded to Ni(lOO) and Pd(lOO) to the structure of metal carbonyl clusters, one finds that the multiply-coordinated CO on palladium has... 

Several mechanistic proposals have been advanced, ranging from metal catalysis to redox cycles with Pd clusters or isolated Pd/PdO species (325-327). It is fair to assume that activation of ethylene occurs by hydrogen abstraction by palladium clusters, whereas H abstraction from acetic acid occurs on PdO (oxidized clusters) ... 

All characterized Pd(I) compounds have Pd-Pd or Pd-M bonds. Accordingly, the paramagnetic behavior anticipated for a d configuration is not found in these multinuclear diamagnetic complexes as the expected unpaired electron is in fact involved in the formation of a metal-metal bond. There are some palladium clusters in formal oxidation states ranging from 0 to -1-1 (metal-metal bonds do not count in the assignment of oxidation states) that will be considered here. 

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. 

Fig. 17 Bottom Pd/X9 clusters used to study the adsorption of a Pd atom (dark sphere) on several metal surfaces. Cluster 1 models the bonding of Pd to an fee hollow site of Al(lll), Rh(lll) or Pd(lll). Cluster II models the adsorption of palladium on a hollow site of W(110). Top calculated 4d electron population for the palladium adatom in the Pd/X9 clusters. Reprinted from ref [32].

In their flow tube reactor studies, Hintz and Ervin found that Pt and Pd cluster anions react with molecular oxygen under addition of multiple O2 units [416]. Whereas no monoxide formation is detected, increasing palladium cluster fragmentation with cluster size is observed upon reaction with O2 (loss of metal atoms for Ptg and Pt4 ). The observation of cluster fragmentation... 

In further work of the group of Ervin, the activity of palladium cluster anions in the CO oxidation catalysis was examined and it was found that Pd also efficiently catalyzes the CO combustion reaction. The palladium clusters, however, are reported to exhibit more fragmentation than the platinum clusters, consistent with the weaker meal-metal bond strength in palladium relative to platinum [22]. 

The Reactivity of Free Palladium Clusters and Atomic Metal Ions... 

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

Size and shape requirements of metal cluster models for reaching convergence of the chemisorption energy constituted the main topic of our study [255]. To reach convergence, we proposed to use symmetric models that can be accurately treated for clusters comprising of up to more than hundred atoms. A series of octahedral and cuboctahedral palladium clusters was considered, from Pdss to... 

It is noteworthy that besides highly dispersed palladium clusters, ionic Pd was also detected by ESR in Pd/Li-AbOa catalysts prepared by anchoring PdCb onto alumina modified with butyl lithium. These catalysts were highly active and selective in the hydrodechlorination of substituted aromatic compounds. The coexistence of Pd and Pd° in the Pd/Li-AbOa catalysts suggests the possible formation of mixed metal ion - metal nanocluster ensemble sites. 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

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