Palladium aggregation

Pt or Pt/Au particles have also been hosted in the hydrophobic holes of nonionic surfactants, e.g., polyethylene monolaurate [46a,62]. The structure and diffusive dynamics of a colloidal palladium aggregate sol have been studied under dilute and semidilute conditions by high-resolution small angle X-ray scattering and X-ray photon correlation spectroscopy. When the sizes of the aggregates determined in the static structure and as derived from the diffusive dynamics at low concentration are consistent with each other. 

Such a difference in terms of product selectivity was attributed to the complete absence of any acidic sites on the carbon nanotubes sur ce and also to the absence of micropores which could induce re-adsorption and consecutive reaction [16]. The presence of micropores could artificially increase the contact time and as a consequence, modify the hydrogenation pathway. The influence of the support nature on the electronic properties of the metallic phase eould also be put forward to explain these results. Depending on the metal-support interaction, the metal particles could exhibit different exposed faces and as a consequence, significantly modify the chemisorption of the reactant on their surface. According to the interaction between the C=C bond and the laces exposed by the palladium particles, the residence time and the desorption of the intermediate could be different and thus, lead to a different selectivity. The presence of palladium aggregates on the activated charcoal as compared to the individual palladium dispersion on the CNTs could be the illustration of this difference in exposed crystalline feces. 

The structure of a non-hydrolyzed dimer, cA-[(2,2 -bipyridine)palladium(II)]2(/i-l,3-N03)2 2+ is shown in Figure 45.521 The dimerized m-( 2.2-bi pyridine) Pd11 units aggregate into a dimer-to-dimer linear conformation through direct metal—metal interaction522 and tt-tt stacking.523 Notably, the dimer was first reported to be linked by double nitrato-bridges (/x-l,3-N03)2 in a cofacial... 

The palladium phthalocyanine (67), developed by Mitsui Toatsu and Ciba58,59 is one of the leading phthalocyanine infrared absorbers for CD-R (Compact Disk-Rewritable) (see Chapter 9.13). Bulky groups (R) reduce undesirable molecular aggregation, which lowers the extinction coefficient and hence the absorptivity and reflectivity. Partial bromination allows fine tuning of the film absorbance and improves reflectivity. The palladium atom influences the position of the absorption band, the photostability and the efficiency of the radiationless transition from the excited state.58 It is marketed by Ciba as Supergreen.60... 

The palladium(II) phthalocyanines are satisfactory singlet oxygen generators. Thus, f>A values for tetra-t-butylphthalocyanine ((27), with stated replacement for Mg) are 2H, 0.22 Zn, 0.34 Pd, 0.54, i.e., increasing in line with the heavy atom effect.180 For palladium(II) tetra-t-butylphthalocyanine ((27), Pd instead of Mg) in benzene at 290 K, 4>f= 0.048 and X = 0.49.180 However, palladium(II) 2,3-dihydroxy-9,16,23-tri-t-butylphthalocyanine was synthesized by a mixed condensation (1 9 mix of appropriate dinitriles) as a likely amphiphilic sensitizer, but did not show bioactivity in an enzyme assay, possibly because of aggregation.180... 

Like the palladium(II) complexes, the platinum(II) porphyrins show appreciable phosphorescence even in aqueous media at room temperature in one study,169 singlet oxygen quantum yields ranged from 0.1 to 0.9 and were strongly influenced by dimerization/aggregation. Platinum(II) 5,10,15,20-tetrakis(/>-carboxyphenyl)porphyrin and platinum(II)coproporphyrin-I ((36) for Pd read Pt) have been studied as phosphorescent labels of antibodies for use in time-resolved microscopy.189... 

Addition of pvp significantly extends catalyst lifetime (Scheme 2). This is a well-known protocol whereby the Pd nanoparticles are wrapped up in the polymer chains, which prevents their aggregation into catalytically inactive palladium black. 

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. 

Leitner et al. have synthesized the PEG-modified silica stabilized and immobilized palladium nanoparticles for aerobic alcohol oxidation in combination with scC02 as reaction medium under mild conditions, which show high activity and excellent stability under continuous-flow operation [68], ScC02 could diffuse the substrates and products from the active nanoparticles in a gas-like manner. This allows rapid chemical transformation at the active center, ensures efficient removal of the products from the surface, and minimizes the mobility of solid-supported catalytically active species [69]. In this way, catalysts based on palladium nanoparticles together with PEG as stabilizing matrix could avoid aggregating and forming less active and selective Pd-black [20, 60, 70]. 

Recently the sandwich motif has been extended to the case of two cyclic aromatic ligands flanking a small planar aggregate of metal atoms. In Pdsf r 7-C7117)203KPPI14), a triangular unit of palladium(O) atoms each coordinated by... 

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