Nanosized Pd particles

The longtime stabihty of surfactant-coated Pd nanoparticles in w/o microemulsions has been investigated. It has been proven that under suitable conditions, the use of the functionalized surfactant Pd(AOT)2 allows very stable nanosize Pd particles to be obtained and to finely control their average size [229],... 

In the very active field of unmodified nanoparticles recent discoveries have been made on size-selective Fischer-Tropsch catalysts that convert selectively CO and H2 into hydrocarbons there is a strong dependence of activity, selectivity and Hfetime on Co particle size. This topic of unmodified, supported or unsupported, nanoparticles is outside the scope of this chapter [74, 75]. Nevertheless, we mention discoveries made by Degussa, who have patented a process for H2O2 synthesis from molecular oxygen and molecular hydrogen with nanosized Pd particles (6 A) [76]. 

The issue of various adsorption sites featured by supported Pd nanoparticles was also dealt with in our work [256]. There, the adsorption of CO on nanosize Pd particles was studied theoretically by a DF method (BP86) and spectroscopically by means of IR reflection absorption spectroscopy (IRAS) and sum frequency generation (SFG). Three-dimensional crystallites of about 140 atoms (chosen as fragments of fee Pd bulk as justified above), that exhibit (111) and (001) facets, were studied in GGA BP86 calculations. Various types of adsorption sites were inspected three-fold hollow, bridge, and on-top positions at... 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Harada et al. [42] prepared nanosized palladium particles supported on activated carbons using a simple liquid-phase reduction of aqueous Pd complexes with KBH4. They found that the addition of appropriate amounts of NaOH into aqueous solutions of Na2PdCLt, followed by reduction with KBH4, produced highly dispersed Pd particles (less of 5 nm in diameter), irrespective of the carbon support used. The prepared catalysts were used efficiently in the liquid-phase oxidation of benzyl alcohol to benzaldehyde and in the liquid-phase hydrogenation of cinnamaldehyde to obtain the saturated aldehyde. 

Line-width broadening may also be caused by other fast relaxation mechanisms in addition to a small particle size. For example, it is well known that, for spherical particles, radiation losses become more pronounced with increasing radius. In some metals, these relaxation mechanisms are so strong that a well-defined plasmon resonance is not observed, as in Fe, Pd, and Pt. Nanosized particles are interesting because the optical resonance can be designed in. For example, in a nanoshell consisting of a dielectric core surrounded by a metallic outer layer, the relative dimensions of these components can be varied. This, in turn, varies the optical resonance, possibly over several-hundred nanometers in wavelength. 

Palladium nanoparticles vfith a size of a few nanometers supported on carbon are widely used as catalysts, for instance in three-way automotive exhaust catalysts and fuel cells, and can easily be prepared by impregnation of a porous support body with a precursor solution, followed by drying, decomposition of the precursor and, if necessary, reduction. It is well-known that the activity and selectivity of these catalysts for hydrogenation reactions depend on the palladium dispersion for particles sizes in the range 1-10 nm. It is, hence, not surprising that the interaction of Pd with hydrogen, and the infiuence of nanosizing, have been widely studied. 

Metal dithiocarbamates have been investigated as single-source precursors for MOCVD nanosized particles of metal sulfides, such as PtS and PdS,43 PbS,437,4 8 and Bi2S3,439 and tin sulfide thin films.440 In this respect the termochemistry of dithiocarbamates (periodically reviewed441 144) is important. Molybdenum dialkyldithiocarbamates are highly effective antiwear, antiseize, and antifriction additives for lubricating oils,445 and are used as vulcanization accelerators.446... 

In recent years, W/O microemulsions have found numerous applications as microreactors for specific reactions (for comprehensive reviews, see Refs. 94 and 95). Thus, it has been shown that hydrophilic enzymes can be solubilized without loss of enzymatic activity and used to catalyze various chemical and photochemical reactions [96,97]. Other interesting applications involve the polymerization of solubilizates in microemulsions [98] and the preparation of micro-porous polymeric materials by polymerization of single-phase microemulsions [99]. Furthermore, microemulsions have been used as microreactors for the synthesis of nanosized particles for various applications [93,95] such as metal clusters (Pt, Pd, Rh, Au) for catalysis [100,101], semiconductor clusters [102-104] (ZnS, CdS, etc.), silver halides [105], calcium carbonates, and calcium fiuoride [106]. Recently it was shown [107,108] that it is possible to use W/O microemulsions for the control of polymorphism of water-soluble organic compounds. In most of these appUcations, one or more reactants are solubilized within a microemulsion and then a reaction is initiated. Depending on its molecular structure. 

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