Polymer stabilized clusters

The cluster Pd2 is formed in the reduction of Pd(NH3)Cl2 in the presence of polyethyleneimine. About 60% of the Pd-II can be reduced to this form of monovalent Pd until further reduction to colloidal palladium occcurs. The stabilized cluster lives for days. It can be titrated with H2O2 and with methyl viologen, and it reacts with hydrogen sulfide to yield hydrogen  

The absorptions of stabilized oligomeric clusters of copper and of platinuirf have also been detected in the reduction of Cu-II and Pt-II salts, although no definite structures have yet been assigned to them. 

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COLLOID SURFACE CHEMISTRY 6. Polymer Stabilized Clusters... 

Formation of Missing Au SC via Thiolation of Polymer-Stabilized Gold Clusters... 

A method has recently been described for wrapping polymers around metal atoms and very small metal clusters using both matrix and macroscale metal vapor-fluid polymer synthetic techniques. Significant early observations are that (i) the experiments can be entirely conducted at, or close to room temperature, (ii) the resulting "polymer stabilized metal cluster combinations are homogeneous liquids which are stable at or near room temperature, and (Hi) the methodology is easily extended to bimetallic and trimetallic polymer combinations. ... 

An intriguing application of these Pd nanoparticles in basic research concerns the question of the solubility of H2 in such materials relative to bulk palladium [47]. Hydrogen concentration-pressure isotherms of surfactant-stabilized palladium clusters and polymer-embedded palladium clusters with diameters of 2, 3 and 5 nm were measured with the gas sorption method at room temperature. The results show that, compared to bulk palladium, the hydrogen solubility in the a-phase of the clusters is enhanced fivefold to tenfold, and the miscibility gap is narrowed. Both results can be explained by assuming that hydrogen occupies the subsurface sites of the palladium clusters. The Pd-H isotherms of all clusters show the existence of hysteresis, even though the formation of misfit dislocations is unfavorable in small clusters. Compared to surfactant-stabilized clusters, the polymer-embedded clusters show slow absorption and desorption kinetics. Moreover, evidence for a cubic-to-icosahedral transition of quasi-free Pd-H clusters by the hydrogen content was reported [47c]. 

Zuo, X., Liu, H., Tian, J. (2000) Asymmetric hydrogenation of alpha-dike-tones over polymer-stabilized and supported Platinum clusters, J. Mol. Catal. A. Chem. 157, 217-224. 

Density functional theory (DFT) has been applied to the investigation of the heterojunction effect resulting from the introduction of heteroatoms into mono precious metal clusters and the interaction between precious metal clusters and stabilizing polymers in cluster model systems. In the present theoretical study, we attempted to explain the heterojunction effect for these model systems as a first step for understanding the heterojunction effect for catalytic reactions involving precious metal cluster catalysts. 

The whole polymer-stabilized precious metal clusters are too large to investigate by using first principle calculations. Therefore, the monomers of the polymers and similar small molecules are used instead of the real polymers and dendrimers. AU the geometries of model systems were fully optimized with C symmetry. 

Precursors, Reducing Agents, and Polymer Stabilizers for Ag Cluster Synthesis... 

Kinetic studies of competitive alcohol oxidation over Au/TiOj have postulated a unique ensemble in which a carbocation can be stabilized adjacent to a neighboring gold-oxo center [163]. This work proposed that benzylic alcohols oxidize via C-H cleavage at the benzylic position. Electronic effects in PVP-stabilized gold nanoparticles for p-hydroxy benzyl alcohol selox have also been reported [164] XANES, XPS, and FT-IR suggest that anionic 1.5 nm Au clusters are the most catalytically active, with electron donation from the polymer stabilizer creating Au-superoxo species (Figure 2.11). 

Further progress is expected in the development of novel methods for the preparation of polymer-immobilized clusters and nanoparticles. A promising direct synthesis of these products is the dissolution of zero-valent metals into polymer solutions that function as macroligands. Subsequent recrystallization of particles then occurs, much like what occurs in biological systems (Section XI). Stabilization of nanoparticles and clusters during polymerization of micelle-forming monomers could become a significant practical method. 

Hence, realization of the yielding process in amorphous glassy polymers requires clusters stability loss in the mechanical stress field, after which mechanical devitrification of the loosely packed matrix proceeds. Similar criterion was obtained for semicrystalline polymers [24]. 

In Chap. 2, metal nanoparticles (abbreviated MNPs) are introduced and defined as giant clusters of nanometer size with precise geometries resulting from the packing of atoms. - Ideally, their size and dispersity can be controlled during their synthesis. They can be covered with ligands, anionic stabilizers such as halides (see below), polyoxometallates, etc. or various polymer stabilizers that will define their solubility in organic solvent, water or fluorous solvent... 

Polymers at low concentration are often used to stabilize the final colloidal nanoparticles which result from the reduction of metal ions in solution. In some cases, however, small intermediate clusters are stabilized. For example, whereas metallic nanoparticles are formed in the reduction of Ag ions in the presence of sodium polyphosphate at low concentration, (Figure 2), the non-metallic cluster Ag9 is stabilized at higher concentrations of this polymer. The cluster can be enriched up to more than 90% reduction of the Ag" ions. It lives several days upon aging of the reduced solution as its absorptions disappear, the 380 nm absorption of nano-sized particles appears. Clusters are also trapped using sodium polyacrylate as protecting material. ... 

This method is similar to the ligand exchange described in Section 2.1.2 except that the precursor clusters are stabilized by the polymer through much weaker interactions [20]. 

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