Rainer Brinkmann

Nanoscale colloidal metals and alloys stabilized by solvents and surfactants Preparation and use as catalyst precursors

Solvent-stabilized organosols of the early transition metal series, e.g. Ti, Zr, Nb, and Mn, may be prepared by the reduction of the THF adducts or thioether solutions of the corresponding metal halides with K[BEt3H]. Mono- and bimetallic organosols of Group 6–11 metals stabilized by tetraalkylammonium halides may be formed either by the reduction of the metal salts using NR4 hydrotriorganoborates or conventional agents, e.g. H2 or HCO2H, after the pretreatment of the metal salts with NR4X. The chemical reduction of transition metal salts in the presence of hydrophilic surfactants provides straightforward access to nanostructured mono- and bimetallic hydrosols. This synthesis can be performed even in water. Mono- and bimetallic nanoparticles stabilized by lipophilic or hydrophilic surfactants of the cationic, anionic or nonionic type serve as precursors for heterogeneous metal colloid catalysts effective for the hydrogenation and oxidation of organic substrates. Bimetallic precursors, e.g. PtRh, have a synergic effect on the catalytic activity. A comparison of catalytic results and CO chemisorption experiments has revealed that the protecting surfactants still cover the nanoparticle surface after adsorption on supports, which markedly improves the lifetime of the catalysts. Chiral protecting agents may induce enantioselectivity in metal colloid catalysts.

Opals: Status and Prospects

The beauty of opals results from a densely packed, highly ordered arrangement of silica spheres with a diameter of several hundred nanometers. Such ordered nanostructures are typical examples of materials called photonic crystals, which can be formed by known microstructuring methods and by self-assembly. Opals represent a self-assembly approach to these structured media; such an approach can lead to novel materials for photonics, photocatalysis, and other areas. Although self-assembly leads to many types of defects, resulting in the surprising and very individual appearance of natural opals, it causes also difficulties in technological applications of opal systems.

A size-selective synthesis of air stable colloidal magnetic cobalt nanoparticles

A novel, size-selective preparation route leads to air stable ‘monodisperse’ colloidal cobalt nanoparticles via the thermolysis of Co2(CO)8 in the presence of aluminum alkyls. X-ray absorption near edge structure measurements have proved that this preparation pathway provides long term stable zerovalent magnetic Cobalt particles. In addition, these measurements show that the chemical nature of the surfactant used exerts a significant influence on the stability and the local electronic and geometric structure of the analyzed nanoparticles.

A correlation between 13C and 59Co NMR data and the catalytic activity of organocobalt complexes in the synthesis of pyridine derivatives

Abstract The correlation between the 13 C and 59 Co NMR spectra of substituted cyclopentadienylcobalt complexes and their catalytic properties in the synthesis of pyridine derivatives is examined. Since the correlations can be expressed as linear relationships, a direct screening of potential catalysts by NMR is possible.

Highly dispersed metal clusters and colloids for the preparation of active liquid-phase hydrogenation catalysts

Abstract A comparison of the activities of conventionally precipitated metals on charcoal, deposited organometallic clusters and metal colloids for liquid-phase hydrogenation has been made under standard conditions in order to assess the effect of particle size. The strong SMSI effect known for TiO2, supports was simulated on charcoal by doping the surface with small amounts of Ti(O) and subsequent oxygenation.

SELECTIVE OXIDATION OF GLUCOSE ON BISMUTH-PROMOTED PD-PT/C CATALYSTS PREPARED FROM NOCT4CL-STABILIZED PD-PT COLLOIDS

Charcoal-supported Pd-Pt catalysts based on Pd-Pt/NOct4Cl colloidal alloys have superior activity and selectivity in the oxidation of glucose to gluconic acid compared with industrial heterogeneous Pd-Pt catalysts. According to transmission electron microscopy, X-ray diffraction/Debye function analysis, X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and extended X-ray absorption fine structure analysis the chemical coreduction of PdCl2 and PtCl2 in the appropriate ratio with NOct4BEt3H yielded the alloyed Pd-Pt colloids in organic solvents. They are screened by the lipophilic NOct4Cl surfactant layer from coagulation and poisoning. TEM showed colloids of particle sizes in the range from 1.5 to 3 nm.

The preparation of finely divided metal powders and transition metal complexes using “organically solvated” magnesium

Abstract Treatment of commercial magnesium powder in THF with a small amount of anthracene generates a highly active form of magnesium (Mg★). The Mg★ is an excellent in situ reducing agent for transition metal salts, giving highly reactive metal powders of Groups 8–12. In the presence of electron donor ligands, this reduction provides a useful one-step route to organotransition metal complexes. The application of 35 kHz ultrasound during the reaction improves the dispersity of the metal powders and enhances the yields of the complexes.