Werner Brijoux

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.

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.

Application of heterogeneous colloid catalysts for the preparation of fine chemicals

Preprepared nanometals stabilized by surfactants may be used as precursors for a new type of heterogeneous catalyst. Independent of the support these mono– or plurimetallic precursors may be optimized dependent on the size, composition, and structure of the particles. Further, the active metal surface may be shielded against poisons by the protective shell. In addition, doping agents may be used in order to enhance the catalytic performance. The cis–selective partial hydrogenation of 3–hexyn–1–ol giving leaf alcohol, a valuable fragrance, and the selective oxidation of glucose giving sodium gluconate are current examples for the application of supported nanometal colloids in fine chemicals catalysis.

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.

Surfactant-Stabilized Nanosized Colloidal Metals and Alloys as Catalyst Precursors

Publisher Summary Due to their catalytic potential, colloidal mono- and bimetallic materials have, after a period of hibernation, attracted more attention, notably through the contributions of Boutonnet et al. An important progress consists of the preparation of water soluble nanoclusters using hydrophilic P- or N-donors as stabilizer . Further miscellaneous agents have been used for this purpose. The first nanosized metals stabilized by surfactants were reported in 1976. In the course of the research in this area, one has developed new methods for the preparation of stable and very soluble metal organo- and hydrosols having a narrow particle size distribution. The metallic core derived from elements of the periodic table Group 6-11 is protected by lipophilic and hydrophilic surfactant molecules.

EXAFS/XANES, chemisorption and IR investigations of colloidal Pt/Rh bimetallic catalysts

Bimetallic Pt/Rh heterogeneous catalysts based on nanoscaled colloids supported on activated charcoal have been shown to exhibit maximum activity in hydrogenation of butyronitrile at a composition of 10 atom% Pt and 90 atom% Rh in the metallic core. This synergetic effect has to be traced back to the special structure of the bimetallic nanoparticles as elucidated by X-ray spectroscopy and CO chemisorption. The surface of Pt/Rh particles was investigated by CO chemisorption combined with IR spectroscopy. It has been shown that a Pt 10 Rh 90 colloid exhibits the maximum ability to chemisorb CO, compared with other compositions. Based on the detection of the different types of bonding of CO molecules to the metal surface and the EXAFS results, our measurements reveal surface enrichment of the Rh component. The surface structure of the particles, which varies with the elemental composition, and the presence of a Pt-dominated core influence the CO chemisorption ability as well as the hydrogenation efficiency of the Pt/Rh colloid catalysts.

The Cobalt-Catalyzed Synthesis of Pyridine and Its Derivatives

Although quite spectacular results have been obtained in the last few decades in the field of homogeneous transition metal catalyzed transformations of olefins and alkynes [1], reactions which could lead to heterocycles have been partly neglected. An obvious reason for this is that substrates containing heteroatoms such as N, O or S could coordinate the metal and suppress the catalytic activity. Nevertheless, some interesting early examples of transition-metal-catalyzed syntheses of heterocyclic compounds have been reported and these have been reviewed by C. W. Bird [2].