Helmut Bönnemann

Synthesis and Characterization

As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Nanoscopic Metal Particles − Synthetic Methods and Potential Applications

Mono- and bimetallic colloidal particles have gained increasing attention in science and application throughout the last several years. In this contribution, we present a synopsis of the wet chemical syntheses of these materials and survey potential applications in catalysis and materials science. Methods for the characterization of these particles and their surfaces are not reviewed here.

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.

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.

Interaction between core and protection shell of N(butyl)4Cl- and N(octyl)4Cl-stabilized Pd colloids

Abstract We present X-ray absorption near edge structure (XANES) measurements on N(butyl)4Cl- and N(octyl)4Cl-stabilized Pd colloids at both the Pd LIII- and the Cl K-edge. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (HeI) results for these colloids, deposited on silica substrates, are also shown. The results provide detailed insight into the mechanism of bonding between the protection shell and the colloidal core. Both XANES and MIES suggest that the chlorine is present on the inside of the protection shell, located between palladium core and N(alkyl)4 groups forming the protection shell. Moreover, the XANES results suggest a dependence of the equilibrium position of the chlorine between the metal core and the alkyl chain on the length of the alkyl chains. The possible motivation for such an effect is discussed on the basis of different models. MIES, in addition, provides information on the thermal stability of the shell-stabilized Pd colloids.

Noble Metal Nanoparticles Incorporated in Mesoporous Hosts

Noble metal nanoparticle containing [Me]x-MCM-41 were synthesized using surfactant stabilised palladium, iridium and rhodium nanoparticles in the synthesis gel. The materials were characterised with XRD, ICP AES, TA, TEM and nitrogen sorption, which showed that the nanoparticles were present inside the pores of MCM-41. The [Me]x-MCM-41 were found to be active and selective catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene and norbornene.

Surface engineering of Co and FeCo nanoparticles for biomedical application

Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition of metal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metal nanoparticles after surface passivation. The particles are characterized by electron microscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-ray absorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stable magnetic fluids in various organic media and water, exhibiting a high volume concentration and a high saturation magnetization. In view of potential biomedical applications of the particles, several procedures for surface modification are presented, including peptization by functional organic molecules, silanization, and in situ polymerization.

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.