Andrea B. R. Mayer

Formation of noble metal nanoparticles within a polymeric matrix: nanoparticle features and overall morphologies

Colloidal noble metal nanoparticles dispersed within a polymeric, protective matrix and exhibiting tailored nanoparticle features are of growing technological interest. Such nanoparticle features include the nanoparticle size, size distribution, shape, and morphology. This essay describes some options to influence these by the variation of several parameters, such as the metal precursor type, the preparation method and conditions, and the protective polymer. Further variability of these metal-polymer systems is provided by the option to modify the overall morphology, that is, the additional tailoring of the location and distribution of the metal nanoparticles within the polymer matrix. Some examples based on block copolymer micelle systems are described.

Colloidal gold nanoparticles protected by water-soluble homopolymers and random copolymers

Abstract Stable gold colloids were prepared by the in situ reduction of tetrachloroauric acid (HAuCl4) in the presence of protective polymers. Several water-soluble polymers and random copolymers have been investigated for their ability to stabilize such gold colloids. UV-vis spectroscopy was used to follow the in situ reductions and to characterize the resulting gold colloids. The particle sizes and their size distributions were determined by transmission electron microscopy and found to be in the nanometer size range. The most stable colloids were obtained for polymers possessing hydrophobic backbones and for side groups that allow good interactions with the gold precursor ions.

Comparisons between cationic polyelectrolytes and nonionic polymers for the protection of palladium and platinum nanocatalysts

Several palladium and platinum nanocatalysts protected by cationic polyelectrolytes were prepared by the in-situ reduction of palladium chloride, PdCl2, and dihydrogen hexachloroplatinate, H2PtCl6. The particle sizes and size distributions were determined by transmission electron microscopy, and the colloids were further characterized by UV-vis spectroscopy. The catalytic activity of these nanoparticles was qualitatively investigated by the hydrogenation and conversion of cyclohexene as a model reaction and compared to palladium and platinum colloids protected by a selection of water-soluble, nonionic polymers. The results show that the catalytic activity is strongly influenced by the protective polymer chosen, as well as particle size and morphology. The use of cationic polyelectrolytes decreases the catalytic activities significantly, in comparison to several water-soluble, nonionic polymers investigated. The effects depend strongly on the particular metal, as illustrated in this case by differences observed between palladium and platinum.

Colloidal Gold Nanoparticles Protected by Cationic Polyelectrolytes

Abstract Several stable gold colloids were prepared by the in-situ reduction of hydrogen tetrachloroaurate (HAuCl4) in the presence of various cationic polyelectrolytes. Several types of such polyelectrolytes were investigated for their ability to stabilize gold colloids, and UV – VIS spectroscopy was used to follow the in-situ reductions and to further characterize the colloids. The particle sizes and size distributions were determined by transmission electron microscopy (TEM). TEM micrographs and UV-VIS spectra were also used to characterize the stability of the colloids after storage for nine months in air at room temperature. Colloids protected by the cationic polyelectrolytes with ammonium side-groups along a hydrophobic polymer backbone frequently exhibited very good stability.

Colloidal platinum‐polyacid nanocatalyst systems

Various water-soluble polymers, with special emphasis on polyacids, were employed for the stabilization of colloidal platinum nanoparticles which had been prepared by the in-situ reduction of dihydrogen hexachloroplatinate H2PtCl6. The particle sizes, size distributions, and morphologies of the platinum nanoparticles were determined by transmission electron microscopy. In addition, the catalytic activities of the platinum-polymer systems were qualitatively evaluated by the hydrogenation of cyclohexene as a model reaction. The type of polymer (e. g., the use of a polyacid versus a nonionic, water-soluble polymer) cannot only influence the nanoparticle sizes and morphologies and the colloid stabilities, but the catalytic activities as well. In most cases, increased catalytic activities were observed for the platinum catalysts if various polyacids were used as protective matrices. Several influences, such as the particle size and morphology, and the interactions between the polymer and the catalyst nanoparticles have to be considered. Therefore, the selection of the protective polymer is highly important for tailoring the catalytic properties of such metal-polymer catalyst systems. Additional influences may stem from the presence of ions (e. g., from the metal precursor or the counterions of the polymer) or special functions introduced by certain components of the polymer (e. g., units capable of hydrogen transfer). Mehrere wasserlosliche Polymere, unter besonderer Berucksichtigung von Polysauren, wurden fur die Stabilisierung von kolloidalen Platin-Nanopartikeln eingesetzt. Die Platinpartikel wurden durch in-situ Reduktion von Dihydrogenhexachlorplatinat H2PtCl6 hergestellt. Die Partikelgrosen, -grosenverteilungen und -morphologien wurden durch Transmissionselektronenmikroskopie bestimmt. Weiterhin wurden die katalytischen Aktivitaten der Platin-Polymer-Systeme qualitativ durch Hydrierung von Cyclohexen als Modellreaktion bestimmt. Die Art des Schutzpolymeren (z. B. die Verwendung einer Polysaure im Gegensatz zu einem nichtionischen, wasserloslichen Polymeren) kann nicht nur die Grose und Morphologie der Nanopartikel sowie die Stabilitat der Kolloide beeinflussen, sondern auch die katalytische Aktivitat. In den meisten Fallen wurden erhohte katalytische Aktivitaten der Platinkatalysatoren beobachtet, wenn verschiedene Polysauren als Schutzmatrices eingesetzt wurden. Unterschiedliche Einflusse, wie die Partikelgrose und -morphologie sowie die Wechselwirkungen zwischen dem Polymeren und den Katalysatornanopartikeln, sind zu berucksichtigen. Daher ist die Auswahl des Schutzpolymeren auserst wichtig, um die katalytischen Eigenschaften solcher Metall-Polymer-Systeme maszuschneidern. Zusatzliche Einflusse konnen von der Gegenwart von Ionen (z. B. von der Vorstufe der Metallverbindung oder den Gegenionen des Polymeren) stammen. Spezielle Funktionen konnen auch durch entsprechende Bestandteile des Polymeren (z. B. Wasserstofftransfereinheiten) eingefuhrt werden.

Palladium nanocatalysts protected by polyacids

Several colloidal palladium nanocatalysts prepared by the in situ reduction of palladium chloride PdCl 2 , ammonium tetrachloropalladate (NH 4 ) 2 PdCl 4 , and palladium acetate Pd(CH 3 COO) 2 were protected by various water-soluble polymers, with special emphasis on polyacids. The particle sizes, morphologies, and size distributions of the palladium nanoparticles were determined by transmission electron microscopy (TEM), and their catalytic activities were qualitatively tested by the hydrogenation of cyclohexene. The type of the polymer (for example, polyacid versus a nonionic, water-soluble polymer) can influence the nanoparticle sizes and morphologies, as well as colloidal stabilities. For the catalytic activities of these metal-polymer systems, the choice of the protective polymer can be equally important. Lower catalytic activities have been mostly found if polyacids were used as protective matrices for these palladium nanocatalysts. It was found to be important to consider several influences, such as the particle size and morphology, as well as the interaction between the polymer and the catalyst nanoparticle. Thus, the selection of the protective polymer is crucial for the development of tailored metal-polymer catalyst systems. Additional influences may stem from the presence of ions, for example, those from the metal precursor, or the counterions of the polymer side groups.

Platinum nanocatalysts immobilized on latex supports

Several latex dispersions of different hydrophobicity were investigated with respect to their ability to adsorb platinum nanoparticles that had been reduced in their presence. Two reduction methods were tested, specifically the slower method of refluxing the alcoholic solutions and the more rapid method of reaction with KBH 4 . The immobilization of the metal particles and their nanosize dimensions were demonstrated by transmission electron microscopy, and their catalytic activity was tested by the hydrogenation of cyclohexene as a model reaction. Some additional immobilized platinum nanoparticles were prepared in the presence of various protective polymers. This can lead to various advantages with respect to, for instance, the stability and the catalytic properties of these materials. Even in the presence of such additional protective polymers, the platinum nanoparticles remained immobilized for some of the hydrophobic latexes both before and after catalytic hydrogenations.

Immobilization of palladium nanoparticles on latex supports and their potential for catalytic applications

Palladium nanoparticles were reduced in the presence of several latex dispersions possessing different hydrophobicities. Various reduction methods were investigated, specifically the slower methods of refluxing the alcoholic solution and the more rapid reduction by potassium tetrahydridoborate. In several cases the latexes showed the ability to adsorb and immobilize the palladium nanoparticles on their surface. Transmission electron microscopy was employed to show the immobilization of the metal nanoparticles on the latex surfaces, and their nanosize dimensions. The latex-metal dispersions showed catalytic activity for the hydrogenation of cyclohexene as a model reaction. A selection of water-soluble protective polymers was included to explore whether the metal nanoparticles were still immobilized. In the case of the more hydrophobic latexes, the accumulation and immobilization of the metal nanoparticles was preserved both before and after their use as hydrogenation catalysts. Palladium-Nanopartikel wurden in verschiedenen Latexdispersionen unterschiedlicher Hydrophobizitat reduziert. Mehrere Reduktionsmethoden wurden untersucht, speziell die langsamere Methode durch Ruckflus in alkoholischer Losung und die raschere Reduktion durch Kaliumtetrahydridoborat. In einigen Fallen zeigten die Latices die Fahigkeit, die Palladium-Nanopartikel auf ihrer Oberflache zu adsorbieren und zu immobilisieren. Die Immobilisierung der Metallnanopartikel auf den Latexoberflachen und ihre Nanodimensionen konnten mit der Transmissionselektronenmikroskopie gezeigt werden. Die Latex-Metall-Dispersionen besitzen katalytische Aktivitat fur die Hydrierung von Cyclohexen als Modellreaktion. Eine Auswahl von wasserloslichen Schutzpolymeren wurde hinzugefugt, um zu prufen, ob die Metallnanopartikel noch immobilisiert bleiben. Im Fall der hydrophoberen Latices wurden die Akkumulation und die Immobilisierung der Metallnanopartikel sowohl vor als auch nach ihrem Einsatz als Hydrierkatalysatoren beibehalten.

COLLOIDAL SILVER NANOPARTICLES PROTECTED BY WATER-SOLUBLENONIONIC POLYMERS AND “SOFT” POLYACIDS

Stable silver colloids were prepared by the in situ reduction of silver nitrate AgNO3 in the presence of various protective polymers. Several nonionic polymers and polyacids, and one anionic polyelectrolyte were investigated for their ability to stabilize such silver colloids. UV-vis spectroscopy was employed to characterize the colloidally-stable silver samples, and the particle sizes, size distributions, and particle shapes were determined by transmission electron microscopy. The interactions among the silver precursors and colloids, and the polymeric matrices can have an influence on the nanoparticle features, the optical properties, and the long-term colloidal stability of the colloid dispersions.

Metal Nanocatalysts Stabilized in Protective Polymer Matrices

Abstract Palladium and platinum nanoparticles were generated in the presence of a variety of protective polymers. The size and size distribution of the metal particles were determined by transmission electron microscopy, and the catalytic activities of the metal-polymer systems were qualitatively tested by the hydrogenation of cyclohexene as a model reaction. Several groups of protective polymers were examined, including water-soluble homopolymers and random copolymers, cationic polyelectrolytes, amphiphilic block copolymers, and latices as carriers. The catalytic properties were found to be significantly influenced by the type of protective polymer used.