Monodisperse metal studies

Another Important concept introduced by Taylor was that of heterogeneity of surface-active centers.(25-26) This stemmed from observation of R. N. Pease that minute amounts of carbon monoxide, much smaller than the amount necessary to cover the surface, were sufficient to poison the surface of a copper catalyst. Taylor proposed that there were active centers on the surface while others argued that nickel impurities segregated preferentially on the surface and acted as catalyst. The variation of the heats of adsorption with surface coverage as determined by R. Beebe was used as evidence supporting the concept of active centers. In spite of the contradictory interpretation of the same experimental data, the concept of active centers has been a fruitful one. It inspired Imaginative research in the field of metal and oxide catalysis and has its present day expression in sophisticated surface physics studies. Subsequent work by coworkers of Turkevich at Princeton refined the nature of active centers in monodisperse metal particles and crystalline oxide catalysts. 

Radiation chemistry methods have been proven to be of high potentiality to induce small and size-monodispersed metal clusters and to study the dynamics of nucleation and growth of clusters. 

In the present work, heterogeneous nucleation procedures for the preparation of monodisperse metal nanoparticles are described (Fig. 1). These methods share the common characteristics of using an aqueous medium and regulating particle growth and preventing aggregation by the addition of various polymers. The effect of the molecular mass and the concentration of the polymers on the size of the particles and then-adsorption on the surface of kaolinite supports were studied. The particle size was determined by transmission electron microscopy (TEM) and the metal/... 

As a rule, short nucleation times are the prerequisite for monodisperse particle formation. A recent mechanistic study showed that when Pt(acac)2 is reduced by alkylalu-minium, virtually all the Pt cluster nuclei appear at the same time and have the same size [86]. The nucleation process quickly consumes enough of the metal atoms formed initially to decrease their concentration below the critical threshold. No new metal cluster nuclei are created in the subsequent diffusion-controlled growth stage. 

This approach of using 2D and 3D monodisperse nanoparticles in catalytic reaction studies ushers in a new era that will permit the identification of the molecular and structural features of selectivity [4,9]. Metal particle size, nanoparticle surface-structure, oxide-metal interface sites, selective site blocking, and hydrogen pressure have been implicated as important factors influencing reaction selectivity. We believe additional molecular ingredients of selectivity will be uncovered by coupling the synthesis of monodisperse nanoparticles with simultaneous studies of catalytic reaction selectivity as a function of the structural properties of these model nanoparticle catalyst systems. 

In order to investigate this effect, ordered arrays of metallic nano-islands were fabricated on glass substrates by a process of natural lithography using monodisperse polystyrene nanospheres. The metal particle dimensions were tailored in order to tune the plasmon resonance wavelength to match the spectral absorption of the fluorophore. The fluorophore, Cy5 dye, which is widely used in optical immunoassays and has a medium quantum efficiency ( 0.3), was used in this preliminary study of the plasmonic enhancement effect. 

To date, it has been difficult to produce monodisperse spherical powders from metal alkoxides with low valence, such as Al and Fe, because the hydrolysis and condensation of metal alkoxides in alcohol are too rapid to allow control of particle size, size distribution, and morphology. Many previous studies reported (28-30) that alkoxide-... 

For each noble metal it appears that reaction parameters such as the nature of the precursor, temperature, and time have to be optimized. Recently a study of the electrochemical reduction of noble metals species in EG was undertaken by Bonnet et al. (28). Belter control of the experimental conditions leading to the preparation of monodisperse particles in the nanometer, submicrometer, and micrometer size range for various noble metals is expected from knowledge of the electrochemical fundamentals of the polyol process. 

Two Co-Mo-alumina catalysts obtained from a commercial vendor as either marketed or special research samples were used in this study. The surface area and pore-size distributions (using the mercury penetration technique) were determined by an independent commercial laboratory. The catalyst properties are given in Table II. Note that the monodispersed (MD) and bidispersed (BD) catalysts have the same metallic composition and are chemically similar. 

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