Nanometer-sized metal particles, surface

Particles produced in the gas phase must be trapped in condensed media, such as on solid substrates or in liquids, in order to accumulate, stock, and handle them. The surface of newly formed metallic fine particles is very active and is impossible to keep clean in an ambient condition, including gold. The surface must be stabilized by virtue of appropriate surface stabilizers or passivated with controlled surface chemical reaction or protected by inert materials. Low-temperature technique is also applied to depress surface activity. Many nanoparticles are stabilized in a solid matrix such as an inert gas at cryogenic temperature. At the laboratory scale, there are many reports on physical properties of nanometer-sized metallic particles measured at low temperature. However, we have difficulty in handling particles if they are in a solid matrix or on a solid substrate, especially at cryogenic temperature. On the other hand, a dispersion system in fluids is good for handling, characterization, and advanced treatment of particles if the particles are stabilized. 

Surface plasmons, or surface plasmon polaritons, are surface electromagnetic waves that propagate inside a metal along a metal/dielectric (or metal/ vacuum) interface their excitation by light is surface plasmon resonance (SPR) for planar surfaces or localized surface plasmon resonance (LSPR) for nanometer-sized metal particles. 

Up to now we have considered unreconstructed, defect-free low-index surfaces, where all surface atoms have the same geometric environment. In the real world, large defect-free terraces of low-index surfaces are the exception rather than the rule, and in nanometer-sized metal particles (clusters) such as those found in industrial catalysts a significant fraction of all surface atoms sit at steps, edges or corners and therefore have lower coordination than those in the terraces. There are many indications that such sites are more reactive than terraces [59],... 

Given that two-nanometer sized metal particles cannot reside within the zeolite supercages, we measured only the magnetic moiety of reduced nickel on the outer surface of zeolite matrix. It is seen from these results that the share of metal nickel present as small particles less than 2 nm in size decreases dramatically on diminishing the amount of metal loaded. Of course, these estimates correspond also the concentrations of oxide precursors initially hosted in the supercages but only their bottom limit, and the real content of oxide component present as the cage-hosted nanoclusters may be substantially higher. 

Since the number of atoms on the surface of a bulk metal or metal oxide is extremely small compared to the number of atoms in the interior, bulk materials are often too costly to use in a catalytic process. One way to increase the effective surface area of a valuable catalytic material like a transition metal is to disperse it on a support. Figure 5.1.5 illustrates how Rh metal appears when it is supported as nanometer size crystallites on a silica carrier. High-resolution transmission electron microscopy reveals that metal crystallites, even as small as 10 nm, often expose the common low-index faces commonly associated with single crystals. However, the surface to volume ratio of the supported particles is many orders of magnitude higher than an equivalent amount of bulk metal. In fact, it is not uncommon to use catalysts with 1 nm sized metal particles where nearly every atom can be exposed to the reaction environment. 

The manufacture of heterogeneous catalysts from pre-prepared nanometal colloids as precursors via the so-called precursor concept ll has attracted industrial inter-est.l l An obvious advantage of the new mode of preparation compared with the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors can be tailored for special applications independently of the support. In addition, the metal particle surface can be modified by lipophilic or hydrophilic protective shells, and covered with intermediate layers, e.g. of oxide. The addition of dopants to the precursor is also possible. The second step of the manufacture of the catalyst consists in the simple adsorption of the pre-prepared particles by dipping the supports into organic or aqueous precursor solutions at ambient temperature. This has been demonstrated, e.g., for charcoal, various oxidic support materials, even low-surface materials such as quartz, sapphire, and highly oriented pyrolytic graphite. A subsequent calcination step is not required (see Fig. 1). 

In the first example colloidal crystals are used to template a unique new material, nanostructured porous gold (Velev et al., 1997, 1999). The metallic structure is assembled on the surface of a filter membrane from nanometer-sized gold particles that are templated by colloidal crystals of larger latex microspheres that have been formed by flow over the filter. When the templates are removed chemically or by calcination they leave behind a three-dimensional metallic nanostructure with long-ranged ordering of the pores. The pore size is precisely controlled in the sub-micrometer range by the diameter of the latex microspheres. 

Electrodes consisting of supported metal catalysts are used in electrosynthesis and electrochemical energy conversion devices (e.g., fuel cells). Nanometer-sized metal catalyst particles are typically impregnated into the porous structure of an sp -bonded carbon-support material. Typical carbon supports include chemically or physically activated carbon, carbon black, and graphitized carbons [186]. The primary role of the support is to provide a high surface area over which small metallic particles can be dispersed and stabilized. The porous support should also allow facile mass transport of reactants and products to and from the active sites [187]. Several properties of the support are critical porosity, pore size distribution, crush strength, surface chemistry, and microstructural and morphological stability [186]. 

For time-dependent electrical perturbation, the typical assumption is that the metal nanoparticle behaves as a dielectric, characterized by a frequency-dependent permittivity ( >). Permittivities experimentally determined on bulk sample are almost invariably used. They need to be corrected with terms depending on the particle size. In fact, when the size of the metal particle has the same order of magnitude of the mean free path of conduction electrons in the bulk of the solid (tens of nanometers), it is necessary to take into account the scattering of the electrons at the metal particle surface. This is one aspect of a more general class of phenomena, known as quantum size effects. They are tightly related to the confinement of electrons in the metal particle and hence to the loss of the band structures typical of a bulk metal. Since this phenomenon regards mainly the valence... 

The need for Ertl s approach becomes evident once a sample of an industrial catalyst is put under closer scrutiny. Figure 5.20 shows a high-activity catalyst with a rather large specific surface area comprised of nanometer-sized active particles. Under reaction conditions, these are reduced into metallic iron, covered by a submonolayer of potassium (and oxygen), which acts as an electronic promoter. The configuration of active particles is stabilized against sintering by a framework... 

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