Metallic and Catalytic Particles

Space available in porous glass [487], ultrafine Nafion [488, 489], and metallic membranes [490, 491] has also been utilized for the development of smal particles. Cylindrical micropores in alumina membranes have been used, for example, as templates for the electrodeposition of parallel arrays of gold particles (0.26 pm in diameter, 0.3 pm to 3 pm in length) which were infrared transparent [491] and could be used as chemical sensors [490], 

but by no means least, reference should be made to the use of proteins in nano-fabrication [492]. One approach is illustrated by the fabrication of a 1-nm-thick metal film with 15-nm-diameters holes, periodically arranged on a triangular protein lattice [493]. Advantage was taken of the 10-nm-thick, uniformly porous surface (or S) layer of the crystalline protein obtained from the thermophilic bacterium Sulfolobus acidocaldarius. The protein was adsorbed from a dilute solution onto a molecularly smooth carbon-film surface, metal coated by evaporation, and ion milled to give spatial ordering of holes with the same nanometer periodicity as the protein lattice [493]. 

The most important properties of the different membrane-mimetic compartments are summarized in Table 4. 

Metallic particles have been extensively used as efficient and selective industrial catalysts [494-497]. Their performance strongly depends on the structure and area of the exposed surface in general, the larger the catalytic area the more 

Compartment Dimensionality, size of host Method of preparation Stability Comment Key references 

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Available results on the preparation, characterization, and utilization of metallic and catalytic particles (Sect. 3), semiconductor particles and particulate films (Sect. 4), conductors and superconductors (Sect. 5), magnetism and magnetic particles and particulate films (Sect. 6), and advanced ceramic materials (Sect. 7) will constitute the main body of the monograph. An attempt will be made to cover these materials exhaustively. 

Metallic and Catalytic Particles and Particulate Films in Membrane-Mimetic Compartments... 

The available information on generating metallic and catalytic particles in the different membrane-mimetic compartments is summarized in Table 5 [539-568],... 

Table 5. Metallic and catalytic particles and particulate films in membrane-mimetic compartments... 

Heiz U, Sanchez A, Abbet S, Schneider W-D (1999) Catalytic oxidation of carbon monoxide on monodispersed platinum clusters each atom counts. J Am Chem Soc 121 3214—3217 Henglein A (1989) SmaU-particle research physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem Rev 89 1861-1873... 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

Particularly attractive for numerous bioanalytical applications are colloidal metal (e.g., gold) and semiconductor quantum dot nanoparticles. The conductivity and catalytic properties of such systems have been employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors, and DNA sensors. Advances in the application of molecular and biomolecular functionalized metal, semiconductor, and magnetic particles for electroanalytical and bio-electroanalytical applications have been reviewed by Katz et al. [142]. 

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