Supported metals, small particles dispersion

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Supported metal catalysts are used in a large number of commercially important processes for chemical and pharmaceutical production, pollution control and abatement, and energy production. In order to maximize catalytic activity it is necessary in most cases to synthesize small metal crystallites, typically less than about 1 to 10 nm, anchored to a thermally stable, high-surface-area support such as alumina, silica, or carbon. The efficiency of metal utilization is commonly defined as dispersion, which is the fraction of metal atoms at the surface of a metal particle (and thus available to interact with adsorbing reaction intermediates), divided by the total number of metal atoms. Metal dispersion and crystallite size are inversely proportional nanoparticles about 1 nm in diameter or smaller have dispersions of 100%, that is, every metal atom on the support is available for catalytic reaction, whereas particles of diameter 10 nm have dispersions of about 10%, with 90% of the metal unavailable for the reaction. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20-1000 A) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured both are prerequisites for the mechanistic study of the catalysis on the atomic scale. 

Hoffman (34) argued that if there is a significant difference in atomic size, phase separation will not occur in very small particles. Sinfelt (55) showed that metals that do not form bulk alloys form metal clusters when they are finely dispersed on a support. 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

The various stages of the preparation and thermal treatments of supported metal catalysts arc very schematically illustrated in Fig. 3. A very similar presentation was earlier given by van Delft et al. [16]. Typically, the support is impregnated with a metal salt (sec Section A.2.2.1.1) which serves as the metal precursor and should be well dispersed. Small metal particles may be formed by either direct reduction under mild conditions or by reduction after an intermediate oxidation step. Mild oxidation will lead to thin oxide films spread out on the support or to small oxide particles, where particles and film may also coexist. More severe treatments in oxidative atmospheres can lead to the... 

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