Supported metals, small particles particle size

There Is an extensive literature relating to Infrared studies of CO on supported platinum. However, there is no consistent band pattern or interpretations which can lead to unambiguous assignments for bands in the 2060-2085 cm region as seen in Figure 3. The various interpretations can be divided into those which emphasize the physical state of the supported metal, i.e., particle size and those which attribute small difference in band positions to chemical factors such as the presence of oxygen. On a 10% Pt/Al203, Barth ec al. [8] observed bands at 2060 and 2083 cm. They attribute the 2060 cin band to small particles which do not have extended crystal faces and the 2083 cm band to CO... 

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]. 

A Ni/Si02 catalyst prepared by incipient wetness was shown to consist, initially, of the silica support filled with the nickel salt. On reduction the nickel particles coalesced to form large crystallites because of their weak interaction with the support. Coprecipitated catalysts, however, were composed of layered silicate structures that were more difficult to reduce to metallic nickel. When reduction did take place, though, the reduced crystallites were fixed within the confines of the support so small metal particles with a narrow size range were produced. ... 

The results described in a previous paper [7] and this one indicate that the use of chelated metal precursors for the preparation of heterogeneous catalysts can suitably be extended to mesoporous support materials. The mechanisms underlying the fundamental processes occurring during catalyst preparation appear to be the same for both types of support materials. Therefore no limitations appear to exist to apply a wide variety of other elements into the pores of several types of mesoporous supports, with retention of the unique textural and structural properties of the support materials. Catalysts thus prepared will feature very high dispersions of active phase as well as very small particles with sizes even smaller tlmn on conventional support materials (due to the limiting size of the pores of mesoporous support materials). 

Interestingly, when the particle size of metal nanoparticles becomes less than 2 nm, terraces become so small that they carmot anymore support the presence of step-edge site metal atom configurations. This can be observed from Figure 1.15, which shows a cubo-octahedron just large enough to support a step-edge site. 

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

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. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

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