Metals particle size

Influence of Metal Particle Size in Nickel-on-Aerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity... 

As illustrated in Figure 1.12, three types of behavior can be observed. The most significant surface feature that changes with metal particle size is the ratio of corner, edge, and terrace surface atoms. 

The use of highly dispersed metals at low concentration levels has found wide use In Industry, particularly for electronic and catalytic uses. The desire to optimize the size and mass uniformity of these metal particles Is of particular Interest. Characterization of these materials Is difficult especially when metal particle sizes are on the order of 5 nm or less and concentrations are below 1 wt-Z. Development of highly sophisticated techniques In recent years has provided new approaches to understanding the physical and chemical properties of these materials. Electron microscopy has proven quite valuable In the acquisition of data and subsequent generation of information, which Is necessary to understand the physical-chemical properties of Individual nm-slzed particles. 

No hydride forms for very small particles. Yet when catalysts with such a small metal particle size are stored in air, they are converted to (crystalline) PdO. Reduction of this oxide with hydrogen produces Pd metal, not hydride. 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

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. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

A study concerning the lowermost particle size necessary for admitting a reaction to occur is provided by Amiridis et al. [55] in their investigations of propene hydrogenation. For metal particle sizes greater than 2nm, hydrogenation of simple olefins is considered to be structure insensitive... 

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. 

Table 13.1. Ti02 Content (wt.%), Specific Surface Area, H/Ir Ratios and Metal Particle Size Obtained by TEM of Supported Ir Catalysts.

The reduction phase (phase 1) is slower than the re-oxidation one (phase 2). The C02 formation decreases regularly upon each CO pulse while the re-oxidation is achieved upon the first pulse of 02. This is a rather general phenomenon in catalysis. Oxides (like rare-earth oxides) reduced more slowly than their suboxides may be re-oxidized. It is interesting to note that the reverse phenomenon can be observed with the metals (Pt, Rh and Pd). Their oxides are reduced at a much lower temperature than the metal can be re-oxidized [19-21] even though the nature of support and the metal particle size may change the redox properties significantly [20,22,23],... 

Nitrogen adsorption isotherms were measured with a sorbtometer Micromeretics Asap 2010 after water desorption at 130°C. The distribution of pore radius was obtained from the adsorption isotherms by the density functional theory. Electron microscopy study was carried out with a scanning electron microscope (SEM) HitachiS800, to image the texture of the fibers and with a transmission electron microscope (TEM) JEOL 2010 to detect and measure metal particle size. The distribution of particles inside the carbon fibers was determined from TEM views taken through ultramicrotome sections across the carbon fiber. 

Nevertheless, the comparisons presented between the behavior of ultra-thin and thick film catalysts (cf. Tables XI, XIII, XIV) where adventitious surface contamination is insignificant, makes it clear that metal particle size can be a real determinant to the course of these reactions, and... 

From a consideration of the Figs. 2, 4, 6, and 7 some conclusions can be drawn. The specific activity in the range upward of approximately 70 A will not be very sensitive to changes in metal particle size. The greatest... 

Van Hardeveld and Hartog describe the effect of metal particle size on the properties of a metal on carrier catalyst. They have related the adsorptive and catalytic properties of metal crystals to crystal size and to the structure of the crystal surface. 

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