Particle size and shape effects

As in the case of CO oxidation, the reduction of NO by CO depends on the heterogeneity of the supported model catalyst. The effect of size and shape of the metal particles has been addressed with Pd/MgO(l 00) model catalysts [88, 90, 91, 168]. 

From Fig. 27 the activity seems to increase by decreasing cluster size. In fact if we want to compare the intrinsic activity of clusters with different size the TON is not necessarily a pertinent parameter. Indeed, if the reaction rate depends on the pressure of at least one reactant, the TON would not take into account the fact that the total flux of one reactant joining the clusters is not solely given by the pressure of this reactant, but we have also to consider the flux of the molecules physisorbed on the substrate. This contribution can be up to 10 times larger than the direct flux and it is strongly cluster size dependent (see Section 4). In that case the right parameter to compare the intrinsic activity of the different clusters is the reaction probability (of NO or CO). It is equal to the consumption rate of one reactant divided by 

Mean Size (nm) Density (cm 2) Covered Area (%) Main Exposed Facets 

As in the case of CO oxidation, the NO reduction by CO on supported model catalysts depends on the heterogeneity of these systems that are namely  

The reaction dynamics studies on supported nanoparticles are relatively not numerous. However, two reactions have been studied in detail CO oxidation and NO reduction by CO. From these studies it is clear that some differences exist between extended metal surfaces and supported particles. At least 

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It was not until recently that Chen and Goodman probed the influence of the oxide support material on the intrinsic properties at the metal surface. By covering a titania support with one or two flat atomic layers of gold they eliminated, direct adsorbate-support interactions as well as particle size and shape effects. Their results definitively showed that the electronic properties at the metal surface changed due to charge transfer between the support and the metal. Furthermore, their comparison of one- and two-layer films highlighted the dependence of these effects on the thickness of the metal slab. 

A dimensionless relationship has been proposed to describe the bulk viscosity T (( ), y) of concentrated suspensions taking into account particle size and shape effects [29] ... 

Effect of Filler Particle Size and Shape on the Rheological Properties of Composites... 

Particle size and shape have a strong effect on the rheological properties of materials as well, viscosity among them. 

Diffusion and sedimentation measurements on dilute solutions of flexible chain molecules could be used to determine the molecular extension or the expansion factor a. However, the same information may be obtained with greater precision and with far less labor from viscosity measurements alone. For anisometric particles such as are common among proteins, on the other hand, sedimentation velocity measurements used in conjunction with the intrinsic viscosity may yield important information on the effective particle size and shape. ... 

The need to separate solid and liquid phases is probably the most common phase separation requirement in the process industries, and a variety of techniques is used (Figure 10.9). Separation is effected by either the difference in density between the liquid and solids, using either gravity or centrifugal force, or, for filtration, depends on the particle size and shape. The most suitable technique to use will depend on the solids concentration and feed rate, as well as the size and nature of the solid particles. The range of application of various techniques and equipment, as a function of slurry concentration and particle size, is shown in Figure 10.10. 

I Goodarznia, DN Sutherland. Floe simulation Effects of particle size and shape. Chem Eng Sci 30 407-412, 1975. 

The activity in terms of 1st order rate constant khcalc was calculated in Table 2 from (8) and (9) with effective diffusivity Dejf=5.3-10 6 m2/s and intrinsic rate constant =33000 Nm3/h/m3 = 23 s"1 fitted to the measurements. This simple and useful method models the measured influence of particle size satisfactorily for a first optimization of particle size and shape. The 35% higher activity measured for the 9-mm Daisy compared to the 12-mm Daisy, however, exceeds the 25% expected from (8), and this illustrates the importance of measuring the activity of the actual shape. 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

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]. 

Both experimental and theoretical work has demonstrated that growth rate dispersion exists, and has a measurable effect on the CSD in both batch and continuous crystallization processes. Further understanding of this phenomenon on a fundamental level will be required to develop methods to make use of or control growth rate dispersion and make it a tool in control of particle size and shape. 

Rendon, J.L. Serna, C.J. (1981) IR spectra of powder hematite effects of particle size and shape. Clay Min. 16 375-381... 

AFM has been used in only a few studies to explore the sizes and morphology of airborne particles (e.g., Friedbacher et al., 1995 Posfai et al., 1998). In this case, atomic scale resolution is not used, but rather much lower resolution that provides information on particle sizes and shapes in the micron and submicron size range under ambient conditions. This has the advantage that effects due to the application of vacuum... 

Eversole, J. D., and H. P. Broida, 1977. Size and shape effects in light scattering from small silver, copper, and gold particles, Phys. Rev., B15, 1644-1655. 

Luxon, J. T., D. J. Montgomery, and R. Summitt, 1969. Effect of particle size and shape on the infrared absorption of magnesium oxide powders, Phys. Rev., 188, 1345- 1356. 

Table V. Effect of Rubber Particle Size and Shape in Impact Poly S/AN + 20 wt % Poly B/AN...

Effect of Particle-Size and Shape—The effect of particle-size on consistency is clearly shown in Figures 95 and 96, taken from the investigations of Ward and Kammermeyer (1940) and DeVaney and Shelton, respectively. Note that as the size of the average particle decreases, the consistency of the pulp increases. In other words, pulps composed... 

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