Shape of catalyst particles

The optimum size and shape of catalyst particles is a compromise between contrasting demands ... 

Figure 3.48. An artist impression of possible shapes of catalyst particles present on a support a. spherical particle with only one point contact to support, b. hemispherical particle, strongly bonded to support and partially poisoned, c. metal crystallite, strongly bonded to and partially encapsulated in support, d. complete wetting of the support by the active phase. After Scholten et al, 1985 and Ba.stein cr a/., 1987.

If there is no mass transfer resistance within the catalyst particle, then Ef is unity. However, it will then decrease from unity with increasing mass transfer resistance within the particles. The degree of decrease in f is correlated with a dimensionless parameter known as the Thiele modulus [2], which involves the relative magnitudes ofthe reaction rate and the molecular diffusion rate within catalyst particles. The Thiele moduli for several reaction mechanisms and shapes of catalyst particles have been derived theoretically. 

Precipitated catalysts and supports for impregnation need to be formed into suitably sized particles for use in the reactor. The size and shape of catalyst particles depend on... 

Hie most commonly found shape of catalyst particle today is the hollow cylinder. One reason is the convenience of manufacture. In addition there are often a number of distinct process advantages in the use of ring-shaped particles, the most important being enhancement of the chemical reaction under conditions of diffusion control, the larger transverse mixing in packed bed reactors, and the possible significant reduction in pressure drop. It is remarkable (as discussed later) that the last advantage may even take the form of reduced pressure losses and an increased chemical reaction rate per unit reactor volume [11]. 

Nanocarbon materials have been obtained by different ways with three catalysts Ni, Co and Fe. The metal nanoparticles inside the nanotubes and nanofibers have both fee and bcc structure. Their orientations along the axis of the nanotube (or nanofiber) can be one of the following [100], [110], [111] and [112]. The shape of catalyst particles (fillings) and their twinning are considered as the result of deformation, caused by the action of surrounding graphene shells. 

Myshlyavtsev and co-workers [62] recently tried to describe explicitly adsorbate-induced changes in the shape of catalyst particles by using the solid-on-solid (SOS) model. The results obtained are, however, somewhat artificial from the physical point of view, because the shape of particles predicted on the basis of this model has little in common with crystallites. 

The overall strategy for the scale-up of this study was to select the diameter and the length of the reactor, size and shape of catalyst particles, and the superficial velocity of liquid in such a way that the effects of nonideal flow together with mass and heat... 

Figure 4.17 Optimal shapes of catalyst particle loading (4.148) along the oxygen channel for the indicated values of oxygen stoichiometry.

Table 8.24 shows the effects of size and shape of catalyst particle on activity and pressure drop. It can be seen that the bed pressure drop is relatively small... 

Optimal size and shape of catalyst particles for ammonia synthesis processes. (Chemie Linz). US 3965246 (1975). 

In electrocatalysis, the activated carbons, glassy carbon, and carbon black are the transitional forms used. Carbon black is the product of incomplete combustion or decomposition of organic compounds. The shape of its particles is close to spherical. They contain several carbon atom lattice fragments arranged without order. Various types of carbon black serve as substrates for metal catalysts, the properties of the carbon blacks themselves having a strong elfect on the catalytic activity of the combined catalysts thus obtained. 

TEM analyses of the colloids do not evidence any significant change in the size and shape of the particles after seven days catalytic reactions (Figure 4). No NPs were observed at long times of reaction (up to one week) starting with molecular catalysts. 

Characteristics of a catalyst particle include its chemical composition, which primarily determines its catalytic activity, and its physical properties, such as size, shape, density, and porosity or voidage, which determine its diffusion characteristics. We do not consider in this book the design of catalyst particles as such, but we need to know these characteristics to establish rate of reaction at the surface and particle levels (corresponding to levels (1) and (2) in Section 1.3). This is treated in Section 8.5 for catalyst particles. Equations 8.5-1 to -3 relate particle density pp and intraparticle voidage or porosity p. 

A noteworthy line of research is the application of TEM on models for supported catalysts. Figure 7.6 shows a side view of Au particles (diameters <6 nm) on top of MgO crystals, taken from the work of Giorgio et al. [16]. The picture beautifully shows the shape of the particles together with the lattice fringes characteristic of certain orientations of the particles and the support. In addition, the authors obtained... 

Carbon support plays a vital role in the preparation and performance of catalysts since it influences the shape, size and dispersion of catalyst particles as well as the electronic interactions between catalyst and support [154,155]. 

Modern electron microscopes are very well capable of imaging individual particles, but of course it is impossible to do so, even for a representative fraction of particles in a supported catalyst. Carlsson et al. [18] described an interesting method to obtain the particle geometry distribution, such that the fraction of edge and corner sites in a supported catalyst can be estimated. An assumption must be made on the shape of the particles, for which these authors used the truncated octahedron, and were able to demonstrate the procedure for gold particles on three different supports. 

Fig. 7.18 AFM image of a planar model catalyst consisting of copper particles deposited by spin-coat impregnation on a flat Si02 on Si(100) substrate, after reduction in hydrogen. Because the image is a convolution of the shape of the particle and that of the tip, the particles appear larger than they are. The height, however, is correctly reproduced.

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