Hemispherical active particles

Figure 3. Schematic presentation of the cross section of the diffusion layer of a partially covered inert electrode with hemispherical active particles, where rm is the radius of the microelectrodes, 8 is the diffusion layer thickness of the macroelectrode, Cq and C are the bulk and the surface concentrations of reacting ions, respectively, x is the ratio of the distance between the centers of neighboring particles and the particle diameter, and 8 > r. Reprinted from ref.7 with permission of Elsevier.

Hence, it can be expected that the diffusion layer of hemispherical active particles on the inert substrate will not overlap if the distance between centers of the particles is larger than 4r, as illustrated in Fig. 1.26a. The common diffusion layer of the macroelectrode will be formed at larger times. 

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.

It is likely that both the above mentioned effects - larger particle sizes and increase in airborne activity with height - contributed to the high values of W. At later times, the airborne activity from Chernobyl was mainly submicrometre in size and had equilibrated with the accumulation mode of natural nuclei. Over the period 10-90 d from the emission, 137Cs disappeared from the atmosphere with a half-life of 6 d, or mean life of 9 d (Fig. 2.8). The mass of air in the troposphere is 9000 kg per m2 of the earth s surface, and the average daily rainfall in the northern hemisphere is 3.1 mm. Using these data, it can be deduced that the washout ratio of 137Cs was... 

A schematic presentation of the cross section of the diffusion layer of an inert macroelectrode partially covered with small active hemispherical particles is shown in Fig. 3, taking into account that S rm. 

It is obvious from (57) that the quantity of catalyst in the form of small hemispherical grain required to transform an inert electrode into an active one decreases rapidly with decrease in size of the particles, for one and the same Sw, as well as with the increase of the... 

Spherical gold particles are also less active than hemispherical ones because of the smaller interface perimeter (Table 15.2). This result has been correlated to the perimeter length of the gold-support interface [8], and has contributed to the establishment of the mechanism reported in Fig. 15.2. 

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