Hemispherical particles

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 interesting to visualize what the dispersion values from Table 3.3 mean. Suppose that the Zr02 is present in hemispherical particles. Then we know from Fig. 3.9 that such a particle with a radius of 2V x has the same dispersion as a layer of thickness x. As x is known for all catalysts in Fig. 3.10, we can now calculate how many half spheres there are on 10x10 nm2 of support area. The... 

Of direct interest for photoemission of supported catalysts is that similar increases in the width of d-bands have been observed by Mason in UPS spectra of small metal particles deposited on amorphous carbon and silica substrates [48]. Theoretical calculations by Baetzold et al. [49] indicate that the bulk density of states is reached if Ag particles contain about 150 atoms, which corresponds to a hemispherical particle 2 nm in diameter. Concomitant with the appearance of narrowed d-bands in small particles is the occurrence of an increase in core level binding energies of up to 1 eV. The effect is mainly an initial and only partly a final state effect [48], although many authors have invoked final state - core hole screening effects as the only reason for the increased binding energy. 

Characterization of the polymer indicates that an application of sodium dodecyl sulfonate as anionic surfactant and sodium persulfate as initiator for both stages leads to a hemisphere particle... 

Figure 3.4 Turnover frequencies for CO oxidation at 300 K over spherical and hemispherical particles of Au and Pt supported on Ti02 [21],...

In the Au/Al203/NiAl(100) system, hemispherical particles occur even at low coverage,7 unlike the situation with titania size distribution was narrow, and particles were stable to 600 K, implying low mobility of adsorbed atoms. Paradoxically, on alumina large particles migrate and coalesce faster than small ones, presumably because the metal-support interaction is weaker but with Au/FeO the diffusivity of atoms is higher due to a lower concentration of surface defects. 

As with so many other gold-catalysed reactions, the particle size is important it has been claimed9,20 that only hemispherical particles between 2 and 10 nm in size are suitable and early work recommended 2.3 nm particles as the best.16 The deposition-precipitation (DP) method is favoured because of the intimate contact between metal and support that it gives. Particles smaller than 2 nm appear to shift the reaction mode towards formation of propane,7,9,10,15 which is clearly a waste of both propene and... 

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. 

The ash particles deposited on boiler tubes are initially held in place by surface forces, i.e. van der Waals and electrostatic attraction forces. Van der Waals forces become important when molecules or solid surfaces are brought close together without a chemical interaction taking place. For a hemispherical particle of radius (r) held at a distance of nearest approach (h) from a plane, the resultant force (F) is given by ... 

The peak-to-peak separation of 50 tm and 5 tm microparticle spheres and hemispheres are compared with the case of the planar diffusion. The hemispherical particle exhibits a peak separation of zero at low scan rates, as it behaves as an adsorbed molecule. With increasing scan rates, A( )p increases and finally approaches the value(s) for the planar diffusion model. The larger the diameter of the hemispheres and spheres, the earlier the planar diffusion behavior is reached vdth increasing scan rates, as the curvature of larger particles has a lesser effect on the diffusion. 

Another interesting point from Equation 13.49 is the magnitude of the electrolyte conductivity inside the pore, x depends on the function of the bulk electrolyte conductivity [15], x° ar d the porosity factor in the case of hemispherical particles (silver, copper, and gold, for example) where is less than 0.45 ... 

The gold NPs were well dispersed with diameters of 1-4 nm and attached to the Ti02 support as hemispherical particles by use of the DP method (Fig. 14.3) [25]. 

According to this simple model, the Agl actually forms at the surface and not in the core at all. The Agl phase that emerges onto the external silica surface grows by spherical diffusion in the bulk solution, where the I2 concentration is much higher, and diffusion is unimpeded. This may explain the hemispherical particle shape adopted by many of the Agl particles. It is apparent that such complex morphology changes could not have been predicted from the spectroscopic data shown in Figure 51.17 alone. 

The above computations for the fiee energy changes for composite latex particles have shown that only core-shell, inverted core-shell and hemispherical particles are stable in a thermodynamic sense. All of the other reported morphologies (e.g. sandwich-like , raspberry or confetti-like particles or occluded domains) are non-equilibrium, kinetically controlled structures, prepared... 

TABLE 11.4 Potential shift by a single electron incident on a Pt hemispherical particle (assnming a constant capacitance of 20 iF cm )... 

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