Sintering atomic mechanisms

There is a qualitative distinction between these two types of mass transfer. In the case of vapour phase transport, matter is subtracted from the exposed faces of the particles via dre gas phase at a rate determined by the vapour pressure of the solid, and deposited in the necks. In solid state sintering atoms are removed from the surface and the interior of the particles via the various diffusion vacancy-exchange mechanisms, and the centre-to-cenU e distance of two particles undergoing sintering decreases with time. 

Fig. 19.2. The microscopic mechanism of sintering. Atoms leave the grain boundary in the neck between two particles and diffuse into the pore, filling it up.

Clearly, the sintering kinetics will be different during each of the aforementioned stages. To further complicate matters, in addition to having to treat each stage separately, the kinetics will depend on the specific atomic mechanisms operative. Despite these complications, most, if not all, sintering models share the following common philosophy ... 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

The decrease in IT is caused by small shifts of atoms located in a layer of 3 to 5 atomic diameters near the interface. Such shifts can be clearly observed in monociystals (reconstruction and relaxation phenomena) [12], There are mechanisms based on the decrease of A at V = const with the decrease of dispersion A/V. The results of action of these mechanisms are change of particle and pore shape, decrease of the micropore amount and surface roughness, etc. during sintering, coalescence, etc. 

The philosophy used to develop detailed chemical kinetic mechanisms for gas-phase reactions can, in principle, be extended to treat heterogeneous reactions, provided diffusion is also considered in the final analysis. Clearly, the problem in heterogeneous catalysis is considerably more complex because of the close proximity of a large number of atoms and their collective effect on reaction kinetics and mechanisms, and the inevitable variation of catalyst structure with time—for example, as a result of sintering and poisoning. 

Two mathematical models for sintering mechanisms for supported metals have been put forward (1) a particle migration model where particles migrate over the surface of the support, collide and fuse, causing loss in dispersion (e.g. Ruckenstein et al 1984) and (2) an atomic migration model involving the... 

The mechanisms above allow rapid diffusional transport of atoms along the surface. We discuss the role of surface diffusion in the morphological evolution of surfaces and pores during sintering in Chapters 14 and 16, respectively. 

On the other hand, rate constants for 0.6 and 5% Pt/alumina catalysts sintered in H2 at 973 K (see Table 1) of 0.53 and 0.84 h 1 are not substantially different. This result is not altogether unreasonable, as the number of crystallites per unit area of support surface and the metal surface area would be about the same in both 0.6 and 5% catalysts because of the much lower dispersion of the 5% catalyst. Nevertheless, it is fascinating that these two catalysts sinter at much different relative rates in air (see discussion above), a fact suggesting that different mechanisms (i.e., atomic migration vs. crystallite migration) may be involved in air versus H2 atmospheres as proposed by Wynblatt and Ahn [5J. 

Since the atomic migration and crystallite migration mechanisms have been amply discussed in the previous proceedings on Catalyst Deactivation, the emphasis of the present paper is on the wetting and spreading phenomena, which appear to play a major role in sintering and redispersion. 

The formation of highly dispersed particles or crystallites in the synthesis process of, for example, a supported metal catalyst, is governed by nucleation and growth mechanisms (vide supra) that have been described in the literature [15, 16, 21-23]. For sintering or redispersion (spreading and film formation) to occur, particles or atoms, molecules or clusters of the active... 

Particle migration (up to 8nm particles migrated over 25 nm at 773 K) was also observed for Pt on alumina [43]. The two major mechanisms of sintering of supported Pt crystallites appeared to be (i) short-distance, direction-selective migration of particles followed by either collision and coalescence or by direct transfer of atoms between the two approaching particles, or (ii) localized direct ripening between a few immobile, adjacent particles. 

Rasmussen et al. (2004) SAX Ni-alumina Sintering mechanism + + Distinction between coalescence and atomic migration... 

Sintering of supported solids can occur by two distinct mechanisms particle migration and coalescence as already mentioned above and interparticle transport of atoms or molecules (Ostwald ripening)... 

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