Tamman temperature

Table 3.4 illustrates that the composition of the gas phase easily can lead to sintering. For example, in air regeneration Pt metal crystallites will be transformed in to the oxides, and the Tamman temperature decrea,ses to below 420 K. 

The conductivity measurements show that equilibrium (1) sets in rapidly at temperatures as low as 500°C. Since the melting point of zinc oxide is about 2100°C. and accordingly its Tamman temperature about 900°C., the process under consideration cannot possibly involve the bulk of the crystal because defects could not diffuse rapidly enough through the lattice at such low temperatures. Except at very high temperatures, the defect equilibrium is realized only at the surface of the crystal, that is, in a layer of a few unit cells thickness. 

The stability of a metal crystallite on a support surface towards migration depends on its physical state of matter. At temperatures much lower than the melting point, near the Taroman temperature (0.4 times the melting temperature), metal crystallites are observed to develop liquid-like properties that apparently enhance their ability to migrate [121. This is particularly true in the presence of H2 [25]. Tamman temperatures for typical catalytic metals are 500 to 1100 K and for Pt and Ni, 817 and 690 K respectively [18]. Thus, crystallite migration may become an important mechanism for sintering of these two important metals above 700-800 K. 

All active solids are metastable and the loss of area may occur at temperatures well below the Tamman temperature, at which the classical form of sintering normally begins to become important. As we have seen in Chapter 10, water plays a key role in the ageing of the metastable oxides and oxide-hydroxides (Sing, 1972). This can be a serious problem when oxide adsorbents and catalysts are exposed to water vapour. 

Tg is the glass softening temperature, A5(7 g)the difference between the entropy of a glass and its crystal at Tg, To is the Vogel-Fulcher-Tamman temperature and z is the minimum size of the... 

Fig. 6. Relationship between Tamman temperature and the rate of R1 + R2 exchange for some first row transition metal oxides. Reproduced from reference [17] with permission.

Ponec and co-workersl l have recently demonstrated a relationship between the Tamman temperature and the Ri and R2 exchange rate for some first row transition metal oxides as shown in Fig. 6. In this study, the Tamman temperature plotted is the ratio of the exchange reaction temperature to the Tamman temperature of the oxide. This was taken to be indicative of the importance of diffusion, with surface diffusion occurring in the 0.2-0.5 temperature range and a bulk diffusion beyond 0.5. 

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