Crystallite growth

Bett JAS, Kinoshita K, Stonehart P. 1976. Crystallite growth of platinum dispersed on graphi-tized carbon black n. Effect of liquid environment. J Catal 41 124-133. 

Lateral crystallite growth is permitted by a contingent of amorphous chains. All earlier theories allow longitudinal (along the chain axis) growth only. 

Another sample, after oven drying, was irradiated by microwave radiation from 2 to 20 min. A comparison of the catalyst properties and water-gas shift rates is provided in Table 124. The authors observed important increases in the activity after treatment with sodium borohydride and hydrazine, which they correlated with an improvement in the reducibility (i.e., intensity of low temperature peak in TPR) related to reduction of Au oxide species and partial reduction of Fe oxide. A decrease in the rate for the microwave irradiated catalyst was linked to Au crystallite growth, but the authors indicated that the procedure is still worth exploring, as the technique increased the number of Au species that were reducible at low temperature. 

The 3D crystallites growth mechanism is discussed in Section 7.10. In this case the structure components are 3D crystallites, and a coherent deposit is built by coalescence (joining) of these crystallites. 

There are two basic mechanisms for formation of a coherent deposit layer growth and 3D crystallites growth (or nucleation-coalescence growth). A schematic presentation of these two mechanisms is shown in Figure 7.11. 

Thermally induced deactivation of catalysts is a particularly difficult problem in high-temperature catalytic reactions. Thermal deactivation may result from one or a combination of the following (i) loss of catalytic surface area due to crystallite growth of the catalytic phase, (ii) loss of support area due to support collapse, (iii) reactions/transformations of catalytic phases to noncatalytic phases, and/or (iv) loss of active material by vaporization or volatilization. The first two processes are typically referred to as "sintering." Sintering, solid-state reactions, and vaporization processes generally take place at high reaction temperatures (e.g. > 500°C), and their rates depend upon temperature, reaction atmosphere, and catalyst formulation. While one of these processes may dominate under specific conditions in specified catalyst systems, more often than not, they occur simultaneously and are coupled processes. 

Van Delft et al. [15, 16] have discussed the tendency of monolayer formation versus crystallite growth of a metal phase a on a substrate s. Deliberately ignoring entropy effects, they calculated the difference A in... 

---