Crystallite aggregates

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Figure 7. Electron micrograph of a typical ZSM-20 crystallite aggregate. Regions of both hexagonal ARAB... (BSS) and cubic ABCABC... (FAU) stacking are evident.

The originally very small and linely divided iron carbide crystallites aggregate to form larger crystallites. For this process a decline of activity of about 20% has been reported which is even more pronounced if the amount of the SiO support is lowered Sintering is also accelerated by water. Thus, a higher degree of deactivation is observed for the catalyst near the exit of the reactor where the highest concemratton of product water has to be assumed (15]. 

A criterion for a transition from a two phase state to a monomolecular melt state may be formulated as follows. The total change in free energy (ACy) by removing one crystallite aggregate from a melt... 

Vlllemaire and Agassant interpreted their results as caused by melting of crystallite aggregates. According to the gel destruction temperature hypothesis in Eq. 21 the prehistory effects can only be observed for T >T>T. This is documented by Vlllemaire and Agassant as shown In fig. 9 actually the figure allows estimation of both Tj m nxvii. ww,... 

In this evaluation, the final state Is a state without crystallite aggregates and the starting state Is the state with crystallite aggregates. 

The flexible blocks primarily influence the elastic nature of the product and its low-temperature performance, and they make important contributions towards the hardness, tear strength and modulus. Such strong polar interactions in polyurethane block copolymers can lead to a supramolecular organization into aggregated structures, which may be in the form of glassy domains or crystallites. Aggregation of highly polar blocks depends upon many different structural features and also upon the thermal tensile history of the specimen, and it is for this reason that thermal... 

Fig. 9.13 a TEM microgn jh of 3Y-TZP primary crystallite aggregation (urea hydrolysis at 150 °C for 10 h, calcinations at 450 °C Iot 1 h). b TEM micrograph of 3Y-TZP dense nanoaggregates [35]. With kind permission of John Wiley and Sons... 

SEM and TEM allow the researcher to observe what the LDH crystallites look like in shape and how LDH crystallites aggregate. LDH materials containing inorganic anions most commonly give materials of a hexagonal platelet shape observable by SEM and TEM (109,376,383). Materials with intercalated organic anions, however, have produced materials of varying shapes and sizes, e.g., ribbon- and barlike substances (259). The LDH carbonate particle has a typical diameter of around 1 micrometer. There seems to be very little correlation between cations used, anions used, or method of preparation and the size of particle obtained. 

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