Crystallite surface

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Noncrystalline separating layers of microfibrils are formed from two crystallite surface layers that are side by side in the microfibril and from a central part called the intermediate zones. The latter are mainly made up of tie molecules among which are distinct straightened... 

Figure 7.4 Polymer chain configurations at crystallite surfaces ...

Re-organization of amorphous chain segments adjacent to crystallite surfaces leads to crystallite thickening. These segments align themselves to form additional crystalline unit cells that increase the c dimensions of the crystallites. 

Composition range 30-80% Rh. In this composition range phase separation occurs, and the structure of such Pd-Rh alloy films has been reviewed (Section II). Phase I varied in composition and phase II contained 88 5% Rh. It was proposed that these results could be explained by the preferential nucleation of rhodium so that the crystallites consisted of a phase II kernel surrounded by an outer shell (phase I), the Rh content of which increased with an overall increase in the Rh content of the alloy film. Note the essential difference to the Cu-Ni films (38, 33) discussed in Section IV.A where complete separation into two phases of fixed equilibrium composition is envisaged, and over a wide composition range the crystallite surfaces have the same composition. 

Nelson and Conrad29 have recently confirmed the viscosity behavior observed by Davidson26 and Nickerson and Habrle27 and have drawn a similar conclusion, namely, that after the rapid destruction of about 2 % of the intercrystalline network, hydrolysis occurs mainly on lateral crystallite surfaces. They also show that the apparent degree of crystallinity is reduced by fine grinding of cotton fibers. 

In the present work, a constant composition method has been used to investigate the growth of HAP from solutions of low supersaturation and in the presence of different background electrolytes. The influence of magnesium and strontium ions both on the rate of crystallization and upon the electrokinetic properties of the crystallite surfaces has also been investigated. 

A hydrogen termination of the crystallite surface is assumed in most studies. Calculations show that a small part of the hydrogen can be removed without the formation of DBs, as shown for the transformation of Si29H36 to SijjH [Hi-4-]. Further dehydrogenation, however, leads to the formation of DBs that are efficient recombination centers and thus effectively suppress the luminescence [De2, Hi4, De5]. The observed IR luminescence has been proposed to be due to recombination via DB centers [De3]. 

A further distinction has to be made between reactions taking place within a molecular crystal and reactions of a molecular crystal A with a molecular crystaT B (see Scheme 2 A and B can also be the same crystal) [7bj. Reactions of the former type, or intra-solid reactions, can be either under topochemical control, depending on the proximity of the reactants, or may imply extensive molecular motion within the crystal lattice. Reactions of the second type, or inter-solid reactions, can either take place on the crystallite surface or require molecular diffusion through the lattice. Inter-solid reactions are often accompanied by formation of eutectics. 

Fig. 6 Schematic drawing of ZSM5 catalyst bed deactivation. View of the fused silica reaction tube at about 40 % of catalyst life time. Black zone (I) of deactivated catalyst particles covered with coke ("methanol coke"). Small dark reaction zone (II) in which methanol conversion to 100 % occurs. Blue/grey zone (III) of active catalyst on which a small amount of "olefin coke" produced by the olefinic hydrocarbon product mixture has been deposited on the crystallite surfaces. The quartz particles before and behind the catalyst bed (zones 0) remain essentially white.

These interrelations are consistent with the above model of high temperature deactivation by coke formation through a reaction of coke growth with methanol. However, this mechanism needs coke seeds provided as "olefin coke" on external acidic centers. Development of ZSM5-catalysts for high temperature application with long life time thus concerns minimizing of acid sites on crystallite surfaces. 

Yermakov and Zakharov (13) reported comprehensive work on the difficulties associated with specific quenching techniques which make sure that only the active metal-carbon bond is quenched. There is evidence that surface determinations by the BET method can give incorrect results, but a strong correlation of polymerization rate to crystallite surface (determined by X-ray techniques) was found (see Fig. 12). The authors conclude that the formation of propagation centers in the case of unsupported TiCl3 proceeds with participation of only those surface titanium ions that are situated in special surface regions as outcrops of growth spirals, or on lateral faces (65-69, 85, 86). 

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