Crystallite morphology

100 faces are prevalent and at even higher CH4 concentrations, the deposits become structureless. Another study showed that at temperatures of 900°C and lower, 111 faces are dominant in the crystallite morphology, and at 1000°C or higher, 100 faces are dominant. These results suggest that low substrate temperatures and low CH4 concentrations favor 111 faces, whereas high substrate temperatures and high CH4 concentrations favor 100 faces. Consistent observations have been 

---

As shown in Fig. 2, the catalytic activity of the zeolite prepared by the direct heating method for methanol conversion was higher than that of the zeolite crystallization for 25 days by the standard preparation method. However, deactivation of the catalyst by carbon deposit occurred early in the reaction, just as with the catalyst prepared by the standard method. Differences in crystallite morphology between those prepared by the standard method and the direct heating method would be attributed to the stage of the precursor formation. Therefore, after the precursor formation the rapid heating was adopted as described below. 

Figure 3. Typical crystallite morphology of ZSM-23. (The distance between two scale bars amounts to 1 pm).

Figure 9. Crystallite morphology of ZSM-23 synthesized from a gel with SiC /A t = 70.

Displacement of peak of fusion on curves DTA in area of smaller temperatures specifies that in the modified fibres crystals have mainly morphological form II (the extended circuits of polymers incorporated into crystallites) while in initial PETP crystals mainly have morphological form I (folded structure). Therefore for initial PETP it is observed endothermic effect at temperature 269°C - speaking by fusion flat folded crystallites (morphological form I). At modified PETP - fibers this effect is observed at temperature 245 - 263°C, it speaks fusion of spherallite (the morphological form II). 

In most microscopy studies the catalyst is examined under vacuum. To understand the system, the catalyst should be examined under reactor conditions. Environmental chambers that do not reduce instrument resolution are presently not available. Similarly, pretreatment systems attached to the instrument would extend characterization insight. At the very least, sample delivery systems that minimize air exposure are necessary. For example, air exposure during sample transfer as might occur with a reduced Pd catalyst can alter the crystallite morphology. Such studies would be expected to enhance catalyst development. 

The subject of crystallite morphology in the real world is not so simple. No matter how diligent is the skill for the catalyst preparation, there is inevitably some broad distribution of crystallite sizes. Within those sizes are crystallites having incomplete faces, initially producing an excess of edge sites an extensive discussion on these matters was made by Bert et al ... 

In alkaline earth metal oxides, the (100) surface termination plane, which exposes equal numbers of anions and cations, is prevalent and, as illustrated in Figure 21.2, it can be anticipated that an entire family of different co-ordination sites, of different basicity, can be exhibited. Furthermore, it would be expected that this would lead to a dependence upon crystallite morphology and/or particle size. 

Simulation can also be applied to longer length-scale phenomena. Examples include attempts to model the structural and mechanical properties of catalyst pellets, the mesopore structure of particle aggregates and phenomenological studies of crystallization. Here I mention just two examples, studies of crystallite morphology and quantitation of the effect of pore blockages on effective sorption capacities. 

Crystallite morphology prediction and morphology control are important for several kinds of applications. Zeolite membranes can require alignment of many distinct crystallites, this being facilitated by having uniformly shaped... 

The scanning electron micrographs show that the crystallite morphology of the nickel-modified samples is inhomogeneous. Both large hexagonal prisms (5-10 pm in diameter and 15-30 pm in length) and a considerable amount of smaller particles, mostly spherical (Fig.1) are observed. 

Annealing of a crystalline polymer, either neat or in a blend, increases the degree of crystallinity, changes the crystallite morphology and relieves built-in stresses in the amorphous phase. Table 12.29 shows the gradual increase of HDT with annealing time. Flexural modulus also increases. Similar effects were reported for amorphous polymers such as PS [Nielsen, 1974]. 

Pores can be created by a variety of techniques. In keeping with the sintering methodology, pores can be created by control of crystallite morphology (Nakahira et al. 

The basic crystallites morphology and size is illustrated in Fig. 1 for the two materials. The crystallites are, in both cases, faceted, with a size in the 20 to 70 nm range. Individual crystallites are rare. Most of them form primary clusters in which the basic components are joined by partial sintering. These clusters tend to have a more branch-like shape in the case of the N1 material, being spheroidal in shape in the case of the N2 powder. 

A series of zincosilicates MFI was synthesized from the mixtures of even Zn/Si ratio and various silicon sources. The properties of the resulting samples differed considerably regarding their zinc content, the crystallite morphology and size and catalytic activity. The samples modified with various cations (Ca, Cu, Zn, Al, H) showed some activity for 2-propanol dehydration (no acetone was detected). The samples modified with Al cations showed the highest activity. It is likely that part of Al could attain the framework positions or facilitate a generation of separated acidic OH groups. The lower activity of the H-forms could result from the presence of hydrogen bonds between adjacent hydroxyls. 

---