Alumina crack patterns

Fig. 5.16 Crack patterns on alumina gel for (a) slow and (b) fast convective drying (taken from Pourcel et al. (2007b)).

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

The compounds making up the catalyst sample can be clearly identified in the XRD pattern. Cupric oxide produces the peaks labeled C, zinc oxide the peaks labeled Z, and y-alumina the peaks labeled A in Fig. 12. Not only does the XRD pattern qualitatively identify the phases present in the catalyst, but the quantity of each phase can be determined by measuring the area under selected diffraction peaks relative to a standard. An example of quantitative analysis by XRD is found in the ASTM Standard Procedure D3906-80 for NaY zeolite in a cracking catalyst. 

Stress patterns of titania without additives were always highly irregular with many peaks and rather low stress levels, indicating severe cracking of the top layer [13,31]. Additives are necessary to relax this problem (see below). Alumina-titania membranes with 25-40 mol% titania yield stress (deflection) curves without peaks (and so without observable cracking). 

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