Eggshell distribution

Fig. 21. (a) 2-D Ti maps, (b) their 1-D central cross sections, and (c) the 1-D profiles of hexa-chloroplatinate dianion distributions obtained by electron probe analyzer measurements. S2 and S3 identify different porous alumina pellets, both prepared with an eggshell distribution of hcxachlorop-latinate dianion. S2 and S3 differ in terms of their nominal diameter and their pore size and surface area characteristics. Reprinted from reference (67), with permission from American Chemical Society, Copyright (2000). 

Catalyst preparation and inspection by microscopy. Preparation by impregnation with ammonium iron citrate and iron nitrate resulted in a homogeneous iron distribution as determined by light microscopy. Ammonium iron EDTA as a precursor yielded an eggshell distribution of the iron compound. Finely divided material deposited on the support was observed with Transmission Electron Microscopy in all catalysts. Tn addition to this, some material deposited next to the support was observed in catalysts ex nitrate. It was therefore decided to focus on the catalysts prepared with ammonium Fe(ITI) citrate. 

In regard to the metal distribution of the metal crystallites, one can differentiate between uniform and eggshell distribution as shown in Figure 15.4. Between those extreme types, other distribution patterns are possible, such as broad eggshell distribution. Which metal location is required depends on the specific cafalytic reaction and needs to be determined in detail during the process... 

Some studies of potential commercial significance have been made. For instance, deposition of catalyst some distance away from the pore mouth extends the catalyst s hfe when pore mouth deactivation occui s. Oxidation of CO in automobile exhausts is sensitive to the catalyst profile. For oxidation of propane the activity is eggshell > uniform > egg white. Nonuniform distributions have been found superior for hydrodemetaUation of petroleum and hydrodesulfuriza-tion with molybdenum and cobalt sulfides. Whether any commercial processes with programmed pore distribution of catalysts are actually in use is not mentioned in the recent extensive review of GavriUidis et al. (in Becker and Pereira, eds., Computer-Aided Design of Catalysts, Dekker, 1993, pp. 137-198), with the exception of monohthic automobile exhaust cleanup where the catalyst may be deposited some distance from the mouth of the pore and where perhaps a 25-percent longer life thereby may be attained. 

If the support is devoid of functional groups apt to interact with the metal precursor, there are not chemical forces facilitating the metal uptake. Under these conditions, metal uptake is driven by absorption forces and can still occur, but it is controlled by simple diffusion. This situation can favor an eggshell radial distribution of the metal precursor over a homogeneous one [31]. 

Careful selection of metal precursors and of competitors helps to obtain a large number of metal distribution profiles on the support surface between the periphery and the grain center [50, 95, 111, 124-127], depending on the strength of their interaction with the support. Four main types of profiles [50, 124-127] serve to describe all the others uniform, film or eggshell, internal ring or egg white, central or egg yolk (Fig 6). 

Other factors, by metal type, amount of metal used, the degree of metal dispersion, the location of metal on support or metal-support interaction. On the other hand the concentration of metal across the diameter of catalyst grains also important. There are three cases of concentration distribution eggshell (metals only on the inside of support), eggwhite (the maximum concentration of metal between one-half and one-quarter of grain diameter) and eggyolk (the maximum concentration of metal within one-half of grain diameter). At the same metal content considerable differences were observed in the concentration distribution in the catalysts. 

In contrast to traditional energy sources, microwave, laser, and sonic energy can be delivered to specific solid sites or small areas by fine-tuning frequencies and energy levels. It is especially suitable for preparation of nano-structured catalyst. For example, inherent eggshell catalysts can be produced with sonochem-ical preparation by which nano-dispersed metal particles are formed with instantaneous decomposition of metal solution by high-intensity irradiation of ultrasound in local area. In the preparation of M02C on ZSM-5 with ultrasound at 20 kHz, narrowly distributed particles of about 2nm in diameter are uniformly dispersed on the outer surface of the ZSM-5 support. ... 

The conversion data (reported in Fig. 5 as the time required to obtain a 90% conversion of acetophenone) for Pd/C catalysts prepared with and without ultrasoimd have been compared to the data, obtained in the same plant and with the same operating conditions, from two commercial samples, a sample (1) characterized by an eggshell metal distribution and a second one (2) characterized by a penetrated metal distribution. All the samples, both commercial and prepared in the laboratory, have the same metal loading (5% Pd wt/wt) and are supported on the same kind of active carbon. The sonicated sample shows a higher activity than all the catalysts tested, both in Fischer-Tropsch synthesis and in the hydrogenation of acetophenone. 

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