Egg-shell distribution

Fig. 6 (a) 2-D 7) maps, (b) their 1-D central cross sections, and (c) the 1-D profiles of hexachloroplatinate dianion distributions obtained by electron probe analyzer measurements. S-2 and S-3 identify different porous alumina pellets, both prepared with an egg-shell distribution of hexachloroplatinate dianion the dianion is located towards the external surface of the pellet). S-2 and S-3 differ in terms of their nominal diameter and their pore-size and surface-area characteristics. Reprinted with permission from ref. 24. Copyright (2000) American Chemical Society. 

Fig. 13.6, 5 strong adsorption leads to an egg shell distribution with aqueous solutions on alumina and acetone solutions on carbon. The use of aqueous solutions with carbon and acetone solutions with alumina promotes a weaker adsorption and a more uniform metal distribution. The data in Fig. 13.7 show that as the polarity of the solvent decreases the distribution of platinum on a carbon support becomes more diffused. ... 

The preparation of egg shell catalysts from more weakly adsorbed speeies that would normally give a uniform distribution has been accomplished by using volumes of the impregnating solution which are smaller than the pore volume of the support. A half pore volume, for instance, can give an egg shell distribution when a full pore volume gives a uniform profile. 

Incipient wetness impregnation will lead to an egg-shell distribution when an acid platinum hexachloride solution that is not highly concentrated is impregnated into y-alumina. As dealt with above, an egg-white distribution of platinum is often preferred. Maatman [6], van den Berg and Rijnten [13], and Michalko [22] describe an egg-white distribution that can be achieved by addition of acids to the impregnating platinum hexachloride solution. In addition to inorganic acids, such as, nitric... 

Accordingly, van den Brink [37] demonstrated that impregnation with a solution of iron(III) ammonium citrate and iron(III) gluconate resulted after drying in a uniform distribution of the active precursor within the support body. Transmission electron microscopy of the subsequently calcined catalysts displayed the presence of very small iron oxide particles, homogeneously dispersed over the silica. With impregnation of iron(ni) ammonium EDTA, the pH of the impregnating solution is important. A solution of a pH of 5.3 produced an egg-shell distribution, and a solution of pH 7.1, a faint egg-shell. However, solutions of a pH of above 8.5 led to uniform... 

The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating 0H (or H" ") ions with (ii) the use of a basic (or an acidic) reacting carrier may give rise to accelerated precipitation of the active component at the edge of the carrier body (egg-shell distribution). 

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