Egg-shell profile

The presence of other materials in the impregnating solution can have a marked effect on the location of the metal within the support particle. These additives have been conveniently divided into three classes. Class 1 additives consist of simple inorganic electrolytes which influence the electrostatic interactions at the solution-support interface. Simple salts such as sodium nitrate, sodium chloride, or calcium chloride do not adsorb strongly enough on alumina to compete with platinum salts for adsorption. Fig. 13.9a 0 shows the concentration profile of platinum on an alumina particle when the impregnation of chloroplatinic acid was done in the absence of any additives. This a somewhat diffused egg shell profile. Fig. 13.9b shows the adsorption profile for the catalyst prepared by impregnation in the presence of an amount of sodium nitrate equimolar to the chloroplatinic acid. Here the amount of platinum adsorbed decreases while the adsorption profile approaches a uniform distribution. It is... 

The regenerated and coked catalyst can be described by the same kinetic model the regenerated catalyst has a high initial activity and deactivation by the deposition of coke, the coked catalyst has a constant activity and is not further deactivated by additional coke deposition. It is tentatively concluded that most of the additional coke is deposited on the coke already present in an egg-shell profile on the catalyst. 

For both PNiMo, and PN1M02. and due to phosphomolybdates degrade totally or partially during adsorption, the active phase would probably be more easily formed between Ni and Mo than for the PNiMoj and PNiMo4 cases, in which the FK remains intact. However, the latter catalyst could be more active than the rest for reactions where diffusion of reactants towards the pellet interior controls the reaction rate because egg shell profiles are present. 

For catalysts prepared with P, the Mo concentration along the sphere radius diminishes continuously from the surface to the center. The catalyst prq>ared from dimethylformamide exhibits an egg shell profile. Then, the use of different precursors and solvents provides an adequate way for obtaining controlled profiles in these impregnated catalysts. 

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. 

In the same context the phenomenon of egg shell reduction has been discussed since thin egg shells are correlated with changed protein profiles of the biocrystalline layer. 

Class 3 additives are materials such as phosphoric acid and citric acid that can compete with the metal for adsorption sites. While Class 1 and Class 2 additives can control the depth and amount of metal adsorbed leading either to uniform or egg shell catalysts. Class 3 species interfere with platinum adsorption and can give entirely different adsorption profiles. This approach is used, specifically, for the preparation of egg white and egg yolk type catalysts. Fig. 13.11 shows that the platinum distribution is displaced from the surface of the... 

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. 

If the rates of the chemical steps 3-5 are comparable or higher than the transport processes 1, 2 and 6, 7, significant concentration profiles of and A2 inside the catalyst particle or in the surrounding layer will occur. If the intrinsic rates are very high as compared to the diffusion process in the pores, the reaction will take place only near the external surface, and the observed transformation rate will be controlled by the external mass transfer. The same situation is observed for non-porous pellets or so-called egg-shell catalysts, where the active phase is placed in a layer near the outer pellet surface. If the intrinsic reaction rate is comparable with the diffusion rate within the pores, a pronounced concentration profile of the reactant within the pellet will develop. 

Effect of support size The size of the catalytic support varies from very small particles (-10 xm in slurry reactors and -100 p,m in fluidized beds) to large ones (-1 cm in packed beds, 10-100 cm in monoliths). When the size of the support is increased, the effect of back-diffusion is reduced, leading to the accumulation of metal at the external surface. This effect is illustrated in Figure 16.11, which shows that the final metal profile changes from egg-shell to nearly uniform when the size of the porous support is decreased. 

It must be pointed out that the catalyst prepared in DMF shows a profile different from the others it is egg shell type, i.e., it has a high concentration on the surface and near-zero value in the rest of the pellet (Figure 5d). This is due to interactions between FK and DMF which result in a structure having a greater ionic radius, as it was previously mentioned, thus avoiding deep penetration into the pores. 

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