Catalyst egg yolk

Oxidation kinetics over platinum proceeds at a negative first order at high concentrations of CO, and reverts to a first-order dependency at very low concentrations. As the CO concentration falls towards the center of a porous catalyst, the rate of reaction increases in a reciprocal fashion, so that the effectiveness factor may be greater than one. This effectiveness factor has been discussed by Roberts and Satterfield (106), and in a paper to be published by Wei and Becker. A reversal of the conventional wisdom is sometimes warranted. When the reaction kinetics has a negative order, and when the catalyst poisons are deposited in a thin layer near the surface, the optimum distribution of active catalytic material is away from the surface to form an egg yolk catalyst. 

The optimum distribution of catalytic material on the support depends on the kinetic order of the reaction catalyzed. When platinum is used for CO and hydrocarbon oxidation in automotive catalysis, the reverse of the normal wisdom is in order—platinum should be distributed toward the interior of the support layer to form a new type of egg yolk catalyst. 

It is possible to shape catalyst bodies, in which the catalytically active substances are not distributed over the complete bulk, but rather located in concentric areas. Many fluidized-bed catalysts, for instance, are spheres in which the active phase represents the core, and the shell is a porous, protective layer to prevent attrition. Bodies exhibiting the active phase in the core are denoted as egg-yolk catalysts. For fixed-bed applications, so-called egg-shell structures are more convenient, in which the catalytic material is located at the external surface, whereas the core is nonreactive. 

Two precipitation reactions have been designed to check the generalization concerning the egg-yolk catalysts, viz. the liquid-phase reduction of Cu and of Ag" " with N2H and CH2O, respectively (for details see Table 4). Both reactions generate H" " ions and it is predicted that a basic carrier, such as 7-AI2O3 at sufficiently low pH, will lead to an egg-yolk distribution. Indeed, the line scans shown in Figs. 10 and 11 prove the metals to be concentrated in the inner part of the extrudates. 

Egg shell", uniform and "egg yolk" Ni/Al203 catalysts [63] behave very differently for alkyne —> alkene (—> alkane) hydrogenations... 

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... 

There can be obtained catalysts with egg-yolk, egg-shell, and homogeneous metal distributions. 

The exchange of labelled sulphur can be promoted by enzyme catalysts, instead of heating. Bird egg yolk and the cysteine desulphydrase that it contains catalyse the exchange of sulphur-35 from Na2 S to L-cysteine, L-cystine and L-cysteic acid. In a typical experiment, 150 ml of a buffer solution containing 2 millimoles of cysteine-HCl, 2 millimoles of Naa S and 500 mg of cysteine desulphydrase preparation is incubated at 38°C for 15 h. A mixture of 74-4% cystine- S and 25-3% cysteine- S is obtained. L-Cystine- S is subsequently reduced electrolytically to cysteine- S. The total yield of L-cysteine- S obtained by isotope exchange is 70%. 

Assume that the rate constant ky is independent of position, for example, the catalyst is neither an eggshell nor an egg yolk design. Then the solution to this ordinary differential equation, subject to the boundary conditions of Eqns. (9-4a) and (9-4b), is... 

A novel method to apply deposition precipitation which makes use of transient concentration gradients will be illustrated by the synthesis of egg-yolk type molybdenum-on-silica catalysts. Two methods have been elaborated to start and restrict the deposition of Mo at the centre of the silica spheres (cf. Table 3). Note that in all experiments the volume of the solution exceeds the carrier pore volume considerably (wet impregnation ). Tlie first method uses relatively dilute solutions and mild reaction conditions (pH, temperature) such that a reaction starts at the centre of the spheres and terminates spon—... 

The second generalization related to the egg-shell catalysts has been confirmed by repeating the copper precipitation under identical conditions with (acidic) silica spheres. Visual inspection of the precipitation process revealed the copper oxide to be concentrated at the outer surface of the spheres. Practical application of this method to produce egg-shell catalysts, however, involves the problem of precipitation in the liquid separate from the carrier. Therefore, the method seems to be especially suited for preparing egg-yolk type catalysts. 

It has been shown that deposition precipitation via redox chemistry can be used for the production of egg-yolk type catalysts. The synthesis of Mo/Si02, CU/AI2O3 and Ag/Al203 catalysts displaying an egg-yolk type distribution has been described to illustrate this novel preparation route. 

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