Adsorbed poisons

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

As discussed in Chapter 3, the controversial argument about the strongly adsorbed poisonous spedes of methanol partial oxidation is almost settled, resvilting in the condusion that COad is at least one of the msgor adsorbates. Therefore, both for understanding the mechanisms of methanol oxidation and for searching for new catalysts, it is very important to understand the behavior of COad under various conditions. 

The effectiveness of an adsorbed poison depends on the ratio x0/x, wherex0 is the amount of poison per active centre necessary to inhibit catalytic action, and xt is the number of active centres. 

This competitive adsorption drives the platinum deeper into the extrudates, and when sufficient HC1 is added, the PtCl 2 ) adsorption reaction is moderated to such an extent that a reasonably homogeneous distribution is obtained. One can also add acids that adsorb even more strongly than chloroplatinic acid, e.g. oxalic or citric acid, with which Pt profiles such as shown in Fig. 9.6 can be prepared. Profile (b) can be useful when a strongly adsorbing poison is present in the stream... 

Indications for the homogeneous nature of the surfaces of some catalysts have been obtained by Maxted and associates (80) in a series of poisoning experiments in which the rate of the hydrogenation of various organic substances over platinum, palladium, and nickel decreased linearly with increasing amounts of adsorbed poisons, according to... 

It is important to know whether adsorbed poisons such as S, C, P. and A extend their negative effects to the first or to the nth. next neighboring atoms orKl their range of interaction on the metal surface (or in the bulk if they are dissolved). The decrease of saturation magnetization of Ni due to atom sorption (by sorption we mean either adsorption or absorption) can help us in speculating on the range of irtteraction. In table 1 are summarized some data available in the literoture concerning some elements in interaction with Ni-based catalysts. 

In order to circumvent the complexity of this system of equations, and to obtain some qualitative picture of the poisoning process, one may consider the realistic but simpler case of a strongly and rapidly adsorbing poison 268). Under diffusion-controlled poisoning, then, the pore can be divided into two zones Region (A) extending to a depth Xp is completely poisoned and catalytically inactive. Region (B) consists of a fully active pore, for X > Xp. Thus,... 

Other solid cocatalysts that are sometimes used include strong solid Lewis acids. For example, calcined silica-alumina, or sulfate- or fluoride-treated alumina or silica-alumina, can adsorb poisons in the reactor, including redox by-products such as formaldehyde. It is surprising how much more activity Cr /silica can produce when tested together with one of these materials. 

In the following, examples of the ensemble control by means of adsorbed poisons are discussed with the the emphasis on steam reforming of methane on sulfur passivated nickel catalysts. The conclusions for ensemble control will be compared with data for catalytic reforming on PtRe(S) catalysts and for the impact of chlorine on partial oxidation of methane on Pd-catalysts. 

Care is necessary when using carbon to clarify specimens for toxicological analysis, particularly those that may contain alkaloids or other strongly adsorbable poisons. 

Various effects are possible when activated carbon is present during fermentation.1 Apart from any direct catalytic action, indirect effects may occur. Carbon may accelerate biological activity by adsorbing poisons, or may retard it by adsorbing nutrients necessary for the growth of microorganisms. Reactions may also be retarded by the adsorption of enzymes. In some cases, the course of the reaction is altered and different products are formed. 

Now. considering the simultaneous presence of a nonreactive but adsorbable poison, P, in the feed at a partial pressure, Pp, another adsorption term is established ... 

Activated charcoal usually is prepared as a mixture of at least 50 g (about 10 heaping tablespoons) in a glass of water. The mixture is then administered either orally or via a gastric tube. Because most poisons do not appear to desorb from the charcoal if charcoal is present in excess, the adsorbed poison need not be removed from the GI tract. Charcoal also may adsorb and decrease the effectiveness of specific antidotes. 

An early analysis by Wheeler [4] treated poisoning in an idealized pore, and can be generalized to a catalyst particle as shown in Chapter 3. Fundamenul to his development, and the others of this section, is the assumption that the catalytic site that has adsorbed poison on it is completely inactive. If Cp, is the concentration of sites covered with poison the fraction of sites remaining active, called the deactivation or activity function, is represented by... 

If, however, tightly absorbed catalyst poisons are present it is easy to show that small pores can completely destroy Type I selectivity. In Section VII, 2, we showed that poisons preferentisdly adsorbed on the pore mouth can reduce the rate of a fast reaction to the rate of diffusion through the poisoned pore mouth. (See eq. 79.) Since diffusion rates of similar molecules are about the same, we could expect the rates of A — B + C and X — Y Z to he reduced to about the same level on catalysts with poisoned pore mouths, regardless of their intrinmc relative rates. Thus the combination of tightly adsorbed poisons and small pores can completely destroy Type I catalyst selectivity, causing reaction rates to be diffusion controlled. 

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