Adsorption of poisons

Figure 10.5 shows the basic concept of the particle-level MR that gives (i) selective addition of reactants to the reaction zone and (ii) selective removal of products from the reaction zone. In the first case, if the diffusivity of one reactant (A) is much higher than that of the other components (B), the reactant (A) selectively diffuses into a catalyst particle through a membrane. Undesired reactions or the adsorption of poisons on the catalysts can be prevented. In the second case, the reaction has a hmited yield or is selectivity controlled by thermodynamics. The selective removal of the desired product from the catalyst particle gives enhancement of selectivity when the diffusivity of one product (R) is much greater than that of the other products (S). 

Poisoning curves for porous catalysts. Curve A is for a porous catalyst with hT very small and poison distributed homogeneously. Curve B is for large hT with the poison distributed homogeneously. Curves C and D correspond to preferential adsorption of poison near the pore mouths. For curve C, hT = 5, and for curve D, hT = 20. 

Schematic representation of the preferential adsorption of poison near the mouth of a pore. [From A. Wheeler, Adv. Catalysis, 3 (249), 1951. Used with permission of Academic Press.]...

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/AljO in benzene hydrogenation. 

Adsorption of poisons/toxins The universal antidote for use in reducing the effects of poisoning by the oral route is composed of activated charcoal, magnesium... 

In purifying mixtures of gas-forming substances, activated carbon is utilized in cigarette filters (mainly in Japan), in gas-mask filters, in venting filters for car petrol tanks (mainly in the USA and Japan) and in air conditioning units for the purification of ambient air. Industry also utilizes activated carbon for the recovery of a multiplicity of volatile solvents from the ambient air, for adsorption of poisonous or noxious gases (e.g. hydrogen sulfide) and as catalysts or catalyst supports. 

Chemisorption studies usually involved about 230 mg of (>203. We estimate total uncertainties in weighing, adsorption of poisons, and so on, as amounting to about 0.05 mg. 

Finally, it has been found that promotion of palladium by bismuth not only increases the selectivity in aldehyde but also limits the deactivation of the catalysts. Similar results have been published in the past decade on the partial oxidation of alcohols with similar catalytic systems (ref. 13) Various interpretations on the role of bismuth have been suggested among them, resistance of Pd/C against overoxidation and surface orientation of the reactant suppressing the formation and strong adsorption of poisoning intermediates are also problably the main reasons of the improved performances in the oxidation of p-cresol. 

Promotion and poisoning, therefore, appear not to be opposite sides of the same coin. Although promotion can be attributed to a lowering of the work function, so that electrons can be donated easily to an adsorbate (thereby weakening internal molecular bonds), adsorption of poisons has little effect on the work function. Instead there is substantial reduction in the LDOS at the Fermi level, and the surface becomes less reactive towards incoming molecules. 

As has already been mentioned, the degree of adsorption of poisons varies greatly with the amount of the catalyst and with its specific surface.. It may however be of interest to give some values for the factors Ki and K2 which were observed for various poisons, in the presence of, 0.05 g. of platinum in a liquid system having a volume of 10 cc., under conditions similar to those of Table VI. This partition applies of course only for concentrations of poison up to the region of inflection in the adsorption graphs. 

Fig. 9. Preferential adsorption of poison near mouth of pore. A rapidly and tightly held poison will completely cover the length ah nearest the pore mouth, leaving the inner length (1 — a)L free of poison.

Sometimes the adsorption of poisons can be diminished owing to the cathodic potential to the catalyst. 

Recent work involved stepped Pt surfaces with controlled adsorption of Bi on step edges [73]. For the stepped singlecrystal surfaces Pt(554), Pt(332), and Pt(221), with nine-, five-, and three-atom wide terraces, respectively, the enhancement factor for HCOOH oxidation increases as the terrace width decreases. Bi appears to block adsorption of poison on reactive (110) oriented step sites and decreases the reaction ensemble size, which increases the rate on narrow (111) terraces. 

Copper catalysts are very sensitive to poisonous compounds, especially when they are used in low-temperature processes, because adsorption of poison is thermodynamically favored. The significant poisons for copper catalysts in methanol production are sulfur and chlorine. Sulfur compounds - for example, H2S - form copper sulfides ... 

The factor (s) relates to the nature and structure of the toxic elements, and the effective volume of the moving at space for the poison the life-factor t) of adsorption depends largely on the nature of the toxic elements and their molecular structure. As the life-adsorption of poison is much longer than that of the reactant molecule, so even a very low concentration of poison still can inhibit the adsorption of the reactant molecule and lead to the catalyst losing its activity as the poison accmnulates on the catalyst s surface. 

As in the case of other solid catalysts, the deactivation of solid acid and base catalysts occurs during use for catalytic reactions by the following mechanisms. Adsorption of poisonous compounds Basic molecules are poisons or inhibitors for acidic... 

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