Poisoning curves

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. 

Sulfur poisoning curves were obtained by doping the feed with thiophene. Thiotolerance (TT), which is defined as the residual activity value in the poisoning curves (TT = a ), was used for determining the relative sensitivity of the catalysts to sulfur poisoning. The catalyst... 

Fio. 8. Form of a typical poisoning curve. The figure shows the depression of the activity of a supported platinum catalyst, by increasing amounts of thiophene, in the liquid-phase hydrogenation of crotonic acid. 

Poisoning curve for a platinum catalyst, poisoned by mercury ions, in the decomposition of hydrogen peroxide. 

When ho is small (surface completely available) this ratio becomes 1 — a, since the hyperbolic tangent terms become equal to their ailments. This, of course, is the usual classical case of non-selective poisoning. When ho is very large (say 1(X), surface only about 1 % available) over most of the poisoning curve the hyperbolic tangent terms will equal unity and we find... 

This equation shows that for fast reaction in which a poison is distributed homogeneously, activity will fall less than linearily with poison concentration. This type of poisoning might be called anti-selective. Physically this occurs because the reaction uses more of the internal surface of the less active poisoned catalyst. That is, the slower reaction on a poisoned catalyst penetrates deeper into the catalyst pellet. In Fig. 8 we plot the poisoning curves for these two limiting cases ho very large (Curve B) and ho very small (Curve A). For intermediate values of ho, intermediate curves would be obtained. We next consider the... 

Fig. 10. Effect of poison and pore size on apparent activation energy. Plots of observed reaction rate vs. 1/T for a hypothetical catalyst having 11,000 kcal. intrinsic activation energy (e.g., nickel in ethylene hydrogenation) but prepared with different pore sizes and poisoned to varying extent with poison preferentially adsorbed near the pore mouth. Curve A large pores, no poison. Curve B fairly large pores, 90% poisoned (hioo = 0.1, a = 0.9) Curve C Small pores, no poison. Curve D Moderate size pores, 50 % poisoned (h o 0.5, a — 0.5). Curve E small pores, 50 % poisoned (hno = 2, = 0.5). The horizontal portions of D and E correspond to diffusion controlled reaction.

Fig. 8.27 Plot of —t poisoning curve of AllO at 7.5 eind 15.0MPa ...

Figure 5.60 Calibration curves for the diarrhetic shellfish poisons in (i) standard solutions in methanol (O), and (11) standard solutions in poison-free scallop extract solutions ( ) (a) pectenotoxin-6 (b) okadaic acid (c) yessotoxin (d) dinophysistoxin-1. Reprinted from J. Chromatogr., A, 943, Matrix effect and correction by standard addition in quantitative liquid chromatographic-mass spectrometric analysis of diarrhetic shellfish poisoning toxins , Ito, S. and Tsukada, K., 39-46, Copyright (2002), with permission from Elsevier Science.

The simplest case is represented by curve 1. The activity depends linearly on the number of unpoisoned active sites. The interpretation of curves 2 and 3 is less obvious. In the former case the interpretation might be that the least active sites are poisoned first, whereas in the latter case the most active sites are poisoned preferentially. Mass-transfer limitations also play a role in poisoning behaviour. If, for example, the poison is deposited in the outer shell of the catalyst particles, a decrease in catalytic activity can be expected as qualitatively described by curve 3 in Fig. 3.37. 

Often poisoning described by curve 1 in Fig. 3.37 is referred to as nonselective poisoning, whereas the deactivation according to curve 3, which implies disproportionately large deactivation, is called selective poisoning. Explanations of. selective poisoning are ... 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

This relation is plotted as curve A in Figure 12.11 and represents the classical case of nonselec-tive poisoning in which the apparent fraction of the activity remaining is equal to the fraction of the surface remaining unpoisoned. This same result is evident from equation 12.3.112 by recognizing that both effectiveness factors are unity for this situation. 

This relation is plotted as curve Bin Figure 12.11. Smith (66) has shown that the same limiting forms for are observed using the concept of effective dififusivities and spherical catalyst pellets. Curve B indicates that, for fast reactions on catalyst surfaces where the poisoned sites are uniformly distributed over the pore surface, the apparent activity of the catalyst declines much less rapidly than for the case where catalyst effectiveness factors approach unity. Under these circumstances, the catalyst effectiveness factors are considerably less than unity, and the effects of the portion of the poison adsorbed near the closed end of the pore are not as apparent as in the earlier case for small hr. With poisoning, the Thiele modulus hp decreases, and the reaction merely penetrates deeper into the pore. 

Now consider the other extreme condition where diffusion is rapid relative to chemical reaction [i.e., hT( 1 — a) is small]. In this situation the effectiveness factor will approach unity for both the poisoned and unpoisoned reactions, and we must retain the hyperbolic tangent terms in equation 12.3.124 to properly evaluate Curve C in Figure 12.11 is calculated for a value of hT = 5. It is apparent that in this instance the activity decline is not nearly as sharp at low values of a as it was at the other extreme, but it is obviously more than a linear effect. The reason for this result is that the regions of the catalyst pore exposed to the highest reactant concentrations do not contribute proportionately to the overall reaction rate because they have suffered a disproportionate loss of activity when pore-mouth poisoning takes place. 

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