Effects of catalyst poisons

We now take the simple cubic model above and allow for the competitive adsorption and desorption of a second species Q. Thus the model becomes 

One feature of this extension is that there are now two different adsorbed species whose concentrations are independent. In tefms of the fractional surface coverages, we have the condition 

The rate equations for the surface concentrations of the two species can be written as 

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The deleterious effects of catalyst poisoning when carrying out asymmetric hydrogenations at low catalyst loading caimot be overemphasised. In order to eliminate the possibility that the substrate synthesis introduced inhibitory impurities, an alternative synthetic protocol was examined (Scheme 7.4). The use of a brominating agent and an expensive palladium catalysed step in the initial route could limit the development of this as an economically favourable process and this was further motivation to examine alternative routes to the hydrogenation substrate. 

L.A. Arma, B.J. McCoy and J.M. Smith, Effect of catalyst poisoning on adsorption and surface reaction rates in liquid-phase hydrogenation, Ind.Eng.Chem.Res., 29(1990)1050. 

The papers included in this symposium cover the full gamut of problems that had to be addressed. The physical and chemical stability of the catalysts had to be significantly improved over known catalysts in order to meet the 50,000 mile life requirement prescribed by the regulations. The effects of catalyst poisons such as lead, sulfur, phosphorus, etc. were also critical in relation to the limits of deposition that could be tolerated while maintaining catalyst effectiveness. The nature of the catalyst support or substrate became significant in relation to its interaction with the metallic components of the catalyst—adherence, distribution, and reactivity at high temperature. 

Figure 3. Temporary effect of catalyst poisons on silica-based catalysts. (Reproduced from Ref. 10 Copyright 1968, American Chemical Society.)...

Steels are less susceptible to hydrogen cracking above room temperature, with iron becoming a better catalyst for the reaction Had + H + e" H2. Hence, more hydrogen escapes as molecular H2 and less adsorbed H is available to enter the metal, contrary to the effects of catalyst poisons, which retard the above reaction. 

For low activity runs caused by catalyst poisons the ethylene concentration of the fragment (equal size) should be expected to be high and therefore melt index to be low. Actually, it does not seem that catalyst poisons influence the yield - melt index curve of fig. 4. So the effect of catalyst poisons does not support this explanation. 

What are the effects of catalyst behaviour, e.g. aging, poisoning, disintegration, activation, regeneration ... 

The metals in the FCC feed have many deleterious effects. Nickel causes excess hydrogen production, forcing eventual loss in the conversion or thruput. Both vanadium and sodium destroy catalyst structure, causing losses in activity and selectivity. Solving the undesirable effects of metal poisoning involves several approaches ... 

Nickel in the feed is deposited on the surface of the catalyst, promoting undesirable dehydrogenation and condensation reactions. These nonselective reactions increase gas and coke production at the expense of gasoline and other valuable liquid products. The deleterious effects of nickel poisoning can be reduced by the use of antimony passivation. 

Behm RJ, Jusys Z. 2006. The potential of model studies for the understanding of catalyst poisoning and temperature effects in polymer electrolyte fuel cell reaction. J Power Sources 154 327-342. 

The Influence of Catalyst Poisoning Processes on Catalyst Effectiveness Factors... 

Two limiting cases of the behavior of catalyst poisons have been recognized. In one, the poison is distributed uniformly throughout the pellet and degrades it gradually. In the other, the poison is so effective that it kills completely as it enters the pore and is simultaneously removed from the stream. Complete deactivation begins at the mouth and moves gradually inward. 

Many industrial processes are mass-transfer limited so that reaction kinetics are irrelevant or at least thoroughly disguised by the effects of mass and heat transfer. Questions of catalyst poisons and promoters, activation and deactivation, and heat management dominate most industrial processes. 

Fig. 12. Effect of self-poisoning (i hr at 450 C, by the reaction mixture) on hexane/H2 reactions. AT is the temperature increase necessary to achieve the same overall conversion after poisoning over that before poisoning. AT is plotted as a function of the average particle size of various Pt/SiO2 catalysts. From V. Ponec et al, in Catalyst Deactivation, p. 93, Elsevier, Amsterdam (1980).

For large values of the Thiele modulus the fraction 0(1 - ) will usually be sufficiently large that F= (1 + 0 )". Curve 3 in Fig. 3.12 depicts selective poisoning of active catalysts near the particle exterior and is the function represented by equation 3.59. Curve 4 describes the effect of selective poisoning for large values of the Thiele modulus. For the latter case the activity decreases drastically, after only a small amount of poison has been added. 

Special care has to be taken, however, that the quinoline titer truly represents the minimum amount of catalyst poison. In most cases this type of base is adsorbed by inactive as well as active sites. Demonstration of indiscriminate adsorption is furnished by the titration results of Roman-ovskii et al. (52). These authors (Fig. 13) showed that introduction of a given dose of quinoline at 430°C in a stream of carrier gas caused the activity of Y-zeolite catalyst (as measured by cumene conversion) to drop with time, reach a minimum value, then slowly rise as quinoline was desorbed. The decrease in catalytic activity with time is direct evidence for the redistribution of initially adsorbed quinoline from inactive to active centers. We have observed similar behavior in carrying out catalytic titrations of amorphous and crystalline aluminosilicates with pyridine, quinoline, and lutidine isomers. In most cases, we found that the poisoning effectiveness of a given amine can be increased either by lengthening the time interval between pulse additions or by raising the sample temperature for a few minutes after each pulse addition. 

Here we shall briefly summarize the effects of individual poisons on various catalytic reactions taking place on automotive catalysts. There are three main catalytic processes oxidation of carbon monoxide and hydrocarbons and reduction of nitric oxide. Among secondary reactions there are undesirable ones which may produce small amounts of unregulated emissions, such as NH3, S03 (6), HCN (76, 77), or H2S under certain operating conditions. Among other secondary processes which are important for overall performance, in particular of three-way catalysts, there are water-gas shift, hydrocarbon-steam reforming, and oxygen transfer reactions. Specific information on the effect of poisons on these secondary processes is scarce. 

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