Poisoning effects

Minute amounts of halide have a powerful poisoning effect upon the catalyst it is advisable to distil the benzyl cyanide from Raney nickel. 

The poisoning effect of molecules such as CO and PF3 (p. 495) arises simply from their ability to bond reversibly to haem in the same manner as O2, but much more strongly, so that oxygen transport is prevented. The cyanide ion CN can also displace O2 from oxyhaemoglobin but its very much greater toxicity at small concentrations stems not from this but from its interference with the action of cytochrome a. 

MEA, DEA mono- or di-ethanola-mine in aqueous solution no poisoning effect... 

A sophisticated quantitative analysis of experimental data was performed by Voltz et al. (96). Their experiment was performed over commercially available platinum catalysts on pellets and monoliths, with temperatures and gaseous compositions simulating exhaust gases. They found that carbon monoxide, propylene, and nitric oxide all exhibit strong poisoning effects on all kinetic rates. Their data can be fitted by equations of the form ... 

The poisoning effect of hydrogen when dissolved in palladium was for the first time properly described and interpreted by Couper and Eley (29) in 1950 in their study of the fundamental importance of the development of theories of catalysis on metals. The paper and the main results relate to the catalytic effect of an alloying of gold to palladium, on the parahydrogen conversion. This system was chosen as suitable for attempting to relate catalyst activity to the nature and occupation of the electronic energy... 

A similar reaction was studied by Kowaka Jfi) who investigated the catalytic activity of palladium and its alloys with silver in the hydrogenation of ethylene. The author alluded to the poisoning effect of hydrogen pretreatment of the palladium catalyst. 

The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm). It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. 

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

In studies on the para-hydrogen conversion rate on nickel and its alloys with copper other authors also noted the poisoning effect of the sorbed hydrogen. Singleton (53) mentioned the poisoning of nickel film catalysts by the slow-sorbed hydrogen. Shallcross and Russell (54) observed a similar phenomenon for nickel and its alloys with copper at — 196°C. At higher... 

On the basis of the related experimental evidence and its discussion one can regard the poisoning effect of the hydride hydrogen in nickel and its alloys with copper as normally accompanying the heterogeneous recombination of hydrogen atoms on these catalysts at lower temperatures. 

As mentioned previously in the introduction to the present review the ability to form the hydride phase is not characteristic solely of palladium or nickel. It would be of interest, therefore, to verify the results on the poisoning effect of hydride formation in the case of nickel or palladium by comparing with the other transition 3d, 4d, and 5d metals and the rare earth (4f) metals. 

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

Figure 2.43 shows the effect of S and P on the rate of methane production132 on Ni(100) at 600 K.The observed smaller poisoning effect in the case of P is in agreement with the less pronounced effect of P on the adsorption of the reactants (Section 2.3). 

From a catalytic viewpoint this is the most important phenomenological parameter for quantifying the promoting or poisoning effect of a given coadsorbed species i (e.g. 02 F, Na+, H+) on the rate of a catalytic reaction. Similarly to the case of classical promotion (eq. 2.19), it is defined from ... 

A similar poisoning effect with MeBr was discovered using Et2 All coinitiator. Poisoning by MeBr was studied using MeCl/MeBr mixtures with f-BuBr and H20 initiators at different temperatures. Results are shown in Figs. 3 and 4. As with Mel,... 

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