Mechanisms of poisoning

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

Other potential poisons include zinc, manganese, chlorine, and bromine. A number of metals may be deposited on the catalysts from engine erosion and wear, including copper, chromium, nickel, and iron. The mechanism of poisoning has been reviewed by Maxted (134) and by Butt (135). 

This study shows that Mel, and, for some systems, MeBr are potent poisons in isobutylene polymerization. The extent of poisoning increases in the presence of less reactive initiator and by decreasing temperatures. The mechanism of poisoning is discussed in Section VIII. 

The detailed mechanism of poisoning by silica, however, is unknown. We have suggested that an interaction of silicic acid generated by hydrolysis of silica or silica-alumina reacts with the active sites on the additive lowering the overall capacity of the material to form sulfates. More work must be done to show the chemistry of this mechanism. 

The poisoning action of the heavy metals is in accord with their known tendencies to decompose hydrocarbons, at cracking temperatures, to carbon and hydrogen. The mechanism of poisoning is believed not to consist of inactivation of the active centers of the catalyst, but rather a superimposition of heavy-metal-type cracking upon the normal cracking... 

Reactor Model A variation of the pseudo-homogeneous 1-D plug flow model with shell progressive mechanism of poisoning (SPM) is proposed. This model accounts for the intraparticle-diffusional resistance using a pore mouth poisoning mechanism. The... 

R. F. Uormsbecber, A. V. Peters, J. M. Maselli, Vanadium poisoning of cracking catalysts mechanism of poisoning and design of vanadium tolerant catalyst system, J. Catal, 100, (1986), pp. 130-137,... 

Rieders F, Brieger H. 1953. Mechanism of poisoning from wax crayons. JAMA 151 1490. 

So, what can be learnt from a tour through the chapters of this book What makes a good poison Are the most potent molecules made by man or fashioned by nature Is enough known about the mechanisms of poisoning to construct effective antidotes, or even a imiversal antidote (a true mithridate), a panacea to all toxic ills Engaging aspects to ponder over by oneself and perhaps discuss with colleagues. 

The Mechanisms of Poisoning. The curves in Figure 8 depict typical deactivation characteristics of oxidation catalysts poisoned by lead and... 

The poison called Compound 1080 (pronounced ten-eighty ) is sodium fluoroacetate. Ranchers who want to protect their sheep from attacks by coyotes put the poison just outside the ranch fence. When the coyotes eat this poison, they die. The mechanism of poisoning by Compound 1080 is the same as that by plant poisons. 

Useful information on the mechanism of poisoning and promotion for a simple model catalyst has come from the effective medium theory of Norskov and coworkers . Their method involves... 

In the case of chlorine, the mechanism of poisoning does not proceed by a surface chemisorption reaction but by a bulk chemical reaction with the potassium promoter. A similar calculation to that shown above, but with respect to the maximum chlorine content, may also be carried out. Let us assume that the catalyst has a potassium content of 1 wt%. 

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