Poisons and Inhibitors

Poisons ndInhibitors. Catalyst poisons and inhibitors can come from the fuel, the lube oil, from engine wear and corrosion products, and from air ingestion. There are two types of catalyst poisons one poisons active sites, the other is a masking agent. 

Innes, W. B., Catalyst Carriers, Promoters, Accelerators, Poisons, and Inhibitors, Chapter 6 in Catalysis, Volume I, edited by P. H. Emmett, Reinhold, New York, 1954. 

IS, and especially poisons and inhibitors, can have strong effects at very low surface coverages (a few percent or less of surface area). Except for inhibitors, they have a cumulative effect. 

Catalyst poisons and inhibitors are usually added inadvertently to the reaction mixture by the use of impure solvents or substrates. Promoters, however, are generally added deliberately to enhance catalyst activity and/or reaction selectivity. 

The performance targets, such as reactivity of the reaction mixture, selectivity, extension of a production campaign or reproducibility, are controlled by quite a number of parameters. The most important parameters are temperature, pressure, catalyst and promoters, poisons and inhibitors, silicon composition and structure, particle size distribution of solids, dust removal from fluidized-bed reactor, homogeneity of fluidized-bed and the purity of chloromethane. 

In Eley-Rideal and Langmuir-Hinshelwood type of kinetic rate expressions, the effect of poisons and inhibitors on the reaction rate is accounted for by allowing part of the catalyst surface to become covered with the poisoning compound and so unavailable for desirable reactions. For example, consider the decomposition reaction of A —> B + C that occurs in the presence of an inhibitor, I. If it is assumed that the inhibitor does not participate in the reaction but that it does occupy active catalyst sites and if the surface decomposition is rate controlling then the observed rate of reaction in Langmuir-Hinshelwood terms is given by [13]... 

Catalyst poisoning is a chemical effect. Catalyst poisons form strong adsorptive bonds with the catalyst smface, blocking active centers. Therefore, even very small quantities of catalyst poisons can influence the adsorption of reactants on the catalyst. The term catalyst poison is usually applied to foreign materials in the reaction system. Reaction products that diffuse only slowly away from the catalyst surface and thus disturb the course of the reaction are referred as inhibitors. Table 5-42 lists some catalyst poisons and inhibitors and the way in which they act. 

Table 5-42 Catalyst poisons and inhibitors in chemical processes T41]...

CO poison and inhibitor strong chemisorption, reduction to methane accelerates sintering... 

To overcome the problems of gas-sensor failure due to poisoning of the elements, several manufacturers have used specific filter devices in front of the elements. For example, filters to remove alkyl lead compounds from petrol vapour have included paradichlorobenzene and fibrous silica (3). However, the most commonly used material is based on active charcoal. Such filters (4) effectively absorb many common poisons and inhibitors including silicones, alkyl lead compounds and halogen-containing species. However, along with the absorption of potentially harmful vapours, carbon filters will also absorb hydrocarbons. This effect in practice limits the use of such filters to detectors for C3 and lower hydrocarbons. They cannot be employed in general flammable gas detectors. 

Elucidating Mechanisms for the Inhibition of Enzyme Catalysis An inhibitor interacts with an enzyme in a manner that decreases the enzyme s catalytic efficiency. Examples of inhibitors include some drugs and poisons. Irreversible inhibitors covalently bind to the enzyme s active site, producing a permanent loss in catalytic efficiency even when the inhibitor s concentration is decreased. Reversible inhibitors form noncovalent complexes with the enzyme, thereby causing a temporary de-... 

R SiH and CH2= CHR interact with both PtL and PtL 1. Complexing or chelating ligands such as phosphines and sulfur complexes are exceUent inhibitors, but often form such stable complexes that they act as poisons and prevent cute even at elevated temperatures. Unsaturated organic compounds are preferred, such as acetylenic alcohols, acetylene dicarboxylates, maleates, fumarates, eneynes, and azo compounds (178—189). An alternative concept has been the encapsulation of the platinum catalysts with either cyclodextrin or in thermoplastics or siUcones (190—192). 

The most important factors affecting performance are operating temperature, surface velocity, contaminant concentration and composition, catalyst properties, and the presence or absence of poisons or inhibitors. 

Chemical Reactivity - Reactivity with Water Reacts vigorously with water, generating phosphine, which is a poisonous and spontaneously flammable gas Reactivity with Common Materials Can react with surface moisture to generate phosphine, which is toxic and spontaneously flammable Stability During Transport Stable if kept dry Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Cholinesterases (ChEs), polymorphic carboxyles-terases of broad substrate specificity, terminate neurotransmission at cholinergic synapses and neuromuscular junctions (NMJs). Being sensitive to inhibition by organophosphate (OP) poisons, ChEs belong to the serine hydrolases (B type). ChEs share 65% amino acid sequence homology and have similar molecular forms and active centre structures [1]. Substrate and inhibitor specificities classify ChEs into two subtypes ... 

Activators and inhibitors regulate not the amount of enzyme protein but the activity ( efficiency ) of that which is present. Two principal mechanisms of control are (i) competitive and (ii) allosteric. Competitive control (inhibition) occurs when a compound which is structurally similar to the true substrate binds to the active site of the enzyme. This is how a number of drugs and poisons bring about their effect. For example, a group of therapeutic drugs called statins are used to treat heart disease because by inhibiting a key enzyme called HMGCoA reductase, they reduce the hepatic synthesis of cholesterol and therefore the plasma concentration of that lipid. 

Inhibitors may act reversibly or irreversibly to limit the activity of the enzyme. Irreversible inhibitors are enzyme poisons and indeed many of them are poisonous in the common sense of the word cyanide for example, is an irreversible inhibitor of one of the cytochromes in oxidative phosphorylation. 

Enzymes in humans work best at temperatures of 37°C. When temperatures climb too high, the efficiency of an enzyme can be greatly reduced. This is one example where an increase in temperature can retard the reaction rate. Another problem is that certain substances can disrupt enzymes by blocking active sites and preventing the substrate from bonding with the enzyme. Substances that disrupt enzymes are known as inhibitors. Many poisons and drugs fit in this category. 

Reversible competitive inhibition is also common. This impairment of an enzyme is due to the fact that the inhibitor (drug or poison or even natural material) looks like the normal substrate for the enzyme. Using the abbreviations E, S and I for enzyme, substrate and inhibitor, respectively,... 

FIGURE 12—28. Icon for the cholinesterase inhibitor physostigmine. This agent is used intravenously as a short-acting cholinesterase inhibitor to reverse anticholinergic poisoning and is in testing in an oral sustained-release formulation for Alzheimer s disease. 

There is actually a whole spectrum of possible phenomena, between very strong irreversible poisoning and normal competition between molecules for a given site (this competition leading to a decrease of activity). Inhibitors, as defined above, correspond to a behaviour intermediate between those just mentioned. The sensitivity to inhibitors can thus be expressed either as in the case of true poisons, or as in the case of competition. In the latter formulation, for example, the value or relative value, of the adsorption coefficient could characterize the inhibitor. 

Compatibility with Human Health. Many biochemical factors of resistance against insects and pathogens can constitute a health hazard for human consumers (19,61,109,131). But in the tomato plant, the use of proteinase inhibitors, phenolics, and/or PPO as bases of resistance appears compatible with human health because a) tomato fruit is not consumed in quantities sufficient to lead to acute or chronic poisoning, and b) at the time of ripening these substances are substantially or completely absent from the fruit (132 Felton and Duffey, unpublished data). 

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