Poisoning susceptibility

This term refers to the sensitivity of a catalyst to poisoning under specified conditions. Two other terms are typically used to describe poisoning susceptibility. Poisoning resistance is the degree to which a catalyst resists deactivation, i.e., a catalyst which deactivates slowly is more resistant to poisoning than one that deactivates rapidly. Poisoning tolerance is defined typically as either the ultimate amount of poison a catalyst can adsorb and... 

Apparently there is not a clear relationship between the catalyst nature and the coke poisoning susceptibility but the tendency to retain and withstand higher metal poisoning of some catalysts over others is clear. Catalysts with pore size near to 120 A present the maximum metals retention for this type of feeds, but a better definition of the texture needs more information and is the reason of further research at ICP. 

Slow response time susceptible to poisoning susceptible to hydrogen induced aging effects... 

Based on the poison susceptibility of a Johnson Matthey platinum-rhodium catalyst oxidizing ethane at 480°C, the severity of poisons tested, on a molar basis, is in the following order (Jung and Becker, 1987) ... 

Experiments involving the use of dimethyl sulphate should be carried out by students only under immediate supervision. Not only is the vapour of dimethyl sulphate highly poisonousy but the cold liquid itself is absorbed easily through the skin, with toxic results individual susceptibility to ditnethyl sulphate poisoning varies and may be very high. If the sulphate is splashed on to the hands, wash immediately with plenty of concentrated ammonia solution in order to hydrolyse the methyl sulphate before it can be absorbed through the skin (see p. 528). 

Wrought or extmded lead—teUurium (0.035—0.10 wt %) aUoys produce extremely fine grains. The binary aUoy is, however, susceptible to recrysta11i2ation. The addition of copper or sUver reduces grain growth and retains the fine grain si2e. Because teUurium is a poison for sealed lead—acid batteries, the teUurium content of lead and lead aUoys used for such purposes is usuaUy restricted to less than 1 ppm. 

Most of them are generally classified as poisons. Exceptions to this rule are known. A notable one is 4-dimethyl aminopyridine (DMAP) (24), which is widely used in industry as a superior acylation catalyst (27). Quaternary salts of pyridines are usually toxic, and in particular paraquat (20) exposure can have fatal consequences. Some chloropyridines, especially polychlorinated ones, should be handled with extra care because of their potential mutagenic effects. Vinylpyridines are corrosive to the skin, and can act as a sensitizer for some susceptible individuals. Niacin (27), niacinamide (26), and some pyridinecarbaldehydes can cause skin flushing. 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

The goal of Haber s research was to find a catalyst to synthesize ammonia at a reasonable rate without going to very high temperatures. These days two different catalysts are used. One consists of a mixture of iron, potassium oxide. K20, and aluminum oxide. Al203. The other, which uses finely divided ruthenium, Ru. metal on a graphite surface, is less susceptible to poisoning by impurities. Reaction takes place at 450°C and a pressure of 200 to 600 atm. The ammonia... 

Cells and isolated enzymes are often susceptible to poisoning at low levels of chemical contamination. It is, therefore, necessary to carry out expensive purification of substrates (feedstocks) and water used in bioprocesses. 

In addition to having the required spedfidty, lipases employed as catalysts for modification of triglycerides must be stable and active under the reaction conditions used. Lipases are usually attached to supports (ie they are immobilised). Catalyst activity and stability depend, therefore, not only on the lipase, but also the support used for its immobilisation. Interesterification reactions are generally run at temperatures up to 70°C with low water availability. Fortunately many immobilised lipases are active and resistant to heat inactivation under conditions of low water availability, but they can be susceptible to inactivation by minor components in oils and fats. If possible, lipases resistant to this type of poisoning should be selected for commercial operations. 

Acute poisoning of humans by freshwater cyanobacteria as occurs with paralytic shellfish poisoning, while reported, has never been confirmed. Humans are probably just as susceptible as pets, livestock, or wildlife but people naturally avoid contact with heavy waterblooms of cyanobacteria. In addition, there are no known vectors, like shellfish, to concentrate toxins from cyanobacteria into the human food chain. Susceptibility of humans to cyanobacteria toxins is supported mostly by indirect evidence. In many of these cases, however, if a more thorough epidemiological study had been possible these cases probably would have shown direct evidence for toxicity. 

Fish are generally more susceptible to poisoning than microscopic plants or other animals as such they are a good indicator species. A summary of the concentrations of selected substances at which toxic effects have been detected in bacteria, algae, crustacia and protozoa is given in Table 16.14. 

When adsorbed on graphite or Au, the catalytic characteristics of the FeCu form are very similar to those of the Fe-only analog, with about 50 mV smaller overpotential, afourfoldsmaUerfraction of O2 reduced to 02 /H02( av > 3.95 Fig. 18.19)and three- to hvefold lower susceptibility to poisoning by CO, CN, and NJ. 

Allergenic plants causing skin eruption by contact include poison ivy (Rhus radicans L.), poison oak (Rhus toxicodendron L.), poison sumac (Rhus vernix L.), and stinging nettle (Urtica dioica L.). In the United States, poison ivy and poison oak cause nearly 2 million cases of skin poisoning and skin irritation annually, for a loss of 333,000 working days. In addition, these weeds cause 3.7 million days of restricted activity among those people who are susceptible to the toxins (1). 

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