Irreversible poisoning

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-... 

Strong centres, forming anion radical even from nitrobenzene molecule are poisoned irreversibly, however, their presence is not necessity for the preservation of catalytic activity. Taking into consideration that regenerated MgO which is not able to ionize nitrobenzene molecule is still active in its reduction by hydrogen transfer and that only a few from reduced nitro compounds form ion radicals on catalyst surface one can ascertain that ion radicals formation is not necessary step in nitroarenes (or nitroparaffins) activation. Probably, one-electron donor sites take part only in activation of alcohol what was demonstrated by us earlier. 

The phenomenon of catalyst poisoning may be divided into three types according to the different effect reversible poisoning, irreversible poisoning and selective poisoning. 

Chlorides may be found in natural gas, particularly associated with offshore reservoirs. Modified alumina catalysts have been developed to irreversibly absorb these poisons from the feed gas. 

Acute benzene poisoning results in CNS depression and is characterized by an initial euphoria followed by staggered gait, stupor, coma, and convulsions. Exposure to approximately 4000 ppm benzene results in complete loss of consciousness. Insomnia, agitation, headache, nausea, and drowsiness may persist for weeks after exposure (126). Continued inhalation of benzene to the point of euphoria has caused irreversible encephalopathy with tremulousness, emotional lability, and diffuse cerebral atrophy (125). In deaths arising from acute exposure, respiratory tract infection, hypo- and hyperplasia of sternal bone marrow, congested kidneys, and cerebral edema have been found at autopsy. 

In many of the other processes that use base metal catalysts, irreversible poisoning of the catalyst occurs as a result of deposition of metal contaminants from the process feedstock onto the catalyst surface. These catalysts are not considered to be regenerable by ordinary techniques. 

Toxic heavy metals and ions, eg, Pb, Hg, Bi, Sn, Zn, Cd, Cu, and Fe, may form alloys with catalytic metals (24). Materials such as metallic lead, ziac, and arsenic react irreversibly with precious metals and make the surface unavailable for catalytic reactions. Poisoning by heavy metals ordinarily destroys the activity of a precious-metal catalyst (8). 

It has been reported that below about 370°C, sulfur oxides reversibly inhibit CO conversion activity. This inhibition is greater at lower temperatures. CO conversion activity returns to normal shortly after removal of the sulfur from the exhaust (44). Above about 315°C, sulfur oxides react with the high surface area oxides to disperse the precious-metal catalytic agents and irreversibly poison CO conversion activity. 

Interest in the ZGB model arises due to its rieh and eomplex irreversible eritieal behavior. In faet, in two dimensions and for the asymptotie regime (t — cxd), the system reaehes a stationary state whose nature solely depends on the parameter 7 - For Fa < Fia = 0.3874 (Fa > F2A = 0.5250) the surfaee beeomes irreversibly poisoned by B (A) speeies, while for Fia < Fa < F2A a steady state with sustained produetion of AB is observed. Fig. 2 shows plots of the rate of AB produetion (J ab) and the surfaee eoverage with A ( a) and B ( s) speeies versus Fa. So, just at Fja and F2A the ZGB model exhibits IPTs between the reaetive regime and poisoned states, whieh are of seeond and first order, respeetively. Experimental evidenee of a first-order transition-like behavior has been reported for the eatalytie oxidation of earbon monoxide on Pt(210) and Pt(lll) [19], as shown, e.g., in Fig. 3. 

Negleeting CO desorption, as in the standard ZGB model, the CO-poi-soned state is irreversible sinee there is no possibility of removing CO from the surfaee. So, CO desorption has to be eonsidered in order to avoid the fully CO-poisoned state. The adsorption and desorption of X then drives the system from a state with high eoneentration of adsorbed CO to the reaetive state and baek. This proeess ean be understood with the aid of Fig. 8. At low X eoverage only the reaetive state is stable. Inereasing X eoverage eauses site bloeking and eonsequently the adsorption of both CO and O2 is redueed. 

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Hydrogen chloride is a permanent irreversible poison to the metha-nation activity of C150-1-03 even though most of it is not picked up by the catalyst but is observed in the effluent gas. Only 0.02-0.04% was found on the discharged catalyst, but any amount of chloride in the feed gas is detrimental to catalyst activity. 

CBs, like OPs, act as inhibitors of ChE. They are treated as substrates by the enzyme and carbamylate the serine of the active site (Figure 10.8). Speaking generally, car-bamylated AChE reactivates more rapidly than phosphorylated AChE. After aging has occurred, phosphorylation of the enzyme is effectively irreversible (see Section 10.2.4). Carbamylated AChE reactivates when preparations are diluted with water, a process that is accelerated in the presence of acetylcholine, which competes as a substrate. Thus, the measurement of AChE inhibition is complicated by the fact that reactivation occurs during the course of the assay. Carbamylated AChE is not reactivated by PAM and related compounds that are used as antidotes to OP poisoning (see Box 10.1). 

Ultradeep desulfurization of fuel oils is used for producing not only clean fuels but also sulfur-free hydrogen used in fuel-cell systems, in which the hydrogen can be produced potentially through the reforming of fuel oils. Fuel-cell systems must be run with little-to-no sulfur content, because sulfur can irreversibly poison the precious metal catalysts and electrodes used [12]. 

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