Poisoning chemical adsorption

This SO2 chemical adsorption consumes the anion vacancies leading to the inhibition of a-02 adsorption. Additionally, the formation of sulfite and sulfate species over the surface is possible via a subsequent reaction of chemisorbed SO2 with Fe O and Fe O2. Although those sulfite and sulfate species formed after 3 h poisoning under 80 ppm SO2 were not detected by XRD (line (b) in Figure 19) possibly due to their limited amounts, their formation was confirmed by FTIR (line (c) in Figme 20) ... 

Contrary to the original assumption which linked the inhibition of bone resorption solely to physico-chemical adsorption of the BP onto the bone surface, there is new evidence indicating cellular mechanisms of BP action, although a prerequisite of the biological activity is still the adsorption. The bisphosphonates deposited on the bone surface poison the bone-resorbing osteoclasts after being internalized by them. 

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

Mercury is emitted from the mercury cell process from ventilation systems and by-product streams. Control techniques include (1) condensation, (2) mist elimination, (3) chemical scrubbing, (4) activated carbon adsorption, and (5) molecular sieve absorption. Several mercury cell (chloralkali) plants in Japan have been converted to diaphragm cells to eliminate the poisonous levels of methyl mercury found in fish (9). 

Poisoning of the catalyst by presence of a catalytic poison may be either due to chemical reaction between catalyst and poison (e.g. Fe + H2S Fe + H2) or poison may render surface of the catalyst unavailable for adsorption of reactants. 

Catalysts are porous and highly adsorptive, and their performance is affected markedly by the method of preparation. Two catalysts that are chemically identical but have pores of different size and distribution may have different activity, selectivity, temperature coefficient of reaction rate, and response to poisons. The intrinsic chemistry and catalytic action of a surface may be independent of pore size, but small pores appear to produce different effects because of the manner and time in which hydrocarbon vapors are transported into and out of the interstices. 

If the catalyst surface is slowly modified by chemisorption on the active sites by materials which are not easily removed, then the process is frequently called poisoning. Restoration of activity, where possible, is called reactivation. If the adsorption is reversible then a change in operating conditions may be sufficient to reactivate the catalyst. If the adsorption is not reversible, then we have permanent poisoning. This may require a chemical retreatment of the surface or a complete replacement of the spent catalyst. 

A differentiation in the activities of surfaces may likewise be witnessed in a variety of chemical and catalytic processes. Thus we find that charcoal will undergo slow autoxidation when exposed to air it will also cataljrtically accelerate the oxidation of a number of organic subtances such as oxalic acid. By processes of selective poisoning of the charcoal it can be demonstrated quite readily that the portion of the surface which can accelerate the oxidation of, oxalic acid is but a small portion of the surface which is available for say the adsorption of methylene blue and that but a minute fraction of the surface (less than 0 5 % foi a good sugar charcoal) is capable of undergoing autoxidation. 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

In chapter 12 we discussed a model for a surface-catalysed reaction which displayed multiple stationary states. By adding an extra variable, in the form of a catalyst poison which simply takes place in a reversible but competitive adsorption process, oscillatory behaviour is induced. Hudson and Rossler have used similar principles to suggest a route to designer chaos which might be applicable to families of chemical systems. They took a two-variable scheme which displays a Hopf bifurcation and, thus, a periodic (limit cycle) response. To this is added a third variable whose role is to switch the system between oscillatory and non-oscillatory phases. 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... 

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