Poisons, adsorption

Competitive reactions and essentially competitive hydrogenations were often used to discuss the extent of the electronic transfers induced by poison adsorption. For instance, two model molecules with different electronic densities are chosen, e.g., benzene and toluene. In this case the electronic donor properties of the methyl group increase the electronic density on the insaturated bonds. During competitive hydrogenation of benzene and toluene, sulfur adsorption poisons the two reactions but is less toxic for toluene than for benzene hydrogenation (94-96). Sulfur, by its adsorption as an electron acceptor, is able to decrease the electronic density of the unpoisoned metallic surface area and could favor the adsorption of the reactant with the highest donor properties enhancing the hydro-... 

As shown above the adsorptive poisoning of these catalysts by S02 was highly temperature dependent. Since the warm up period is usually very short, a catalyst which uptakes S02 at that period, but rapidly recovers its high activity when the automobile runs normally would be a promising candidate for use in autoexhaust purification. 

Low temperature carbon monoxide sensors based on the reversible carbon monoxide adsorptive poisoning of precious metal electrodes are also being developed by Los Alamos National Laboratory. The addition of metals such as ruthenium to the platinum electrode material greatly improves the hydrogen oxidation kinetics in the presence of CO. An amperometric sensor that senses the CO inhibition of the hydrogen oxidation can be fabricated from a platinum electrode, a proton conductor and a platinum ruthenium alloy electrode. While the... 

FIGURE 3.9 Change in surface coverage versus time for the irreversible adsorption (poisoning) of Pt by CO gas. Starting from an initial CO surface coverage (Oq) at time f = 0, approaches a value of 1 (completely poisoned surface) at long times. 

Sweetening. Another significant purification appHcation area for adsorption is sweetening. Hydrogen sulfide, mercaptans, organic sulfides and disulfides, and COS need to be removed to prevent corrosion and catalyst poisoning. They ate to be found in H2, natural gas, deethanizer overhead, and biogas. Often adsorption is attractive because it dries the stream as it sweetens. 

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). 

FIG. 7 Log-log plots of the interface width (w ) versus the Monte Carlo time t, measured at different adsorption probabihties using channels of width L = 30. Data were obtained during the displacement of an A-poisoned phase by the reactive regime. From top to bottom the probabihties are 0.5192, 0.5202, 0.5211, 0.5215, and 0.5238. 

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. 

Eight variants of the DD reaction mechanism, described by Eqs. (21-25) have been simulated. The simplest approach is to neglect B2 desorption in Eq. (22) and the reaction between AB species (Eq. (25)). For this case, an IPT is observed at the critical point Tib, = 2/3. Thus this variant of the model has a zero-width reaction window and the trivial critical point is given by the stoichiometry of the reaction. For Tb2 < T1B2 the surface becomes poisoned by a binary compound of (A -I- AB) species and the lattice cannot be completely covered because of the dimer adsorption requirement of a... 

The discussion in the previous section suggests that adsorption of pyridine on the catalyst is a necessary prerequisite for the formation of 2,2 -bipyridine but as platinum catalysts, which are poisoned by... 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

There is a complication in choosing a catalyst for selective reductions of bifunctional molecules, For a function to be reduced, it must undergo an activated adsorption on a catalytic site, and to be reduced selectively it must occupy preferentially most of the active catalyst sites. The rate at which a function is reduced is a product of the rate constant and the fraction of active sites occupied by the adsorbed function. Regardless of how easily a function can be reduced, no reduction of that function will occur if all of the sites are occupied by something else (a poison, solvent, or other function). 

Although benzene is not a poison such as H2S and HC1, it does depress activity by reforming and adsorption onto the catalyst. At high levels it can produce carbon. 

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