Poisoning adsorptive

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-... 

Poisoning effects at Pt indicated that both the UPD and the OPD H coverages declined with increasing extents of poison adsorption, in a more or less parallel way (136). 

Once we employ the Langmuir - Kinshelwood analysis, the first effect intro duces the poison adsorption tenn into the denominator of the reaction rate expression. The second effect produces the deactivation rate equation as a function of the reactant concentration as well as the concentration of the poison. This introduces a fraction of unpoisoned active sites into the reaction rate equation ... 

Stability Catalyst replacement between batches to overcome rapid poisoning Possibility of continuous renewal. Must have good attrition resistance Essential for fixed bed operation. Plug flow may establish a poison adsorption front... 

The location of the potential sites of poison adsorption or, for ethane hydrogenolysis, hydrogen adsorption, must be specified. For single crystals, such information may be available from LEED studies. The poisoning entities may occupy a sublattice relative to the metal atoms. For instance, H may be adsorbed on the hollow sites centered among 4 atoms in a square lattice. 

Bi-Pt pairs were assumed to be responsible for the catalytic effect, without poison formation. Figure 7 shows a comparison between the experiment and the calculated curve. The agreement is surprisingly good considering the crude assumptions that adatoms cover one surface site and that they are randomly distributed. Bi is known to inhibit three H adsorption sites on Pt(lll) [75]. Nevertheless, the simulation data seem to corroborate the electronic effect for the Bi-Pt system. The data for Pt(lll) and Pt(lOO) revealed the role of substrate in determining the effect of Bi in suppressing poison adsorption (the electronic effect for Pt(l 11), but a third body for Pt(lOO)). In addition, on the same surface (Pt(lOO)), Bi can suppress poison formation by a third-body effect and can catalyze... 

Slow decay of adsorbents due to irreversible adsorption of trace conponents or thermal deactivation of active sites is also common. When this occurs, operating conditions must be adjusted accordingly. Because of this poisoning, adsorption processes, which use surface phenomena, are often much more sensitive to trace chemicals than distillation and other separation techniques that rely on bulk properties. An occasional wash step or extreme regeneration step maybe needed. A short life for the sorbent, which can be a problem in biological operations, often makes the process uneconomical. Long-term pilot plant tests with the actual feed from the plant are useful to determine the seriousness of these problems. 

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