Irreversible adsorption, experimental measurements

The problem of decrease in catalyst activity due an irreversible adsorption of poison was solved numerically using a single point collocation approximation. The numerical results are compared with experimental data obtained by measuring concentration changes due to thiophene poisoning of Ni/AljO in benzene hydrogenation. 

This brief review is limited to consideration of platinum group metals in aqueous solutions. Thermodynamic approaches should work in other protic solvents. However these solvents are always of organic nature, and the experimental measurements of any thermodynamic quantities are complicated by irreversible dissociative adsorption of organic species. Another interesting field, still separated, is platinum electrochemistry in aptotic media. 

The mechanism described above, with irreversible adsorption of reactants and irreversible desorption of the Mari explained very satisfactorily all the experimental data of Tamaru et and Boudart et for the decomposition rate of ammonia at high temperatures and low pressures on tungsten and molybdenum respectively with simultaneous measurement of the surface concentration of N by Auger electron spectroscopy. Disturbing the stationary state by flashing desorption of the metal catalysts, the rate of returning to steady state of the two postulated irreversible steps of adsorption and desorption could be evaluated separately. 

It is reported (149) that hydrogen adsorption is irreversible. The above mechanism would predict irreversible H2 loss, but of only half of the chemisorbed hydrogen. Experimental details, thoroughly documented in the later papers, are scanty in the discussion of chemisorption in (745), where the work was originally reported. However, it seems likely that the gas restored to the reaction mixture by the atom/atom recombination mechanism would either have been pumped off before the heating to 300 C, or would have been discounted as a background in the measurement of the gas desorbed on heating. Thus the partial desorption predicted by the above mechanism is probably consistent with the measured results (145, 149). 

The chemisorption of sulfur from mixtures of H,S and H2 has been widely studied we have discussed some of the results. Nevertheless, introduction of irreversible and reversible adsorbed sulfur, which is in line with adsorption stoichiometries varying from more than 1 to 0.4 sulfur atom by accessible platinum atom, shows that different adsorbed species are involved in sulfur chemisorption. In fact, electrooxidation of adsorbed sulfur on platinum catalysts occurs at two different electrochemical potentials (42) in the same way, two different species of adsorbed sulfur were identified on gold by electrochemical techniques and XPS measurements (43,44). By use of 35S (45) it was pointed out that, according to the experimental conditions, reducible PtS2 or nonreducible PtS mono-layers can be created. 

The whole discussion of polymer adsorption so far makes the fundamental assumption that the layer is at thermodynamic equilibrium. The relaxation times measured experimentally for polymer adsorption are very long and this equilibrium hypothesis is in many cases not satisfied [29]. The most striking example is the study of desorption if an adsorbed polymer layer is placed in contact with pure solvent, even after very long times (days) only a small fraction of the chains desorb (roughly 10%) polymer adsorption is thus mostly irreversible. A kinetic theory of polymer adsorption would thus be necessary. A few attempts have been made in this direction but the existing models remain rather rough [30,31]. 

The desorption of solvents from soil has not been extensively measured. In the application of advection-dispersion models to predict solute movement, it is generally assumed that adsorption is reversible. However, the adsorption of the solutes in T able 17.1.1 may not be reversible. For example, hysteresis is often observed in pesticide adsorption-desorption studies with soils. The measurement and interpretation of desorption data for solid-liquid systems is not well understood.Once adsorbed, some adsorbates may react further to become covalently and irreversibly bound, while others may become physically trapped in the soil matrix. The non-singularity of adsorption-desorption may sometimes result from experimental artifacts. ... 

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