Adsorption proton exchange

Kurbatov plots (jj ) have often been employed to determine the net metal ion/proton exchange, x> from adsorption data. Although Kurbatov constants are convenient curve-fitting parameters, they are insensitive to the variation of x with pH and adsorption density and should be dispensed with for use in adsorbate partitioning calculations, particularly when high adsorption densities are expected ( 9 ). 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

However, fi adsorption on catalysts is also supported by deuterium exchange studies of thiophene. When thiophene and D2 are passed over Mo-based HDS catalysts at temperatures below which substantial HDS occurs, the thiophene protons exchange with deuterium. This exchange occurs much more rapidly at the 2- and 5-positions than at the 3- and 4-sites. When CpRu(jj -T)+ (equation 9) is dissolved in CD3OD in the presence of OD at 23 °C, the 2,5-hydrogens of thiophene... 

A significant problem in surface complexation models is the definition of adsorption sites, The total number of proton-exchangeable sites can be determined by rapid tritium exchange with the oxide surface (25). Although surface equilibria are usually written in terms of one surface site, e.g. Equations 5, 6, 8, 9, adsorption isotherms for many ions show that the number of molecules adsorbed at maximum surface coverage (fmax) is less than the total number of surface sites. For example, uptake of Se(VI) and Cr(VI) ions on Fe(0H)3(am) at T ax 1/3 and 1/4 the total... 

The utilization of RF-GC methodologies can be extended in the study of the surface properties of various solids and related processes. Thus, in a recent work, the effect of the presence of hydrogen in the adsorptive behavior of a Rh/Si02 catalyst was studied, as the selective oxidation of CO in a rich hydrogen atmosphere is a process of great technological and environmental interest, because it is related to the development of proton exchange membrane fuel cells. 

Over FAU zeolites in presence of air and water, dichloromethane is selectively transformed into formaldehyde and chlorhydric acid. NaX is more active than NaY, the protonic exchange of this latter zeolite causing a large decrease in activity. The activity was shown to be related to the heat of dichloromethane adsorption. 

In cases where the particle charge is the result of adsorption of ionic surfactants, its sign depends on the absorbability of the surfactant cation and aifion. The most probable mechanism as elucidated by Fowkes and Pugh is the adsorption of the surfactant as an ion pair (neutral molecule) followed by a proton exchange reaction the direction of which depends on the relative acidity/basicity of the surfactant and the surface. The final step in the charge generation process is the dissociation... 

The simple surface hydration and proton exchange enables the metal cation complexes also to adsorb due to ligand exchange. Equally with the solvent association and condensation processes this adsorption may lead to the formation of extended gel structures and surface precipitation. However, as the surface site distribution and surface potential influence these processes, the physicochemical conditions (pS, pH, pi, pe) where they occur do not match those for the solution species. ... 

A particularly interesting example on the interplay between the proton exchange for the surface hydroxyls and the adsorption or desorption of solution species is provided by silica. As mentioned above, it is known that the hydrolysis of silica species is minimal at 6 < pH < 7. It is therefore expected that the number of >SiOH groups (Equation 8.102a) increases when the pH scale descends and the number of >SiO groups (Equation 8.102b) increases when the pH scale ascends. Moreover, it is expected that the stability of silica sols would be the least at 6 < pH < 7. As shown in Figure 8.31 this is exactly what is observed. - ... 

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