Competitive adsorption

The kinetics of the ethylene oxidation are rather complicated as they depend not only on ethylene and oxygen pressure but also on the concentration of the reaction products. These influence the rate by adsorption competition with the reactants. Moreover, different forms of adsorbed oxygen may occur on the catalyst surface. Consequently, the rate equations proposed in the literature consist of either Langmuir—Hinshelwood and Eley—Rideal types or power rate models with non-integer coefficients. Power rate models are less appropriate as their coefficients inevitably depend on the reaction conditions. 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

The way sulfur treatment is applied gives rise to pronounced differences in selectivities21. A decrease in the overall alkene yield was observed when 1,3-butadiene was hydrogenated on presulfided Pd. In contrast, much improvement in selective alkene formation from isoprene was achieved when sulfur was present in the feed. In the latter case adsorption competition was suggested to account for the favorable effect of sulfur. 

However, the sulfurization of the surface palladium has a detrimental effect on the consecutive hydrogenation (Fig. 17) the olefin yield is lower on sulfided palladium. Such a result is not in accordance with the literature cited above and this difference can be explained by differences in experimental conditions the improved consecutive selectivities were obtained with the sulfur present in the feedstock, i.e., in adsorption competition, when the detrimental effect on the consecutive hydrogenation was pointed out on partly sulfided palladium without sulfur compounds in the feedstock. The selectivity for olefin production in the hydrogenation of iso-prene is increased by the presence of sulfur in the substrate (Fig. 18). 

Ardizzone, S., Cappelletti, G., Mussini, P.R., Rondinini, S. and Doubova, L.M. (2002) Adsorption competition effects in the electrocatalytic reduction of organic halides on silver. J. Electroanal. Chem. 532, 285-293. 

Clearly, protection of enteric bacteria against phages and colicins occurs by two distinct processes—one involving specific direct adsorption competition for common outer membrane receptors and a second, nonspecific, noncompetitive mechanism involving a cell-mediated event invoked by iron. 

These siderophores also protect E. coli against the B-group colicins, but by a mechanism involving cell-mediated utilization of their ferric ion rather than by an adsorption competition for a cell surface receptor. b Trisodium salt. c (65, 66). 

The retention model by Cecchi and co-workers also quantitatively faced the prediction of the retention behavior of neutral and zwitterionic analytes in IPC. According to the electrostatic models, at odds with clear experimental data [1,50,52,53], the retention of a neutral solute is not dependent on the presence and concentration of a charged IPR in a chromatographic system. Equation 3.23 is very comprehensive if Ze is zero [50], it simplifies since ion-pairing does not occur (C2= C3 = 0). Adsorption competition models the retention patterns of neutral analytes in IPC and the slight retention decreases of neutral analytes with increasing HR concentration may be quantitatively explained [50,53]. 

Even if chain length is the key parameter, the ligand bonding density (usually above 2.5 umol/m ) may be very influential in determining overall stationary phase hydrophobicity. When the monolayer capacity, theoretically estimated by [L]j, increases as a result of increased bonding density, adsorption competitions are less operative and enhanced retention is expected. It should be noted that ligand bonding density can be calculated on the basis of the column carbon load and the total surface area of the column. 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

The mechanism given above places no restrictions on the source of the reversible poison. Consequently, the poisoning can be due not to an adsorption competition between the reactant and a diluent but to an adsorption competition between the reactant and one or more of the reaction products. When this occurs the products will determine the kinetics in the flow type and static systems where appreciable conversion is allowed. Under these conditions the kinetics may be expressed by equations similar to equation (6), and the order will be determined by the magnitude of constants similar to H which depend upon the various velocity constants and adsorption equilibrium constants of the heterogeneous reaction. 

If the charge status of analyte and the HR is the same, a decrease in retention is observed because of electrostatic repulsion between solute and charged stationary phase, and because of adsorption competition. 

If the analyte is uncharged, a very weak decrease in retention is usually observed, primarily because of adsorption competition for the stationary phase. 

As a conclusion, the adsorption competition between butadiene and butene is actually in favour of butadiene on Pd(lll), making this catalyst highly selective in butenes for this hydrogenation reaction. This is not true for Pt(l 11) which is poorly selective. Moreover, one can remark that the more open (110) faces of fee metals are more active for the butadiene conversion into butenes than the close packed (111) faces [29, 33]. Some striking results are given in Table 3. 

This section reports studies of the effect of strongly interacting adsorbates on the sorption of other adsorbates representing the same class (small cations, small anions, surfactants, polymers), e.g. sorption of copper is studied in absence and in the presence of other heavy metal cations at otherwise identical conditions (solid to liquid ratio, pH, equilibration time, etc.). A few examples of adsorption competition between anions or cations of inert electrolytes are also presented. This limitation does not imply that actual adsorption competition occurs only between adsorbates representing the same class. 

Apparently the presence of another species representing the same class should lower the uptake of the species of interest. First the two species compete for surface sites (the number of surface sites available for sorption of the species of interest is lower in the presence of competitor). Moreover, with ionic species, the presence of the competitor in the interfacial region leads to less favorable electrostatic conditions for the adsorption of the species of interest. It should be emphasized that these mechanisms of adsorption competition are effective only when a significant amount of the competitor is adsorbed. For instance, adsorbed species contribute to electrostatic potential of the surface, but the unadsorbed ions in solution do not. 

Also surface precipitation can result in sorption synergism, namely, the uptake of the cation of interest by coprecipitation with hydroxide or basic salt of the competitor can be higher than the adsorption on the original adsorbent. This example illustrates the dependence of the adsorption competition/synergism on the experimental conditions coprecipitation can only occur at sufficiently high concentration of the competitor. 

Faur-Brasquet, C., Kadirvelu, K., and Le Cloirec, P. (2002). Removal of metal ions from aqueous solution by adsorption onto activated carbon cloths adsorption competition with organic matter. Carbon, 40, 2387—92. 

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