Adsorbed competitive

Vibrational Spectroscopic Studies of Adsorbate Competition During Carbon Monoxide Adsorption on Platinum Electrodes... 

Both types of species adsorb competitively with CO, leading to a decrease in the amount of adsorbed CO and a shift of v(CO) to lower frequency. The latter effect is consistent with the decrease in dynamic coupling expected for a uniformly reduced CO coverage. 

Figure 6 shows the TPR spectra of adsorbed CO on nickel. The CO was desorbed mostly as the molecular form, whereas the amounts of desorbed carbon dioxide and methane were quite small. Thus, most of the CO adsorbed on nickel is in an undissociated state, and the extent of its adsorption is fairly weak, as the desorption is completed below 200 C. In contrast, the adsorption of methyl acetate on nickel is stronger than those of other reactants or products, as evaluated from the retention time in the nickel-activated carbon column shown in Table III. This fact suggests that most of the nickel is covered by methyl acetate and reaction products, and the coverage of adsorbed CO is quite low under the reaction conditions when the partial pressure of CO is close to that of methyl acetate. The carbonylation is therefore accelerated by increasing the CO/AcOMe ratio which increases the coverage of CO adsorbed competitively with methyl acetate. 

It is clear from the data presented in Table XIV that H2S, though adsorbed competitively with dibenzothiophene, is not a major inhibitor for dibenzothiophene adsorption. Dibenzothiophene was shown to be preferentially adsorbed relative to biphenyl on both the cr and r sites. It is surprising that no adsorption of H2S was noted on the hydrogenation site (r) since it is known to be a strong inhibitor for many aromatic hydrogenations. A... 

Similarly, when two or more adsorptives adsorb competitively on a surface, the adsorption isotherm for adsorptive i at a given temperature is a function of the equilibrium partial pressures of all of the adsorptives. In the case of adsorption from a liquid solution, an adsorption isotherm for any preferentially adsorbed solute may be similarly defined in terms of the equilibrium concentration of the respective solution component, but the isotherm usually... 

Several mechanisms were considered and rejected as being inconsistent with the observed kinetics,11 but finally a two-site mechanism was found acceptable, on one of which both reactants adsorb competitively and on another of which only hydrogen is dissociatively adsorbed. Reaction led to... 

There should not be independent adsorption sites for each reactant they should adsorb competitively, carbon monoxide much more strongly than hydrogen. 

Should the adsorption follow the Langmuir format, with A and P adsorbing competitively on the same sites, then... 

The adsorbed concentrations can now be related to the bulk concentrations by a suitable isotherm such as that of Langmuir. If the reactants (and possibly the products) adsorb competitively on the surface sites... 

As seen previously, adsorption processes result in the decrease of surface energy. As mentioned in Section 1.3.1, adsorption is an increase in the concentration of a gaseous or dissolved substance at the interface of a condensed and a gaseous or liquid phase, respectively, due to the operation of surface forces. The main feature of the adsorption is that the adsorbed particles occupy the free sites of the interface. When two or more different substances are adsorbed, competitive adsorption occurs. In aqueous solution, competitive adsorption takes place in all cases because water molecules cover the total surface of the solid. The water concentration, however, is usually much greater than the dissolved substances, so the change in water concentration can be neglected. In the thermodynamic equations, the parameters characterizing water are included in other thermodynamic parameters. 

Of the cases described so far, the presence of a third compound had the strongest effect on the selectivity of hydrogenation of two olefinic substrates in the pair olefin-unsaturated alcohol. This influence appeared both in cases where the third compound was unsaturated, was adsorbed competitively and reacted on the catalyst surface, and in cases where the third compound was represented by an inert solvent not undergoing competitive adsorption (was not entering equations of the Langmuir-Hinshelwood type, which were degraded to pseudo-zero order) and obviously operated through interactions of molecules from the bulk phase with adsorbed molecules of the substrate. 

Takoudis et al. (1981) proposed a model for a bimolecular Langmuir-Hinshelwood surface reaction with two empty sites in its reaction step. The two chemisorbed species were assumed to adsorb competitively on the surface. The two dimensional model with reaction rates as parameters were shown to exhibit oscillations. Bifurcation of this model was also discussed. Takoudis et al. (1982) described a procedure for obtaining necessary and sufficient conditions for the existence of periodic solutions in surface reactions with constant temperature. An analytic method for the analysis of bifurcation to periodic solutions was developed. 

If two species are adsorbed competitively, the appropriate Langmuir isotherms are... 

On the other hand, for 8 and 9, the difference (qs - Qc) between the TT-electron density of the sulfur atom and that of the carbon atom with the highest TT-electron density (C-3) is small. Furthermore, the bond between C-2 and C-3 of these compounds is adsorbed competitively with the C-S bond. If this situation were not the case, these two molecules would chemisorb by a multistep mechanism (28). According to the multistep mechanism, thiophene molecules are chemisorbed in such a way that the bond between C-2 and C-3 is coordinated to an anion vacancy at the surface, and the adjacent sulfur center interacts with a contiguous sulfur atom on the surface. Therefore, the mechanism by which the three- and four-ring compounds are desulfurized is quite different from that of the one- and two-ring compounds, namely, 8 and 9, respectively. 

NH3 synthesis in the absence of poisons is relatively well understood on the atomic level and a wealth of experimental information is available [7]. Experimental information on NH3 synthesis under partial poisoning by oxygenic compounds is much less abundant and the mechanism on the atomic level is still poorly understood. Stoltze and Ndrskov [8] assumed as the simplest conceivable mechanism that 0- adsorbs competitively with the other intermediates. At constant temperature, gas-phase composition and pressure, the relative loss in activity is equal to(l- o) [8]. 

In Table IV the unsuccessful Hammett correlations of the hydrogenation rate of some benzene derivatives are summarized. A significant fact is that all these cases are reactions in solution, whereas Table III contains gas phase reactions. There are two possible causes of scatter of the points. First, the solvent may retard the reaction by diifusional resistance to the transport of hydrogen into the liquid phase, or, second, the solvent may adsorb competitively on the catalysts and block its surface. 

ICI catalyst showed that ethene adsorbed competitively with the ethyne, but the results required two types of site (see Table 9.6) (or two modes of chemisorption of the ethyne) to explain them. Their properties did not however match any of those proposed by Webb. Type X, in the majority, adsorbed both hydrocarbons, but ethene was favoured by a factor of 2200 Type Y adsorbed ethene only, perhaps because of its high concentration. The main source of the ethane was confirmed as ethene, since in the reaction with deuterium the main product was ethane-d2- When the pressure of ethyne was varied in the presence of excess ethene, its rate of removal (and that of formation of dimers) passed through a maximum, while that of ethane formation feU to zero at an ethyne pressure of 2 kPa (see Figure 9.7). The ethane rate was almost independent of the ethene pressure. Extensive work by Borodzinski and colleagues led " to detailed proposals for the identity of two types of site, designated A and E, that were thought to be created as the carbonaceous overlayer developed, and a third type (E ) that may play a role on certain supports. Type A sites, in the majority, were small, so that only ethyne and hydrogen could adsorb on them, the former perhaps as vinylidene (>C=CH2),... 

B adsorbs competitively at the binding site A and B adsorb on different... 

The relation between minimum anion activity necessary to inhibit pitting of 18-8 stainless steel in a solution of given CT activity follows the relation log (CT) = k log (anion) -i- const. The same relation apphes to inhibition of pitting of aluminum. The equation can be derived assuming that ions adsorb competitively in accord with the Freundlich adsorption isotherm [26]. 

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