Strongly Bound Adsorbates

For strongly bound adsorbates, all translational and rotational degrees of freedom present in gas phase are assumed to be frustrated and converted into vibrational modes between adsorbates and the surface. Assuming that the PV contribution to the internal energy is small (see Ref. [11] and references therein), the state properties can be calculated from the vibrational contributions only, according to the following  

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In this picture, the kinetic barriers hindering the exchange between the two adlayers are related to the presence of metastable, but rather strongly bound, adsorbed species (Hupd and OHad), which cannot be removed easily, and which block the surface for adsorption of the respective other species. The nonequilibrium situation is also reflected in the shape of the corresponding peaks A and A, where the anodic one (A) is less sharp and extends over a larger potential range. 

The fact that products are not detected may be due to an IP effect as described above. For n > 4 hydrogen uptake increases progressively as the number of constituent atoms increases, and it is estimated that saturation has occurred for clusters containing more than ten atoms. Hydrogen chemisorption on platinum surfaces is generally weak, and even the most strongly bound adsorbed state on a stepped surface is fully desorbed by 450 The expected... 

To compute the entropy and heat capacity from statistical thermodynamics, one has to consider the type of adsorption, depending on how strongly the adsorbate binds to the surface. In general, adsorbates that bind weakly to surfaces, for example, closed-shell adsorbates such as H O and CO, have a low barrier for surface diffusion, which makes them highly mobile on the surface. On the other hand, strongly bound adsorbates have a high barrier for surface diffusion and are assumed to be immobile on the surface. As a result of differences in adsorption, the statistical... 

Physisorption, i.e. physical adsorption, from a selective solvent to a selective surface is a common and simple way to prepare thin polymer films on solid surfaces [108, 109]. This method is usually employed for reversible immobilization of polymer chains to surfaces. However, due to the rather weak interactions between the substrate and block copolymers (mostly van der Waals forces or hydrogen bonding), the polymer brushes are likely to desorb. Desorption can occur upon exposure to good solvents or the polymer chains can be displaced by other, more strongly bound adsorbates. Under certain conditions, the films might also be thermally unstable. 

If component A is much more strongly bound (adsorbed) to the catalyst than C is, or the rates differ significantly, ie, l fe3CB/fe4 fe2CB/fe i l and ... 

If an adsorbed chemical group (anchor) is more strongly bound to the surface than a solvent molecule would be at that site, an equiHbrium expression may be written for the displacement of solvent by adsorbate. Adsorption is particularly strong if the chemical nature of the adsorbed group is similar to that of the particle surface for example, in aqueous systems perfluoroalkane groups adsorb weU on polytetrafluoroethene particles and aromatic polyethene oxides adsorb weU on polystyrene. 

Although we include adsorption here following the chapter on mass transfer, we should be clear that it is a very specific process in its fullest fundamental meaning. Adsorption is the process by which molecules in the fluid phase in contact with a solid move to the solid surface and interact with it. Once at the solid surface these molecules may be reversible or irreversible adsorbed, that is, they may come back off the surface to the fluid phase with their full molecular integrity intact, or they may be so strongly bound that the rate of removal is for all purposes close enough to zero to be considered zero. 

To further demonstrate the power of the kinetic lattice gas approach we review briefly the work on precursor-mediated adsorption and desorption [60,61]. We consider an adsorbate in which, in addition to the most strongly bound chemisorbed (or physisorbed) adsorbed state, the adparticles can also be found in intrinsic or extrinsic precursor states. One introduces three sets of occupation numbers, , = 0 or 1, = 0 or 1, and /, = 0 or 1, depending... 

Note that the dissociation proceeds with a much lower barrier on the stepped surface. As the structure diagrams show, at all stages in the dissociation the species are more strongly bound on the stepped surface, for reasons discussed in connection with Eq. (87). However, the transition state is most affected, because two N atoms are bound to four metal atoms in the transition state on a perfect surface, whereas that on the stepped surface consists of five metal atoms. As noted above, geometries in which atoms bind to different metal atoms are always more stable than when the two adsorbate atoms share one metal atom. Hence, dissociation is favored over step sites, and if a surface contains such defects they may easily dominate the kinetics. 

In this paper it has been shown that IR spectroscopy remains one of the most incisive tools for the study of both strong and weak bonding at surfaces. In addition to being able to study surface species structure in the chemisorbed layer, it is possible to obtain dynamical information about more weakly-bound adsorbates as they llbrate and rotate on the surface. These motions are controlled by local electrostatic forces due to polar surface groups on the surface. 

Therefore, we arrive at the same conclusion for the mechanism of COad oxidation in the lower potential regime as for Pt-free Ru(OOOl), postulating that at potentials E < 0.55 V, only strongly bound OHad/Oad species are present in the mixed COad + OHad/Oad adlayer, which are not reactive towards CO2 formation, while for E > 0.55 V, additional, weakly adsorbed OHad/Oad species are formed, which can react with the (likewise destabilized) COad- Similar to COad oxidation on a Ru(OOOl) surface, the reaction starts by dissociative adsorption of H2O on the Ru(OOOl) surface (no shift in the onset potential). In this case, however, the Pt islands can accelerate the reaction by accepting the Hupd resulting from a homolytic dissociation process. Thus, we tentatively propose a mechanism for CO oxidation at potentials between the reaction onset up to the bending point (see also Lin et al. [1999]), which is... 

It has been shown in Section 2.1.4 that methanol adsorbate formed from dilute solutions on a porous Pt surface, consists of COad and COHad in a ratio CO COH of ca. 20-30% [14]. The results of isotopic exchange with bulk CO seem to indicate that only the fraction present as COad can be desorbed and replaced by bulk CO. Probably the same arguments as in the case of pure COad can apply. COHad seems to be more strongly bound to the Pt surface and cannot be desorbed. 

One important restriction of the applicability of all the above-mentioned conclusions should be always kept in mind—that only the properties of strongly bound particles were studied. Always when the second gas was introduced, it reacted with the chemisorbed layer of the first one, the gas phase being pumped off beforehand. This need not be a serious restriction with the hydrogen layer at 78°K and the oxygen layers both at 78° and 300°K, where only a few percent are desorbed during evacuation. However, in the case of hydrogen at room temperature, as much as approximately 25% of the adsorbed amount can be desorbed by mere pumping off the gas phase (19). 

A simple example of the redox behaviour of surface-bound species can be seen in Figure 2.17, which shows the behaviour of a bare platinum electrode in N2-saturated aqueous sulphuric acid when a saw tooth potential is applied. There are two clearly resolved redox processes between 0.0 V and 0.4 V, and these are known to correspond to the formation and removal of weakly and strongly bound hydride, respectively (see section on the platinum CV in chapter 3). The peak currents of the cathodic and anodic reactions for these processes occur at the same potential indicating that the processes are not kinetically limited and are behaving in essentially an ideal Nernstian fashion. The weakly bound hydride is thought to be simply H atoms adsorbed on top of the surface Pt atoms, such that they are still exposed to the... 

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