Chemisorption surface complex

Demuth JE (1977) Chemisorption of C2H2 on Pd(l 11) and Pt(l 11) formation of a thermally activated olefinic surface complex. Chem Phys Lett 45 12-17... 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

It is also termed chemisorption (especially for gases), inner sphere adsorption and, in the case of ligands, ligand exchange. The binding constants, Kf and for the surface complexes show the same stability trend as do the constants for the equivalent complexation reactions in solution. 

Chuvylkin et al. (54) have used this approach to discuss EPR signals arising from weak R02 surface complexes in a number of systems where the g tensor does not fit the pattern expected [Eq. (6) and Fig. 3] from the ionic model. This is not discussed quantitatively, but they conclude that the appearance of covalently bonded oxygen is impossible without a favorable orientation of appropriate electronic orbitals. A similar covalent bonding approach has been considered theoretically for the chemisorption of oxygen on silicon surfaces (55). Examples of weakly bonded oxygen are given in Section IV,E. 

Fig. 84 (Upper) Schematic presentation of surface complexation (preoxidation) by a nucleophilic reagent N and final oxidation of the silver particle by oxygen. (Lower) Chemisorption of a complex AgN and final reduction of the silver ions in chemisorbed AgN by excess electrons deposited by reducing radicals. In both parts of the figure, the changes in the position of the Fermi level in the colloidal silver particles are indicated [531]...

Contrary to the case of Equation (49), the chemisorbed COs-complexes are, in this case, electrically quasineutral. Depending on the position of the Fermi level, the COs surface-complex will have a more negative or positive overcharge. It will become a carbonate ion only when a cation migrates from the interior to the surface. Recent investigations into the chemisorption of CO2 on spinel, ZnO, and ZnO-Cr2O.3 catalysts by Kwan... 

The species first to be expected from the chemisorption of methane on a metal surface is a surface-attached methyl group. If, as seems probable, the threefold axis of the CH3 group is perpendicular to the metal surface (C3v overall symmetry of the surface complex), the expected completely symmetrical modes are eCH3 s, <5CH3 s, and eCM (M = metal). According... 

When ethylene is adsorbed on bare nickel at 35° C. or on either bare or hydrogen-covered nickel at 150° C., the intensity of the C—H bands, shown as A of Fig. 3, is small compared with those of the associated chemisorbed ethylene shown in Fig. 2. When the species represented by A is treated with H2 at 35° C., the band intensities increased as is shown in B of Fig. 3. This behavior shows that A is due to a dissociatively chemisorbed ethylene in which the number of hydrogens per carbon is low (7). The species obtained by dissociative chemisorption will be referred to as a surface complex. It is doubtful whether the surface complex has a specific stoichiometric composition. Rather it appears that the carbon-hydrogen ratio will depend on the severity of the dehydrogenation conditions. In some cases it appears that a surface carbide, which has no hydrogens, is obtained. Even in this case the carbons appear to be easily rehydrogenated to adsorbed alkyl groups. 

The exact stoichiometry of the Pt-H surface complex is still a matter of debate since it depends on the size of the metal particle. For many supported Pt catalysts, an assumption of 1 H atom adsorbing for every 1 Pt surface atom is often a good one. Results from chemisorption can be used to calculate the dispersion of Pt, or the fraction of exposed metal atoms, according to ... 

The reversibility of phenol adsorption [341], which is of great importance in adsorbent regeneration (or reactivation) [343,347-349,352,355,453,454] was first discussed in detail by Magne and Walker [360]. They reported that weakly adsorbed (physisorbed) phenol can become chemisorbed in the course of time or by increasing the temperature. On the basis of the finding that chemisorption (on a commercial activated carbon) was inhibited by the presence of oxygen surface complexes, the authors concluded that the sites responsible for phenol chemisorption are carbon sites of the active surface area, i.e., oxygen-free sites... 

Surface complexes between As(V) or As(III) and B- and C-type hydroxyls are observed in IR studies but have not been verified by XAFS spectroscopy, probably because hydrogen bonding dominates the interaction between As and B- or C-type OH moieties (Sun and Doner, 1996) Fig. 8 d, e, and f). Hydrogen bonding is a long-range attractive force between atoms that does not involve direct bonding, and therefore would be classified as physisorption as opposed to chemisorption, which involves the formation of a chemical bond. 

The characteristic chemisorption data obtained for the various supported catalyst samples are summarized in Table 2. It is evident that, the 02-net adsorption (3n) has in general lower magnitude for catalyst samples supported on pure silica and silica-rich support and higher magnitudes for catalyst samples supported on alumina and alumina-rich supports. However, these adsorption values decrease markedly as the CoPc content increases on one and the same support (97.1 SA) being related most probably to the mode of surface complex dispersion. 

In the case of the simplest mechanism of repeated adsorption-desorption events of the unaltered molecules, the retention time and the peak shape are insensitive to the composition of the carrier gas — now we would add provided that it constantly modifies the column surface. Some of the more complex mechanisms of migration are indistinguishable from the outside because they are analogously insensitive. It is, for instance, simple chemisorption, when the initial electronic structure of the adsorptive substantially changes upon adsorption, but is restored at the desorption stage. Such is also the microscopic history of metallic adatoms in metal columns. Another case is the chromatography of molecular halides in columns loaded with alkali halides the adsorbed state is a surface complex between the two halides see Sect. 1.5.1. In both examples the structure of the original adsorption sites is not necessarily restored. It is, of course, unimportant in the experiments with tracers. 

Surface Complex Formation on Carbonates. There are various possibilities for functional groups on the surface of carbonates, sulfides, phosphates, and similar compounds. By using a very simple approach similar to the one used for hydrous oxides (chemisorption of H20), one could postulate surface groups for carbonates (e.g., FeC03) as shown in List III. 

A variety of distinct but interrelated phenomena may be involved in the adsorption process of metal ions onto activated carbons adsorption (physical adsorption or chemisorption), surface precipitation, ion exchange, and surface complexation. The metal sorption is often not the result of one mechanism but of several reactions. The mechanisms involved and their degree of importance seem to depend on the materials and the operating conditions used. 

There is in principle no limit to the size of a molecule that may bind metal ions. What we often see is that as molecules get larger they become less soluble, which places a limit on their capacity to coordinate to metal ions in solution. However, what we also now know is that even solids carrying groups capable of coordinating to metal ions can adsorb metal ions from a solution with which they are in contact onto the solid surface, effectively removing them from solution through complexation (a process called chemisorption). Thus complexation is not restricted to the liquid state, and will occur in the solid state as discussed earlier, it also can occur in the gas phase. 

The complexity of the chemisorption — surface reaction — desorption processes taking place on the catalytic surfaces. 

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