Anion-cation pair sites

In addition to the observation that the total uptake was only about 20% of the BET monolayer, in agreement with the previously discussed gravimetric studies, the data showed that when the CO2 pressure was increased at constant CO pressure, the amount of adsorbed CO decreased. Similarly, increasing the pressure of CO decreased the amount of adsorbed CO2. These results are consistent with adsorption on anion-cation pair-sites, where CO adsorbs on a cation and interacts with a neighboring anion, and where CO2 adsorbs as a bi dentate carbonate species. For competitive adsorption on a fixed number of surface sites, the coverages are given by the following expressions ... 

It is possible, however, that available pair-sites may be blocked by the adsorption of CO2 or H2O. For example, CO2 may adsorb on these sites, forming bidentate carbonate species. Although such adsorbed species may suppress the regenerative mechanism over anion-cation pair-sites, these species may be important for adsorptive WGS mechanisms. The general conclusion, however, is that the adsorption isotherms for CO and COo and for H2 and H2O are useful for probing the pair-sites necessary Tor WGS. 

The above ideas that anion-cation pair sites are the surface sites for CO and CO2 adsorption on magnetite was verified directly by Udovic and Dumesic (43 ). These authors prepared films of magnetite on polycrystalline iron foils and varied the oxidation state of the surface by vacuum-annealing at different temperatures. In short, it was shown by Auger electron spectroscopy and X-ray photo-... 

We now introduce a Fourier transform procedure analogous to that employed in the solution theory, s 62 For the purposes of the present section a more detailed specification of defect positions than that so far employed must be introduced. Thus, defects i and j are in unit cells l and m respectively, the origins of the unit cells being specified by vectors R and Rm relative to the origin of the space lattice. The vectors from the origin of the unit cell to the defects i and j, which occupy positions number x and y within the cell, will be denoted X 0 and X for example, the sodium chloride lattice is built from a unit cell containing one cation site (0, 0, 0) and one anion site (a/2, 0, 0), and the translation group is that of the face-centred-cubic lattice. However, if we wish to specify the interstitial sites of the lattice, e.g. for a discussion of Frenkel disorder, then we must add two interstitial sites to the basis at (a/4, a/4, a]4) and (3a/4, a/4, a/4). (Note that there are twice as many interstitial sites as anion-cation pairs but that all interstitial sites have an identical environment.) In our present notation the distance between defects i and j is... 

Hummel and Luthjens [398] formed electron—cation pairs in cyclohexane by pulse radiolysis. With biphenyl added to the solvent, biphenyl cations and anions were formed rapidly on radiolysis as deduced from the optical spectra of the solutions. The optical absorption of these species decreased approximately as t 1/2 during the 500 ns or so after an 11ns pulse of electrons. The much lower mobility of the molecular biphenyl anion (or cation) than the solvated electron, es, (solvent or cation) increases the timescale over which ion recombination occurs. Reaction of the solvated electron with biphenyl (present in a large excess over the ions) produces a biphenyl anion near to the site of the solvated electron localisation. The biphenyl anion can recombine with the solvent cation or a biphenyl cation. From the relative rates of ion-pair reactions (electron-cation, electron—biphenyl cation, cation—biphenyl anion etc.), Hummel and Luthjens deduced that the cation (or hole) in cyclohexane was more mobile than the solvated electron (cf. Sect. 2.2 [352, 353]). 

Faced with the challenge of ion pair binding, there are three basic ion pair receptor designs contact ion pair receptors (including cascade complexes) in which the anion and cation are bound as a contact ion pair, ditopic receptors with individual, well-separated anion and cation binding sites, and zwitterion receptors, Figure 5.2. We will discuss some examples of each type of complex in the following sections. 

The skeletal nitrogen atoms in cyclophosphazenes possess a lone pair of electrons and, hence, they have long been viewed as potential donor sites to bind a proton or to form complexes with electron-acceptor molecules. The possibility of formation of anion-cation complexes by release of a halogen ion to a Lewis acid and charge-transfer complexes has also been studied. In addition, some cyclopho-sphazene derivatives form crystalline inclusion clathrates with a variety of guest molecules. Allcock (21, 22) has reviewed these aspects in detail. 

The existence of such species has been confirmed by IR spectroscopy for CO2 interactions with several metal oxide surfaces.(23-26) Finally, hydrogen can adsorb either heterolytically (H+ on an anion and H- on a cation) or reductively (forming two hydroxyl groups) on oxides (21). Depending upon relative surface populations, desorption is possible in each of these fashions. For heterolytic adsorption, pair-sites are again indicated. 

Zr) for NjO decomposition. The actual oxidation states of the metals are investigated with XPS [21]. The possible active sites for the reaction may also be related to lattice oxygen anion defects formed after cation incorporation [22]. For high Pb Zr ratio catalysts, such as in the case of Pb Zr = 1 1, the catalytic activity is substantially reduced. This can be attributed to change of ZrOj structure to perovskite type and thus to demolishment of cation pairs with multiple oxidation states that are essential for facilitating the decomposition of NjO gas [23-25]. 

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