Face Specificity

Polycrystalline electrodes are still used in many electrochemical experiments. For this reason, polycrystalline metals are still included in compilations of pzc as in this chapter, although the physical significance of such a quantity is ambiguous (see Section I). 

The correct pzc of single-crystal faces of Cu was obtained576,578,587 only after a really oxide-free surface was produced, although unsuccessful attempts are still reported.597 The pzc values for the three main faces of Cu show the correct sequence with the crystallographic orientation, i.e., (Ill) (100) (110). These three values are still insufficient, however, to give definite evidence in a plot such as Fig. 12 of the characteristic pattern of the dependence on the crystallographic orientation. 

Pb also crystallizes in the fee system and therefore the same dependence of EamQ on the crystallographic orientation should be expected. Quite surprisingly, Ecm0 varies in the sequence (112) (110) (100) (111),135 i.e., exactly the other way round. Although the authors of the measurements do not remark on this apparent anomaly, a possible explanation can be sought in the surface mobility of Pb atoms at room temperature, which may lead to extensive surface reconstruction phenomena. It doesn t seem possible to clarify this aspect for the time being, since the most recent studies on the pzc of Pb single-crystal faces date back almost 20 years. 

Zn and Cd both crystallize in the hep system. The basal (0001) face is the most dense and for both metals the Ea=o of this face is more positive than for the lateral faces.156,850 While Zn was among the first metals to be studied as single crystals, Cd has been among the last and is being actively investigated. 9 

For Sn, the variation of Eaa0 with the crystal face is negligible.621 Finally, Bi and Sb have been studied for several decades in Tartu671 and an extensive number of different crystal faces have been investigated thus far.28,725 Being semimetals, Bi and Sb show anomalies in the correlation of ff=0 with the surface atomic density254 which can be explained in terms 

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A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

It is clear from Eq. (27) that owing to the crystal face specificity of 0, Eaao is expected to vary with the crystallographic orientation as well. Moreover, since the interfacial term X results from interfacial molecular interactions, it must be face-specific also. For a well-defined metal surface, Eq. (27) becomes... 

Equation (32) suffers from the same shortcomings as Eq. (27). In particular, d/dT must be known independently for the same metal sample as the one used as an electrode. Moreover, in view of the crystal-face specificity of ff=o, its temperature coefficient is also expected to depend on the crystallographic orientation. Being a differential quantity, dEa=JdT is an even more delicate experimental quantity than Eaa0 itself. 

For the same reasons, data on single-crystal faces for metals such as Zn, Sb, Bi, Sn, and Cd have not been plotted in Fig. 15. In order to indicate the probable position of d-metal surfaces, the line described by Eq. (64) has also been drawn in Fig. 15. It is interesting that all the points for sd-metals fall between the sp- and the d-metal groups. The crystal face specificities of Eas0 for Sb and Bi are complicated by their semimetallic nature. In any case, no data on 0 exist for a series of faces of these elements (only electrochemical work functions are available).28,864... 

TFe data of Popov et alm for Ag contradict the above sequence. They found that pentanol adsorbed more strongly on Ag(100) than on Ag(l 11). Similarly, Cd(0001) adsorbs less strongly than pc-Cd.661 The data for Sb and Bi are to some extent contradictory since the trend is broadly correct but with scatter, which is attributed to the crystal face specificity of space-charge effects.153 For instance, adsorption of cyclohexanol on Bi conforms to the sequence (011) > (101) > (211) > (001) >(111), while the capacitance at a - Ovaries in the sequence (001) > (011) > (211) > (101) > (111). Thus only the faces (001), (211), and (111) are in the expected order. Surprisingly, the Cd data of Lust etal. show similarities with those of Naneva etal.,212 although capacitances disagree. Thus the order of cyclohexanol adsorbability is (1010) > (0001) while the capacitance varies in the order (1010) > (1120) > (0001), i.e., the other way round. In these cases one might wonder whether the G(M-B) term is really independent of face. 

Almost all that is known about the crystal face specificity of double-layer parameters has been obtained from studies with metal single-crystal faces in aqueous solutions. Studies in nonaqueous solvents would be welcome to obtain a better understanding of the influence of the crystallographic structure of metal surfaces on the orientation of solvent molecules at the interface in relation to their molecular properties. 

Crystal face specificity of the potential of zero charge, 21... 

Potential of zero charge cont.) contribution of the solvent, 158 Conway and Colledan, and the determination of, 34 on copper, and aqueous solution, 89 crystal phase and, 44 crystal face specificity of, 21 and the crystal surface specificity, 152 DeLevie, on the effect of the density of broken bonds on, 75 dependence upon crystal phase, 154 dependence upon time of measurement, 150,151... 

The reasons for the crystallite size effect are not known. There are several possibilities. If the selective oxidation reaction is crystal face specific, then the size effect is due to the fact that different proportions of various crystal faces are present on crystallites of different sizes. It is known that small crystallites supported on silica are more difficult to reduce than large crystallites. This different reducibility can contribute to the size effect. It is possible that the charge transfer ability of small crystallites is different. Since charge transfer is involved in the activation of gaseous oxygen which is active in degradation of surface intermediates, this could also be a contribution. It is clear that further work is needed to distinguish the possibilities. 

Oxidative dehydrogenation to acetaldehyde is not face specific, dehydration is on apical (001) and (101) planes... 

Interesting information exists about the crystal-face specificity/reactivity [75]. In ethene hydrogenation the (001) face of Ni is inactive, because this face, under the standard reaction conditions applied, is completely covered by carbonaceous deposits. The (111) and (Oil) faces are both active and differ by less than a factor of 2 in their activity. The data just mentioned are just another example of a frequently encountered phenomenon the crystal face specificity or the particle size sensitivity of a reaction is induced by a side reaction and is not caused by the reaction in question [76]. 

The two carbonyl groups of symmetrical diketones are distinguishable, with the carbonyl group undergoing reduction doing so with enantiotopic specificity. Some acyclic and monocyclic examples are shown in Scheme 9.- 2,44.49.50 Once more, enantiomeric products can be selected by the use of organisms with opposite enantiotopic face specificities, as shown for the reduction of (19) to (/ )- or (S)-(20) (Scheme 10). ... 

Regiospeciflcity combined with diastereotopic face specificity is seen in the reduction of the trione (60) to (61) (Scheme 26). ° In Scheme 27 the reduction is quite regiospeciflc, but both diastereotopic feces of the exocyclic ketone function of (62) are attacked to give the erythro-(63) and threo- 64) hydroxy ketone products. The degree to which regio specificity and diastereotopic face specificity can be controlled with different enzymes is shown in the bile acid reductions of Scheme 28. ° ... 

Enantiomeric and diastereotopic face specific reductions are also readily effected on racemic bicyclic ketones. An illustration of the broad structural range that is amenable to enzyme-catalyzed transformation in this way is given in Scheme 38. While 2-decalones, such as ( )-(81)- 83), and the related heterocyclic analogs ( )-(85) are good substrates for HLADH, the 1-decalone ( )-(84) is not. However, by changing enzymes to MJADH, ( )-(84) becomes a good substrate.Similarly, TBADH is a highly satisfactory catalyst for stereospecific reduction of ( )-(86), but will not accept its dimethyl... 

Even higher multiplicities of specificity combinations are possible. In Scheme 49, conversion of (115) to (116) involves regiospecific reduction of an enantiotopic carbonyl group concurrently with enantiotopic face specificity. This transformation can also be achieved using Saccharomyces species, with the level of (116) produced being enhanced by the addition of unsaturated carbonyl compounds, such as acrolein. ... 

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