Crystal faces systems

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. 

Figure 12. (a) Dependence of the potential of zero charge, Eaw0, on the crystallographic orientation for the metals Cu, Ag, and Au, which crystallize in the fee system. From Ref. 32, updated, (b) (pg. 155) Correlation between Eam0 of single-crystal faces of Cu, Ag, and Au, and the density of broken bonds on the surface of fee metals. From Ref. 32, updated. 

Experiments at present are concentrated on sd-metals and Pt-group metals. The sp-metals, on which theories of the double layer have been based, are somewhat disregarded. In some cases the most recent results date back more than 10 years. It would be welcome if double-layer studies could be repeated for some sp-metals, with samples prepared using actual surface procedures. For instance, in the case of Pb, the existing data manifest a discrepancy between the crystalline system and the crystal face sequence of other cases (e.g., Sn and Zn) the determination of EgaQ is still doubtful. For most of sp-metals, there are no recent data on the electron work function. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

In addition, it sustains CO electro-oxidation at relatively low overpotential, and there are crystal face dependences for both the ORR and CO oxidation. Since Au is also a system that exhibits both particle size and support effects in heterogeneous catalysis, it provides an interesting model system for smdying such effects in electrocatalysis. 

The ability of bimetallic systems to enhance various reactions, by increasing the activity, selectivity, or both, has produced a great deal of interest in understanding the different roles and relative importance of ensemble and electronic effects. Deposition of one metal onto the single-crystal face of another provides an advantage by which the electronic and chemical properties of a well-defined bimetallic surface can be correlated with the atomic structure.5 22 23 Besenbacher et al.24 used this method to study steam reforming (the reverse of the CO methanation process) on Ni(l 11) surfaces... 

An additional difficulty was introduced many years ago by the hypothesis that different crystal faces possess different surface tensions. If, for instance, the prism face has a 7S greater or smaller than the 7S of the basis, then the edge between the two is pulled in two different directions by two unequal forces in the absence of any other force which could stabilize the system. It is usual, however, to hide this defect by stating that a crystal face has its characteristic surface energy, that is, admitting that Ts is not analogous to 7. 

These principles are nicely illustrated by the contrast between the serine-threonine and serine-allothreonine (allothr) systems. The relative orientation of molecular serine vis-a-vis its various crystal faces suggests that allothr can be adsorbed on the homotopic 100 faces as well as on the enantiotopic 011 faces (Figure 21). 

In the orthorhombic point group mm2 there is an ambiguity in the sense of the polar axis c. Conventional X-ray diffraction does not allow one to differentiate, with respect to a chosen coordinate system, between the mm2 structures of Schemes 15a and b (these two structures are, in fact, related by a rotation of 180° about the a or c axis) and therefore to fix the orientation and chirality of the enantiomers with respect to the crystal faces. Nevertheless, by determining which polar end of a given crystal (e.g., face hkl or hkl) is affected by an appropriate additive, it is possible to fix the absolute sense of the polar c axis and so the absolute structure with respect to this axis. Subsequently, the absolute configuration of a chiral resolved additive may be assigned depending on which faces of the enantiotopic pair [e.g., (hkl) and (hkl) or (hkl) and (hkl)] are affected. 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Figure 7.6. Part of a current-time record during a mononuclear layer-by-layer growth of a quasiperfect Ag( 100) crystal face with a circular form in the standard system Ag(100)/6 MAgN03 at T7 = -6 mV and T = 318 K. Surface area A = 3 X 10 cm . Current density i = 1 mA/cm. The current spikes indicate the formation, growth, and decay of new layers. (From Ref. 34, with permission from the Electrochemical Society.)...

The ciystal habit of sucrose and adipic add crystals were calculated from their intern structure and from the attachment energies of the various crystal faces. As a first attempt to indude the role of the solvent on the crystal habit, the solvent accessible areas of the faces of sucrose and adipic add and were calculated for spherical solvent probes of difierent sizes. In the sucrose system the results show that this type of calculation can qualitatively account for differences in solvent (water) adsorption hence fast growing and slow growing faces. In the adipic add system results show the presence of solvent sized receptacles that might enhance solvent interadions on various fares. The quantitative use of this type of data in crystal shape calculations could prove to be a reasonable method for incorporation of solvent effeds on calculated crystal shapes. 

It has also been shown [173, 233, 234] that the catalytic activity of various crystal faces of platinum for the reduction of N03 ions is very different. This means that similar effects should be expected in the case of platinized systems with various preferred crystallographic orientations [235]. 

In Fig. 4.1 we depict three lattice types of the cubic system and the crystal faces with the highest reticular density (the density of lattice points per unit area) in each t5q)e. 

Figure 4.1. Crystal faces with the highest rank in the order of morphological importance (with the highest reticular density) for P. F. and I lattice types of the cubic system (a) P lattice, 100) (b) F lattice. Ill) (c) I lattice. (110).

---