Crystallographic surfaces

Fig. 7. Ionic arrangements on different (a) < 100 >, (b) < 110 >, and (c) < 111 > crystallographic surfaces. The interionic separations are given in nanometers for AgBr and in parentheses for AgCl. Values are larger for AgBr because of the relative ionic radii of bromide and chloride.

When reaction is absent from certain crystallographic surfaces, the formulation of rate equations based on geometric considerations proceeds exactly as outlined above, but includes only the advance of interfaces into the bulk of the reactant particle from those crystallographic surfaces upon which the coherent reactant/product contact is initially established. When reaction occurs only at the edges of a disc or plate-like particle... 

Figure5.9. Hypothetical particle of cubic MgO, exhibiting three crystallographic surfaces. The (100) and (110) surfaces are nonpolar, implying that Mg and O ions are present in equal numbers, but the polar (111) surface can be terminated either by Mg cations or O anions. In practice, MgO crystals predominantly show (100) terminated surfaces. 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

Polycrystalline films deposited on amorphous substrates are of lower crystallographic surface heterogeneity the higher the temperature of annealing subsequent to deposition or the higher the substrate temperature during deposition again, photoelectric work-function data serve to emphasize the point (9,10, 12,18). 

However, electrons are the same independently of the type of material. Thus, how can we understand that different metals behave in different ways How can we understand, for example, that the work function of different metals—say platinum and silver—are different (see Fig. 6.46) Or how to explain the different behavior of different crystallographic surfaces of a given single crystal, say, Ag(l 11) and Ag(100) ... 

The techniques and difficulties involved in the preparation and characterization of single crystal metal surfaces have been considered here in detail because the evaluation of chemical activity in terms of surface structure, particularly of the crystallographic surface structure, is one of the most promising applications of the vacuum microbalance. The preparation of flat, clean, undistorted single crystal samples suitable for surface studies is a difficult and tedious assignment. 

Hie core structures of dislocations are more important during ordinary dissolution than the elastic stress fields. While this conclusion might not be true for very low undersaturations, experimental evidence indicates that it is true for moderate undersaturations. Evidence indicates that dissolution inhibitors act by chemisorbing at specific surface sites namely, at kinks in crystallographic surface steps. 

For the oxidation of tantalum and chromium, apparently no difference in the rate of oxide growth on different crystallographic surfaces has been observed. This could be a result of the initial formation of the oxide. Much more work should be carried out to study the formation of the first few layers of oxide. 

The incorporation of either type of molecule onto a crystallographic surface not only alters the growth rate, it also violates the local symmetry of a surface. For... 

The outer feces of grown crystals tend, in some systems, to be more reactive than faces exposed by cleavage, though the influence of a possible difference in properties of the crystallographic surface exposed must also be considered. At edges and comers of crystals and at steps on the surface, the lack of symmetry in the structure, and hence the reactivity, is generally greater than for the surface constituents as a whole, but the amount of material involved is much less. 

Zinc, cadmium and mercury oxides The decomposition of zinc oxide was studied [45] thermogravimetrically, using sintered material and on (0001) and (1010) crystal faces. Evaporation results in the development of characteristic structures for the different crystallographic surfaces, including a remarkable anisotropy on faces... 

When a Pt(100) surface is subjected to ultrahigh vacuum conditions, the clean surface spontaneously reconstructs [84] from the (1 x 1) structure to the reconstructed state at room temperature by the adsorption of impurities, such as water molecules to the (5x20) structure [84]. The present (lxl) structure means that the state of the sample treated this way, which gave an LEED pattern of the (100) plane without further treatment under vacuum, has a satisfactory initial crystallographic surface structure from an LEED point of view for the electrochemical experiment. 

The crystallographic surface area of bentonite clay is about 750 m2/ g (17), although the actual value that is obtained depends on the exchange cation. Quirk (18) has reported the solid-liquid surface area of bentonites with various alkali and alkaline earth exchange cations dispersed in aqueous solution. The sodium-exchanged form of bentonite has a measured surface area of 700 m2/g, which is close to the theoretical... 

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