Surface crystallograph

It is well known that the 0 of a metal depends on the surface crystallographic orientation.6,65,66 In particular, it is well established that 0 increases with the surface atomic density as a consequence of an increase in the surface potential M. More specifically, for metals crystallizing in the face-centered cubic (fee) system, 0 increases in the sequence (110) <(100) <(111) for those crystallizing in the body-centered cubic (bcc) system, in the sequence (111) < (100) <(110) and for the hexagonal close-packed (hep) system, (1120) < (1010) < (0001). 

M) were typically used for such a comparison to eliminate the influence of possible differences in the inner-layer capacities. However, C of different solid metals, as well as of liquid Ga, In(Ga), and Tl(Ga) alloys have shown such a large variation that this approach can hardly be considered as appropriate. It should be noted that the error in C, which for solid electrodes is much higher than for liquid electrodes, increases with the decrease ofcei further, as shown later (Section II.2 (iv)), the effects of surface crystallographic inhomogeneity also prove especially appreciable.24 67 74... 

Chang SC, Hamelin A, Weaver MJ. 1991. Dependence of the electrooxidation rates of carbon monoxide at gold on the surface crystallographic orientation A combined kinetic-surface infrared spectroscopy study. J Phys Chem 95 5560-5567. 

Hamelin A, Ho Y, Chang SC, Gao X, Weaver MJ. 1992. Surface crystallographic dependence of voltammetric oxidation of polyhydric alcohols and related systems at monocrystaUine gold-acidic aqueous interfaces. Langmuir 8 975-981. 

In reality, as the barrier becomes narrower, it deviates from the square shape. One often used model is the parabolic barrier (dashed line in Fig. 1). When the barrier is composed of molecules, not only is the barrier shape difficult to predict, but the effective mass of the electron can deviate significantly from the free-electron mass. In order to take these differences into account, a more sophisticated treatment of the tunneling problem, based on the WKB method, can be used [21, 29-31]. Even if the metals are the same, differences in deposition methods, surface crystallographic orientation, and interaction with the active layer generally result in slightly different work functions on either side of the barrier. 

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

Since the substrate on which adsorbates are deposited greatly influences the behavior of those adsorbates, it is important to first examine the substrates themselves. We must distinguish between the clean surface and the same when covered with adsorbates, because adsorbates are capable of modifying the geometric (and electronic) structure of the substrate. To enable a convenient comparison. Table 6.1 combines the structures known to us for both clean and adsorbate-covered surfaces, as far as they have been determined with a reasonable degree of precision and reliability by the various surface crystallographic techniques mentioned in Section IV (co-adsorption and molecular adsorption are treated in the next Section). 

In electrochemistry, potential and current measured by electroanalytical methods provide kinetic and potential energy pictures of electrochemical reactions. Measured current and potential are strongly connected to the molecular scale properties of the electrode surface, solvent molecules and ions. Currents and potentials represent how molecules and atoms are distributed near the interface, how they are bonded on the electrode surface, and how they are solvated in the electrolyte solution. The electrochemical properties are also sensitive to the atomic arrangements of the electrode surface crystallographic orientations and defects. 

In terms of structural information, that is the determination of the atomic coordinates of at least some of the species in the surface region, several methods are available. We can class these methods in two main groups scattering techniques (ion and electron) and scanning probe techniques. In several cases -and specifically for the Pt-Sn system - a combination of these methods can provide the complete determination of the crystallographic parameters of the first 2-3 atomic planes of a surface. The surface phases observed in the Pt-Sn system are normally termed in reference to their in-plane periodicity, as observed most often by LEED. For a description of this method and for notes about how the surface crystallographic conventions need to be somewhat modified when applied to the field of alloy surfaces, see the appendix to the present paper. 

In view of this need, we discuss here a variety of electrocatalytic topics, ranging from basic and microscopic concepts to phenomenological principles. Thus, the origin of electrodic reactions, electrosorption, and electrode kinetics are introduced briefly for the benefit of the nonelectrochemist. Since electrocatalytic reactions take place at the electrode surface, attention is given to recent efforts to link catalyst activity with microscopic surface properties. These include surface crystallographic orientation, crystallite size and distribution, adsorbate-adsorbent-support synergism, multiple adsorption states, identification of surface intermediates, and electrocatalytic surface reaction mechanisms. 

The interplay between substrate-adsorbate and adsorbate-adsorbate interactions in the structure of the phase formed in Region II has been analysed. This has been achieved by comparing structural parameters obtained by in situ STM for a number of different surface crystallographic... 

Madaren, J.M., J.D. Pendry, P.J. Rous, D.K. Saldin, G.A. Somorjai, M.A. Van Hove and D.D. Vvedensky, 1987, Surface Crystallographic Information Service A Handbook of Surface Structures (Rcidcl, Dordrecht). 

This is by far the most widely used method of obtaining surface crystallographic information and as such has been extensively reviewed [19— 26]. Details of experimental techniques are included in most of these... 

In this case the grafted organometaUic is completely dealkylated (species 4). These species are usually prepared at moderate temperature (e.g. around 100°C for tin). The atoms are suspected to be localized on parhcular surface crystallographic sites, very likely those which are the most hydrogenolyzing with respect to the surface organometaUic complex which has been grafted. These are called adatoms and they play the role of selective poisons. 

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