Dependence on surface structure

Like formic acid, methanol decomposition has also been used to probe the acid-base properties of metal oxides [70]. However, methoxide decomposition is dependent on surface structure in much the same way as formate decomposition. For example, methanol undergoes parallel dehydration and dehydrogenation reactions on the same crystal surface of zinc oxide [25]. Once again, product selectivity ratios may not necessarily serve as a diagnostic of acid-base properties alone. 

It is usually the case that activated, direct dissociations are strongly dependent on surface structure and another nice example of this is oxygen dissociation on Ag. Here oxygen adsorbs directly over a large barrier on Ag(lll), the close packed fee plane, whereas on (110) there is a much smaller barrier. This is reflected in thermal desorption experiments carried out by Campbell (1985 fig. 14). On the (110) surface recombination of dissociated oxygen atoms is observed at 600 K while the molecularly chemisorbed state is much more weakly bound and desorbs at 200 K. A similar molecular state is seen on Ag(lll), but the dissociated state can hardly be seen at all. 

Figure 2 gives a scheme illustrating some applications in geochemistry and technology where surface reactivity (kinetics of dissolution, catalytic activity, photochemical activity) depends on surface structure, expecially on surface coordination. It has been shown by various spectroscopic techniques [electron-spin resonance (ESR), electron double-resonance spectroscopy (ENDOR), electron-spin echo modulation (e.g., see Motschi, 1987), Fourier transform infrared spectroscopy (Zeltner et al., 1986), and in situ X-ray absorption studies of surface complexes (EXAFS) (Hayes et al., 1987 Brown, 1989)] that inner-sphere... 

K.D. Rendulic, A. Winkler, and H. Kamer. Adsorption Kinetics of H2/Ni and Its Dependence on Surface Structure, Surface Impurities, Gas Temperature, and Angle of Incidence. J. Vacuum Sci. Technol. A 5 488 (1987). 

An important geometric factor that affects differences in activation energies is the structure of the metal surface. This dependence on surface structure will be very different for adsorbates in which a chemical bond is activated over a single metal atom, as a CH bond in CH4, than when a particular surface ensemble of metal atoms is required, as for the dissociation of CO or N2. This topic is described in detail in the final sections of this chapter. 

We discuss in this section the relation between type of adsorbate bond activated and its dependence on surface structure. In heterogeneous catalysis, this has important consequences for the particle size and shape dependence of catalytic reactions, where the rate is controlled by a particular bond activation reaction. 

One should note the remarkable similarity in activation energy dependence on surface structure as we discussed in the previous section for activation of CO. Remarkable is the low activation energy of NO on the (100) surface compared to that on the (111) surface and the closeness in its value compared to that at the step-edge site. Figure 10.32b shows that the NO molecules dissociates on the (100)... 

Adhesion and friction are also dependent on surface structure in the sense of roughness, although the effect can be difficult to predict. Frictional forces often contain an adhesive component, which tends to decrease as roughness increases, but... 

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