Oxygen crystal face dependence

The crystal face dependence and the activity of the various group 8-10 metals can be understood in terms of the metal-oxygen bond strength. On an active metal both O2 and H2 are dissociatively adsorbed, oxides are not formed under reaction conditions, and the heat of adsorption for Oacisis relatively low. 

Lewis Acid Sites. Many other mechanisms (66, 85) are best described in terms of the more general concept of aprotic, or Lewis acidity which is defined in terms of the capacity to donate or share pairs of electrons. Aprotic acid sites are commonly derived from the coordinatively unsaturated cations at crystal edges or adsorbed on crystal faces, from deydration of hydroxylated surfaces, and from deamination or deamination and dehydration of silica-aluminum catalysts or similarly treated clays having extensive tetrahedral substitution (130, 132). Formation of Lewis acid sites by deamination or deamination-dehydration is dependent on inversion of the basal oxygens of the aluminum-substituted tetrahedron away from the surface, in order to expose the aluminum (131). 

Figures 2-4 show a comparison of the ring-disk electrode for the oxygen reduction kinetic data along with the base voltammetry in oxygen-free solutions for each Vi hkl) surface. Clearly, the kinetics of the ORR on VtQikl) surfaces vary with crystal face in a different manner depending on the solution. In perchloric acid solution. Figure 2, the variation in activity at 0.8-0.9 V is relatively small between the three low-index faces, with the activity increasing in the order (100) < (110) (lll). A similar structural sensitivity is observed in KOH, Figure 3, with the activity...

Figure 13.6.1 Cyclic voltammogram for a smooth platinum electrode in 0.5 M H2SO4. Peaks formation of adsorbed hydrogen. Peaks H oxidation of adsorbed hydrogen. Peaks Oq formation of adsorbed oxygen or a platinum oxide layer. Peak Oc reduction of the oxide layer. Point 1 start of bulk hydrogen evolution. Point 2 start of bulk oxygen evolution. The shape, number, and size of the peaks for adsorbed hydrogen depend on the crystal faces of platinum exposed (62), pretreatment of electrode, solution impurities, and supporting electrolyte. See also Figure 13.4.4.

Fig. 8.7. The expected oxygen-containing sturface functional groups on diamond depend on the surface crystal face.

Figure 3 shows the appearance of a crystal rearranged by this reaction. The facets which developed on each face and the consequent rearrangement pattern depended upon the temperature and gas composition. Thus at 350° with 5 % oxygen, strong reflections were obtained from facets of several orientations, but at 400° with 12% % oxygen the (111) reflections predominated. 

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