Oxygen-polar surface, zinc oxide

Temperature-Programmed Decomposition of 2-Propanol on the Zinc-Polar, Nonpolar, and Oxygen-Polar Surfaces of Zinc Oxide... 

Zinc polar plane [0001] where zinc ions are more outwardly positioned than oxygen ions oxygen polar plane [0001], where ions are more outwardly positioned than zinc ions, and the non polar prismatic plane [1010], where both zinc and oxygen ions are on the same plane. They have reported that catalytic decomposition of propan-2-ol was highest on [0001] polar surface of zinc oxide. It is further reported that catalyst morphology and activity is influenced by zinc oxide precursors [8]. 

The most obvious advantages of the oxygen cathode are that it has low weight and infinite capacity. Consequently, prototype D-size cells based on the zinc-air system have been shown to have twice the overall practical capacity of zinc-mercuric oxide cells (and 10 times that of a standard Leclanchd cell) when subjected to a continuous current drain of 250 mA. In the larger industrial cells, energy densities of up to 200 Wh/kg and specific capacities of 150 Ah/dm3 may be obtained. On the other hand, a catalytic surface must be provided for efficient charge transfer at the oxygen cathode, and by its nature the electrode is susceptible to concentration polarization. 

Another factor that alters the galvanic position of some metals is the tendency, especially in oxidizing environments, to form specific surface films. These films shift the measured potential in the noble direction. In this state, the metal is said to be passive (see Chapter 6). Hence, chromium, although normally near zinc in the EMF Series, behaves galvanically more like silver in many air-saturated aqueous solutions because of a passive film that forms over its surface. The metal acts like an oxygen electrode instead of like chromium hence, when coupled with iron, chromium becomes the cathode and current flow accelerates the corrosion of iron. In the active state (e.g., in hydrochloric acid), the reverse polarity occurs that is, chromium becomes anodic to iron. Many metals, especially the transition metals of the periodic table, commonly exhibit passivity in aerated aqueous solutions. 

Presumably, thiol adsorption on these surfaces involves desorption of surface oxygen and hydroxide groups and the thiol seems to be mainly coordinated to the metal atoms in the oxide, as x-ray photoelectron spectroscopy (XPS) indicates, for instance, for thiol adsorption on zinc-terminated ZnO [86,87], where the thiol reduces the oxygen at the surface and forms a zinc-sulfur bond. O-polar ZnO, on the other hand, seems only to adsorb the thiols molecularly at room temperature [87]. 

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