Oxides with the Corundum Structure

Chromium ions at the surfaces of inorganic oxides are characterized by a wide variability of the oxidation state, coordination number, and local structure. Consequently, Cr-based materials are especially attractive as catalysts. Much is known about the catalytic activity of pure Cr203 for various reactions (469), including polymerization of alkenes (470-472), hydrogenation-dehydrogenation of hydrocarbons (473-481), reduction of NO and decomposition of N2O4 (482), and oxidation of organic compounds (483, 484). 

Morphology and Surface Structures Comparison of Dispersed Materials and Crystal Films 

Further sintering at high temperature increases the size of the microcrystals without significant modifications of the shape, as illustrated by SEM images (Fig. 23c). Such observations are indirect evidence suggesting that the distribution of faces is not seriously altered (77), but this interpretation appears to be inadequate because it does not take account of the accurate analysis of the crossing plane fringes and the simultaneous use of simulation methods. 

Although the experimentally determined faces of sintered microcrystals are all neutral, there are many reports indicating that oriented Cr203 (0001) thin films are preferentially formed by epitaxial growth on metal single crystals (281,494-496) well-ordered, nonpolar (1012) surfaces have received far less attention. 

The structures of the three families of faces, terminating the polyhedra of well-sintered samples, as obtained by simply cutting the crystal lattices along the (0112), (2116), and (1120) planes taking into account surface relaxation have been discussed extensively (22) and are summarized only briefly here (Fig. 24). 

---