Simple Insulator Oxides

Martin, R. Dutartre, and J. A. Dalmon, React. Kinet. Catal. Lett., 1981, 

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The most important of these are perovskite structure solids with a formula A2+b4+o3 that can be typified by BaCeC>3 and BaZrCV The way in which defects play a part in H+ conductivity can be illustrated by reference to BaCeCV BaCeC>3 is an insulating oxide when prepared in air. This is converted to an oxygen-deficient phase by doping the Ce4+ sites with trivalent M3+ ions (Sections 8.2 and 8.6). The addition of the lower valence ions is balanced by a population of vacancies. A simple substitution reaction might be formulated ... 

The simple integral oxidation state compounds behave as either insulators or semiconductors. The tetracyanoplatinates of monovalent cations form anionic columnar stack structures with conductivities in the range 10-4—10-9 fi 1 cm-1 and a systematic variation of cry with 1 /electrical conductivity can be increased by as much as a factor of 104 by pressures up to 180 kbar.27... 

Oxide catalysts fall into two general categories. They are either electrical insulators or they can act as semiconductors. Insulator oxides are those in which the cationic material has a single valence so they have stoichiometric M 0 ratios. The simple oxides, MgO, AI2O3 and Si02 and the more complex zeolites, which are aluminosilicates, fall into this category. These materials are not effective as oxidation catalysts and find most use as solid acids or bases.2-3... 

A simple oxide catalyst can be used in either the bulk state or supported on an inert oxide support material. The bulk oxides are usually prepared using a precipitation-calcination sequence similar to those described in Chapter 9 for the preparation of support oxides. " In general, the simple semiconductor oxides are not very good catalysts for synthetic reactions. The insulator oxides, however, can be used as solid acids and bases for a number of reactions. Alumina has been used as an acid catalyst for the vapor phase rearrangement of cyclohexanone oxime to caprolactam (Eqn. 10.9). Modification of the y-alumina surface by the addition of 10-20% of B2O3 increased its activity for this reaction, giving caprolactam in 80% selectivity even after several hours of continuous operation. "... 

Numerous studies have been devoted to bulk oxides, and their properties, both structural and electronic, are now well apprehended. At least, this statement applies to insulating oxides in which correlation effects do not induce a breakdown of the effective one-electron picture. In the remainder of this book, we will mainly focus on these simple oxides, and we will describe how their properties are modified at surfaces, whether they are ideally clean and planar, or contain defects. With the purpose of interpreting these modifications in a unified theoretical framework, we have presented here the basis of a model which stresses the factors governing the electronic structure. These include the relative position of the anions and cations in the periodic table, which fixes the energy difference between the outer levels of the neutral atoms strength of the... 

In the chemical industry, simple aldehydes and ketones are produced in large quantities for use as solvents and as starting materials to prepare a host of other compounds. For example, more than 1.9 million tons per year of formaldehyde, H2C=0, is produced in the United States for use in building insulation materials and in the adhesive resins that bind particle hoard and plywood. Acetone, (CH.3)2C"0, is widely used as an industrial solvent approximately 1.2 million tons per year is produced in the United States. Formaldehyde is synthesized industrial ) by catalytic oxidation of methanol, and one method of acetone preparation involves oxidation of 2-propanol. 

The electronic conductivity of metal oxides varies from values typical for insulators up to those for semiconductors and metals. Simple classification of solid electronic conductors is possible in terms of the band model, i.e. according to the relative positions of the Fermi level and the conduction/valence bands (see Section 2.4.1). 

A set of simple physical and chemical principles can be used to understand and predict the surface reconstruction or faceting, which occurs on oxide surfaces.8,9 These include (1) autocompensation of stable surfaces, (2) rehybridization of the dangling bond charge density, (3) formation of an insulating surface, (4) conservation of near-neighbor bond lengths (which minimize the formation of surface strain fields), and (5) surface kinetic factors. We shall briefly discuss all of these factors governing the surface reconstruction of metal oxide. 

Fig. 2. Schematic illustrations of two back-end surface configurations after CMP. For the conventional technology, a simple oxide surface must be cleaned up, whereas for damascene structures, both outcropping metals and insulator must be processed together. The difficulty is to preserve the metal lines and the diffusion barriers.

The vast majority of metal complexes in the solid state are insulators and do not exhibit any interesting electrical conduction properties because the metal atoms are surrounded by insulating ligands which prevent the passage of carriers from one site to another. This review will be limited to a discussion of the electrical conduction properties of coordination compounds, and will not include simple inorganic compounds with high electrical conductivity such as mixed metal oxides, (3-alumina and TaSe3. 

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