Crystal modifications, oxide systems

The gallium oxide system is somewhat similar to that of aluminum, affording a high-temperature (a) and a low-temperature y-Ga203, each having the same structure as their aluminum counterparts. /9-Ga203 is the most stable crystalline modification (mp 1740 °C) it has a unique crystal stracture with the oxide ions in distorted ccp and Ga in distorted tetrahedral and octahedral sites, with Ga-O distances of 1.83 and 2.00 A, respectively (Figure 11). ... 

Crystalline Tellurium. — Molten tellurium solidifies to a brittle, silvery, crystalline mass, which is easily powdered. The crystalline modification can also be obtained by sublimation of the element or by its slow formation, for example in the gradual decomposition of hydrogen telluride5 or in the slow atmospheric oxidation of an aqueous solution of an alkali telluride.6 When obtained of appreciable size the crystals are generally found to be prismatic, of the trigonal system, and isomorphous with metallic selenium (a c=l 1-3298 a=86-8°).7... 

The problem of lateral modification of HTSC surface layers, and the local electrosynthesis of HTSCs on the surface of patterned substrates including the precursors is very interesting. Such processes can occur, for example, during electrooxidation of metals when the process in its initial stages takes place only on isolated microscopic regions. Thus, Josephon junctions on the surface of Bi-Sn alloys [222] and on ceramic YBCO samples [295,444] were obtained by using electrochemical oxidation without any special local techniques. But it is hard to control such oxidation processes, and sufficient reproducibility cannot be ensured for most systems. Josephson tunnel junctions based on electrochemically synthesized BKBO crystals have been described [445]. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

The modification of BEA zeolite by surface deposition of silica and impregnation with cerium oxide was studied as a tool to improve the selectivity of the reaction. The number of acid sites, particularly the strong ones, on BEA zeolite decreases with increasing amounts of silica deposited on its surface. Moreover, there is no severe pore blocking after deposition. On the contrary, cerium oxide impregnation affords a catalyst with decreased adsorption capacity because part of the cerium oxide is deposited in the channels of the zeolite crystals and blocks the porous system. In addition, cerium oxide modification creates new weak acid sites on the zeolite surface. Silica modification decreases catalytic activity but slightly increases selectivity with respect to all ortho-HAP, para-HAP and para-acetoxyacetophenone, in comparison to the unmodified BEA zeolite, and the stability of the catalyst is also improved after modification. The best reaction results are obtained over 16% cerium-oxide-modified catalyst, the selectivity with respect to the C-acetylated products being increased to about 70% while the conversion remains 60%-80%. 

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