Rock-Salt-Structure Catalysts

The discovery of high Tq superconductivity in La2-xMxCu04 (M = Ba, Sr) (Bednorz and Muller 1987) based on perovskite and rock-salt structures has led to an international effort in superconductivity research over the last decade. The principles that govern superconducting copper-oxide-based materials have enormous significance in the application of these oxides as potential catalysts... 

Magnesium oxide (MgO) is a good example of a catalyst with rock salt structure, that is, NaCl-type structure (Figure 2.8). 

Vibrational spectroscopies give rise to interesting information on the microscopic structure of soUd-solution mixed oxides. For example, the state of vanadium in soUd solution in Ti02 anatase catalysts [59], the partial ordering of cations in comndum-type Fe-Cr oxides [60], the real presence of Ti" in the silicalite framework of TSl catalysts [58] and the solubility of AT ions in the NiO rock-salt structure [61] have been objects of IR spectroscopic studies. 

Ionic Materials - Ionic and quasi-ionic materials are important as catalysts in their own right as well as having widespread use as supports. Unlike metals, where most surfaces have roughly the same surface energy, ionic materials tend strongly to favour the existence of certain faces. Tasker has shown that only those faces which have a net zero dipole perpendicular to the surface will be inherently stable. Thus in the rock-salt structure, (100) and (110) will be stable faces, while (111) is inherently unstable. For ZnO, the polar (0001) and (0001) faces are both inherently unstable. When a surface which is inherently unstable is found to... 

The phase diagrams for the Mo—N system shows at least three phases fee y-Mo2N, face-centered tetragonal y3-Mo2N, and hex S-MoN of the WC structure. Only the fee y-Mo2N has been applied as catalyst (13). The y-Mo2N has a B1 rock salt (NaCl) structure, in which Mo atoms form an fee arrangement and N atoms occupy half of the available interstitial octahedral sites (6). 

The ability to tailor metal oxide systems for physical and chemical applications represents an obvious advantage for improved performance. Interesting examples are cobalt and nickel oxides, which find applications in many oxidation reactions and also as promoters of Mo and W oxide catalysts for the hydrodesulfurization reactions of middle distillates. NiO and CoO can be mixed in all proportions to form homogeneous solid solutions of the type C0jcNii xO, 0 < x < 1, in a well-ordered rock salt crystal structure in the bulk material. When used in redox reactions in alkaline media [38], these solid solutions exhibit electrochemical properties. The reason why CoO and NiO form solid solutions lies in the close match of... 

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