Magnesia structure

Attempts have been made in this laboratory to stabilize the reference catalyst Li/MgO by means of hydrothermal synthesis. On the basis that adding silica to the magnesia structure could induce beneficial effects on Li content and surface area. 

Fig. 16.1. Ionic ceramics, (a) The rocksalt, or NoCl, structure, (b) Magnesia, MgO, has the rocksalt structure. It can be thought of as an f.c.c. packing with Mg ions in the octahedral holes. ( ) Cubic zirconia ZrOj an f.c.c. packing of Zr with O in the tetrahedral holes, (d) Alumina, AljOj a c.p.h. packing of oxygen with Al in two-thirds of the octahedral holes.

Magnesia, MgO, is an example (Fig. 16.1b). It is an engineering ceramic, used as a refractory in furnaces, and its structure is exactly the same as that of rocksalt the atoms pack to maximise the density, with the constraint that like ions are not nearest neighbours. 

In the mid-1950s, alumina-silica catalysts, containing 25 percent alumina, came into use because of their higher stability. These synthetic catalysts were amorphous their structure consisted of a random array of silica and alumina, tetrahedrally connected. Some minor improvements in yields and selectivity were achieved by switching to catalysts such as magnesia-silica and alumina-zirconia-silica. 

Magnesia forms solid solutions with NiO. Both MgO and NiO have face-centered cubic lattices with NaCl-type structures. The similarity between the ionic radii of the metals (Ni2+ = 0.69 A, Mg2+ = 0.65 A) allows interchangeability in a crystal lattice, and thus the formation of solid solutions with any proportion of the two oxides is possible. Such solid solutions are more difficult to reduce than NiO alone. Thus Takemura et al. (I) demonstrated that NiO reduced completely at 230°-400°C (446°-752°F) whereas a 10% NiO-90% MgO solid solu-... 

Silica-magnesia matrices have not yet been properly evaluated as an RCC catalyst matrix. However, such a matrix in conjunction with stabilized zeolite might provide an attractive matrix with a Kaolin-enhanced dual pore structure. Silica-magnesia matrices are notorious for their poor regeneration characteristics. When prepared by the dual pore Kaolin-enhanced method, they might be easier to regenerate and, thereby, open up a new family of residuum catalysts. Such catalysts have not yet been explored. 

Reaction of 118 with ethylene under a pressure of 60 bar gives a new magnesia-cyclic product (121) of which the structure in the solid state was established by X-ray... 

Mg-PSZ represents magnesia, MgO, partially stabilized zirconia, Zr02, ceramics. These ceramics are strong and tough structural materials. The main characteristics of Mg-PSZ ceramics are... 

The substrates used in this study were (0001) sapphire (A1203), (001) magnesia (MgO), and amorphous fused silica (Si02). All substrates were obtained with surface finish to 0.025 / m, and were cleaned with isopropylalcohol (IPA) prior to PA A or APS application. The surfaces and interfaces after peel test were characterized using X-ray photoelectron spectroscopy (XPS). The PMDA-ODA PAA was cast from NMP solution. Figure l shows the structure of the PAA and the thermally imidized PMDA-ODA polyimide. 

Magnesia (MgO), used as an insulator for electrical heating devices, has a face-centered cubic structure like that of NaCl. Draw one unit cell of the structure of MgO, and explain why MgO is more brittle than magnesium metal. 

The Bessemer process is relatively less expensive but does not produce a product of high quality. It is not possible to exercise control over the composition of the product because the conversion occurs so quickly. Furthermore, this process does not effect the removal of phosphorus. The phosphorus pentoxide that is formed during the blow is reduced to phosphorus upon addition of carbon and hence remains as an impurity in the final product. Provision for the removal of phosphorus may be made by the use of the so-called basic Bessemer process, which employs a converter lined with magnesia (MgO), but this practice entails other disadvantages. In the United States, the acid Bessemer process is used exclusively and accounts for about 15% of the steel produced in this country. Steel so produced is used largely as structural steel, as reinforcement for concrete, and in the tinplate industries. 

Recently, Pt clusters supported on hydrotaleite-derived magnesia, Mg(Al)0, were shown to catalyze the aromatization of n-hexane as effectively as a Pt/KL catalyst (8). A high surface area basic oxide was chosen to support the metal clusters in order to investigate the influence of carrier structure on aromatization. In that study, significant quantities of cracked products were produced by both the zeolite and non-zeolite catalysts. Since the zeolite catalyst was prepared in a fashion that resulted in residual acidity being present, observation of cracked products is not surprising. However, cracking reactions over Pt/Mg(Al)0 are not as easily explained. 

Very small gold particles can also be formed on magnesia and brucite (Mg(OH)2).55,83 Au/MgO made by deposition-precipitation contained particles smaller than 1 nm that were claimed to show icosahedral and fee cubo-octahedral structures,170 but this is hard to believe as the diameter of a single gold atom is already 0.29 nm, and lnm particles afflxed at the steps... 

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