Formation of metal oxide

Sihca is reduced to siUcon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous siUcon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum haUdes, siUca can be converted to silane in high yields by reaction with hydrogen (15). SiUcon itself is not hydrogenated under these conditions. The formation of siUcon by reduction of siUca with carbon is important in the technical preparation of the element and its alloys and in the preparation of siUcon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and siUcate. At 800—900°C, siUca is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce siUca to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

Appendix Thermodynamic data for the Gibbs energy of formation of metal oxides... 

Figure 8.19 F.llingham diagram for the free energy of formation of metallic oxides. (After F. D. Richardson and J. H. F. Jeffes, J. Iron Steel Inst. 160, 261 (1948).) The oxygen dissociation pressure of a given M - MO system at a given temperature is obtained by joining on the lop left hand to the appropriate point on the M-MO frec-energy line, and extrapolating to the scale on the right hand ordinate for POi (atm).

Figure 4.9 Ellingham diagram for the free energy of formation of metallic oxides.

The successes of hydrophilicity scales in correlating much data mean that one should not underestimate the importance of gs(dip). A plot of AHf (heat of formation of metal oxide, a measure of hydrophilicity) versus X(M) - X(Hg) shows two lines. Preferential orientation increases with oxygen affinity. Correlations between A Hf and X(M) - X(Hg) exist also for solvents other than water, with the rate of increase of X(M) - X(Hg) with A/f/being stronger in the sequence acetonitrile < H20 < DMSO, with increasing... 

Figure 5.22 Formation of metal oxide films (a) parallel diffusion of cations and electrons (b) counterdiffusion of cations and holes (c) counterdiffusion of anions and electrons (d) parallel diffusion of anions and holes and (e) counterdiffusion of anions and cations.

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

The most important process with regard to metal oxidation and combustion is the formation of metal oxides on the surface of the metal particles. Some metal particles become coated with an oxide layer that surrounds the unreacted metal, whereas in other cases finely divided metal oxides are formed that are expelled from the surface of the metal particle. When solid shells of metal oxides are formed, no additional supply of the oxidizer fragments (or molecules) to the metal particles is possible. The oxidation process is thus interrupted and incomplete combustion occurs. On the other hand, when the metal oxide is expelled from the surface, oxidizer fragments continue to be supplied to the underlying unreacted surface of the metal. 

Inorganics can also be synthesized and used as templates. Thus, controlled siloxane networks were formed when dispersions of alkoxysilanes (such as (MeO)3SiMe) are mixed with the suitable template matrixes. Ultrafine particles of metal oxides can be used as starting materials for the formation of metal oxide films. For instance, a mixture of a double-chained ammonium amphiphile and an aqueous solution of aluminum oxide particles (diameter about 10 100 nm) gives a multilayered aluminum oxide film when calcinated at over 300°C. 

These methods should be clearly distinguished from the formation of metal oxides by the pyrolysis of the corresponding alkoxide vapors (64-66). 

Equations (2.45) through (2.47) are typical oxidation reactions for the formation of metal oxides. The reverse reactions (steps g-a) are the reduction of metal oxides to form metal, such as is found in smelting operations. It should be apparent that the control of the oxygen partial pressure during heating is an important parameter in determining which phases will form. 

Formation of Metal Oxides Which Promote Mechanical Aspects of... 

Owing to reduced salt solubility, the formation of metal oxides and, eventually, the presence of stable solid-matter particles, these are all present in the SCWO processes. These particles can cause equipment-fouling and erosion. However the reduced solubility of salts under supercritical conditions introduces the possibility of a solid fluid separation. 

Solution phase deposition of metal oxides is less established than deposition of metal sulfides. The chemical process for the formation of metal oxides in water can be considered to be hydrolysis of metal cations and dehydration of intermediate hydroxides, for divalent cations described as... 

Many pyrot reactions take place by the formation of metallic oxides whose emissivity varies strongly with wavelength (Ref 9). As long as the emissivity is reasonably constant over the temp range investigated (ie, as long as the grey body... 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

Overvoltage associated with the evolution of oxygen at the anode is even less reproducible than the hydrogen overvoltage because of its greater variability with time and because of the formation of metal oxides on the surface of the anode. Approximate values of minimum overvoltage as measured by Caspari on oxygen evolved from a normal NaOH solution at 25 °C at different metals are contained in Table 14. 

Oxidation of metals occurs at the metal-scale interface and oxygen reduction at the scale-gas interface. This results in the formation of metal oxide scale on the surface. This is analogous to aqueous galvanic corrosion of metals. The oxide layer formed serves the... 

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