Anhydrous oxides

The anhydrous oxide is obtained by ignition ol the hydrated oxide produced. 

The precipitate obtained is in fact colloidal and has no definite composition. Careful drying of the precipitate gives the anhydrous oxide, SnO, which may also be prepared by heating tin(II) ethane-dioate (oxalate) ... 

Niobium Oxides. The solubihty of oxygen in niobium obeys Henry s law to the solubiHty limit of the first oxide phase of 850—1300°C (123). The amount of oxygen in solution in niobium is 1.3 at. % at 850°C and nearly 2 at. % at 1000°C (124). Only three clearly defined anhydrous oxides of niobium have been obtained in bulk, ie, NbO, Nb02, and Nb20. Niobium monoxide, NbO, is obtained by hydrogen reduction of the pentoxide at... 

Ruthenium (IV) oxide [12036-10-1] M 133.1, d 6.97. Freed from nitrates by boiling in distilled water and filtering. A more complete purification is based on fusion in a KOH-KNO3 mix to form the soluble ruthenate and perruthenate salts. The melt is dissolved in water, and filtered, then acetone is added to reduce the ruthenates to the insoluble hydrate oxide which, after making a slurry with paper pulp, is filtered and ignited in air to form the anhydrous oxide [Campbell, Ortner and Anderson Anal Chem 33 58 1961]. 

The presence of the electrolyte is required to provide a pathway for the current and, in urban areas, this is commonly ironfll) sulfate formed as a result of attack by atmospheric SO2 but, in seaside areas, airborne particles of salt are important. Because of its electrochemical nature, rusting may continue for long periods at a more or less constant rate, in contrast to the formation of an anhydrous oxide coating which under dry conditions slows down rapidly as the coating thickens. 

Whereas a film formed in dry air consists essentially of an anhydrous oxide and may reach a thickness of 3 nm, in the presence of water (ranging from condensed films deposited from humid atmospheres to bulk aqueous phases) further thickening occurs as partial hydration increases the electron tunnelling conductivity. Other components in contaminated atmospheres may become incorporated (e.g. HjS, SO2, CO2, Cl ), as described in Sections 2.2 and3.1. 

Nickel occupies an intermediate position in the electrochemical series Ni2 /Ni = -0-227 V, so that it is more noble than Zn and Fe but less noble than Sn, Pb and Cu. Figure 4.21 shows a revised potential-pH equilibrium (Pourbaix) diagram for the Ni-H O system at 25°C. The existence of the higher anhydrous oxides Nij04, NijO, and NiOj shown in an earlier diagram appears doubtful in aqueous systems in the absence of positive identification of such species. It is seen that ... 

Inasmuch as the protonation of the oxide can be favored by the direction of the field at the O/S interface (cf. Fig. 1) at equilibrium, the proton current in this direction should decrease exponentially with increasing anodic polarization, as the field strength is decreasing and can even change sign. Conversly, the deprotonation should be favored, becoming the main mechanism of formation of the anhydrous oxide. 

In conclusion, one can say that most anodic oxide films are of a duplex, or even triplex, character, with only the inner portion being composed of a pure anhydrous oxide. In the duplex films, the outer layer contains anions and often a degree of hydration. There could exist a third thin oxide layer at the surface, again with somewhat different properties, which may have a role in the kinetics of oxide growth. 

A mixture of the anhydride and anhydrous oxide exploded violently on heating. 

Catalytic asymmetric epaxidation (13, 51-53). Complete experimental details are available for this reaction, carried out in the presence of heat-activated crushed 3A or powdered 4A molecular sieves. A further improvement, both in the rate and enantioselectivity, is use of anhydrous oxidant in isoctane rather than in CH2C12. The titanium-tartrate catalyst is not stable at 25°, and should be prepared prior to use at -20°. Either the oxidant or the substrate is then added and the mixture of three components should be allowed to stand at this temperature for 20-30 min. before addition of the fourth component. This aging period is essential for high enantioselectivity. Epoxidations with 5-10 mole % of Ti(0-/-Pr)4 and 6-12% of the tartrate generally proceed in high conversion and high enantioselectivity (90-95% ee). Some increase in the amount of catalyst can increase the enantioselectivity by 1-5%, but can complicate workup and lower the yield. Increase of Ti(0-i-Pr)4 to 50-100 mole % can even lower the enantioselectivity. 

The study of these systems may be divided on practical grounds into (1) investigations of the anhydrous oxides and silicates (taking in the first eight oxides in the list above) (2) investigations involving hydrous silicates, as well as combinations containing both carbon dioxide and water. 

Fig. 30. Spectra of three high area, amorphous, hydrous oxides of titanium compared with that of the crystalhne anhydrous oxide, rutile. One is a straight titania gel, two are coprecipitated with different elements to form mixed metal hydrous oxide gels.

FeCl3,0.3H2O > FeClj > FeOCl. ° The reaction of BBr with anhydrous FeClj or anhydrous oxides of Fe gives anhydrous FeBrj. The structure of (4-EtpyH)[FeBr ] has been reported, the anion being tetrahedral. ... 

The anhydrous oxide absorbs moisture from the air at ambient temperatures forming hydrated oxide. The oxide also absorbs carbon dioxide from air, forming neodymium carbonate. 

Palladium oxide is prepared by heating palladium sponge in oxygen at 350°C. The oxide is obtained as a black powder. The oxide also may be prepared specially for catalytic use by heating a mixture of palladium chloride and potassium nitrate at 600°C and then leaching out water-soluble residue. A hydrated form of the oxide, which is acid soluble can be prepared by precipitation from solution, for example, by hydrolysis of palladium nitrate. The brown hydrated oxide converts to black anhydrous oxide on heating. Its solu-bdity in acids decreases with lowering of water content. 

The pentahydrate Rh203 5H20 is a yellow precipitate soluble in acids partially dissolves in hot water ignites to form anhydrous oxide. 

Another form of the same oxide, in combination with water, ib very abundant as a mineral kDOWn as brown iron ore. It has a brownish color, produces a yellow streak, and, wheD crushed, a yellow powder it Is thus easily distinguished from the anhydrous oxide, which yields a red-brown powder. It is of lower specific gravity than the red, varying in density from 3-8 to 4-2. It usually occurs in a massive state, and is composed of—... 

Relatively little is known about anhydrous oxides of palladium. The only well-characterized simple oxide is PdO, which has the PtS structure with four coplanar Pd-O distances of 2.01A (4, 21). Recently, Guiot (1) presented x-ray evidence suggesting that a new palladium oxide surface compound is formed as an intermediate step when palladium metal is oxidized in air to PdO. However, the stoichiometry of this compound is unknown, since it can be obtained only as a minor constituent, with major amounts of Pd metal and PdO. Higher anhydrous oxides (e.g., Pd02) have been reported, but their existence has never been firmly established. 

The investigations of platinum pyrochlores have demonstrated the effectiveness of high pressure techniques in the synthesis of anhydrous oxides when one or both reactants have limited thermal stability. The bulk of the work reported here represents a continuation of an exploration of metal oxide-platinum oxide systems at high pressure. 

On exposure to water, an anhydrous oxide can become hydrated by physical adsorption of water molecules without dissociation, dissociative chemisorption of water leading to new hydroxy groups, and finally to the formation of superficial oxyhydroxide or hydroxide, such as for MgO [14]. When silica groups are exposed to water for an extended time, their hydroxylation produces polymeric chains of -Si(0H)2-0-Si(0H)2 0H groups which can link up to form three-dimensional silica gel networks. Around 2 nm thick silica gel layers have been observed on silica surfaces prepared by evaporation of silica on mica which were exposed to humid air [70], Thus, it may be postulated that surface groups are present not only in a two-dimensional oxide-liquid interface, but also in a bulk phase of finite thickness extending from the surface into the interior of the solid [71]. 

Amido-phosfhorous and -phosphoric acids are prepared by the action of dry ammonia on the anhydrous oxides or oxyhalides such as phos-phoryl chlorides. The imido-derivatives, in which =- NH replaces = O, can often be obtained from the amido-derivatives by heating. Other methods are the hydrolysis of amido-esters and of phosphorus chloro-nitrides (q.v.). 

The anhydrous oxides, ZnO and CdO, are formed on pyrolysis of the nitrates or carbonates, or by roasting the metals in air. Both oxides can be sublimed at very high temperatures. Both ZnO and CdO tend to have lattice defects leading to coloration. For ZnO the pure white compound turns yellow on heating (deficit of oxygen), while CdO may be brown or even black, depending on how much it has been heated. 

The best known anhydrous oxides are listed in Table 18-E-2 the tetraoxides of Ru and Os are discussed later (Section 18-F-l). The oxides, generally rather inert to aqueous acids, are reduced to the metal by hydrogen, and dissociate on heating. There are mixed metal oxides, e.g., BaRu03, and platinum and palladium bronzes of formula MlPt304 (x = 0-1). Some oxides like MnPt306 have Pt—Pt bonds. Mixed oxides are used for electrodes in H2—02 fuel cells and in the chloralkali process. 

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